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Case study·Standard multi-step (3/5)·No calculator

Difficulty guide

The label describes the problem, not your ability.

1/5 - Foundational

Warm-up setup; checks one basic idea.

2/5 - Routine exam skill

Normal syllabus skill; usually one main rule or interpretation.

3/5 - Standard multi-step

Exam-style problem with multiple steps or a common trap.

4/5 - Hard exam skill

High-end exam problem; combines methods carefully.

5/5 - Challenge

Stretch problem beyond normal exam pressure.

Question

A student has two equations: I: X→Y\mathrm{X\rightarrow Y}, ΔH=−120 kJ\Delta H=-120\text{ kJ}; II: Y→Z\mathrm{Y\rightarrow Z}, ΔH=+40 kJ\Delta H=+40\text{ kJ}.
Total: 5 marks

(a) Add I and II to obtain the net reaction. (1 mark)

(b) Find net ΔH\Delta H. (2 marks)

(c) State why this addition is allowed. (2 marks)

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cbse-chemistry-11.u5.t5-4.case.010

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