Is the enthalpy change for a given chemical change the same whether the reaction takes place in a single stage or via several stages, provided the initial and final conditions are the same.

Introduction:

Hess’s Law (1840) states that for a given chemical change the enthalpy change is the same whether the reaction takes place in a single stage or via several stages, provided the initial and final conditions are the same.

We will test the validity of this law using the reaction between sodium hydroxide and hydrochloric acid.

The reaction between solid sodium hydroxide and dilute hydrochloric acid can be carried out in two ways.

Method 1 NaOH(s) + HCl(aq) NaCl(aq) + H2O(l) ?H?1

Method 2 NaOH(s) NaOH(aq) ?H?2

then NaOH(aq) + HCl(aq) NaCl(aq) + H2O(l) ?H?3

According to Hess’s Law ?H?1 = ?H?2 + ?H?3

In calculating the enthalpy of reaction values in each of the above and following cases, it is assumed that:

a. The density of the solutions is 1 gcm-3

b. The specific heat capacity of the solutions is 4.2Jg-1K-1

c. The specific heat capacity of the polystyrene cup is negligible and may be ignored.

Apparatus:

* Polystyrene cup

* Thermometer

* HCl

* NaOH pellets

* H2O

* Digital weight reader

* Measuring cylinder

Method:

Measurement of ?H?1

Pour 50cm3 of 1M hydrochloric acid into the polystyrene cup and record its temperature as accurately as you possibly can. Weigh out 2g of sodium hydroxide pellets and quickly add these to the acid in your polystyrene cup. Stir and record the maximum temperature reached. Calculate the enthalpy change of this reaction in kJmol-1.

Measurement of ?H?2

Pour 50cm3 of water into an empty polystyrene cup and record its temperature as accurately as you possibly can. Weigh out 2g of sodium hydroxide pellets and quickly add these to the water in your polystyrene cup. Stir and record the maximum temperature reached. Calculate the enthalpy change of this process in kJmol-1.

Measurement of ?H?3

Pour 50cm3 of 1M sodium hydroxide into an empty polystyrene cup and record its temperature as accurately as you possibly can. Measure out 50cm3 of 1M hydrochloric acid into a measuring cylinder and record its temperature. Calculate the average initial temperature of the acid and the alkali. Add the acid to the alkali in your polystyrene cup and record the maximum temperature reached. Calculate the enthalpy change of this reaction inkJmol-1.

Data Collection:

Measurement of ?H?1

Measurement of ?H?2

Measurement of ?H?3

Initial Temperature/ï¿½C ? 0.1ï¿½C

The enthalpy change ?H?1 is given by multiplying the mass of HCl (m) by its specific heat capacity (Cp) and the change in temperature (?T). Since this reaction takes place in solution, which we assume to be mostly water. Hence the mass of the solution will be 50g and the specific heat capacity is given as 4.2J/g/ï¿½C. The calculation for the mass of the solution is as follows:

% Error in ?T =

The enthalpy change ?H?2 is given by multiplying the mass of NaOH (m) by its specific heat capacity (Cp) and the change in temperature (?T). Since this reaction takes place in solution, which we assume to be mostly water. Hence the mass of the solution will be 50g and the specific heat capacity is given as 4.2J/g/ï¿½C. The calculation for the mass of the solution is as follows:

% Error in ?T =

The enthalpy change ?H?3 is given by multiplying the mass of HCl (m) by its specific heat capacity (Cp) and the change in temperature (?T). Since this reaction takes place in solution, which we assume to be mostly water. Hence the mass of the solution will be 100g and the specific heat capacity is given as 4.2J/g/ï¿½C. The calculation for the mass of the solution is as follows:

So using the results found above we see that:

It can be seen that ?H?1 is almost equal to ?H?2 + ?H?3 but due to experimental errors such as heat being lost to the surroundings and not having exact readings they are not equal. Hess’s law has therefore been validated but the results would have been much more accurate if there was an insulating capsule around the polystyrene cup so as to prevent any heat from being lost to the environment and by having more accurate apparatus in terms of measuring cylinders and thermometers.