Energetics

Question 1

State Hess’s Law.

Answer 1

The enthalpy change of a reaction, at constant pressure, is independent of the route taken.

Question 2

Explain why it is important that the reaction mixture is stirred before recording each temperature.

Answer 2

this ensures that all of the solution is at the same temperature

Question 3

Explain, using the idea of bonds, why the value for the enthalpy of hydration for a chloride ion is more negative than that for a bromide ion.

Answer 3

• chloride ions are smaller than bromide ions
• force of attraction between Cl+ ions and δ+ H is stronger
• requires more energy

Question 4

Give the meaning of the term mean bond enthalpy.

Answer 4

enthalpy change at constant pressure needed in breaking covalent bonds into gaseous atoms averaged over a range of different compounds

Question 5

Define the term standard enthalpy of combustion.

Answer 5

The enthalpy change, at constant pressure, when one mole of a substance combusts completely in oxygen
with all reactants & products in standard states.

Question 6

Define the term standard enthalpy of formation.

Answer 6

The enthalpy change when one mole of a substance is formed from its constituent elements with all reactants & products in their standard states under constant pressure.

Question 7

Describe the steps you would take to determine an accurate minimum temperature that is not influenced by heat from the surroundings.

Answer 7

• start a clock when KCl is added to water
• record temperature every minute for 5 minutes
• plot a graph of temperature vs time
• extrapolate back to time of mixing = 0 & determine the temperature

Question 8

Define the term enthalpy of atomisation.

Answer 8

the enthalpy change when 1 mole of gaseous atoms is formed from the element in its standard state

Question 9

Define the term bond dissociation enthalpy.

Answer 9

the enthalpy change when one mole of a covalent bond is broken into two gaseous atoms (or free radicals)

Question 10

Define the term enthalpy of hydration.

Answer 10

the enthalpy change when one mole of gaseous ions become aqueous ions

Question 11

Define the term enthalpy of solution.

Answer 11

enthalpy change when one mole of an ionic solid dissolves in a large enough amount of water to ensure that dissolved ions are well separated and do not interact with one another.

Question 12

Define the term (first) electron affinity.

Answer 12

the enthalpy change that occurs when 1 mole of gaseous atoms gain 1 mole of electrons to form 1 mole of gaseous ions with a –1 charge