Thermochemistry

Thermochemistry:

• The Law of Conservation of Energy – energy can neither be created nor destroyed but it can be changed from one form to the other form.
• Exothermic reaction – a chemical reaction that gives out heat to the surrounding.
• Endothermic reaction – a chemical reaction that absorbs heat from the surrounding.
• Surroundings do not involve in the reactions. Example: water, container, the air, solvent and thermometer.
• Heat of reaction – the heat change when the number of moles of reactants in the chemical equation reacts to form products in standard conditions.
• Standard conditions: temperature (25˚C / 298 K), pressure (1 atm), concentration of solution (1.0 mol dm^-3), reactants and products are at their normal physical states.
• Heat of precipitation – the heat change when one mole of a precipitate is formed from their ions in aqueous solution.
• Heat of displacement – the heat change when one mole of a metal is displaced from its salt solution by a more electropositive metal.
• Heat of neutralisation – the heat change when one mole of water is formed from the reaction between an acid and an alkali.
• Heat of combustion – the heat change when one mole of a substance is completely burnt in oxygen under standard conditions.

Exothermic reaction

1. Chemical energy –> Heat energy
2. The heat energy is transferred to the surrounding.
3. Temperature of the surrounding increases.

Example of Chemical Reactions:

• Respiration
• Burning of metal
• Reaction of an alkaline metals (Group 1) with water
• Reaction of a reactive metal with acid
• Neutralisation reaction between acid and alkali
• Reaction of a carbonate with acid
• Combustion of carbon compound
• Displacement reaction of metals
• Rusting of iron

Example of Physical Processes:

• Freezing process
• Condensation process
• Dissolving an alkali in water
• Dissolving an concentrated acid in water

Endothermic reaction

1. Heat energy –> Chemical energy
2. The heat is absorbed from the surrounding.
3. Temperature of the surrounding decreases.

Example of Chemical Reactions:

• Photosynthesis
• Decomposition of nitrate salts
• Decompositon of carbonates salts
• Reaction between acid with hydrogen carbonates

Example of Physical Processes:

• Melting process
• Boiling process
• Sublimation process
• Dissolving of ammonium salts in water
• Dissolving of potassium salts in water
• Dissolving of thiosulphate  in water

Heat of Reaction

1. Enthalpy (H) – absolute energy content of a substance.
2. Change in energy content (ΔH) – absolute energy content cannot be determined, but ΔH can be determined when the reactants are converted to the products.
3. 1 kJ (kilojoule) = 1000 J
4. The unit ΔH  is kJ

Energy Level Diagram

1. ΔH = H _product – H _reactant = negative value.
2. Example: CH₄(g) + 2O₂(g) –> CO₂(g) + 2H₂O(l) ΔH = -890 kJ
3. The value of ΔH is negative = exothermic reaction.
4. ΔH = H _product – H _reactant = positive value.
5. Example: N₂(g) + 3H₂(g) –> 2NH₃(g) ΔH = +91.8kJ
6. The value of ΔH is positive = endothermic reaction.