Thermodynamics

Question 1

What’s the difference between kinetics and thermodynamics?

Answer 1

Kinetics: Speed of a reaction
Thermodynamics : Energy changes associated with a reaction

Question 2

What is the first law of thermodynamics

Answer 2

The first type of energy we will consider is enthalpy (H).

You cannot just measure an enthalpy, you must measure a change in enthalpy (ΔH).

Question 3

ΔH

Answer 3

amount of heat energy given out or taken in as a reaction proceeds at a particular temperature.
A reaction may be EXOTHERMIC or ENDOTHERMIC
Units = kJ per mole of reactant, i.e. kJmol-1

Question 4

Exothermic

Answer 4

ΔH is negative

Question 5

Endothermic

Answer 5

ΔH is positive

Question 6

Hess’s Law of Constant Heat Summation

Answer 6

The enthalpy change going from reactants to products is the same regardless of which route is taken

Question 7

Entropy

Answer 7

Entropy is harder to understand as it refers to the order (or disorder) in a system.

High entropy = high disorder

You can increase entropy by heating or providing more space for the molecules

The entropy of a liquid is higher than a solid and a gas is higher than a liquid

As T=0 is neutral entropy then entropy
is usually positive (but not always)

Question 8

Gibbs free energy

Answer 8

At constant temperature and pressure (i.e. enthalpy is zero):
ΔG = - T ΔS
Therefore it is another way of expressing
entropy at a constant temperature

Question 9

Van’t Hoff Equation

Answer 9

For a reversible reaction, the equilibrium constant can be measured at a variety of temperatures then plot lnKeq vs. 1/T.

This is the van ‘t Hoff plot and is widely used to estimate the enthalpy and entropy of a chemical reaction.

By measuring the equilibrium constant, Keq, at different temperatures, the Van ‘t Hoff plot can be used to assess a reaction when temperature changes.

Knowing the slope and intercept from the van ‘t Hoff plot, the enthalpy and entropy of a reaction can be easily obtained:

ΔH = - R x slope
ΔS = R x intercept

The van ‘t Hoff plot can be used to quickly determine
the enthalpy of a chemical reaction both
qualitatively and quantitatively.