Exothermic reactions
5 marks · HardThe decomposition of a certain substance is shown by the energy profile diagram below.
State whether this reaction is exothermic or endothermic, and explain your answer using the energy values from the diagram.
Calculate the value of ΔH (in kJ/mol) for this reaction using the energy profile diagram.
The activation energy for the reaction is the energy difference between the reactants and the highest point on the diagram. Use the diagram to state the value of the activation energy for this reaction.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(b) Exothermic; because the products are at a lower energy (3 kJ/mol) than the reactants (8 kJ/mol), so energy is released in the reaction. · must show States exothermic, Correct explanation using diagram numbers
(c) -5 kJ/mol · must show Calculation: ΔH = energy of products - energy of reactants
(d) 6 kJ/mol · must show Activation energy = TS - Reactants
Must show: Explanation using correct energy values
Show mark scheme and worked solution
Marking points
B1 (b) · 1 mark · Correctly identifies the reaction as exothermic using the diagram
B2 (b) · 1 mark · Explains this is because products are at a lower energy (3 kJ/mol) than reactants (8 kJ/mol); must quote values
C1 (c) · 1 mark · Uses correct method: ΔH = energy of products - energy of reactants (follow-through)
C2 (c) · 1 mark · Correct calculation: -5 kJ/mol (accept -5, accept value -4.5 to -5.5 for tolerance)
D1 (d) · 1 mark · States activation energy = 6 kJ/mol (difference from reactants at 8 to TS at 14); accept 5.5–6.5 kJ/mol
Worked solution
b) The reaction is exothermic (B1) because the products are at a lower energy (3 kJ/mol) than the reactants (8 kJ/mol) (B2).
So, ΔH = -5 kJ/mol (C1, C2).
So, activation energy = 6 kJ/mol (D1).