Answers to review questions – Chapter 03

1.State the first and second laws of thermodynamics. (p. 57)

First law—Energy can be neither created nor destroyed but may be changed from one form to another.

Second law—The entropy of the universe must always increase.

2.(a) What does ‘equilibrium’ mean in a chemical reaction? (pp. 57–59)

Equilibrium is a state where the forward and reverse reactions are occurring at the same rate.

(b) Why is it that the most useful reactions for providing energy are those with equilibrium constants far higher or lower than 1.0? (pp. 57–59)

The equilibrium constant (Keq) is calculated from the sum Keq = [products]/[reactants].

For the general reaction

aA+bBcC+dD

the equilibrium expression is:

For Keq values of ~1, neither reactants nor products predominate. For very high (or low) values of Keq one side of the above reaction is greatly favoured and all reactants can be converted to products (or vice versa).

3.Define entropy. What is the G of a reaction? On what factors does G depend? (pp. 57–59)

Entropy is the state of disorder of a system. The disordered state of a collection of gas molecules is considered to be greater than that of those same molecules when in the more ordered liquid (or solid) state.

G is the ‘free energy’ associated with a chemical change. The magnitude of G is a measure of how ‘far’ the system is from achieving equilibrium. When equilibrium is achieved, G = 0.

4.Define oxidation, reduction and hydrolysis. Give an example of each. (pp. 69–70)

Oxidation is the net loss of electrons from a chemical species during a reaction.

Fe2+Fe3++e–

Reduction describes the process where a chemical species has a net gain of electrons.

Cu2++2e–Cu

Hydrolysis describes the reaction where a bond between two nuclei is broken and the components of a water molecule are attached in place of the broken bond.

RCOOR1+H2ORCOOH+R1OH

5.What are:

(a) endergonic reactions? (p. 59)

Endergonic reactions are those that consume energy from their surroundings when they proceed. The dissolution of urea in water is an endergonic reaction that can be used to cool beer cans when the need arises.

(b) exergonic reactions? (p. 59)

Exergonic reactions are those that emit heat energy (or other forms such as light or electrical energy) to the surroundings. Fire is a prime example.

6.What is the basis of the specificity of enzyme action? Explain in terms of enzyme structure why boiling inactivates enzymes. (pp. 60–65)

An enzyme is a catalyst produced by the protein production machinery of a cell. It acts by increasing the rate of particular chemical reactions. The basis for enzyme specificity is molecular shape. The enzyme is able to lower the activation energy of a reaction by holding substrate molecules in particular orientations that lead to much increased reaction rates. Enzymes are proteins produced by the transcription of DNA into RNA and the processing of RNA into proteins. Often they are large structures (molecular weights of 100 000 g mole–1 = 100 kDaltons) that consist of an agglomeration of a small number of lower molecular weight subunits. These subunits are held together by intermolecular forces that are broken as the temperature is increased. The rupture of the intermolecular bonds between the subunits results in a loss of activity.

7.Define ‘substrate’ and ‘active site’ of an enzyme. Describe the forces involved in binding the substrate to the active site. (pp. 60–62)

The compound on which an enzyme acts is called a substrate. Enzymes recognise and bind a molecule through the active site, where the polypeptides are from a specialised region where only the substrate molecules can fit. Exposed R-groups of the amino acids in the active site and their arrangements relative to each other determines the specificity of the enzyme to particular substrates.

8.Match each term with an appropriate item from the list below (a–e): oxidation, reduction, anabolism, catabolism, cofactor, coenzyme, substrate, isomerase, hydrolase, lyase.

(a) Compound acted upon by an enzyme—substrate (p. 60)

(b)Type of enzyme that causes geometric/structural changes in a substrate molecule—isomerase (p. 68)

(c) Metal iron required by many enzymes—cofactor (p. 65)

(d) Conversion of ferrous ions (Fe2+) to ferric ions (Fe3+)—oxidation (p. 69)

(e) The breakdown of compounds in living cells—catabolism (p. 67)

9.What is an electron carrier? Name three electron carriers and indicate their reduced and oxidised forms. (p. 70)

The term ‘electron carrier’ seems to be a misnomer as the chemical species that acts in this way does so by gaining or losing protons. The chemical term for a substance that acts as an electron carrier is a ‘mediator’.

Examples are: NAD (nicotinamide adenine dinucleotide), FMN and FAD (flavin nucleotides), the Cytochromes (a family of compounds designated b, c1, c, a, a3).

Oxidised formReduced form

NADHNAD+

FADH2FAD

FMNH2FMN

10.Calculate the net oxidation-reduction potential and the standard free energy change for each of the following reactions written from left to right. Indicate whether or not the reaction will tend to go spontaneously given the proper conditions. Use the equation G0 = -nFE0.

(a) NADH + H+ + 1/2O2 NAD+ + H2O (pp. 58–59)

NADH2+NAD++2H++2e–(oxidation reaction) 0.32 V

0.5 O2+2H++2e–H2O(reduction reaction) 0.82 V

Net reaction

NADH2++0.5 O2NAD++H2OE = (0.82 + 0.32) = 1.14 V

G= –nFE

wheren=2

F=Faraday constant, 96485 Coulombs mole–1

E=1.14 V

G=–2.2 × 105 J mol–1

A negative and large value for G means that this reaction is spontaneous as written, and a long way from equilibrium.

(b)Coenzyme Qox + 2H+ + 2{cytochrome a {Fe2+}]  Coenzyme Qred + 2{cytochrome a {Fe3+}] (pp. 58–59)

CoenzymeQox+2H++2e- Coenzyme Qred(oxidation reaction) –0.17 V

Cytochrome a(Fe2+)Cytochrome a(Fe3+)+ e-(reduction reaction) –0.29 V

Net reaction (multiply the cytochrome reaction by 2)

CoenzymeQox + 2H+ + Cytochrome a(Fe2+)  Coenzyme Qred + Cytochrome a(Fe3+) E = –0.46 V

G= –nFE

wheren=2

F=Faraday constant, 96485 Coulombs mole–1

E=–0.46 V

G=8.9 × 104 J mol–1

A positive and large value for G means that the reaction is spontaneous in the reverse direction (as written) and a long way from equilibrium.