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

(0620 Extended, 5070 required) Collision theory and activation energy are 0620 Supplement content (6.2 S5-S7); 0620 Core candidates need only the effects of each factor on rate and that a catalyst speeds up a reaction and is unchanged at the end.

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

The “Effect on rate” column is 0620 Core; the “Because” column (collision theory) is 0620 Extended and 5070 required.

(0620 Extended, 5070 required) 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.
• (0620 Extended, 5070 required) Lower Eₐ.
• Do not change the position of equilibrium or the yield — only how fast it is reached (0620 Extended, 5070 required).
• 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

0620 Core: some reactions are reversible (⇌), and changing conditions can reverse a reaction (heating hydrated copper(II) sulfate or cobalt(II) chloride; adding water to the anhydrous salt). The rest of this section is (0620 Extended, 5070 required) (0620 6.3 Supplement 3-4).

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

Le Chatelier’s principle

(0620 Extended, 5070 required)

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

(0620 Extended, 5070 required) The Haber and Contact processes (equations, sources, conditions and the reasoning behind the conditions) are 0620 Supplement content (6.3 S5-S11).

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

• (0620 Extended, 5070 required) Say collisions are more frequent, not “more collisions”.
• (0620 Extended, 5070 required) Only temperature increases collision energy.
• A catalyst does not increase yield.
• (0620 Extended, 5070 required) Equilibrium needs a closed system.
• (0620 Extended, 5070 required) “Shifts to the right” is not enough — say towards products/reactants and why.
• (0620 Extended, 5070 required) 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.
• (0620 Extended, 5070 required) 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. (0620 Extended, 5070 required) In an exothermic forward reaction, what does raising temperature do to yield?
3. (0620 Extended, 5070 required) Does a catalyst change the equilibrium position?
4. (0620 Extended, 5070 required) Why is 450 °C used in the Haber process rather than a lower temperature?
5. (0620 Extended, 5070 required) State two features of dynamic equilibrium.
6. (0620 Extended, 5070 required) Why is 200 atm used in the Haber process rather than a much higher pressure?
7. (0620 Extended, 5070 required) State the source of nitrogen and of hydrogen used in the Haber process.
8. (0620 Extended, 5070 required) 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.