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3.9: Energy in Chemical Reactions - Biology

3.9: Energy in Chemical Reactions - Biology


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Slow Burn

These old iron chains give off a small amount of heat as they rust. It occurs when iron and oxygen go through a chemical reaction similar to burning, or combustion. The chemical reaction that occurs when something burns obviously gives off energy. You can feel the heat, and you may be able to see the light of flames. The rusting of iron is a much slower process, but it still gives off energy. It's just that it releases energy so slowly you can't detect a change in temperature.

What Is a Chemical Reaction?

A chemical reaction is a process that changes some chemical substances into others. A substance that starts a chemical reaction is called a reactant, and a substance that forms as a result of a chemical reaction is called a product. During the reaction, the reactants are used up to create the products.

Another example of a chemical reaction is the burning of methane gas, shown in Figure (PageIndex{2}). In this chemical reaction, the reactants are methane (CH4) and oxygen (O2), and the products are carbon dioxide (CO2) and water (H2O). As this example shows, a chemical reaction involves the breaking and forming of chemical bonds. Chemical bonds are forces that hold together the atoms of a molecule. Bonds occur when atoms share electrons. When methane burns, for example, bonds break within the methane and oxygen molecules, and new bonds form in the molecules of carbon dioxide and water.

Chemical Equations

Chemical reactions can be represented by chemical equations. A chemical equation is a symbolic way of showing what happens during a chemical reaction. For example, the burning of methane can be represented by the chemical equation:

[ce{CH_4 + 2O_2 ightarrow CO_2 + 2 H_2O}]

The arrow in a chemical equation separates the reactants from the products and shows the direction in which the reaction proceeds. If the reaction could occur in the opposite direction as well, two arrows pointing in opposite directions would be used. The number 2 in front of O2 and H2O shows that two oxygen molecules and two water molecules are involved in the reaction. If just one molecule is involved, no number is placed in front of the chemical symbol.

Role of Energy in Chemical Reactions

Matter rusting or burning are common examples of chemical changes. Chemical changes involve chemical reactions, in which some substances, called reactants, change at the molecular level to form new substances, called products. All chemical reactions involve energy. However, not all chemical reactions release energy, as rusting and burning do. In some chemical reactions, energy is absorbed rather than released.

Exergonic Reactions

A chemical reaction that releases energy is called an exergonic reaction. This type of reaction can be represented by a general chemical equation:

[mathrm{Reactants ightarrow Products + Energy}]

Besides rusting and burning, examples of exothermic reactions include chlorine combining with sodium to form table salt. The decomposition of organic matter also releases energy because of exergonic reactions. Sometimes on a chilly morning, you can see steam rising from a compost pile because of these chemical reactions (see Figure (PageIndex{3})). Exergonic chemical reactions also take place in the cells of living things. In a chemical process similar to combustion, called cellular respiration, the sugar glucose is "burned" to provide cells with energy.

Endergonic Reactions

A chemical reaction that absorbs energy is called an endergonic reaction. This type of reaction can also be represented by a general chemical equation:

[mathrm{Reactants + Energy ightarrow Products}]

Did you ever use a chemical cold pack like the one in the picture below? The pack cools down because of an endergonic reaction. When a tube inside the pack is broken, it releases a chemical that reacts with water inside the pack. This reaction absorbs heat energy and quickly cools down the contents of the pack.

Many other chemical processes involve endergonic reactions. For example, most cooking and baking involves the use of energy to produce chemical reactions. You can't bake a cake or cook an egg without adding heat energy. Arguably, the most important endergonic reactions occur during photosynthesis. When plants produce sugar by photosynthesis, they take in light energy to power the necessary endergonic reactions. The sugar they produce provides plants and virtually all other living things with glucose for cellular respiration.

Activation Energy

All chemical reactions need energy to get started. Even reactions that release energy need a boost of energy in order to begin. The energy needed to start a chemical reaction is called activation energy. Activation energy is like the push a child needs to start going down a playground slide. The push gives the child enough energy to start moving, but once she starts, she keeps moving without being pushed again. Activation energy is illustrated in Figure (PageIndex{5}).

Why do all chemical reactions need energy to get started? In order for reactions to begin, reactant molecules must bump into each other, so they must be moving, and movement requires energy. When reactant molecules bump together, they may repel each other because of intermolecular forces pushing them apart. Overcoming these forces so the molecules can come together and react also takes energy.

Review

  1. What is a chemical reaction?
  2. Identify reactants and products in a chemical reaction.
  3. List three examples of common changes that involve chemical reactions.
  4. Define a chemical bond.
  5. What is a chemical equation? Give an example.
  6. Our cells use glucose (C6H12O6) to obtain energy in a chemical reaction called cellular respiration. In this reaction, six oxygen molecules (O2) react with one glucose molecule. Answer the following questions about this reaction.
    1. How many oxygen atoms are in one molecule of glucose?
    2. Write out what the reactant side of this equation would look like.
    3. How many oxygen atoms are in the reactants in total? Explain how you calculated your answer.
    4. How many oxygen atoms are in the products in total? Is it possible to answer this question without knowing what the products are? Why or why not?
  7. Answer the following questions about the equation you saw above: CH4+ 2O2 → CO2 + 2H2O
    1. Can carbon dioxide (CO2) become transformed into methane (CH4) and oxygen (O2) in this reaction? Why or why not?
    2. How many molecules of carbon dioxide (CO2) are produced in this reaction?
  8. Is the evaporation of liquid water into water vapor a chemical reaction? Why or why not
  9. Why do bonds break in the reactants during a chemical reaction?
  10. Contrast endergonic and exergonic chemical reactions. Give an example of each.
  11. Define activation energy.
  12. Explain why all chemical reactions require activation energy.
  13. Heat is a form of ____________ .
  14. In which type of reaction is heat added to the reactants?
  15. In which type of reaction is heat produced?
  16. If there was no heat energy added to an endothermic reaction, would that reaction occur? Why or why not?
  17. If there was no heat energy added to an exothermic reaction, would that reaction occur? Why or why not?
  18. Explain why a chemical cold pack feels cold when activated.
  19. Explain why cellular respiration and photosynthesis are “opposites” of each other.
  20. Explain how the sun indirectly gives our cells energy.

Explore More

Watch the video below to learn more about activation energy.


What reaction must occur to release the energy in atp

1) The answer is: Energy must have been absorbed from the surrounding environment.

There are two types of reaction:

1) Endothermic reaction (ΔH > 0, chemical reaction that absorbs more energy than it releases). In endothermic reactions heat is reactant.

Because products have higher energy, this example is endothermic reaction.

2) Exothermic reaction (chemical reaction that releases more energy than it absorbs).

2) The answer is: A reversible reaction contains a forward reaction, which occurs when reactants form products, and a reverse reaction, which occurs when products form reactants.

For example, balanced reversible chemical reaction: N₂ + 3H₂ ⇄ 2NH₃.

Nitrogen and hydrogen are reactants and ammonia is product of reaction. Reaction goes in both direction. Ammonia is synthesized from nitrogen and hydrogen and ammonia decomposes on nitrogen and hydrogen.

The amount of substance of reactants and products of reaction do not change when chemical reaction is in chemical equilibrium.

In a chemical reaction, chemical equilibrium is the state in which both reactants and products are present in concentrations which not change with time. Speed of direct and irreversible chemical reaction are equal

3) The answer is: The reaction rate of the forward reaction would increase in order to decrease the number of particles.

According to Le Chatelier's Principle the position of equilibrium moves to counteract the change.

The equilibrium shift to the right, so more product (ammonia) will be produced.

There are less molecules of ammonia than molecules of hydrogen and nitrogen. For every two molecules of ammonia, there are four molecules of hydrogen and nitrogen.

4) The answer is: The increase in energy will cause the reactants' particles to move faster, which will increase their temperature and lead to a faster reaction rate.

The reaction rate is the speed at which reactants are converted into products.

The collision theory states that a certain fraction of the collisions (successful collisions) cause significant chemical change.

The successful collisions must have enough energy (activation energy).

Chemical bonds are broken and new bonds are formed.

Particles are in constant, random motion and possess kinetic energy, molecules faster and have more collisions.

5) The answer is: When a reaction is at chemical equilibrium, a change in the system will cause the system to shift in the direction that will balance the change and help the reaction regain chemical equilibrium.

For example, chemical reaction: heat + NH₄⁺ + OH⁻ ⇄ NH₃ + H₂O.

According to Le Chatelier's Principle, the position of equilibrium moves to counteract the change, because heat is increased, system consume that heat, so equilibrium is shifted to right, by decreasing concentration of reactants and increasing concentration of product.

A. Decreasing the temperature decreases the kinetic energy of the reactants, and the reaction goes more slowly.

C. Reactants must collide, with proper orientation, with energy greater than or equal to the activation energy for a reaction to occur.

D. Increasing the amount of reactants increases the number of collisions, and the reaction goes faster.

F. The energy of a collision between atoms or molecules must be greater than or equal to the activation energy (Ea) for bonds to be broken.

Decreasing the temperature decreases the kinetic energy of the reactants, so fewer molecules have enough kinetic energy to get over the Eₐ barrier, and the reaction goes more slowly.

Reactants must collide, with proper orientation, with energy greater than or equal to Eₐ for a reaction to occur.

Increasing the concentration of reactants increases the frequency of collisions, so the number of successful collisions increases.

The energy of a collision between atoms or molecules must be greater than or equal to Eₐ for bonds to be broken.

B is wrong. If Eₐ is low, more of the molecules can get over the energy barrier, and the reaction rate is fast.

E is wrong. If the energy of the products is higher than the energy of the reactants, the products must have gained energy. The reaction is endothermic.

G is wrong. Eₐ is the energy difference between the energy of the transition state and that of the reactants.


This team of ants is breaking down a dead tree. A classic example of teamwork. And all that work takes energy. In fact, each chemical reaction - the chemical reactions that allow the cells in those ants to do the work - needs energy to get started. And all that energy comes from the food the ants eat. Whatever eats the ants gets their energy from the ants. Energy passes through an ecosystem in one direction only.

Chemical reactions always involve energy. Energy is a property of matter that is defined as the ability to do work. When methane burns, for example, it releases energy in the form of heat and light. Other chemical reactions absorb energy rather than release it.

Exothermic Reactions

A chemical reaction that releases energy (as heat) is called an exothermic reaction. This type of reaction can be represented by a general chemical equation:

Reactants &rarr Products + Heat

In addition to methane burning, another example of an exothermic reaction is chlorine combining with sodium to form table salt. This reaction also releases energy.

Endothermic Reaction

A chemical reaction that absorbs energy is called an endothermic reaction. This type of reaction can also be represented by a general chemical equation:

Reactants + Heat &rarr Products

Did you ever use a chemical cold pack? The pack cools down because of an endothermic reaction. When a tube inside the pack is broken, it releases a chemical that reacts with water inside the pack. This reaction absorbs heat energy and quickly cools down the pack.

Activation Energy

All chemical reactions need energy to get started. Even reactions that release energy need a boost of energy in order to begin. The energy needed to start a chemical reaction is called activation energy. Activation energy is like the push a child needs to start going down a playground slide. The push gives the child enough energy to start moving, but once she starts, she keeps moving without being pushed again. Activation energy is illustrated in Figure below.

Activation Energy. Activation energy provides the &ldquopush&rdquo needed to start a chemical reaction. Is the chemical reaction in this figure an exothermic or endothermic reaction?

Why do all chemical reactions need energy to get started? In order for reactions to begin, reactant molecules must bump into each other, so they must be moving, and movement requires energy. When reactant molecules bump together, they may repel each other because of intermolecular forces pushing them apart. Overcoming these forces so the molecules can come together and react also takes energy.


What is the Relation of Energy in Chemical Reactions?

All chemical reactions involve the exchange of energy. You will recall that compounds are made up of atoms which are held together by energy forces called chemical bonds. The amount of residual energy in compounds is variable, of course, but we must conceive of a compound as a reservoir of energy.

You will also recall that the second law of thermodynamics implies that a closed system is a losing proposition, from an energy viewpoint. In other words, there is a decrease in total energy content and organization, or we sometimes say, that there is an increase in randomness.

Since chemical reactions involve the exchange of energy, a closed chemical reaction displays this tendency. We might say that it exemplified the second law. In any given chemical reaction, the state of the participating compounds which represents the lowest enthalpy and the highest entropy is the stables form of that reaction. In a closed system the reaction gets it will proceed in the direction of greatest stability.

Thus, chemical reactions may be predicted on the basis of energy relationships. In actual practice, reactions are characterized by the changes in H and S.

These changes in total energy and in organization are symbolized by Hand S, respectively. More precisely, chemical reactions are usually characterised in relation to energy changes by considering the amount of free energy which results from the reaction.

By free energy, we mean energy available for doing useful work, and we symbolize it as F. The change in free energy resulting from a chemical reaction would then be symbolized as delta F. This amount of free energy can be determined by considering the delta H and delta S that were introduced above.

For example, consider the following generalized reaction: A + B -» C + D. If //represented by A and B on the left side of the equation is greater than H represented by C and D on the right side, then the reaction will proceed readily from left to right, inasmuch as C and D represent a more stable condition.

In this case, energy will be lost to the environment so that the total energy change is negative, that is, it is lost from the system, in consideration of the energetic involved, the reaction may be written as: A + B -» C+ D -» AH If the converse were true, then energy would have to supplied from some external source and the reaction would be A + B-»C+D – AH

Now consider S and AS. If delta His zero in the reaction immediately above, which means that there is no change in total energy in the reaction, and if energy is equally distributed between C and D on the right but unequally distributed between A and B on the left, there is a difference in S.

The left side of the equation would thus represent more organization, or less entropy, and the right side would represent a higher S. Left to itself, the reaction would tend to go from left to right, since a high S represents greater stability.

In this situation, of course, energy would have to be supplied in order for the reaction to occur, since reactions tend toward randomness if they are completely closed. To view the matter from a different standpoint, entropy is a function of temperature.

To illustrate, let us consider the physical states of water, that is, its existence as a solid, a liquid, or a gas. Steam is the least organized state of the three, whereas ice represents the greatest state of organization. Since the physical states of water are temperature-dependent, then temperature must be considered in determining changes in entropy from one state to another. The mathematical expression used is a product of temperature and entropy change, or Tdelta S.

Stability changes in a chemical reaction are also dependent upon Hand TS, which are variables. As enthalpy decreases, stability increases. However, the very reverse is true of entropy. As entropy decreases, stability decreases. In other words, changes in stability are inversely proportional to changes in enthalpy and directly proportional to changes in entropy.

It should be obvious from the discussion presented above that reaction exhibiting a negative free energy change result in a more stable condition, and those reactions exhibiting a positive free energy change result in a less stable condition.

Now let us consider the following generalized reaction: A + B-> C+F

This means that in the reaction free energy is lost from the system to the environment. Consequently, the reaction will proceed in the direction of C, which represents the greatest stability with reference to this system. Such a reaction is called an exergonic reaction.

Now consider this reaction: A + B-> C+ F

In order for this reaction to take place, energy will have to be supplied from the environment to the system. In this reaction C represents a less stable condition than A + B. Such a reaction is termed an energetic reaction.

It is generally true that in a living system decomposition reaction are exergonic, characterized by -AF, while synthetic reactions are endergonic, characterized by + AF. In a living system, metabolism is so ordered that the exergonic reactions are coupled to the endergonic reactions, thus supplying the necessary energy for uphill synthetic processes.

Perhaps this explanation of energy relationship in chemical reactions seems unduly complex, and indeed, it is not an easy subject. We shall find these concepts extremely useful, however, in understanding many of the life processes.


7.3 Phase Changes

Learning Objective

Depending on the surrounding conditions, normal matter usually exists as one of three phases: solid, liquid, or gas.

A phase change A physical process in which a substance goes from one phase to another. is a physical process in which a substance goes from one phase to another. Usually the change occurs when adding or removing heat at a particular temperature, known as the melting point or the boiling point of the substance. The melting point The temperature at which a substance goes from a solid to a liquid (or from a liquid to a solid). is the temperature at which the substance goes from a solid to a liquid (or from a liquid to a solid). The boiling point The temperature at which a substance goes from a liquid to a gas (or from a gas to a liquid). is the temperature at which a substance goes from a liquid to a gas (or from a gas to a liquid). The nature of the phase change depends on the direction of the heat transfer. Heat going into a substance changes it from a solid to a liquid or a liquid to a gas. Removing heat from a substance changes a gas to a liquid or a liquid to a solid.

Two key points are worth emphasizing. First, at a substance’s melting point or boiling point, two phases can exist simultaneously. Take water (H2O) as an example. On the Celsius scale, H2O has a melting point of 0°C and a boiling point of 100°C. At 0°C, both the solid and liquid phases of H2O can coexist. However, if heat is added, some of the solid H2O will melt and turn into liquid H2O. If heat is removed, the opposite happens: some of the liquid H2O turns into solid H2O. A similar process can occur at 100°C: adding heat increases the amount of gaseous H2O, while removing heat increases the amount of liquid H2O (Figure 7.2 "The Boiling Point of Water").

Water is a good substance to use as an example because many people are already familiar with it. Other substances have melting points and boiling points as well.

Second, the temperature of a substance does not change as the substance goes from one phase to another. In other words, phase changes are isothermal A process that occurs at constant temperature. (isothermal means “constant temperature”). Again, consider H2O as an example. Solid water (ice) can exist at 0°C. If heat is added to ice at 0°C, some of the solid changes phase to make liquid, which is also at 0°C. Remember, the solid and liquid phases of H2O can coexist at 0°C. Only after all of the solid has melted into liquid does the addition of heat change the temperature of the substance.

For each phase change of a substance, there is a characteristic quantity of heat needed to perform the phase change per gram (or per mole) of material. The heat of fusion The amount of heat per gram or per mole required for a phase change that occurs at the melting point. (ΔHfus) is the amount of heat per gram (or per mole) required for a phase change that occurs at the melting point. The heat of vaporization The amount of heat per gram or per mole required for a phase change that occurs at the boiling point. (ΔHvap) is the amount of heat per gram (or per mole) required for a phase change that occurs at the boiling point. If you know the total number of grams or moles of material, you can use the ΔHfus or the ΔHvap to determine the total heat being transferred for melting or solidification using these expressions:

heat = n × ΔHfus (where n is the number of moles) or heat = m × ΔHfus (where m is the mass in grams)

For the boiling or condensation, use these expressions:

heat = n × ΔHvap (where n is the number of moles) or heat = m × ΔHvap (where m is the mass in grams)

Remember that a phase change depends on the direction of the heat transfer. If heat transfers in, solids become liquids, and liquids become solids at the melting and boiling points, respectively. If heat transfers out, liquids solidify, and gases condense into liquids.

Example 4

How much heat is necessary to melt 55.8 g of ice (solid H2O) at 0°C? The heat of fusion of H2O is 79.9 cal/g.

We can use the relationship between heat and the heat of fusion to determine how many joules of heat are needed to melt this ice:

heat = m × ΔHfus heat = (55 .8 g ) ( 79 .9 cal g ) = 4,460 cal

Skill-Building Exercise

How much heat is necessary to vaporize 685 g of H2O at 100°C? The heat of vaporization of H2O is 540 cal/g.

Table 7.4 "Heats of Fusion and Vaporization for Selected Substances" lists the heats of fusion and vaporization for some common substances. Note the units on these quantities when you use these values in problem solving, make sure that the other variables in your calculation are expressed in units consistent with the units in the specific heats or the heats of fusion and vaporization.

Table 7.4 Heats of Fusion and Vaporization for Selected Substances

Substance ΔHfus (cal/g) ΔHvap (cal/g)
aluminum (Al) 94.0 2,602
gold (Au) 15.3 409
iron (Fe) 63.2 1,504
water (H2O) 79.9 540
sodium chloride (NaCl) 123.5 691
ethanol (C2H5OH) 45.2 200.3
benzene (C6H6) 30.4 94.1

Looking Closer: Sublimation

There is also a phase change where a solid goes directly to a gas:

This phase change is called sublimation. Each substance has a characteristic heat of sublimation associated with this process. For example, the heat of sublimation (ΔHsub) of H2O is 620 cal/g.

We encounter sublimation in several ways. You may already be familiar with dry ice, which is simply solid carbon dioxide (CO2). At −78.5°C (−109°F), solid carbon dioxide sublimes, changing directly from the solid phase to the gas phase:

Solid carbon dioxide is called dry ice because it does not pass through the liquid phase. Instead, it does directly to the gas phase. (Carbon dioxide can exist as liquid but only under high pressure.) Dry ice has many practical uses, including the long-term preservation of medical samples.

Even at temperatures below 0°C, solid H2O will slowly sublime. For example, a thin layer of snow or frost on the ground may slowly disappear as the solid H2O sublimes, even though the outside temperature may be below the freezing point of water. Similarly, ice cubes in a freezer may get smaller over time. Although frozen, the solid water slowly sublimes, redepositing on the colder cooling elements of the freezer, which necessitates periodic defrosting. (Frost-free freezers minimize this redeposition.) Lowering the temperature in a freezer will reduce the need to defrost as often.

Under similar circumstances, water will also sublime from frozen foods (e.g., meats or vegetables), giving them an unattractive, mottled appearance called freezer burn. It is not really a “burn,” and the food has not necessarily gone bad, although it looks unappetizing. Freezer burn can be minimized by lowering a freezer’s temperature and by wrapping foods tightly so water does not have any space to sublime into.

Concept Review Exercises

Explain what happens when heat flows into or out of a substance at its melting point or boiling point.

How does the amount of heat required for a phase change relate to the mass of the substance?

Answers

The energy goes into changing the phase, not the temperature.

The amount of heat is a constant per gram of substance.

Key Takeaway

Exercises

How much energy is needed to melt 43.8 g of Au at its melting point of 1,064°C?

How much energy is given off when 563.8 g of NaCl solidifies at its freezing point of 801°C?

What mass of ice can be melted by 558 cal of energy?

How much ethanol (C2H5OH) in grams can freeze at its freezing point if 1,225 cal of heat are removed?

What is the heat of vaporization of a substance if 10,776 cal are required to vaporize 5.05 g? Express your final answer in joules per gram.

If 1,650 cal of heat are required to vaporize a sample that has a heat of vaporization of 137 cal/g, what is the mass of the sample?

What is the heat of fusion of water in calories per mole?

What is the heat of vaporization of benzene (C6H6) in calories per mole?

What is the heat of vaporization of gold in calories per mole?

What is the heat of fusion of iron in calories per mole?

Answers


Light energy is trapped by phototrophs during photosynthesis, in which it is absorbed by bacteriochlorophyll and other pigments and converted to chemical energy for cellular work. The energy is required by the bacterium for synthesis of cell wall or membrane, synthesis of enzymes, cellular components, repair
mechanism, growth and reproduction.

Some change of energy occurs whenever bonds between atoms are formed or broken during chemical reactions. When a chemical bond is formed, energy is required. Such a chemical reaction which requires energy is called an endergonic reaction (energy is directed inward). When a bond is broken, energy is released. A chemical reaction that release energy is an exergonic reaction (energy is directed outward).

During chemical reaction energy is either released or absorbed and the quantum of energy liberated or taken up is useful energy and is referred to Free Energy Change (ΔG) of the reactions.

High Energy Phosphate

Adenosine Tri-Phosphate (ATP) is the principal energy carrying molecule of all cells and is indispensable to the life of the cell. It stores the energy released by some chemical reactions, and it provides the energy for reactions that require energy. ATP consists of an adenosine unit composed of adenine, ribose with three phosphate groups. In ATP and some other phosphorylated compounds, the outer two phosphate groups are joined by an anhydride bond.

Some of the other high energy nucleotides involved in biochemical processes are given in Table 4.1.

Table 4.1: High energy nucleotides involved in biosynthesis

Name of the Nucleotide

Biosynthesis

Nutrients are broken from highly reduced compounds to highly oxidized compounds within the cells. Much of the energy released during oxidation reduction reactions is trapped within the cell by the formation of ATP. A phosphate group is added to ADP with the input of energy to form ATP.

ATP + H2O → ADP + pi(ΔG° = – 7.3 K cal/mol)
ATP + H2O → AMP + ppi(ΔG° = – 10.9 K cal/mol)

ATP is ideally suited for its role as an energy currency. It is formed in energy trapping and energy generating processes such as photosynthesis, fermentation, and aerobic respiration. In bacterial and archeal cells, most of the ATP is formed on the cell membrane, while in eukaryotes the reactions occur primarily in the
mitochondria (Figure 4.2).

Oxidation – Reduction Reactions

Oxidation is the removal of electrons (e – ) from an atom or molecule and is often an energy producing reaction. Reduction of a substrate refers to its gain or addition of one or more electrons to an atom or molecule. Oxidations and reduction are always coupled. In other words, each time one substance is oxidized, another is simultaneously reduced.
F2 + 2e – → 2F –
H2 + 2e – → 2H + + 2e –
NAD + + 2H + + 2e – ⇄ NADH + H + .


Enthalpy Change

Enthalpy (H) - the total amount of thermal energy in a substance.

Enthalpy change (ΔH) - the energy released to or absorbed from the surroundings during a chemical or physical change.

As long as pressure remains constant, the enthalpy change of the chemical system is equal to the flow of thermal energy in and out of the system.

ΔH system = | qsystem |

For a chemical reaction, the enthalpy change, ΔH, is given by the equation -

ΔH = Hproducts - Hreactants

When the products of a reaction have a greater enthalpy than the reactants, ΔH will be positive. The system absorbs thermal energy from its surroundings and the reaction is endothermic.

On the other hand, if the enthalpy of the products is less than that of the reactants, ΔH will be negative. In this case, the system releases thermal energy to its surroundings and the reaction is exothermic.

Molar Enthalpy Change

Molar enthalpy change (ΔHr) - the enthalpy change associated with a physical, chemical, or nuclear change involving 1 mol of a substance SI units – J/mol.

To write the balanced equation for the molar enthalpy change of formation of a product, the coefficient of that product must always be 1. Other substances in the equation may have fractional coefficients as a result.

The quantity of energy involved in a change (the enthalpy change, ∆H, expressed in kJ) depends on the quantity of matter that undergoes the change.

To calculate an enthalpy change, ∆H, for some amount of substance other than 1 mol, you need to obtain the molar enthalpy value, ∆Hr, from a reference source, and then use the formula

where n is the amount and ∆Hr is the molar enthalpy change of the reaction.

Representing Molar Enthalpy Changes

Potential energy diagram - a graphical representation of the energy transferred during a physical or a chemical change.

( a ) The condensation reaction of 1 mol of water vapour is exothermic. The reactant has a higher potential energy than the product.

( b ) The vaporization reaction of liquid water to water vapour is endothermic. The reactant has a lower potential energy than the product.


EQ: What is a chemical reaction?

We did a reading/vocabulary activity to review the difference between chemcial and physical changes. Click Here to do that activity.

Next, we played Kahoot to practice recognizing the difference between physical and chemical changes. Click here to review that Kahoot.

Next, we took notes on chemical reactions. The power point for that is Energy &amp Metabolism16.

We then practiced balancing chemical equations. Blocks 1 and 4 had to finish the worksheet (see me if you need a copy.)

Exit Ticket at the end of class:

  • What are the reactants and products in this equation? Is it balanced? Explain how you know?
  • CH4 + O2 –> CO2 + H2O

Potential and Kinetic Energy

When an object is in motion, there is energy associated with that object. Think of a wrecking ball. Even a slow-moving wrecking ball can do a great deal of damage to other objects. Energy associated with objects in motion is called kinetic energy (Figure 5). A speeding bullet, a walking person, and the rapid movement of molecules in the air (which produces heat) all have kinetic energy.

Now what if that same motionless wrecking ball is lifted two stories above ground with a crane? If the suspended wrecking ball is unmoving, is there energy associated with it? The answer is yes. The energy that was required to lift the wrecking ball did not disappear, but is now stored in the wrecking ball by virtue of its position and the force of gravity acting on it. This type of energy is called potential energy (Figure 5). If the ball were to fall, the potential energy would be transformed into kinetic energy until all of the potential energy was exhausted when the ball rested on the ground. Wrecking balls also swing like a pendulum through the swing, there is a constant change of potential energy (highest at the top of the swing) to kinetic energy (highest at the bottom of the swing). Other examples of potential energy include the energy of water held behind a dam or a person about to skydive out of an airplane.

Figure 5 Still water has potential energy moving water, such as in a waterfall or a rapidly flowing river, has kinetic energy. (credit “dam”: modification of work by “Pascal”/Flickr credit “waterfall”: modification of work by Frank Gualtieri)

Potential energy is not only associated with the location of matter, but also with the structure of matter. A spring on the ground has potential energy if it is compressed so does a rubber band that is pulled taut. On a molecular level, the bonds that hold the atoms of molecules together exist in a particular structure that has potential energy. Remember that anabolic cellular pathways require energy to synthesize complex molecules from simpler ones and catabolic pathways release energy when complex molecules are broken down. The fact that energy can be released by the breakdown of certain chemical bonds implies that those bonds have potential energy. In fact, there is potential energy stored within the bonds of all the food molecules we eat, which is eventually harnessed for use. This is because these bonds can release energy when broken. The type of potential energy that exists within chemical bonds, and is released when those bonds are broken, is called chemical energy. Chemical energy is responsible for providing living cells with energy from food. The release of energy occurs when the molecular bonds within food molecules are broken.


Bioprocess Engineering

Bioprocess Engineering: Kinetics, Sustainability, and Reactor Design, Second Edition, provides a comprehensive resource on bioprocess kinetics, bioprocess systems, sustainability, and reaction engineering. Author Dr. Shijie Liu reviews the relevant fundamentals of chemical kinetics, batch and continuous reactors, biochemistry, microbiology, molecular biology, reaction engineering, and bioprocess systems engineering, also introducing key principles that enable bioprocess engineers to engage in analysis, optimization, and design with consistent control over biological and chemical transformations.

The quantitative treatment of bioprocesses is the central theme in this book, with more advanced techniques and applications being covered in depth. This updated edition reflects advances that are transforming the field, ranging from genetic sequencing, to new techniques for producing proteins from recombinant DNA, and from green chemistry, to process stability and sustainability.

The book introduces techniques with broad applications, including the conversion of renewable biomass, the production of chemicals, materials, pharmaceuticals, biologics, and commodities, medical applications, such as tissue engineering and gene therapy, and solving critical environmental problems.

Bioprocess Engineering: Kinetics, Sustainability, and Reactor Design, Second Edition, provides a comprehensive resource on bioprocess kinetics, bioprocess systems, sustainability, and reaction engineering. Author Dr. Shijie Liu reviews the relevant fundamentals of chemical kinetics, batch and continuous reactors, biochemistry, microbiology, molecular biology, reaction engineering, and bioprocess systems engineering, also introducing key principles that enable bioprocess engineers to engage in analysis, optimization, and design with consistent control over biological and chemical transformations.

The quantitative treatment of bioprocesses is the central theme in this book, with more advanced techniques and applications being covered in depth. This updated edition reflects advances that are transforming the field, ranging from genetic sequencing, to new techniques for producing proteins from recombinant DNA, and from green chemistry, to process stability and sustainability.

The book introduces techniques with broad applications, including the conversion of renewable biomass, the production of chemicals, materials, pharmaceuticals, biologics, and commodities, medical applications, such as tissue engineering and gene therapy, and solving critical environmental problems.



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