WJEC AS Chemistry Unit 1.7 - Simple Equilibria and Acid-Base Reactions

Reversible reaction

• a reaction which can go in either direction depending on the conditions

• when you have a reversible reaction in a closed system, a dynamic eqilibrium is reached

Dynamic Equilibrium

in a reversible reaction the rate of forward reaction is equal to rate of backward reaction

Equilibrium

• rate of forward reaction=rate of backward reaction

• changes occur on a molecular level only

• concentration of all reactants and products remains constant, therefore no observable overall change

• Conditions; to reach equilibrium, the reaction must be a closed system (all chemicals kept in), therefore rates of reaction equal

Position of Equilibrium

• Proportion of products and reactants in an equilibrium mixture

• Changed if conditions changed in a way which changes the rate of the forward or backward reactions

• can be changed by changing the reaction conditions

• Influenced by concentration, temperature and pressure

• May result in colour change

• Lies to the left = more reactants than products, therefore higher reactant concentration

• Lies to the right = more products than reactants, therefore high product concentration

Le Chatelier’s Principle

If a system at equilibrium is subjected to a change then the position of equilibrium to shift to minimise/oppose that change

Effect of concentration change on equilibrium

• Increase concentration of a reactant;

  • position of equilibrium will move so that the conc of this reactant decreases by reacting with other reactants and turning into products

  • position of equilibrium moves to the right

  • more products formed

• Decreasing the concentration of a reactant;

  • position of equilibrium will move so that the conc of this reactant increases again

  • More products will react to replace the reactant that’s been removed

  • The position of equilibrium moves to the left

• Increasing the concentration of a product;

  • moves to the left as it shifts to decrease the conc of prodycst

  • more reactancts formed

Effect of temperature on position of equilibrium

• If enthalpy change is negative;

  • Reaction is exothermic

  • Temperature increase; position of equilibrium moves to the left

• If enthalpy change is positive;

  • reactions is endothermic

  • Temperature increase; position of equilibrium moves to the right

• An increase in temperature moves the position of the equilibrium in the endothermic direction

Effect of a catalyst on position of equilibrium

• decreases time it takes to reach equilibrium by lowering the activation energy

• increases rate of forward and backward reactions to the same extent

• does not affect the position of equilibrium

Equilibrium Constant, K_c

• Solids are never included

• Not affected by pressure or concentration, only temperature

• Use square brackets around formulae (represents conc in moldm^-3)

• Units vary. Must state units. Can have not units

• Greater value; the further the positions of equilibrium lies to the right

• Smaller value; the further the positional of equilibrium lies to the left

Acids

• Donors of H⁺ (aq)

• can be described as strong/weak or concentrated/dilute

Bases

• acceptors of H⁺ (aq)

• When disolved in water, it is called an alkali. Common ion to all alkalis = hydroxide ion

Strong vs Weak Acid

• A strong acid is fully dossociated/ionised in an aqueous solution. The more easily/quicker an acid can donate H+, the stronger the acid

• A weak acid is only partially disscoiated in aqueous solution

Concentrated vs Dilute acid

• A concentrated acid consists of a large quantity of acid and a small quantity of water

• A dilute acid consists if a large quantity of water and a small quantity of acid

• Possible to have a dilute solution of a strong acid or a concentrated solution of a weak acid

Monobasic acid

an acid that has 1 replacable hydrogen ion, such as HCl

Produces 1 mole of H+(aq) per 1 mole of acid

Hydrogen ion concentration = the concentration of the acid

Dibasic acid

An acid that has 2 replacable hydrogen ions such as H2SO4

Produces 2 moles of H+(aq) per 1 mole of acid

Hydrogen ion concentration =2x concentration of the acid

Relationship between pH and H+(aq) concentration

• acidity of a solution; a measure of the conc of the aqueous hydrogen ion

• pH=-log[H+] where [H+] is the conc of H+ in mol dm-3

• [H+]=10^-pH

Acid-based titrations