AC16: Entropy and free energy
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What is enthalpy
A measure of the total energy content of a system
What is entropy
A measure of how dispersed the energy is the system is/ how disordered the particles are in the system
When there’s increased disorder among particles in a system, what does this do to the entropy
Increases entropy
Does a solid have a high or low entropy state
Low, since the particles have very little freedom of movement, so are highly ordered
Does a gas have a high or low entropy state
High, since the particles are free to move around, so gas is a highly disordered state
Will the entropy change be positive or negative for a vaporisation
Positive since gas has a greater entropy than a liquid, since particles have a greater freedom to move
Is the entropy change of a dissolution, dissolving, positive or negative
Positive, when solid dissolves the particles can move around more so they’re less ordered so the solution will have a higher entropy
If 4 moles of gas produce 2 moles of gas, is the entropy change positive or negative
Negative, since the particles become more ordered/ decrease in the disorder
How to calculate the entropy change
Entropy of products - entropy of reactants
What are the units of entropies
JK^-1mol^-1
What is the equation for free energy
Enthalpy change - (Temperature(K) x entropy change)
For a reaction to be feasible, what must the free energy be less than
Zero
For free energy, G, to be negative, so reaction is feasible, what must enthalpy and entropy change be like
Enthalpy change must be negative Entropy change must be positive
For free energy to be positive, what must enthalpy and entropy changes be like
Enthalpy change = positive Entropy change= Negative
When calculate free energy, what things must you be careful about
• Units, ensuring all are in kJ mol^-1 • Temperature in Kelvin not celsuis
To manipulate free energy value, what can be changed
The temperature, Enthalpy and entropy change are fixed values
What must an endothermic reaction be accompanied by in order for the reaction to be feasible
An entropy increase
If the free energy is negative will the reaction definitely happen
No, because the reaction could be exceedingly slow, so has a high activation energy
The enthalpy and entropy change of a reaction both have a negative sign. Discuss how the feasibility of this reaction will change as the temperature increases (2 marks)
Feasibility AND ΔG Reaction becomes / is less feasible / not feasible AND ΔG increases OR ΔG becomes / is less negative / more positive OR ΔG > 0 OR ΔH − TΔS > 0 OR ΔH − TΔS becomes / is less negative / more positive OR ΔH > TΔS ✔ OR TΔS becomes / is more negative than ΔH ✔ Effect on TΔS TΔS becomes more negative OR TΔS decreases OR −TΔS becomes more positive OR −TΔS increases OR magnitude of TΔS increases OR | TΔS | increases ✔
Define the term enthalpy change of hydration (2 marks)
(enthalpy change for) 1 mole of gaseous ions OR 1 mole of hydrated ions / aqueous ions ✔ gaseous ions forming aqueous / hydrated ions ✔