• Heat is a form of energy that flows from something at a higher temperature to something at a lower temperature.
• Temperature is a measure of the average kinetic energy of particles.
• The ‘system’ is our chemical reaction and the ‘surroundings’ is everything else in the Universe
• In an exothermic reaction, heat energy is transferred from a system (chemical reaction) to the surroundings– the surroundings get hotter.
• In an endothermic reaction, a system (chemical reaction) takes in heat energy from the surroundings– the surroundings get cooler.
• ∆H for an exothermic reaction is negative.
• ∆H for an endothermic reaction is positive.
• Total energy is conserved in a chemical reaction.
• The term stability is usually used to describe the relative energies of reactants and products in a chemical reaction. If the products have less energy than the reactants then they are more stable.
• Standard enthalpy change of reaction (∆Hr ) is the enthalpy change (heat given out or taken in) when molar amounts of reactants, as shown in the stoichiometric equation, react together under standard conditions to give products.
• Standard enthalpy change of combustion (∆Hc ) is the enthalpy change when one mole of a substance is completely burnt in oxygen under standard conditions.
• Enthalpy change of neutralisation (∆Hn) is the enthalpy change when one mole of water molecules are formed when an acid (H+) reacts with an alkali (OH−) under standard conditions
• Enthalpy change of solution (∆Hsol) is the enthalpy change when one mole of solute is dissolved in excess solvent to form a solution of ‘infinite dilution’ under standard conditions
• Standard enthalpy change of formation (ΔHf ) is the enthalpy change when one mole of a substance is formed from its elements in their standard states under standard conditions.
• Standard state refers to the pure substance at 100 kPa and a specified temperature (assume 298 K unless another temperature is specified).
• ΔHf for any element in its standard state is zero/The enthalpy for any element in its natural state is zero (such as O2, C2, H2)
• ΔHr=ΣΔHf(products) − ΣΔHf(reactants)

5.3 bond enthalpy

• Bond enthalpy is the enthalpy change when one mole of covalent bonds, in a gaseous molecule, is broken under standard conditions.
• Bond enthalpies are used for reactions occurring in the gaseous state.