📝 The enthalpy changes

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When a chemical reaction occurs, energy is transferred to or from the surroundings. In an exothermic reaction, heat is given out to the surroundings, so the products have less energy than the reactants. In an endothermic reaction, heat is absorbed from the surroundings, so the products have more energy than the reactants. Energy changes in chemical reactions that lead to heating or cooling are called enthalpy changes (ΔH). Exothermic enthalpy changes are shown as negative values (–). Endothermic enthalpy changes are shown as positive values (+). Standard enthalpy changes are compared under standard conditions of pressure, 10⁵ Pa (100 kPa), and temperature, 298 K (25 °C).
Enthalpy changes can be calculated experimentally using the relationship: enthalpy change = – mass of liquid × specific heat capacity × temperature changes ΔH = – mcΔT. The standard enthalpy change of formation is the enthalpy change when one mole of a compound is formed from its elements under standard conditions.The standard enthalpy change of combustion is the enthalpy change when one mole of a substance is burnt in excess oxygen under standard conditions. The standard enthalpy change of atomization is the enthalpy change when one mole of gaseous atoms is formed from the element in its standard state under standard conditions. The standard enthalpy changes of hydration and solution can be defined in terms of one mole of a specified compound reacting completely. The standard enthalpy change of neutralisation can be defined in terms of one mole of water formed when hydrogen ions and hydroxide ions react.
Hess’s law states that ‘the total enthalpy change for a chemical reaction is independent of the route by which the reaction takes place’. Hess’s law can be used to calculate enthalpy changes for reactions that do not occur directly or cannot be found by experiment. Hess’s law can be used to calculate the enthalpy change of a reaction using the enthalpy changes of formation of the reactants and products. The calculation can be condensed into a simple formula:
∆H = Σ [∆H_f (products)] – Σ [∆H_f (reactants)]
The enthalpy change for a reaction can always be calculated if the enthalpy of combustion of reactants and products are known. The calculation can be condensed into a simple formula:
∆H = Σ [∆H_c (reactants)] – Σ [∆H_c (products)]
Bond breaking is endothermic; bond making is exothermic. Bond energy is a measure of the energy needed to break a covalent bond. Average bond energies are often used because the strength of a bond between two particular types of atom is slightly different in different compounds. Hess’s law can be used to calculate the enthalpy change of a reaction using the average bond energies of the reactants and products.
ΔH_r= ΔH₁ + ΔH₂
ΔH_r= enthalpy change for bonds broken + enthalpy change for bonds formed.

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