Higher Chemistry | Unit 3 Part 2

Enthalpy

The measure of the chemical energy in a substance.

Exothermic reactions

- Reactions which give out heat, notated by -ΔH.

- May need excess heat energy to be removed to prevent extreme temperature rises and the reaction becoming out of control.

Endothermic reactions

- Reactions which take in heat, notated, by +ΔH.

- May need to be supplied with heat energy to maintain the reaction rate, which can be costly.

Describe ways you can calculate the enthalpy change of a reaction:

- Molar bond enthalpies, for reactions in the gaseous phase.

- Eh = cmΔT and ΔH = Eh/n, when heat is transferred to water in an experiment.

- Hess’ Law, where multiple routes for a reaction are given.

Molar bond enthalpy

- The energy required to break one mole of bonds in a diatomic molecule. Measured in kJ mol^-1.

- Bonds forming release energy and so are exothermic reactions (-ΔH).

- Bonds breaking absorb energy and so are endothermic reactions (+ΔH).

Mean molar bond enthalpy

The average energy required to break one mole of bonds for a bond which occurs in a number of compounds. Measured in kJ mol^-1.

Enthalpy of combustion

The energy released when one mole of a substance is burned completely in oxygen. Always exothermic.

Hess’ Law

- The enthalpy change for a chemical reaction depends on the initial and final states of the reaction, and is independent of the route taken.

- ie. ΔH1 = ΔH2 + ΔH3

Equilibrium

- The point in a reversible reaction where the rate of the forward reaction equals the rate of the reverse reaction.

- Equilibrium can only be reached when the reaction occurs in a closed system (ie. a sealed container) so no reactants or products are lost. It is achieved whether the reaction starts from the reactants or products side.

- Concentration of products and reactants remain constant, though not necessarily at equal concentrations.

Dynamic equilibrium

Equilibrium in which the reaction has not stopped (ie. reactants and products are still being formed).

Position of equilibrium

- The relative concentration of reactants and products.

- Lying to the right favours the products and the forward reaction, so there is a higher concentration of product than reactant.

- Lying to the left favours the reactants and the reverse reaction, so there is a higher concentration of reactant than product.

Chatelier’s principle

When a system is in equilibrium, the equilibrium position will move to counteract any change made to the system.

Concentration of reactant on equilibria

- Increasing reactants shifts equilibrium to the right, towards products.

- Decreasing reactants shifts equilibrium to the left, towards reactants.

Concentration of products on equilibria

- Increasing products shifts equilibrium to the left, towards reactants.

- Decreasing products shifts equilibrium to the right, towards products.

Pressure on equilibria

- Increasing pressure shifts equilibrium to the side with fewer numbers of gaseous moles.

- Decreasing pressure shifts equilibrium to the side with greater numbers of gaseous moles.

Temperature on equilibria

- Increasing temperature shifts equilibrium towards the endothermic direction.

- Decreasing temperature shifts equilibrium towards the exothermic direction.

Catalyst on equilibria

No effect on equilibria, but allows equilibria to be reached quicker at a lower temperature (ie. reaction is sped up.)

Chromatography

- A technique used to separate components present in a mixture, and consists of a stationary and mobile phase.

- Different molecules have different attractions for these phases depending on their polarities and/or size.

Rf value

- Distance travelled by spot / distance travelled by solvent

- The closer the value is to 1, the further it has travelled.

Volumetric analysis

The use of a solution of accurately known concentration in a quantitative reaction to determine the concentration of another substance.

Self-indicating titration

A titration which turns a different colour when its end point has been reached (eg. potassium permanganate goes from purple to colourless once reduced).