Energy Considerations

Question 1

1) What is the first thermodynamic law?

Answer 1

• Energy cannot be created or destroyed, only transformed

Question 2

2) What does ΔH mean?

Answer 2

• Enthalphy change ΔH of a chemical reaction is the sum of energy used when bonds are broken/ energy released when bonds are formed
• This is a change in bond energy : energy released when a bond is formed or required to break a bond

Question 3

3) Define exothermic and endothermic reactions

Answer 3

-ve ΔH : exothermic, heat is lost to surroundings

+ve ΔH : endothermic, heat taken up by molecules (less likely than exo)

Question 4

4) Define entropy and so the 2nd thermodynamic law

Answer 4

• entropy, S, is the level of disorder/ number of ways something can be arranged
• 2nd law: all processes must increase the entropy of the universe
• +ve ΔS reactions are favorable as they increase entropy (e.g. release of CO2 or heat energy)

Question 5

5) What is ΔG, Gibbs’ free energy change, and state the equation?

Answer 5

• this is useful, ‘free’ energy -> available from a reaction
• A portion of enthalpy is used to increase entropy (products) and a portion is available to do work (ΔG)
• Equation: ΔG = ΔH - TΔS
  [ T in Kelvin and ΔG in kJmol^-1]

Question 6

6) Define endergonic and exergonic reactions

Answer 6

-ve ΔG = spontaneous reaction is exergonic
[this means the reaction CAN occur spontaneously but still depends on rate of reaction, e.g. glucose oxidation is -ΔG but occurs at a very slow rate w/o enzymes]
+ve ΔG = not spontaneous is endergonic

Question 7

7) How does ΔG value change as the reaction proceeds towards equilibrium?

Answer 7

• At equilibrium, there is no net change in ratio of reactants or products, as forwards and backwards reactions are occurring at the same rate
• so ΔG = 0

Question 8

8) How is the STANDARD free energy change (ΔG°’) different from free energy change?

Answer 8

• Standard conditions of 1mmol conc, 298 K and pH 7

not under standard conditions in the cell

Question 9

9) How is the direction of reversible reactions determined, and how does ΔG°’ change for irreversible reactions?

Answer 9

• Le Chateliers principle: determined by the concentrations of reactants and products
  > in both directions, the ΔG°’ required is similar/same so either direction is able to occur
  > irreversible: ΔG°’ for the backwards reaction is very high so does not occur

Question 10

10) State the equation which can be used to calculate ΔG°’ and how to calculate the equilibrium constant of a reaction using this

Answer 10

ΔG = ΔG°’ + RTln ([AB]/[A][B])
[R=8.314 gas constant, T in Kelvin]

-> at equilibrium, ΔG = 0 so can be simplified to:
ΔG°’ = -RTlnKa [Ka = equilibrium constant]

Question 11

11) What does a more negative ΔG°’ value mean?

Answer 11

• a greater value of Ka as there is a greater concentration of [AB] than [A][B]

Question 12

12) How can an energetically unfavorable reaction be achieved?

Answer 12

• Coupling it to a favorable reaction (often ATP hydrolysis as ATP is an energy carrier)
• the ΔG free energy from the favorable reaction drives the unfavorable reaction (net free energy change from both coupled reactions is less than 0, ΔG = -ve)

Question 13

13) Give an example of an unfavorable reaction coupled with a favorable reaction

Answer 13

• Unfav: glutamate + NH4+ glutamine + H20
  [ ΔG°’ = +15 kJmol-1, not spontaneous ]
• Fav : ATP + H2O -> ADP + Pi + H+
  [ ΔG°’ = -30 kJmol-1, spontaneous ]

-Coupled: glutamate + ATP + NH4+ -> glutamine + ADP + Pi [ΔG°’ = -15kJmol-1, spontaneous]