Research Question:

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)     ΔHθ1

Method 2     NaOH(s)                 NaOH(aq)                                  ΔHθ2

        then     NaOH(aq)  +  HCl(aq)               NaCl(aq)  + H2O(l)  ΔHθ3

According to Hess’s Law     ΔHθ1 = ΔHθ2 + ΔHθ3

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

  1. The density of the solutions is 1 gcm-3
  2. The specific heat capacity of the solutions is 4.2Jg-1K-1
  3. The specific heat capacity of the polystyrene cup is negligible and may be ignored.
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Apparatus:

  • Polystyrene cup
  • Thermometer
  • HCl
  • NaOH pellets
  • H2O
  • Digital weight reader
  • Measuring cylinder

Method:

Measurement of ΔHθ1

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 ΔHθ2

Pour 50cm3 of water into an empty polystyrene cup and record its temperature as accurately ...

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