Year 10 Science · Unit 2 · Lesson 6
Challenge Worksheet
Learning Goals
Find the mistake
A student wrote this answer
"Synthesis reactions always release energy because combining substances makes new, more stable bonds, this always releases heat. Decomposition reactions always absorb energy because you need to put energy in to break things apart. This is a rule that applies to all chemical reactions without exception. So if a reaction is exothermic, it must be a synthesis reaction, and if it's endothermic, it must be decomposition."
1. Identify the specific mistake(s) in this student's answer. Explain clearly what the student has misunderstood about the relationship between reaction type and energy change.
2. Provide one specific counterexample of a synthesis reaction that is endothermic, AND one specific counterexample of a decomposition reaction that releases energy. For each, name the reactants and products and briefly explain the energy change.
3. The real determinant of whether a reaction is exothermic or endothermic is the relative bond energies of reactants and products. Write a corrected version of the student's explanation that accurately describes what determines whether a reaction releases or absorbs energy.
1. The decomposition of hydrogen peroxide (2H₂O₂ → 2H₂O + O₂) releases energy when catalase enzyme is present. Yet the student's rule says decomposition always absorbs energy. Use the concept of activation energy and catalysts to explain why this reaction can be both a decomposition reaction AND exothermic.
2. Incitec Pivot, an Australian company, manufactures nitrogen fertilisers using the Haber process (synthesis of ammonia). The process requires high pressure and high temperature but uses an iron catalyst. Explain why a catalyst is used, and whether the use of a catalyst changes whether the reaction is exothermic or endothermic.
Wrap Up
In one sentence, what was the main idea of this lesson?