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Manipulating Enthalpy Change in Chemical Equations

Key Concepts

  • If a chemical equation is reversed, the sign of ΔH is also reversed.

  • The value of ΔH given as kJ mol-1 refers to kJ per 1 mole of reactant or product as written in the equation.

Reversing Chemical Equations

If a chemical equation is reversed, the sign of ΔH is also reversed.

  • The equation for the formation of ammonia (NH3) from nitrogen gas and hydrogen gas is given below.
    N2(g) + 3H2(g) → 2NH3(g) + 92.4 kJ mol-1
    Energy is a product of the reaction, energy is released during the reaction. Another way to write this equation is:
    N2(g) + 3H2(g) → 2NH3(g)       ΔH = - 92.4 kJ mol-1
    The equation given above is exothermic (ΔH is negative), 92.4 kJ of energy per mole of N2(g) is released.

  • This equation can be reversed to show the decomposition of ammonia into nitrogen and hydrogen gas. This will require an input of 92.4 kJ of energy per mole of nitrogen gas, that is, the reaction will be endothermic.
    2NH3(g) + 92.4 kJ mol-1 → N2(g) + 3H2(g)
    Energy is a reactant, energy is absorbed during the reaction. Another way to write this equation is:
    2NH3(g) → N2(g) + 3H2(g)       ΔH = + 92.4 kJ mol-1
    The decomposition of ammonia is an endothermic reaction (ΔH is positive), 92.4kJ of energy per mole of nitrogen gas is absorbed by the reaction.

Manipulating the Enthalpy Change Term

The value of ΔH given as kJ mol-1 refers to kJ per 1 mole of reactant or product as written in the equation.

N2(g) + 3H2(g) → 2NH3(g)       ΔH = - 92.4 kJ mol-1

  • 92.4 kJ of energy is released for every 1 mole of N2(g)

  • 92.4 kJ of energy is released for every 3 moles of H2(g)

  • 92.4 kJ of energy is released for every 2 moles of NH3(g) produced.
  1. How much energy is released if only 1 mole of ammonia (NH3) gas is produced?

    92.4 kJ of energy is released in the production of 2 moles of ammonia gas
    Half as much energy will be released if only half the amount of ammonia gas is produced
    ½ x 92.4 = 46.2 kJ of energy will be released in the production of 1 mole of ammonia.

  2. How much energy is released if 10 moles of nitrogen (N2) gas and 30 moles of hydrogen (H2) gas is used in the reaction?

    92.4 kJ of energy is released for every 1 mole of N2(g)
    10 times as much energy will be released if 10 times the amount of reactants are used
    10 x 92.4 = 924kJ of energy will be released

  3. How much energy is released if 5 moles of hydrogen (H2) gas and 5/3 mole of nitrogen (N2) gas is used in the reaction?

    92.4 kJ of energy is released for every 3 moles of H2(g)
    The amount of energy released for every mole of hydrogen gas used = 92.4 ÷ 3 = 30.8 kJ mol-1 of H2(g)
    The amount of energy released for 5 moles of hydrogen gas = 5 x 30.8 = 154 kJ


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