Rates of Reaction and Reversible Reactions: Revision Notes

Condensed for the final weeks. For the full explanation, use the Rates of Reaction and Reversible Reactions study guide.

Collision theory

A reaction occurs only when particles collide with sufficient energy (the activation energy, Eₐ) and in the correct orientation.

Rate increases if collisions become more frequent, more energetic, or both.

The four factors — and the correct explanation for each

Factor	Effect on rate	Because
Temperature ↑	Faster	Particles move faster → more frequent collisions and a greater proportion exceed Eₐ
Concentration ↑	Faster	More particles per unit volume → more frequent collisions
Pressure ↑ (gases)	Faster	Same effect as concentration — particles closer together
Surface area ↑	Faster	More particles exposed → more frequent collisions
Catalyst	Faster	Provides an alternative pathway of lower activation energy

Only temperature changes the energy of collisions. The others change frequency only — a distinction examiners test directly. At Extended/O Level standard, a temperature rise’s dominant effect is the greater proportion of collisions exceeding Eₐ, not just the extra frequency — this is why a modest temperature rise speeds a reaction up far more than frequency alone would suggest.

Catalysts

• Not used up; chemically unchanged at the end.
• Lower Eₐ; do not change the position of equilibrium or the yield — only how fast it is reached.
• Enzymes are biological catalysts.

Measuring rate

rate = amount of product formed / time
     = amount of reactant used  / time

Methods: gas volume collected (syringe), mass loss (balance), or time for a cross to disappear (turbidity). Each method suits a different reaction: gas-collection or mass-loss for reactions producing a gas, turbidity for reactions producing a precipitate that blocks a view of a cross beneath the flask.

On a graph, the steeper the curve the faster the rate. The curve levels off when a reactant is used up; a larger final volume means more product, not a faster rate.

Reversible reactions and equilibrium

Dynamic equilibrium — forward and reverse rates are equal, concentrations remain constant, in a closed system. Both reactions continue; nothing stops.

Le Chatelier’s principle

If a change is applied, the position of equilibrium shifts to oppose it.

Change	Shift
↑ Concentration of a reactant	Towards products
↑ Temperature	Towards the endothermic direction
↑ Pressure	Towards the side with fewer gas moles
Catalyst added	No shift — equilibrium reached sooner

The two industrial processes

HABER   N2 + 3H2 <=> 2NH3     exothermic forward
        450 C, 200 atm, iron catalyst

CONTACT 2SO2 + O2 <=> 2SO3    exothermic forward
        450 C, 200 kPa (2 atm), vanadium(V) oxide

Both use a compromise temperature: lower would give a higher yield but too slowly to be economic. Pressure is also a compromise: higher pressure would push both equilibria further towards the product, but very high pressures are expensive to generate and maintain, and for the Haber process present genuine safety risks — 200 atm is already a compromise, not the highest pressure achievable.

Raw materials: Haber’s nitrogen comes from the air, hydrogen from methane. Contact’s sulfur dioxide comes from burning sulfur or roasting sulfide ores, oxygen from the air.

Exam traps

• Say collisions are more frequent, not “more collisions”.
• Only temperature increases collision energy.
• A catalyst does not increase yield.
• Equilibrium needs a closed system.
• “Shifts to the right” is not enough — say towards products/reactants and why.
• Quoting the wrong catalyst for Haber vs Contact — iron for Haber, vanadium(V) oxide for Contact — the pressures and temperatures also differ between the two.
• Explaining temperature’s effect using only “more collisions,” without mentioning the greater proportion exceeding activation energy.
• Forgetting that the whole of the Haber and Contact process content — equations, sources, conditions and the equilibrium reasoning behind them — is Supplement in 0620 and simply required for every 5070 candidate; 0620 Core does not need any of it.

Self-test

1. Why does powdering a solid speed up a reaction?
2. In an exothermic forward reaction, what does raising temperature do to yield?
3. Does a catalyst change the equilibrium position?
4. Why is 450 °C used in the Haber process rather than a lower temperature?
5. State two features of dynamic equilibrium.
6. Why is 200 atm used in the Haber process rather than a much higher pressure?
7. State the source of nitrogen and of hydrogen used in the Haber process.
8. State the source of the sulfur dioxide and oxygen used in the Contact process.

Answers: 1. It increases surface area, exposing more particles, so collisions are more frequent. 2. Decreases it — the equilibrium shifts towards the endothermic (reverse) direction to oppose the rise. 3. No — it speeds up forward and reverse equally, so equilibrium is reached sooner at the same position. 4. A compromise: lower temperature would give a higher yield but the rate would be uneconomically slow. 5. Forward and reverse rates are equal; concentrations of reactants and products remain constant; the system is closed. 6. A much higher pressure would push the equilibrium further towards ammonia, but very high pressures are expensive to generate and maintain and present safety risks, so 200 atm is a compromise rather than the highest pressure achievable. 7. Nitrogen comes from the air; hydrogen comes from methane. 8. Sulfur dioxide comes from burning sulfur or roasting sulfide ores; oxygen comes from the air.