L3 - Energy Considerations

Question 1

State the first law of thermodynamics:

Answer 1

Energy cannot be created or destroyed but can be transformed from one form to another.

Question 2

Define: the study of thermodynamics

Answer 2

It is the study of energy transformations.

Question 3

Forms of energy in reaction: Bond energy

Answer 3

Mexo and Bendo Making bonds releases energy and breaking bonds requires energy. The stronger the bond, the more energy released/required. This is known as bond energy.

Question 4

Bond energy of C-H, C-C, C=C, C=O, Hydrogen bonds and Van der Waals Interactions.

Answer 4

C-H –> 412 C-C –> 348 C=C –> 610 C=O –> 740 Hydrogen bonds –> 12-30 Van der Waals Interactions –> 0.4-4

Question 5

Define: Enthalpy change ΔH

Answer 5

Sum of energy that is used to break and make bonds.

Question 6

Define: Exothermic

Answer 6

ΔH-ve Heat lost to surroundings Reaction more likely to occur. Oxidation of glucose has ΔH of -2813.

Question 7

Define: Endothermic

Answer 7

ΔH+ve Heat taken from surroundings to molecule. Some reactions do occur but not all.

Question 8

State the second law of thermodynamics:

Answer 8

All processes must increase the entropy of the universe. These reactions are more favourable +ve ΔS.

Question 9

Cells and thermodynamics: How do they increase entropy of universe?

Answer 9

Cell maintains organisation. By making large macromolecules, you decrease in entropy (2 to 1). This decreases entropy inside cell but increase entropy of universe by:

1. Releasing heat into environment
2. Releasing other small molecules such as CO2 into environment.

Question 10

Define: Gibbs free energy

Answer 10

The useful energy available for a reaction. Overall energy from breaking and making bonds.

Question 11

Gibbs equation:

Answer 11

ΔG=ΔH-TΔS

T is temp in kelvin.

S has kJK^-1mol^-1

ΔG and ΔH is kJmol^-1

Question 12

Define: Exergonic

Answer 12

ΔG -ve and reaction can occur spontaneously. Releases energy.

Question 13

Define: Endergonic

Answer 13

ΔG +ve and reaction cannot occur spontaneously. requires energy.

Question 14

Explain the relationship between ΔG of a reaction and direction of the reaction.

Answer 14

ΔG doesn’t tell us about the rate of a reaction. Large -ve value means that it could take many years for reaction to occur. (due to high E_a) Usually enzymes are used to lower E_a allowing reaction to occur.

Question 15

ΔG and Equilibrium:

Answer 15

At equilibrium, ΔG=0. Reactant and product made is constant. For example, Y to X. Formation of X is favoured. ΔG = -ve Formation of Y is not. ΔG = +ve ΔG and direction of reaction depends on conc of reactants and products.

Question 16

Standard States

Answer 16

ΔG°’ is the free energy change of a reaction under standard conditions of: 1M conc of reactants and products. 25°C (298K) pH=7 (biological systems) ΔG° has pH = 0 for chemists.

Question 17

Factors that affect direction of a reaction:

Answer 17

1. Volume of conc
2. Ratio of conc of products and reactants.

More reactants then reaction goes to right. More products then reaction goes to left.

ΔG also determines the direction of the reaction.

Question 18

ΔG and ΔG°’ Equation:

Answer 18

ΔG = ΔG°’ + RTlnK_a

At equilibrium ΔG=0 so: ΔG°’ = -RTlnK_a

R=8.314JK^-1mol^-1 → 0.008314kJK^-1mol^-1

T in kelvin.

ΔG°’ in kJmol^-1

K_a in kJmol^-1

As ΔG°’ increases (more exergonic/negative) then more product is formed so K_a is higher.

K_a = Products/reactants

Question 19

Energetically unfavourable reactions

Answer 19

They can be coupled with favourable reactions, usually hydrolysis of ATP.

Glutamate+NH4⁺ + ATP → Glutamine+ADP+Pi Glutamate + NH4(+) = Glutamine + H2O

ΔG°’ = +15 kJmol^-1

H2O + ATP = ADP + Pi

ΔG°’ = -30 kJmol^-1

Use enzymes to couple them.