Thermodynamics

Question 1

What happens when a solid ionic lattice dissolves in water?

Answer 1

Bonds between the ions break (lattice enthalpy of dissociation)
Bonds between the ions and water are made (enthalpy change of hydration)

Question 2

How do you calculate the enthalpy change of a reaction using mean bond enthalpies?

Answer 2

Total energy absorbed (by bonds broken) - total energy released (by bonds made)

Question 3

Why may the value calculated using mean bond enthalpies be different?

Answer 3

Because it is an average value, and the same bond enthalpy varies from molecule to molecule, sometime within the same molecule.

Question 4

What is entropy a measure of and what is its symbol?

Answer 4

Disorder, S

Question 5

What three factors affect entropy?

Answer 5

Physical state, dissolution and number of particles.

Question 6

Give the three entropy equations.

Answer 6

DeltaS(total) = deltaS(system) + deltaS(surrounding)
DeltaS(system) = S(products) - S(reactant)
DeltaS(surroundings) = -deltaH divided by temperature
(Joules per mole divided by Kelvin)

Question 7

What is the free energy equation?

Answer 7

DeltaG = deltaH - TdeltaS

Question 8

What is enthalpy change?

Answer 8

Heat energy transferred in a reaction at constant pressure.