Learning focus
Explain electrochemical and energetic processes using ions, electrons, balanced equations, observations, energy pathways and quantitative evidence.
Apparatus
A polystyrene cup in a beaker provides insulation and support. A lid reduces heat exchange and evaporation. A thermometer measures temperature and stirring distributes energy more evenly.
Procedure
Measure reactants, record initial temperature, mix quickly, stir and record the highest temperature for an exothermic reaction or lowest for an endothermic process. Repeat trials improve reliability.
Temperature change
For an exothermic process, Delta T is maximum minus initial. For an endothermic process, the magnitude is initial minus minimum, while the reaction classification determines the sign of Delta H.
Fair comparison
Use the same total volume or mass, concentrations, apparatus and starting temperature when comparing reactions. Temperature change alone is not a fair energy comparison if quantities differ.
Worked example
Initial 22.0 °C and maximum 35.5 °C give a rise of 13.5 °C. This indicates an exothermic reaction.
Practical or data skill
Carry out or analyse an acid-alkali temperature experiment. Record readings at regular intervals and plot a graph.
Examination tip
The thermometer measures the surroundings represented by the mixture, not the chemical system directly.
Review questions and suggested answers
Question 1
Why use a polystyrene cup?
Suggested answer
To reduce thermal energy transfer to the wider surroundings.
Question 2
Which temperature is used for an exothermic reaction?
Suggested answer
The maximum temperature reached.
Question 3
Why stir?
Suggested answer
To make the temperature more uniform.
