Enthalpy

• Enthalpy has the symbol H
• Enthalpy is all the heat energy stored within a chemical system.
• Exothermic reactions have a negative enthalpy change whereas endothermic reactions have a positive enthalpy change.
• Activation Energy is the minimum energy required for particles to successfully collide and react.
• We always determine enthalpy change under standard conditions.
• Standard conditions: 298K and 100kPa
• The standard enthalpy change of combustion is the enthalpy change when one mole of a substance is completely burnt in an excess of oxygen in standard conditions.
• The standard enthalpy change of neutralisation is the enthalpy change when one mole of water is formed in a neutralisation reaction under standard conditions.
• The standard enthalpy change for a strong acid and a strong base is always the same.
• Some energy is required to fully dissociate weak acids so this lowers the value of enthalpy change.
• Weak bases require more energy to fully ionise than acids so even further lower the enthalpy change.
• The standard enthalpy change of formation is the enthalpy change when one mole of a compound is formed from its elements under standard conditions.
• Standard enthalpy change of reaction is the enthalpy change when a reaction takes place in the molar quantities shown by it's balanced equation under standard conditions.
• Standard enthalpy change of reaction is only used when the reaction doesn't fit into any of the other categories.
• Bendo Mexo
• Bond breaking is endothermic
• Bond making is exothermic
• Bond enthalpy is the energy needed to break one mole of a specific bond in a molecule in it's gaseous state.
• Bond enthalpies are only an average as the same bond can have different enthalpies depending on the other atoms in the molecule.
• Combust a liquid fuel, use the thermal energy released to heat a known mass of water and measure the temperature change of said water.
• Measure mass of burner and fuel, before experiment, using balance. Measure your starting temperature of water. Remove cap from burner and immediately light the wick. Stir water whilst heating and measure the temp change of water. Extinguish flame and immediately replace cap. Measure final mass of fuel and final temp of water. Calculate the mass of fuel used and the respective temp change.
• q = m c T
• specific heat capacity of water is 4.18 J/g/mol
• H = -[ mcT / 1000n ]
• The Published value of standard enthalpy change of combustion would be much higher as alot of heat is dissipated to the surroundings rather than the water in our experiment.
• Leaving the spirit burner uncapped would cause fuel to evaporate making it seem as if more fuel has been burnt, so the reaction appears less exothermic.
• In incomplete combustion, less thermal energy is released than complete combustion.
• Bomb calorimeters used to improve accuracy.
• Hess' Law states that if a reaction can occur via multiple different pathways, the total enthalpy change for each of these pathways will all equal one another.