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 Form 4 Chemistry: Energy changes in chemical and physical processes lessons

Heat of neutralization and neutralization reactions involving weak acids or weak alkalis

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Answer Text:
2. Heat of neutralization;
- Is the heat evolved when acid and a base react to form and mole of water.
- Alternatively;
- It is the heat evolved when one mole of hydrogen ions from an acid reacts with one mole of hydroxide ions from an alkali to form\ give one mole of water.
Equation:#OH^(-)(aq) + H^(+)(aq) H_2O(l)#
- Neutralization reactions are exothermic.
- Are determined by:
- measuring the temperature rise produced when a known volume of acid is neutralized by a known volume of alkali.
Examples.
(a). Strong acids reacting with strong alkalis. The ΔH is always about – #57KJMol^(-)# and higher than that for weak acids – weak alkalis.
Reason:-
- Strong acids and alkalis are already fully ionized in aqueous solution and no heat is lost in ionizing the acid or the alkali.
Consider:
- Reaction between sodium hydroxide and hydrochloric acid
#NaOH(aq) to Na^(+) (aq) + OH^(-) (aq)#
# HCl(aq) to H^(+) (aq) + Cl^(-) (aq)#
On reacting; #OH^(-) (aq) + H^(+) (aq) to H_2O(l)#
Diagrammatically (energy level diagram)
figchanges22720201233.JPG
(b). Neutralization reactions involving weak acids or weak alkalis.
- The ΔH is lower than expected, e.g. only #– 52Kjmol^(-)#; and hence lower than that for strong acidsstrong alkalis.
Reason:
- Weak acids and weak alkalis are NOT fully ionized in aqueous solutions and some heat is used in ionizing them.
Consider; reaction between ethanoic acid and ammonia
solution.
figchanges22720201228.JPG
Therefore; ΔH is given by; ΔH = ΔHI + ΔHII = 5 + (-57) = #-52KJMol^(-1)#
Note: -
- Dibasic acids e.g. #H_2SO_4# contains two replaceable hydrogen atoms hence on incomplete neutralization, they form two moles of water.
- Therefore, H neutralization = #1/2# x ΔH reaction.