Entropy Flashcards
Why can’t the driving force behind chemical reactions be explained by enthalpy changes alone?
The driving force behind chemical reactions cannot be explained by enthalpy changes alone as it does not make sense for chemicals to end up in a less stable, higher energy state in endothermic reactions.
What is entropy?
The entropy (S) of a given system is the number of possible arrangements of the particles and their energy in a given system.
In other words, it is a measure of how disordered or chaotic a system is.
When a system becomes more disordered, its entropy will…?
Increase
When a gas is formed in a chemical reaction, entropy…?
Increases
When a solid turns into a liquid in a reaction, entropy…?
Increases
In the thermal decomposition of calcium carbonate (CaCO3), does entropy increase or decrease?
Entropy increases as a gas molecule is formed.
CaCO3(s) → CaO(s) + CO2(g)
The CO2 gas molecule is more disordered than the solid reactant (CaCO3), as it is constantly moving around.
As a result, the system has become more disordered and there is an increase in entropy.
Which system is more energetically favourable and why - a system with higher or lower entropy?
The system with the higher entropy will be energetically favourable (as the energy of the system is more spread out when it is in a disordered state).
What does it mean when a reaction is described as feasible?
Chemists talk about reactions being feasible or spontaneous.
What they mean is that reactions take place of their own accord, in other words, they are energetically favourable.
e.g. cups fall off tables and spontaneously break into many pieces, never the other way around.
hot objects always cool and spread their heat into the surroundings, never the other way around.
What doesn’t feasibility take into account?
Feasibility takes no account of the rate of reaction and states only what is possible, not what actually happens. A feasible reaction might be incredibly slow, such as the rusting of iron.
What is the unit for entropy?
J K-1 mol-1
How can the standard entropy change of a reaction be calculated?
The standard entropy change (ΔS) for a given reaction can be calculated using the standard entropies (S) of the reactants and products.
ΔSꝋ = ΣSproductsꝋ - ΣSreactantsꝋ
What equation is used to calculate whether a reaction is feasible or not?
The Gibbs equation
ΔGꝋ = ΔHreactionꝋ - TΔSsystemꝋ
When ΔGꝋ is negative, the reaction is…?
Feasible and likely to occur
When ΔGꝋ is positive, the reaction is…?
Not feasible and unlikely to occur
What is meant by free energy?
- Some energy used to change the entropy of the system.
- Leftover energy used to do useful work = free energy.
- Negative ΔG = energy left over, which means the reaction is feasible
