Learning focus
Explain electrochemical and energetic processes using ions, electrons, balanced equations, observations, energy pathways and quantitative evidence.
Axes and levels
The vertical axis represents energy or enthalpy and the horizontal axis represents reaction progress, not time. Reactants begin at a higher level than products.
Activation-energy arrow
Draw Ea from the reactant energy level to the top of the curve. The curve rises because energy is needed before bonds can rearrange.
Enthalpy arrow
Draw Delta H vertically from reactant level down to product level. The downward change corresponds to a negative value.
Interpretation
The profile shows that energy released during bond making exceeds energy absorbed during bond breaking. The net difference leaves the system and warms the surroundings.
Worked example
Reactants at 80 kJ/mol and products at 25 kJ/mol give Delta H = 25 – 80 = -55 kJ/mol.
Practical or data skill
Sketch an exothermic profile from numerical reactant, product and peak energies. Label all four required features.
Examination tip
Reaction progress is not a time axis.
Review questions and suggested answers
Question 1
Where are products in an exothermic profile?
Suggested answer
Below the reactants.
Question 2
What is the sign of Delta H?
Suggested answer
Negative.
Question 3
What does the peak represent?
Suggested answer
The activation-energy barrier or high-energy transition arrangement.
