Entropy and Gibbs free energy

In terms of energy, what happens when a chemical reaction occurs?

• Energy changes occur, generally in the form thermal energy

• The energy change is either endothermic or exothermic

What is an endothermic reaction?

• Heat energy is absorbed during the reaction

• Positive enthalpy change

• More energy is used to break bonds than is released in making bonds

What is an exothermic reaction?

• Heat energy is given off during the reaction

• Negative enthalpy change

• More energy is released making bonds than is used to break

How does ionic radius affect lattice enthalpy?

• A smaller ionic radius means the ions are closer together

• As a result, the positive and negative ions are more strongly attracted together

• Lattice enthalpy is more exothermic

Explain how ionic charge affects lattice enthalpy

• A greater ionic charge means greater electrostatic attraction between positive and negative ions

• Lattice enthalpy is more exothermic

What is Hess’ law?

The enthalpy change of reaction is independent of the route it takes

What is entropy?

The measure of disorder of a system or degree of randomness

Define the term ‘surroundings’ (in enthalpy changes)

Anything other than the chemical reactants and products

What is the ‘system’?

The reactants and products of a chemical reaction

In terms of enthalpy, when is a system more stable?

A system is more stable when its energy is spread out/in a more distorted state

The higher the entropy, the more stable the system

How does entropy change when a substance changes state?

Entropy increases from Solids to liquids to gas

This is because the system becomes more distordered

How does entropy change as temperature changes?

As temperature increases, the particles in a substance gain kinetic energy and become more disordered. As a result, entropy increases

How does entropy change during a reaction in which the number of gas molecules changes?

• Entropy increases if the number of gas molecules increases

• This is because the gaseous state has the highest entropy as there is more disorder in the system

How do you calculate the entropy change of the system (given the standard entropies of reactants and products)?

∆S system= ∑S products - ∑S reactants

When will ∆S (standard entropy change) be positive?

When disorder increases

E.g. state change from solid to liquid/ liquid to gas, increase in temperature, increase in number of gaseous molecules

What is Gibbs free energy?

This overall change in energy during a reaction is Gibbs free energy, ∆G

This energy change consists of entropy change (∆S) and enthalpy change (∆H) at a given temperature (T)

What is the Gibbs free energy equation?

∆G = ∆H - T∆S system

What is the equation for calculating total entropy change?

∆S total = ∆S system - (-∆H reaction/T)

What does the sign of ∆G say about the feasibility of a reaction?

∆G has to be negative for the reaction to be feasible/spontaneous

If ∆G is positive, the reaction is not feasible/spontaneous at the given temperature

If ∆H is negative and ∆S is positive, how will changing temperature affect the spontaneity of the reactoin?

∆G is always negative regardless of temperature so the reaction is spontaneous at all temperatures

If ∆H is positive and ∆S is negative, how will changing temperature affect the spontaneity of the reaction?

∆G is always positive regardless of the temperature so the reaction is never spontaneous at any temperature

If ∆H and ∆S are positive, how will changing temperature affect the spontaneity of the reaction?

∆G is the only negative at high temperatures so the reaction will more be spontaneous at low temperatures

If ∆H and ∆S are negative, how will changing the temperature affect the spontaneity of the reaction?

∆G is only negative at low temperatures so the reaction will not be spontaneous at high temperatures

Give the equation for that link Gibbs free energy change and standard electrode potential

∆G= -nFEcell

∆G is the standard Gibbs free energy in J mol

n is the number of moles of electrons transferred in the cell reaction

F is the Faradays constant

E-cell is the standard cell potential

How do you know chemical equation goes to completion in regards to the value of E-cell?

If the value of E-cell is more positive then +0.6V then the reaction goes to completion

If the value of E-cell is more negative then -0.6V there is no reaction or minimal reaction

What is value of ∆G reaction if the system is in equilibrium?

∆G reaction would be zero

What is the value of ∆G reaction if products dominate the reaction?

∆G reaction has a fairly low negative value

What is the value of ∆G reaction if reactants dominate the reaction?

∆G has a slightly positive value

When can a chemical reaction be considered complete in term of the value of ∆G reaction?

If ∆G reaction is high and negative

The reaction cannot be feasible if the value of ∆G reaction is high and positive