Physical Chemistry

Exothermic reactions

Exothermic reactions release heat to the surroundings, making the temperature rise.

Endothermic reactions

Endothermic reactions absorb heat, making the surroundings cooler.

Calorimetry experiments

Calorimetry experiments measure temperature changes during reactions such as combustion, neutralisation, dissolving, and displacement.

Q = mcΔT

The formula Q = mcΔT is used, where Q is the heat energy (J), m is the mass (g), c is the specific heat capacity (J/g°C), and ΔT is the temperature change.

Molar enthalpy change (ΔH)

ΔH = Q ÷ moles reacted, which gives the energy change per mole.

Energy level diagrams

In an exothermic reaction, products are lower in energy than reactants because energy is released; in an endothermic reaction, products are higher in energy as energy is absorbed.

Activation energy

The activation energy is shown as the peak of the curve in energy level diagrams.

Bond-breaking

Bond-breaking is endothermic because energy is needed to separate atoms.

Bond-making

Bond-making is exothermic because energy is released when new bonds form.

Enthalpy change calculation

ΔH = (energy needed to break bonds) - (energy released when bonds form).

Exothermic reaction energy

If more energy is released than absorbed, the reaction is exothermic.

Endothermic reaction energy

If more energy is absorbed than released, the reaction is endothermic.

Factors affecting reaction rate

Surface area, concentration, pressure, temperature, and catalysts affect reaction rate by changing how frequently and energetically particles collide.

Collision theory

Reactions happen when particles collide with enough energy (activation energy) and the correct orientation.

Catalyst

A substance that speeds up a reaction without being used up, by providing an alternative pathway with lower activation energy.

Reversible reaction

A reaction indicated by the symbol ⇌, meaning it can go both forward and backward.

Dynamic equilibrium

A state where the forward and reverse reactions occur at the same rate, keeping the concentrations of reactants and products constant.

Effect of temperature on equilibrium

Increasing temperature shifts equilibrium toward the endothermic direction; decreasing shifts it toward the exothermic side.

Effect of pressure on equilibrium

Increasing pressure shifts equilibrium toward the side with fewer moles of gas.

Catalyst and equilibrium position

A catalyst speeds up both forward and reverse reactions equally, helping the system reach equilibrium faster without changing the final balance of reactants and products.