Easy summary for enthalpy changes a level chemistry

Easy summary for enthalpy changes a level chemistry 

learning outcomes You should be able to: ■

■ explain that some chemical reactions are accompanied by exothermic or endothermic energy changes, mainly in the form of heat energy, and that energy changes can be exothermic or endothermic ■

■ explain and use the terms enthalpy change of reaction and standard conditions with reference to enthalpy changes of: formation, combustion, hydration, solution, neutralisation and atomisation ■

■ explain and use the term bond energy ■

■ calculate enthalpy changes from experimental results, including the use of the relationship: enthalpy change, ∆H = –mc∆T

■ apply Hess’s law to construct simple energy cycles and carry out calculations, in particular: – determining enthalpy changes that cannot be found by direct experiment – calculating average bond energies ■

■ construct and interpret a reaction pathway diagram in terms of the enthalpy change of reaction and the activation energy

Summary ■ 

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, 105Pa (100 kPa), and temperature, 298K (25 °C). 

■ Enthalpy changes can be calculated experimentally using the relationship: enthalpy change = –mass of liquid × specific heat capacity × temperature change ΔH = –mcΔT 

■ The standard enthalpy change of formation (ΔH —O f ) is the enthalpy change when one mole of a compound is formed from its elements under standard conditions. 

■ The standard enthalpy change of combustion (ΔH —O c ) is the enthalpy change when one mole of a substance is burnt in excess oxygen under standard conditions. 

■ The standard enthalpy change of atomisation (ΔH —O at) 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. 

■ Hess’s law can be used to calculate the enthalpy change of formation of a compound using the enthalpy changes of combustion of the reactants and 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.