Enthalpy changes of neutralisation for strong and weak acids

Question 1

the standard enthalpy change of neutralisation is

Answer 1

the enthalpy change when one mole of water is produced by the neutralisation of an acid with an alkali when measured at 100kPa and a specified temperature, usually 298K

Question 2

the standard enthalpy change of neutralisation of a strong acid with a strong base will always have

Answer 2

a similar value of around -57.6 kJ mol-1

note that this is exothermic

Question 3

the general equation for the reaction of a weak acid, HA, with a strong base such as soium hydroxide is

Answer 3

HA(aq) + OH-(aq) → A-(aq) + H2O(l)

the majority of the HA molecules are undissociated in this reaction
this means more energy could be required to dissociate these molecules, which is why the ∆neutH for weak acids against a strong base is less than a strong acid with a strong base because less heat energy is given out, instead used to dissociate more acid molecules.

Question 4

dissociation is an …………….. process

Answer 4

endothermic (+∆H)

Question 5

hydration is an …………….. process

Answer 5

exothermic (-∆H)

Question 6

when acid molecules dissociate in the endothermic process, the ions formed are then ……………………..

Answer 6

hydrated by the H2O molecules present in the aqueous solution,so heat energy is released as hydration is an exothermic process

Question 7

two equations used to represent the dissociation of a weak acid, HA, and the subsequent hydration of the ions are:

Answer 7

HA(aq) → H+(g) + A-(g) ∆H = +xkJ mol-1
DISSOCIATION (endothermic)
H+(g) + A-(g) → H+(aq) + A-(aq) ∆H = -ykJ mol-1
HYDRATION (exothermic)

If the magnitude of x is greater than y, then the overall heat energy change will be endothermic