GCSE Chemistry • Topic 6

Rates of Reaction & Equilibrium

Understanding reaction speed (Core) and the balance of reversible reactions (Higher Tier).

📌 What You’ll Learn

  • Measuring rates via decrease in reactants or increase in products
  • Collision Theory: Temperature, Concentration, and Surface Area
  • The role of catalysts (they are not used up!)
  • Dynamic Equilibrium in closed systems
  • Le Chatelier’s Principle and shifting the balance (Higher Tier)

🧠 How to Study

  • Always use the word "Frequency" for collisions
  • Identify which factor (temp, conc, SA) controls the rate from a graph
  • Remember: Catalysts help reach equilibrium faster but don't shift it
  • Practice shifting equilibrium based on heat absorption logic
1. Measuring Rates of Reaction (Calculations & Graphs)

Calculating Rate

Rate can be measured by the decrease in reactants or the increase in products over time.

$$ \text{Rate of Reaction} = \frac{\text{Amount of Reactant used OR Product formed}}{\text{Time}} $$

Interpreting Graphs

  • Steeper line: Faster reaction speed.
  • Horizontal line: Reaction has stopped. Why? The limiting reactant has been completely used up.
  • Mean Rate: Overall change divided by total time.
  • Tangent (Higher Tier): Slope of the tangent at a specific point gives the instantaneous rate.

Q1: If 50 $cm^3$ of gas is produced in 20 seconds, what is the mean rate?

$$ 50 / 20 = 2.5 \, \text{cm}^3\text{/s} $$

2. Collision Theory (Core Factors)

Successful Collisions

Particles must collide with energy $\ge E_a$ for a reaction to occur.

[Image illustrating collision theory factors: concentration, surface area, and temperature]
Factor Why it increases the rate? (Exam-Safe Language)
Temperature Particles move faster, increasing frequency of collisions, and a greater proportion of particles have energy $\ge E_a$.
Concentration / Pressure More particles in the same volume, increasing the frequency of collisions.
Surface Area For the same mass of solid, a powder has more particles exposed, increasing the frequency of collisions.
Catalyst Provides an alternative pathway with lower $E_a$. Catalysts are not used up during the reaction.
[Image showing effect of a catalyst on activation energy in a reaction profile]
⚠️ Exam Trap

You MUST use the word frequency or "per second" for collision rates. Just saying "more collisions" is too vague for full marks.

3. Reversible Reactions & Equilibrium (Higher Tier)

Dynamic Equilibrium

Occurs in a closed system when the forward and reverse reactions happen at the same rate and concentrations remain constant.

  • Energy: If forward is exothermic, reverse is endothermic (same energy amount).
  • Catalysts: A catalyst does not change the position of equilibrium; it only helps the system reach equilibrium faster.

Q1: True or False: At equilibrium, the concentration of reactants must be equal to the concentration of products.

False. Concentrations are constant, not necessarily equal.

4. Le Chatelier’s Principle (Higher Tier Only)

Shifting the Balance

The system shifts to counteract any change you apply.

Applied Change System's Response (Equilibrium Shift)
Increase Temp Increasing temp adds heat; system shifts in Endothermic direction to absorb it.
Increase Pressure Pressure changes only affect gases. System shifts to side with fewer gas molecules.
Increase Concentration System shifts away from that substance to use it up.
[Image summarizing Le Chatelier's Principle: effects of temperature, pressure, and concentration]

Q1: $A(g) + 2B(g) \rightleftharpoons C(g)$. If pressure is increased, where does equilibrium shift?

To the right (towards C), because there is only 1 molecule of gas on the right compared to 3 on the left.

⚡ Quick Revision Checklist

Rate Mastery:
  • Rate = Change / Time (units matter)
  • Frequency of successful collisions
  • "Greater proportion" (Temperature logic)
  • "Same mass" (Surface area logic)
Equilibrium Logic:
  • Closed system is mandatory
  • Le Chatelier (counteract logic)
  • Catalysts: Reach faster, don't shift
  • Pressure effects gas only

🚫 Brutal Exam Trap Summary

1. Don't say temp increases $E_a$; it increases the proportion of particles that have it.
2. Don't forget that a catalyst is not used up—this is a classic 1-mark recall.
3. Don't use "more collisions"; use "increased frequency of successful collisions".
4. Don't assume catalysts change the yield in equilibrium; they only change the speed to get there.

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