1.7 Simple Equilibria and Acid-Base Reactions

WJEC A-Level Chemistry

What is dynamic equilibrium?

when the rate of forward reaction equals the rate of reverse reaction

What is homogenous equilibrium?

all products and reactants are in the same state

What is a heterogenous equilibrium?

not all products and reactants are in the same state

What is Le Chatelier’s principle?

if a change is introduced into a system in equilibrium, the system responds by opposing the change

Factors affecting equilibrium

• concentration

• temperature

• pressure

• catalyst

Cu²⁺ + 4Cl^- ⇌ CuCl₄^2-

blue + . ⇌ green

What is observed if the concentration of Cl^- is increased?

1. Cl^- has been added so the system wants to decrease the amount of Cl^-

2. The rate of forward reaction increases

3. Equilibrium shifts in favour of products

4. A green colour is observed

2SO₂ + O₂ ⇌ 2SO₃

→ -ΔH exothermic

← +ΔH endothermic

What happens to amount of SO₃ if temperature is increased?

1. Temperature has been increased so the system wants to decrease the temperature

2. The rate of reverse endothermic reaction increases

3. Equilibrium shifts in favour of reactants

4. Amount of SO₃ decreases

N₂ + 3H₂ ⇌ 2NH₃

4 moles ⇌ 2 moles

What happens to amount of ammonia if pressure is increased?

1. Pressure has been increased so the system wants to decrease the pressure

2. The rate of forward reaction increases

3. Equilibrium shifts in favour of products

4. Amount of ammonia increases

Effect of catalysts

• no effect on position of equilibria

• only reduces time of reaction

Equilibrium constant formula

aA + bB ⇌ cC + dD = Kc = [C]^c x [D]^d

. [A]^a x [B]^b

How to work out Kc units

What is an acid?

proton (H⁺) donor

What is a base?

proton (H⁺) acceptor

base vs alkali

base = insoluble, alkali = soluble base

strong vs weak acid

an acid is stronger if it can dissociate completely

_ oxide + acid → (acid+base)

salt + water

_ hydroxide + acid → (acid+alkali)

salt + water

_ carbonate + acid →

salt + water + carbon dioxide

metal + acid →

salt + hydrogen

What is an amphoteric nature?

a substance that can act as an acid or a base e.g. water

What happens to pH if concentration increases?

pH decreases

pH =

-log [H⁺]

[H⁺] =

10^-pH

Haber process

• makes ammonia (alkali)

• N₂ + 3H₂ ⇌ 2NH₃

• exothermic, ΔH = -92 kjmol^-1

Haber process compromised conditions

• 200 atm (expensive+dangerous if higher)

• 450°C (reaction too slow if lower)

• Fe catalyst

Contact process

• makes sulfuric acid

• 2SO₂ + O₂ ⇌ 2SO₃

• exothermic, ΔH = -196 kjmol^-1

Contact process compromised conditions

• 1-2 atm (expensive+dangerous if higher)

• 400°C-450°C (reaction too slow if lower)

• V₂O₅ catalyst

pH of seawater

slightly alkaline

How is pH of seawater maintained?

maintained by buffering action of dissolved carbon dioxide, hydrogencarbonate, and carbonate ion

• resists change in pH

Increasing CO₂ affect on seawater

• increasing concentration = decrease in pH

• fears that increased acidity will be detrimental to marine life e.g. corals