Energetics

Energetics

• Enthalpy is the heat energy stored in a chemical system- enthalpy changes can be measured from temperature changes when a chemical reaction takes place
• Enthalphy change is the heat energy change at constant pressure
• units are KJmol-1
• Exothermic - heat energy given out to surroundings, temperature of surroundings INCREASES and enthalpy change is NEGATIVE
• exothermic reactions- combustion of fuels, respiration
• Endothermic- heat energy is taken in from the surroundings, temperature of surroundings DECREASES and enthalphy change is POSITIVE
• Endothermic reactions often require the input of energy, examples: thermal decomposition of CaCO3 and photosynthesis
• Activation energy - minimum energy required to start a reaction by the breaking of bonds (energy needed to put in)
• most endothermic reactions must be heated continously in order to provide the necessary energy.
• Bond enthalpy - energy required to break one mole of a given covalent bond in the molecules in the gaseous state, bond enthalpy values are ALWAYS POSITIVE because bond breaking requires energy (endothermic). 
• Bond enthalpies give an indication of the relative strength of a covalent bond-(the greater the value, the stronger the bond)
• Mean bond enthalpy - energy required to break a covalent bond, averaged for that type of bond in a range of different compounds.
• enthalpy change = (energy released when new bonds form) - (energy required…