Heat is evolved during neutralization of an acid with an alkali. Known quantities of the standard solutions of an acid and alkali are mixed and the change in temperature is noted and from this, the enthalpy of neutralization is calculated.
Enthalpy of neutralization is the heat evolved when one gram equivalent of the acid is completely neutralized by one gram equivalent of a base in dilute solutions. Neutralization involves bond formation between H+(aq) and OH–(aq) to form water molecule. For example, in the neutralization of HCl and NaOH
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l), ΔH = –57 KJ
Experiment: To estimate the enthalpy of neutralization of hydrochloric acid and sodium hydroxide.
Polythene bottle (250 ml) with mouth, Rubber cork (with two holes) to fit in the mouth of polythene bottle, thermometer (1/10th degree), stirrer with a cork and a handle, magnifying glass and 100 ml graduated cylinder.
dil HCl, dil NaOH
How To Perform Experiment
(i) Take polythene bottle fitted with a thermometer (1/10th degree) and stirrer.
(ii) Remove the cork and by means of a graduated cylinder add 100 ml of the 0.5 N acid solution into the bottle and cork the bottle.
(iii) Fix the thermometer in the cork in such a way that its bulb is just near the bottom of the bottle.
(iv) Place 100 ml of 0.5 N sodium hydroxide solution in a polythene beaker.
(v) Note the temperature of each of the two solutions after every one minute till solutions attain same temperature. If the temperature of bottle the solutions is not the same, make it so by stirring the solution with lower temperature.
(vi) Note this temperature. Let it be t1°C.
(vii) When both the solutions acquire the same temperature (t°C), transfer 100 ml of NaOH solution from the polythene beaker into the acid solution in the polythene bottle quickly without any loss of time. Ensure that there is no splashing.
(viii) Immediately fit the cork having the thermometer and the stirrer in the mouth of the polythene bottle and stir gently.
(ix) Note the temperature after every one minute till it becomes constant. Note down the maximum temperature attained.
- (i) Temperature should be noted very carefully. Greater the accuracy with which the temperatures are measured, smaller will be the percentage error.
- (ii) The final temperature should be noted after stirring the mixture well.