Chapter 16

Equilibrium

How do chemical reactions occur?

Collision Model• Molecules react by colliding into one

another.

– This explains why reactions proceed faster at higher concentrations.• Higher concentration of reactants leads to greater

production of product. (More effective collisions)• Effective collisions are collisions that break the bonds of

reactants allowing products to form.

• Also, more effective collisions occur at higher temperatures(higher kinetic energy).– More collisions• Higher temperature = Higher kinetic energy (more

collisions.• Also, these collisions are more energetic. So, it is more

likely that these higher energy collisions will break the bonds needed to reach the activated complex.

Activation Energy (Ea)The energy needed to break bonds

Enzymes and other catalysts lower Ea

Catalyst- a substance that speeds up a reaction without being consumed.

Enzymes- catalysts produced inour bodies.

Catalysts act by lowering Ea

Factors that Affect Reaction Rate

1. Concentration2. Temperature3. Presence of a Catalyst

The Equilibrium Condition

Equilibrium usually implies balance or steadiness.

BUTIn chemistry there is another

definition.

Equilibrium the exact balancing of two processes, one of

which is the opposite of the other.

Reversible reactions are often in equilibrium.

Reversible Reactions

• When writing reversible reactions use an arrow that points in both directions.

N₂ + 3H₂ ↔ 2NH₃

N₂ + 3H₂ ↔ 2NH₃

As N₂ and H₂ are first combined the reaction for the formation of NH₃ can be observed.

Until, finally the process appears to stop.

What has happened?

Actually, we have reached chemical equilibrium.

Chemical EquilibriumA dynamic state where the concentrations of all reactants and products remain constant.

A + B ↔ C + D

At chemical equilibrium the rate of the forward reaction equals the rate of the reverse reaction.

Le Chatelier’s PrincipleStates that:

When a change is imposed on a system at equilibrium, the position of the equilibrium shifts in a direction that tends to reduce the effect of that change.

Changing concentration of one of the reactants or products affects equilibrium.

H₂O + CO ↔ H₂ + CO₂

If CO is added to the system, the [CO] will increase and additional product will be produced. This is often called a shift to the right. Right meaning products.

If instead, additional H₂ is added the opposite will then occur. This will cause a shift to the left(reactants).

Changing volume of the container will also cause a shift.

N₂ + 3H₂ ↔ 2NH₃ reactants = 4 mol gas products = 2 mol gas

• Since we know the volume of one mole of any gas at STP, (22.4L) as volume decreases the equilibrium will shift toward the lower number of moles of gas. In this case it shifts to the right, toward products.

• If volume was increased, instead, it would shift the opposite direction. To the left, toward reactants.

The effect of a change in temperature

First write the equation including any energy needed or released.

2SO₂ + O₂ ↔ 2SO₃ + energyEnergy released = exothermic

If heat is added (temperature increase), the equilibrium should shift away from the product of energy. That would mean a shift to the left. A shift toward reactants.

Equation with energy as a reactant

N₂ + O₂ + energy ↔ 2NOEnergy needed = endothermic

If heat is added (temperature increase), the equilibrium should shift away from the reactant of energy. That would mean a shift to the right. A shift toward products.