Revision:Enthalpy

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What are the thermodynamic standard conditions?

298K, 1 atm pressure, concentration of 1mol per dm3

What is enthalpy change?

When the heat content stored in a chemical system is either:

lost by a chemical system = heat is gained by surroundings (Exotherimic) or; gained by a chemical system = heat is lost by surroundings (Endothermic)

Enthalpy change is usually measured in KJ per mol.

Define standard enthalpy of neutralisation

The heat change when a known concentration of acid and base react in their molar quantities by removing 1 proton, forming 1 mole of water in a specified equation under standard conditions.

Why is enthalpy change for exothermic reactions negative?

Because energy has been released to the surroundings. The products therefore have a lower enthalpy than the reactants and the final enthalpy is less than the initial enthalpy.

Classify as endothermic or exothermic

• NaOH dissolves in water & temp. of solution rises - Exothermic
• Ammonium Chloride dissolves in water and temp. of surroundings drop - Endothermic
• H and O combine explosively into water - Exothermic
• Liquid water condenses into ice at 0 C - Exothermic
• Liquid Nitrogen boils spontaneously at room temp.- Endothermic

Why is standard enthalpy changes of combustion always negative?

Because heat is always involved, the reaction is exothermic and therefore heat is lost to the surroundings. The energy required to break bonds is always less than the energy released when bonds are formed.

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