Hess' Law of Heat Summation

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Hess' Law of Heat Summation

Hess' Law states that:

"The enthalpy change for any reaction depends on the products and reactants and is independent of the pathway or the number of steps between the reactant and product".

BASICALLY: Hess' Law states "the heat evolved or absorbed in a chemical process is the same whether the process takes place in one or in several steps" ­­>This is also known as the "law of constant heat summation". All it means is that no matter how many steps the chemical reaction proceeds through, the total heat evolved or absorbed is the added heat values of each step and will always be a constant figure for that process. For example: When phosphoric acid (a tri­hydrogen acid) is neutralized with a base, the hydrogens are neutralised in 3 steps: STEP 1: H3P04 + NaOH ­> NaH2PO4 + H2O ­gives X amount of heat STEP 2: NaH2PO4 + NaOH ­> Na2HPO4 + H2O ­gives Y amount of heat STEP 3: Na2PO4 + NaOH ­> Na3PO4 +H2O ­gives Z amount of heat Therefore, the total heat of reaction ( Hrxn) is equal to:

There are two (2) ways to calculate the heat of reaction using Hess' Law:

1­ Equation Method (Algebraic Method) 2­ Heat of Formation Method (Summation Method)

Equation (Algebraic) Method
Strategy: ­add equations for reactions with known enthalpies so that their net result is the desired reaction

Sample Problem 1: Use the equations below to determine the enthalpy of reaction for the decomposition of hydrogen peroxide:

H2O2(l) H2O(l) + 1/2 O2(g)

The equations are: 1­ H2(g) + O2(g) H2O2(l) 2­ H2(g) + 1/2 O2(g) H2O(l)

Hf = ­188 kJ

Hf = ­286 kJ

Solution:

Sample Problem 2: Use the following equations: 1­ 1/2 N2(g) + 3/2 H2(g) NH3(g) 2­ 1/2 N2(g) + O2(g) NO2(g) 3­ H2(g) + 1/2 O2(g) H2O(g)

Hf = ­46.0 kJ
Hf = +34.0 kJ Hf = ­242.0 kJ

to calculate the enthalpy of reaction

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