Discussion
-Calculations
a)
b) The maximum change in temperature is produced when 20cm3 of acid is mixed with 20cm3 of sodium hydroxide.
NaOH(aq) + HCl(aq) → H2O(l) + NaCl(aq)
or NaOH(aq) + CH3COOH(aq) → H2O(l) + CH3COOHNa(aq)
The mole ratio of sodium hydroxide to acid is 1:1. Therefore, the concentration of sodium hydroxide is also 1.00M.
c) Energy change of hydrochloric acid = mc∆T
= (20+20) x 4.18 x (27.1-33.8)
= -1.12024 kJ
Enthalpy change of neutralization of hydrochloric acid = -1.12024(0.020)
= -56.012 kJmol-1
Energy change of ethanoic acid = mc∆T
= (20+20) x 4.18 x (27.1-33.5)
= -1.07008 kJ
Enthalpy change of neutralization of ethanoic acid = -1.07008(0.020)
= -53.504 kJmol-1
d) The enthalpy change of neutralization of hydrochloric acid is greater than that of ethanoic acid. As ethanoic acid is a weak acid which partially ionizes in water, some energy given out by neutralization is absorbed to dissociate the unionized acid molecules. Therefore the enthalpy change is smaller than that of hydrochloric acid.
a)
b) The maximum temperature change is produced when 24cm3 of acid is mixed with 25cm3 of sodium hydroxide solution.
no. of moles of acid reacted = 1 x 24/1000
= 0.024 mol
concentration of sodium hydroxide solution = 0.02425/1000
= 0.96 M
c) Energy change of hydrochloric acid = mc∆T
= (24+25) x 4.18 x (27.0 – 34.4)
= -1.5157 kJ
Enthalpy change of neutralization of hydrochloric acid = -1.12024(0.96 x 25/1000)
= -63.15 kJmol-1
Energy change of ethanoic acid = mc∆T
= (24+25) x 4.18 x (27.0 – 34.3)
= -1.4952 kJ
Enthalpy change of neutralization of hydrochloric acid = -1.12024(0.96 x 25/1000)
= -62.299 kJmol-1
d) Also, The heat given out by hydrochloric acid is higher than that of ethanoic acid because some energy is absorbed by the acid during the neutralization to ionize weak ethanoic acid molecule.
- Suitability of determining the concentration of NaOH and enthalpy change of neutralization
In determining the concentration of sodium hydroxide solution, Method 2 is more suitable. In this method, 2.0cm3 of acid is added to sodium hydroxide in each interval. The slope of the curve plotted by method 2 is smoother and more accurate than method 1 because temperature change is measured at a smaller interval. The concentration found is more accurate.
However, Method 1 is more suitable in determining the enthalpy change of neutralization. In Method 2, we have to measure the temperature change many times. A lot of heat is lost to the surroundings, so the percentage error in calculating the enthalpy change of neutralization is much larger than that of Method 1.
-Relationship between the enthalpy change of neutralization and different pairs of acid-base
For the experimental results, the enthalpy change of neutralization between hydrochloric acid and sodium hydroxide is greater than that of ethanoic acid.
In neutralization: H+(aq) + OH-(aq) → H2O(l)
Ethanoic acid is a weak acid. It slightly ionizes to give some H+ for neutralization. Energy is absorbed from neutralization process to disassociate the unionized acid molecules for neutralization to proceed.
However, hydrochloric acid is a strong acid which completely ionizes to give H+ for neutralization. Energy will not be absorbed from neutralization process. Hence, the reaction between hydrochloric acid and sodium hydroxide will be more exothermic than that of ethanoic acid.
To summarize, the stronger the acid, the greater the enthalpy change of neutralization.
-The reasons why titration in Method 2 must be carried out quickly
In method 2, very small amount (2.0 cm3) of acid is added to the solution each time. The heat loss will be very significant as the ratio of acid added to the reaction mixture is small. In order to reduce the heat loss and the percentage error in calculations, the titration must be carried out quickly to obtain more accurate results.
-Sources of error
- Heat loss to surroundings
Heat is lost due to evaporation, convection and conduction. The value of enthalpy change of neutralization calculated will be smaller than the actual value. Better insulation, such as wrapping the polystyrene cup with cotton wool, can be taken to reduce the error.
- Some heat may be absorbed by thermometer, stirrer and the polystyrene cup
In order to reduce the error, the heat capacities of apparatus should be taken into account in calculations.
- The specific heat capacity of solutions is not equal to that of water
- Density of the solutions is not equal to that of water
Conclusion
The enthalpy change of neutralization of HCl is -56.012 kJmol-1 whereas the enthalpy change of neutralization of CH3COOH is -53.504 kJmol-1.
The objective of the experiment has been fulfilled as the enthalpy changes of neutralization between different pairs of acid-base were found.
