THEORY:
Enthalpy of neutralization is the amount of heat change that takes place when 1 gm equivalent of an acid is neutralized by 1 gm equivalent of a base in dilute solutions to form one mole of water.
The enthalpy of neutralization of a strong acid with a strong base is almost constant and is equal to 56 KJ or 13.6 K calories. It is because of the fact that the dilute solutions of strong acids and strong bases are completely ionized, and the reaction is always the same, no matter which strong acids and alkalis are used.
H+ (aq) + OH– (aq)→ H₂O (1) ∆H = -56 KJ
But the enthalpy of neutralization is not constant in the cases of neutralization of a weak acid by a strong base, or a weak base by a strong acid or a weak acid by a weak base.
APPARATUS:
1) Plastic beaker or Styrofoam cup
2) Beaker
3) Thermometer reading to 0.1°C
CHEMICALS:
1) (N) hydrochloric acid
2) (N) sodium hydroxide
PROCEDURE:
i) Pour 100 ml of (N) hydrochloric acid from a measuring cylinder into a plastic beaker or Styrofoam cup.
ii) Rinse the measuring cylinder with water two or three times, and pour 100 ml of (N) sodium hydroxide from the same cylinder into another beaker.
iii) Note the steady temperature of the acid solution with a thermometer (reading to 0.1°C). Rinse the thermometer and also note the steady temperature of the alkali solution. The temperature of both the solutions should be the same, if not, take the mean of the two as the initial temperature.
iv) Pour the sodium hydroxide solution quickly but without splashing into the hydrochloric acid solution in the plastic beaker.
v) Briefly stir the mixture with the thermometer itself and record the highest temperature reached.
RESULTS:
Temperature of hydrochloric acid = t1°C
Temperature of sodium hydroxide t₂°C
Initial temperature of the solutions =t1+ t2 / 2 =t°C (say)
Highest temperature after mixing = tf
Approximate mass of the mixed solution = 200 gm
Specific heat of the mixed solution = 1 cal per gm per °C
CALCULATIONS:
Heat evolved in the reaction = heat gained by the mixture
= mass of the solution x sp. heat of the solution x change in temp.
= 200 x 1 x (tf -t) cals.
Hence, heat evolved when 100 ml of (N) HCl is neutralized
= 200 x 1 x (tf -t) cals
;. Heat evolved when 1000 ml of (N) HCL is neutralized
= 200 (tf -t) x 1000/100
= 2000 (tf -t) cals.
N.B. The final result will turn out to be slightly less than the expected theoretical value (13.6 kcals) since our measurements are only approximate.