Topic 8 - Energetics I

• Enthalpy change is heat energy change. Enthalpy is generally measured under standard conditions of 100KPa and 298K.
• When bonds are broken in a reaction, energy is taken in from the surroundings. When bonds are formed, energy is given out.
• When energy is taken in from the surroundings, the enthalpy change is positive. When energy is released to the surroundings, the enthalpy change is negative.
• Overall enthalpy change = energy to break bonds + energy to make bonds
• Enthalpy changes can be shown on an energy level diagram. These also indicate is a reaction is endothermic or exothermic.
• In endothermic reactions, more energy is needed to break bonds than make new ones. Therefore, the overall change in enthalpy is positive and heat is taken in from the surroundings.
• In exothermic reactions, more energy is needed to make new bonds than break existing ones. Therefore, heat is given out and change in enthalpy is negative.
• Enthalpy change of reaction is defined as the enthalpy change when quantities of substances in standard states react completely under standard conditions.