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Chemistry · Rate and Extent of Chemical Change

Rate of Reaction and Calculations

Spec 5.6.1.1 📙 Higher
📖 In-Depth Theory

What is Rate of Reaction?

The RATE OF REACTION measures how quickly reactants are used up or products are formed.
FAST reactions: explosions, burning, acid + reactive metal.
SLOW reactions: rusting, paint drying, decay.
Rate can be measured by monitoring:
The DECREASE in mass of reactants over time.
The INCREASE in mass or volume of products over time.
The DECREASE in concentration of a reactant.
The INCREASE in concentration of a product.
The amount of GAS PRODUCED per unit time.
The CHANGE IN COLOUR or TURBIDITY (cloudiness) over time.

Calculating Rate of Reaction

RATE OF REACTION formula:
Rate = quantity of reactant used or product formed ÷ time
Units depend on what is measured:
If measuring VOLUME of gas: rate = cm³/s or cm³/min.
If measuring MASS: rate = g/s or g/min.
If measuring CONCENTRATION: rate = mol/dm³/s.
From a GRAPH:
The GRADIENT (slope) of a graph of quantity vs time = the rate at that point.
Steep gradient → fast rate.
Flat gradient → slow rate (or reaction finished).
The gradient DECREASES over time as reactants are used up.
EXAMPLE:
A reaction produces 60 cm³ of gas in 120 seconds.
Rate = 60 ÷ 120 = 0.5 cm³/s
MEAN RATE vs INSTANTANEOUS RATE:
MEAN RATE = total quantity ÷ total time (average over the whole reaction).
INSTANTANEOUS RATE = gradient of a tangent to the curve at a specific point in time.

Measuring Rate Experimentally

Common experimental methods for measuring rate:
GAS COLLECTION:
Collect gas produced over water in an inverted measuring cylinder, or in a gas syringe.
Measure volume of gas collected at regular time intervals.
Plot volume vs time → gradient = rate.
Example: Mg + 2HCl → MgCl₂ + H₂ (measure H₂ collected).
MASS LOSS:
Place reaction flask on a balance.
CO₂ escapes → mass decreases.
Record mass at regular intervals.
Example: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂ (measure mass loss).
COLOUR CHANGE / TURBIDITY:
For reactions involving a colour change — measure colour intensity with a colorimeter.
For precipitation reactions — observe when a mark under the flask becomes invisible.
Example: sodium thiosulfate + HCl → sulfur precipitate clouds the solution.
Time taken for the cross to disappear → inversely proportional to rate.
⚠️ Common Mistake

The rate DECREASES over time in a closed system as REACTANTS ARE USED UP — fewer reactant particles mean fewer collisions. The graph flattens out when the reaction is complete (all reactants used). A steeper initial gradient = faster initial rate.

📐 Variables
rateRate of reaction (rate) is measured in cm³/s or g/s ()
📐 Key Equations
Rate = quantity of product formed (or reactant used) ÷ time
Rate = gradient of graph of quantity vs time
📌 Key Note

Rate = quantity ÷ time. Units: cm³/s, g/s, mol/dm³/s. Rate = gradient of quantity-vs-time graph. Steeper = faster. Rate decreases over time (reactants used up). Mean rate = total ÷ total time. Instantaneous rate = tangent gradient at a point.

🎯 Matching Activity — Rate of Reaction Concepts

Match each term to its correct definition. — drag the symbols on the right to match the component names on the left.

Rate of reaction
Drop here
Steep gradient on graph
Drop here
Flat gradient on graph
Drop here
Mean rate
Drop here
Instantaneous rate
Drop here
How quickly reactants are used up or products are formed — quantity ÷ time
Fast rate — large quantity produced in short time
Gradient of tangent drawn at a specific point on the curve
Total quantity produced ÷ total time taken for reaction
Slow rate or reaction complete — little change per unit time
⚽ FIFA Worked Examples
Rate Calculation

A reaction between calcium carbonate and HCl produces 84 cm³ of CO₂ in 60 seconds. Calculate the mean rate of reaction.

F

Rate = quantity of product ÷ time

I

Rate = 84 cm³ ÷ 60 s

F

Rate = 1.4

A

Rate = 1.4 cm³/s

⭐ Higher Tier Only

Draw tangents to curves to find instantaneous rate at a given time. Compare initial rates by comparing gradients at t=0 for different experiments. Interpret rate-concentration graphs — linear relationship indicates rate directly proportional to concentration.

🧪 Required Practical

🔬 RP6 (Chemistry) — Investigate the effect of concentration on the rate of reaction between sodium thiosulfate and hydrochloric acid (cross disappears). Alternatively: rate of reaction between marble chips and HCl by mass loss or gas collection.

Know the method, variables, equipment and how to analyse results.

🎯 Test Yourself
Question 1 of 2
1. A reaction produces 120 cm³ of gas in 4 minutes. What is the mean rate of reaction in cm³/s?
2. On a graph of gas volume vs time, what does a steeper gradient indicate?
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