Calculating entropy changes

Question 1

How can entropy change be calculated when all value for S are known?

Answer 1

• adding all the entropies of the porducts and subtracting the sum of the entropies gives the value for ΔS

Question 2

What determines the feasibility of a reaction?

Answer 2

• enthalpy change
• entropy change

Question 3

How are the factors that determines feasiblity are combined?

Answer 3

• two factors combines in a quantity called the Gibbs free energy, G

Question 4

What does the value of ΔG tell us about the feasibility of a reaction?

Answer 4

• if it is negative then the reaction is feasible

Question 5

What is the equation for ΔG?

Answer 5

• ΔG = ΔH - TΔS
  
  • ΔH = enthalpy change
  • T = temp
  • ΔS = entropy change

Question 6

Why does ΔG depend on temp

Answer 6

• some reactions may be feasible at one temperature and not at another
• an endothermic reaction can become feasbile when temperature is increased if there is a large enough entropy change

Question 7

What happens when ΔG=0?

Answer 7

• the reaction is just feasible
• you can calculate at what temp the reaction is feasible from this
• althought the reaction does not flip to feasible
  
  • in a closed system an equibrium exists around this temperature in which both products and reactants are present

Question 8

What does it mean that a reaction is thermodynaimically unstable but kinetically stable?

Answer 8

• reaction is feasible at that temp
• the reaction is so slow that in practice it doesn’t take place at all

Question 9

Reaction is feasible at 900k and not 1300 k , why?

Answer 9

• -TS term is less -ve