Rate of Reaction and Calculations | Chemistry

📖 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

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Steep gradient on graph

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Flat gradient on graph

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Mean rate

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Instantaneous rate

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Slow rate or reaction complete — little change per unit time

How quickly reactants are used up or products are formed — quantity ÷ time

Total quantity produced ÷ total time taken for reaction

Fast rate — large quantity produced in short time

Gradient of tangent drawn at a specific point on the curve

⚽ 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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🧪 Rate of Reaction and Calculations

What is Rate of Reaction?

Calculating Rate of Reaction

Measuring Rate Experimentally

🔬 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.

Mr. Badmus AI