Thermodynamics

Question 1

Enthalpy and enthalpy change

Answer 1

• Enthalpy is the measure of the heat content of a substance

- Enthalpy change is the change in heat content (in K) at a constant temperature

Question 2

Standard Conditions

Answer 2

• 100Kpa

- 298K (25 celsius)

Question 3

Types of reaction

Answer 3

• Endothermic = - enthalpy change, i.e bond making

- Exothermic = + enthalpy change i.e bond breaking

Question 4

Hess’s law

Answer 4

• Enthalpy change is the same no matter what reaction route is taken

Question 5

Solving enthalpy of solution questions

Answer 5

• create a Hess’ law triangle of reactions with one route of Enthalpy of Solution of the solid into the water, and another that is Lattice Enthalpy of Dissociation of the solid, then Enthalpy of Hydration of the gas into the water

Question 6

Born Haber cycles

Answer 6

• Only way to measure lattice enthalpy
• Includes all enthalpy changes in formation of an ionic compound from solid to gaseous atoms to gaseous ions
• separate steps for every enthalpy change, endothermic reactions have upwards arrows and exo. have downwards arrows

Question 7

Experimental vs Theoretical

Answer 7

• Theoretical enthalpy calculations use a formula that assume all ions are perfectly spherical
• Experimental calculations use Born-Haber cycles that provide slightly differing results due to the covalent character included in the ionic bonds distorting the spherical shape
• the greater the difference between Theo. and Exp. values, the larger the amount of covalent character, which weakens the bond

Question 8

Entropy

Answer 8

• Entropy = S = J(mol^-1)(K^-1), essentially the measure of how ordered the structure of a substance is i.e how much do the atoms move, how much energy do they have. More order = less S
• Entropy change = S(products) - S(reactants)
• there is a tendency for entropy to increase and for everything to gradually go from more to less ordered
• Increase temperature, no. particles and S increaes
• changes in state signal large increases in enthalpy from solid->liquid->gas

Question 9

Gibbs Free energy

Answer 9

• change in Gibbs free energy (G) = Change in enthalpy- temperature*change in entropy
• favourable reactions have decrease in H, increase in S
• if G is less than or equal to 0 the reaction is feasible, reactions become feasible when the temperature is such that G=0 exactly, which can be calculated if you know H and S