Heat of neutralization and neutralization reactions involving weak acids or weak alkalis
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)(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 ammoniasolution.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.