AQA GCSE FOUNDATION Chemistry: Temperature Changes in Reactions

AQA GCSE Foundation Chemistry: Temperature Changes in Reactions

This tutorial will guide you through understanding temperature changes in chemical reactions, focusing on how to measure these changes and relate them to bond energies.

1. Measuring Temperature Changes:

• Materials: You will need a polystyrene cup, a thermometer, a graduated cylinder, and the reactants for your experiment.
• Procedure:
  1. Measure out a specific volume of your reactants using the graduated cylinder.
  2. Place the reactants in the polystyrene cup.
  3. Record the initial temperature of the reactants using the thermometer.
  4. Gently stir the mixture and record the temperature every 30 seconds.
  5. Continue monitoring the temperature until it reaches a constant value.
  6. Plot your results on a graph with time on the x-axis and temperature on the y-axis.

2. Interpreting Temperature Changes:

• Exothermic Reactions: Reactions that release heat energy into the surroundings are called exothermic reactions. They result in an increase in the temperature of the surroundings. This is because the energy released from the formation of new bonds is greater than the energy required to break the old bonds.
• Endothermic Reactions: Reactions that absorb heat energy from the surroundings are called endothermic reactions. They result in a decrease in the temperature of the surroundings. This occurs because the energy required to break the old bonds is greater than the energy released from the formation of new bonds.

3. Calculating Energy Changes:

• Formula: The energy change (q) can be calculated using the formula: q = mc?T where:

  • q = energy change (in Joules, J)
  • m = mass of the solution (in grams, g)
  • c = specific heat capacity of the solution (in J/g°C)
  • ?T = change in temperature (°C)

• Specific Heat Capacity: The specific heat capacity of a substance is the amount of energy required to raise the temperature of 1 gram of the substance by 1 degree Celsius. For water, the specific heat capacity is approximately 4.18 J/g°C.

4. Bond Energies and Temperature Changes:

• Bond Breaking: Breaking existing bonds requires energy input and is an endothermic process.
• Bond Formation: Forming new bonds releases energy and is an exothermic process.
• Overall Energy Change: The overall energy change in a reaction is the difference between the energy required to break bonds and the energy released when new bonds are formed.
  • Exothermic Reactions: Energy released (bond formation) is greater than energy required (bond breaking).
  • Endothermic Reactions: Energy required (bond breaking) is greater than energy released (bond formation).

5. Comparing Heat Absorption and Release:

• Graphical Analysis: By plotting your temperature data on a graph, you can visually compare the heat absorption and release of different reactions.
• Steeper Slope: A steeper slope on the graph indicates a greater change in temperature, implying a larger heat change (either absorption or release).

6. Example:

Let's say you are investigating the reaction between sodium hydroxide and hydrochloric acid. You mix 50 g of each solution in a polystyrene cup and observe a temperature increase from 20°C to 35°C.

• Calculating energy change:

  • m = 100 g (mass of the solution)
  • c = 4.18 J/g°C (specific heat capacity of water)
  • ?T = 15°C (change in temperature)
  • q = (100 g) x (4.18 J/g°C) x (15°C) = 6270 J

• Conclusion: Since the temperature increased, this indicates an exothermic reaction. The calculated energy change of 6270 J represents the amount of heat energy released by the reaction.

7. Safety Precautions:

• Always wear appropriate safety goggles when handling chemicals.
• Handle chemicals with care and avoid contact with skin.
• Work in a well-ventilated area.
• Dispose of chemicals properly as instructed by your teacher.