AS/A-level Chemistry - Enthalpy Changes

• An Enthalpy Change is the heat energy exchange with surroundings at constant pressure.

Do you know the difference between exothermic and endothermic reaction in AS/A-level Chemistry?

• During an exothermic reaction, heat energy is released to the surroundings.
• An energy loss from the chemicals is balanced by the same energy gain to the surroundings, which rise in temperature.
• In an exothermic reaction, ΔH is negative.

• During an endothermic reaction, heat energy is taken in from the surroundings.
• Any energy gain to the chemicals provided by the same energy loss from surroundings, which fall in temperature.
• In an endothermic reaction, ΔH is positive.

Standard Conditions

• A Pressure of 100 kPa/1 Atmosphere
• A stated temperature: 298k (25 degrees c)
• A concentration of 1 mol dm-3 (for aqueous solutions)
• E.g. The state of water at 298k and 1 atmosphere is a liquid.

• The standard enthalpy of combustion is when 1 mol of substance is burnt in excess oxygen under standard conditions of 298k and 1 atmosphere pressure.
• All Hydrocarbons burn with excess oxygen form carbon dioxide and water; however, hydrocarbons burnt in limited oxygen form carbon monoxide and water.
• E.g.  C2H4 (g) + 3O2 (g) --> 2CO2 (g) + 2H20(l)

• The Standard enthalpy of formation is when 1 mol of substance is formed from its constituent elements under standard conditions of 298k and 1 atmosphere pressure.
• For an element, the standard enthalpy change of formation is defined as zero.
• E.g. H2(g) + ½ O2(g) --> H20(l)

• The standard enthalpy change of neutralisation is the energy change when an acid and base form 1 mole of water under standard conditions of 298k and 1 atmosphere pressure.
• HCl(aq) +NaOH(aq) --> NaCl(aq) + H20(l)

References:
1. https://igcseandialchemistry.com/enthalpy-change-reaction/

That's the end of the topic!

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