Investigating The Rate of a Reaction
Section: 6. Chemical Reactions | Syllabus: Cambridge AS Level Physics 9702
How to Measure Rate of Reaction Rate of reaction can be measured by monitoring how quickly reactants are used up OR how quickly products are formed over time. Rate = Change in amount / Time taken There are several methods to measure rate, depending on the type of reaction.
Method 1: Measuring Gas Volume Used when the reaction produces a gas . Measure the volume of gas produced over time. Two common setups: Gas syringe method - gas pushes piston in syringe, read volume directly Upturned measuring cylinder method - collect gas over water in inverted cylinder Example reactions: Marble chips (CaCO₃) + hydrochloric acid → CO₂ gas Magnesium ribbon + hydrochloric acid → H₂ gas Hydrogen peroxide decomposition → O₂ gas Advantages: Easy to set up, accurate measurements, can plot graph in real time Disadvantages: Gas syringe can stick, some gas may escape initially Gas Syringe Experiment - Detailed Method Apparatus needed: Conical flask Gas syringe (0-100 cm³) Delivery tube and bung Stopwatch/timer Reactants (e.g., acid and marble chips) Method: Set up apparatus with gas syringe connected to flask via delivery tube Add measured volume of acid to flask Add measured mass of marble chips and quickly seal with bung Start stopwatch immediately Record volume of gas in syringe every 10-30 seconds Continue until no more gas is produced (reaction complete) Repeat experiment with different conditions (temperature, concentration, surface area) Method 2: Measuring Mass Loss Used when a gas escapes from the reaction vessel.
The mass of the flask decreases as gas is lost. Setup: Place conical flask on a balance (with cotton wool in neck to prevent spray escaping) Record mass at regular time intervals Mass decreases as gas escapes Example reactions: Marble chips + acid → CO₂ escapes (mass decreases) Magnesium + acid → H₂ escapes (mass decreases) Advantages: Very accurate, simple setup, easy to see when reaction complete Disadvantages: Not suitable if gas is toxic (must be collected, not released), balance must be sensitive Important: Use cotton wool plug (not sealed bung) to allow gas to escape while preventing spray loss.
Method 3: Measuring Change in Transparency (Precipitation) Used when a precipitate (solid) forms in solution, making it cloudy/opaque. Setup - The "Disappearing Cross" Method: Draw a black cross (X) on paper Place conical flask on top of cross Add reactants and start timer Look down through flask from above Time how long until cross is no longer visible (obscured by precipitate) Classic example reaction: Sodium thiosulfate + hydrochloric acid → sulfur precipitate (yellow/cloudy) Na₂S₂O₃(aq) + 2HCl(aq) → 2NaCl(aq) + SO₂(g) + S(s) + H₂O(l) Advantages: Simple, cheap equipment, good for comparing rates Disadvantages: Subjective (different people judge "disappeared" differently), only gives ONE time measurement per experiment, less accurate Method 4: Measuring Color Change Used when reactant or product is colored .
Use a colorimeter or visual observation. Examples: Iodine clock reaction (blue-black color appears) Bromine water decolorization Permanganate reactions (purple → colorless) Method: Time how long until a specific color change occurs.
Investigating Effect of CONCENTRATION Typical experiment: Marble chips + hydrochloric acid Variables: Independent variable: Concentration of acid (e.g., 2.0, 1.5, 1.0, 0.5 mol/dm³) Dependent variable: Volume of CO₂ gas produced OR time taken Control variables: Temperature, mass/size of marble chips, volume of acid Method: Measure 50 cm³ of 2.0 mol/dm³ HCl into flask Add 5g of marble chips (medium size) Measure gas volume every 30 seconds for 5 minutes Repeat with 1.5, 1.0, and 0.5 mol/dm³ HCl Plot graph of volume vs time for each concentration Expected results: Higher concentration → steeper gradient → faster rate All graphs plateau at same height if acid is in excess Investigating Effect of TEMPERATURE Typical experiment: Sodium thiosulfate + hydrochloric acid Variables: Independent variable: Temperature (e.g., 20°C, 30°C, 40°C, 50°C, 60°C) Dependent variable: Time for cross to disappear Control variables: Concentration of both solutions, volumes used Method: Heat sodium thiosulfate solution to required temperature using water bath Pour into flask on top of cross on paper Add acid and start timer Time how long until cross disappears Repeat at different temperatures Calculate rate = 1/time Expected results: Higher temperature → shorter time → faster rate Rate approximately doubles per 10°C rise Investigating Effect of SURFACE AREA Typical experiment: Marble chips + hydrochloric acid Variables: Independent variable: Surface area (large chips, small chips, powder) Dependent variable: Volume of CO₂ produced OR time taken Control variables: Temperature, concentration of acid, MASS of marble chips, volume of acid Method: Use same mass (e.g., 5g) of marble chips in different sizes Add to same volume and concentration of acid Measure rate using gas syringe method Compare initial rate…
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