Unit 2.1

• An enthalpy change is the change in energy which occurs during a chemical reaction
• The lower the energy of a compound, the more stable it is.
• Enthalpy change values are given under standard conditions. at 25 degrees C (298K) and 1atm (101KPa).
• The enthalpy change of formation is the enthalpy change when one mole of a compound is formed from elements in their natural state.
• The enthalpy change of combustion is the enthalpy change when one mole of a substance is completely burnt to form its products.
• Hess's Law states that the total enthalpy change for a reaction is independent of the route taken.
• The enthalpy change of a solution can be calculated using the equation( ΔH=-mcΔT÷n )where m is mass of solution (g), c is specific heat capacity of water (4.18JgK), ΔT is temperature change and n is number of moles os substance not in excess
• A positive value means endothermic
• A negative value means exothermic
• E=mcΔT is the equation to find heat (J) using temperature change, mass of the substance and the specific heat capacity of water.
• The rinciples of conservation of energy states that energy cannot be destroyed or created, only transferred from one form to another.
• Enthalpy change of formation can be calculated by the equation: total enthalpy of products - total enthalpy of reactants.
• Enthalpy change of combustion can be calculated by the equation: total enthalpy of reactants - total enthalpy of products
• Reactions which result in enthalpy changes include: neutralisation, combustion, displacement and ionisation.