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Required Practical: Effect of Concentration

Chemistry: AQA GCSE Higher, topic 6.1.3

Effect of Concentration on the Rate of Reaction (Gas Volume Method)

This experiment aims to investigate how altering the concentration of a solution influences the rate at which a gas is produced during a chemical reaction.

Apparatus and materials

Apparatus and materials

  • Magnesium ribbon (cut into equal lengths)
  • Sulfuric acid solutions of different concentrations (e.g. 1.0 mol/dm3 and 1.5 mol/dm3)
  • 100 cm3 conical flask
  • Gas syringe with delivery tube
  • Rubber bung
  • Stop clock
  • Measuring cylinder
  • Safety goggles and a lab coat
Variables

Variables

  • Independent variable:
  • Concentration of the acid
  • Dependent variable:
  • Volume of gas produced over time
  • Control variables:
  • Length and surface area of magnesium ribbon
  • Volume of acid used
  • Temperature
  • Use of the same equipment setup for all trials
Method

Method

  • Set up the apparatus.
  • Measure a fixed volume of dilute sulfuric acid and pour it into the conical flask.
  • Add one strip of magnesium ribbon to the acid and quickly seal the flask with the bung.
  • Start the stop clock as soon as the magnesium is added.
  • Record the volume of hydrogen gas collected in the syringe every 10 seconds until no more gas is produced.
  • Repeat the procedure using a different concentration of acid.
Analysis

Analysis

  • Record time (s) and gas volume (cm3) in a results table.
  • Plot a graph of gas volume against time for each acid concentration.
  • Compare the gradients (slopes):
  • The steeper the line, the faster the rate of reaction.
  • To calculate the mean rate of reaction =

    TotalMassOfGasProduced(cm3)ReactionTime(s)\dfrac{Total Mass Of Gas Produced (cm^3)}{Reaction Time (s)}
Expected Results

Expected Results

  • Higher acid concentrations will produce hydrogen gas more quickly.
  • The same total volume of gas may be produced, but in less time.
  • This supports the hypothesis that a higher concentration increases the rate of reaction.
Hazards and precautions

Hazards and precautions

  • Dilute sulfuric acid:
  • Acid may irritate the skin and eyes
  • Wear goggles and rinse immediately if contact occurs
  • Reaction mixture fizzing:
  • May cause acid spray or foam to escape
  • Use a conical flask and avoid leaning over the mixture
  • Hydrogen gas:
  • Flammable if exposed to sparks or flame
  • Ensure there are no open flames nearby

Investigating the Rate of Reaction (Colour Change Method)

There are many ways to carry out this practical. This experiment investigates how temperature affects the rate of a chemical reaction by observing the rate at which a solution changes colour.

Apparatus and materials

Apparatus and materials

  • Measuring cylinders
  • Conical flask
  • Stop clock
  • Sheet of white paper with a black cross drawn on it
  • Bunsen burner or hot water bath (to adjust temperature)
  • Thermometer
  • Solution of choice
  • Dilute acid of choice
Variables

Variables

  • Independent variable:
  • Temperature of the sodium thiosulfate solution
  • Dependent variable:
  • Time taken for the cross to become invisible
  • Control variables:
  • Concentration and volume of both solutions
  • The same black cross and viewing method
  • Same observer for consistency
Method

Method

  • This reaction uses sodium thiosulfate and hydrochloric acid.
  • Pour 50 cm3 of sodium thiosulfate solution into a conical flask.
  • Place the flask on top of a piece of white paper with a black cross drawn in the centre.
  • Measure 10 cm3 of dilute hydrochloric acid in a separate cylinder.
  • Add the acid to the flask, swirl, and start the clock.
Method (continuation)

Method (continuation)

  • Observe the solution and note the time when the cross can no longer be seen.
  • Record the temperature of the reaction mixture.
  • Repeat the experiment at different starting temperatures:
  • 20°C, 30°C, 40°C, 50°C
Analysis

Analysis

  • Calculate 1000 ÷ time (s) for each temperature to estimate the reaction rate.
  • Plot a graph of:
  • Rate of reaction (1/s) on the y-axis
  • Temperature (°C) on the x-axis
  • Draw a smooth curve of best fit.
  • The line should show that higher temperatures lead to faster reactions due to more energetic particle collisions.
Explanation

Explanation

  • When temperature increases, particles move more quickly and collide more often with enough energy to react.
  • This means the reaction occurs faster, and the sulfur precipitate forms in a shorter time.

Test yourself on Required Practical: Effect of Concentration

  • What should be done immediately after adding the magnesium ribbon to the acid?

  • What does a steeper gradient on a graph of gas volume against time show about the reaction?

  • Using the formula for mean rate of reaction, what happens to the calculated rate if the same volume of gas is produced in less time?

  • A graph of gas volume against time shows two lines for different acid concentrations. One line has a much steeper gradient than the other. Which trial most likely used the higher concentration of acid?

  • A student compares two trials using different concentrations of sulfuric acid, but the same length of magnesium ribbon in each. Based on the expected results, what should be true about the total volume of gas produced in each trial?

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