Entropy change, ΔS and Gibbs free energy change, ΔG

Question 1

What is entropy

Answer 1

-entropy (S) of a given system is the number of possible arrangements of the particles and their energy in a given system
-it is a measure of how disordered a system is
-a system with a higher entropy will be energetically more stable (as the energy of the system is more spread out when it is in a disordered state)

Question 2

Entropy is greater if

Answer 2

-there are more ways of arranging the energy in a molecule or atom
-there are more ways of arranging the molecules or atoms in a given volume

Question 3

unit for standard molar entropy, S

Answer 3

J K-1 mol-1

Question 4

The entropy change when melting

Answer 4

-entropy increases when a substance melts
-increasing the temperature of a solid causes the particles to vibrate more
-the regularly arranged lattice of particles changes into an irregular arrangement of particles
-these particles are still close to each other but can now rotate and slide over each other in the liquid
-as a result, there is an increase in disorder

Question 5

The entropy change when boiling

Answer 5

• entropy increases when a substance boils
  -the particles in a gas can now freely move around and are far apart from each other
  -the entropy increases significantly as the particles become very disordered

Question 6

The entropy change when condensing and freezing

Answer 6

-the entropy decreases when a substance condenses or freezes
-the particles are brought together and get arranged in a more regular arrangement
-the ability of the particles to move decreases as the particles become more ordered
-there are fewer ways of arranging the energy so the entropy decreases

Question 7

entropy change when dissolving

Answer 7

-entropy also increases when a solid is dissolved in a solvent
-the solid particles are more ordered in the solid lattice as they can only slightly vibrate
-when dissolved to form a dilute solution, the entropy increases as:
-particles are more spread out
-an increase in the number of ways of
arranging the energy

Question 8

entropy change when crystallizing

Answer 8

-crystallisation of a salt from a solution is associated with a decrease in entropy
-the particles are spread out in solution but become more ordered in the solid

Question 9

entropy change and temperature change

Answer 9

-if we carry reactions above standard temperature, an increase in temperature makes the entropy change of surroundings more positive
-if we carry out reactions below standard temperatures, a decrease in temperature makes the entropy change of surroundings less positive

Question 10

entropy change during a reaction in which the number of gaseous molecules increases

Answer 10

-the greater the number of gas molecules, the greater the number of ways of arranging them, and thus the greater the entropy
-for example in the decomposition of calcium carbonate (CaCO3)
CaCO3(s) → CaO(s) + CO2(g)
-the CO2 gas molecule is more disordered than the solid reactant (CaCO3) as it can freely move around whereas the particles in CaCO3 are in fixed positions in which they can only slightly vibrate
-the system has therefore become more disordered and there is an increase in entropy

Question 11

entropy change during a reaction in which the number of gaseous molecules decreases

Answer 11

-a decrease in the number of gas molecules results in a decrease in entropy causing the system to become less energetically stable
-for example in the formation of ammonia in the Haber process
N2(g) + 3H2(g) ⇋ 2NH3(g)
-before the reaction occurs, there are four gas molecules (1 nitrogen and 3 hydrogen molecules) in the reactants
-after the reaction has taken place, there are now only two gas molecules (2 ammonia molecules) in the products
-since there are fewer molecules of gas in the products, there are fewer ways of arranging the energy of the system over the products
-the system has become more ordered causing a decrease in entropy
-the reactants (N2 and H2) are energetically more stable than the product (NH3)

Question 12

ΔSsystemꝋ =

Answer 12

ΣΔSproductsꝋ - ΣΔSreactantsꝋ

Question 13

Define spontaneous changes

Answer 13

changes that tend to continue to happen naturally once started

Question 14

entropy and equilibrium

Answer 14

-at equilibrium the total entropy change of forward reaction equals the total entropy change of the backward reaction
-under standard conditions the overall entropy change is zero

Question 15

What is Gibbs free energy

Answer 15

we use Gibbs free energy to determine whether a chemical reaction is likely to be spontaneous

Question 16

ΔGꝋ =

Answer 16

ΔHreactionꝋ - TΔSsystemꝋ
Note: The units of ΔGꝋ are in kJ mol-1
The units of ΔHreactionꝋ are in kJ mol-1
The units of T are in K
The units of ΔSsystemꝋ are in J K-1
mol-1 (and must therefore be converted
to kJ K-1 mol-1 by dividing by 1000)

Question 17

Gibbs free energy in exothermic reactions and if the ΔSsystemꝋ is positive

Answer 17

-In exothermic reactions, ΔHreactionꝋ is negative
-Both the first and second term will be
negative
-Resulting in a negative ΔGꝋ so the reaction
is feasible
-Therefore, regardless of the temperature, an
exothermic reaction with a positive
-ΔSsystemꝋ will always be feasible

Question 18

Gibbs free energy in exothermic reaction and if the ΔSsystemꝋ is negative:

Answer 18

-In exothermic reactions, ΔHreactionꝋ is negative
-The first term is negative and the second term is positive
-At high temperatures, the -TΔSsystemꝋ will be very large and positive and will overcome ΔHreactionꝋ
-Therefore, at high temperatures ΔGꝋ is positive and the reaction is not feasible
-The reaction is more feasible at low temperatures, as the second term will not be large enough to overcome ΔHreactionꝋ resulting in a negative ΔGꝋ
-This corresponds to Le Chatelier’s principle which states that for exothermic reactions an increase in temperature will cause the equilibrium to shift position in favour of the reactants, i.e. in the endothermic direction
-In other words, for exothermic reactions, the products will not be formed at high temperatures
-The reaction is not feasible at high temperatures

Question 19

Gibbs free energy in endothermic reactions and if the ΔSsystemꝋ is negative

Answer 19

-In endothermic reactions, ΔHreactionꝋ is positive
-Both the first and second term will be positive
-Resulting in a positive ΔGꝋ so the reaction is
not feasible
-Therefore, regardless of the temperature,
endothermic with a negative ΔSsystemꝋ will
never be feasible

Question 20

Gibbs free energy in endothermic reactions and if the ΔSsystemꝋ is positive

Answer 20

-In endothermic reactions, ΔHreactionꝋ is positive
-The first term is positive and the second term
is negative
-At low temperatures, the -TΔSsystemꝋ will
be small and negative and will not overcome
the larger ΔHreactionꝋ
-Therefore, at low temperatures ΔGꝋ is
positive and the reaction is less feasible
-The reaction is more feasible at high
temperatures as the second term will
become negative enough to overcome the
ΔHreactionꝋ resulting in a negative ΔGꝋ –This again corresponds to Le Chatelier’s principle which states that for endothermic reactions an increase in temperature will
cause the equilibrium to shift position in favour of the products
-In other words, for endothermic reactions, the products will be formed at high temperatures
-The reaction is therefore feasible