GCSE Combined Science Chemical energy changes questions by topic

Three reactions, three thermometers: label each change exothermic or endothermic.

1. (a)Methane burning on a hob.[1]
2. (b)Calcium carbonate breaking down when heated strongly in a furnace.[1]
3. (c)State the sign of ΔH for an exothermic reaction.[1]
4. (d)State what happens to the temperature of the surroundings during an endothermic reaction.[1]

This question is about reaction profiles.

1. (a)State whether the products sit above or below the reactants on an endothermic profile.[1]
2. (b)State what ΔH represents.[1]
3. (c)Give one everyday use of an endothermic change.[1]
4. (d)Give one everyday use of an exothermic change.[1]

A student carries out the temperature-change practical for the first time. The instructions are jumbled: record the peak temperature; add 5 cm³ of alkali and stir; measure 25 cm³ of acid into the polystyrene cup; record the acid’s starting temperature.

1. (a)Write the four steps in the correct order.[2]
2. (b)Name the container used instead of a glass beaker and state why it is chosen.[2]

Sketch data for a reaction: starting energy 100 kJ/mol, highest point 260 kJ/mol, ending energy 60 kJ/mol.

1. (a)State whether the reaction is exothermic or endothermic.[1]
2. (b)Calculate ΔH.[1]
3. (c)State what happens to ΔH if the reaction is repeated with a catalyst.[1]
4. (d)State what happens to the peak if a catalyst is added.[1]

Draw or describe an endothermic reaction profile, then answer about the peak.

1. (a)Describe the positions of the reactants, the products and the ΔH arrow.[2]
2. (b)Name the quantity measured from the reactants up to the top of the curve and define it.[2]
3. (c)State the effect of a catalyst on that quantity.[1]

Two students run the temperature-change practical. Student A uses a polystyrene cup with a lid and measures a maximum rise of 7.2 °C; Student B uses an open glass beaker and measures 5.1 °C.

1. (a)Calculate how much larger Student A’s rise is.[1]
2. (b)Explain why Student B’s result is lower.[2]
3. (c)Suggest one further improvement to Student B’s method.[2]

Hydrogen bromide can be made directly from its elements in the gas phase: H₂ + Br₂ → 2HBr.

1. (a)Explain, in terms of bonds, why a reaction is exothermic overall.[2]
2. (b)Using bond energies H–H = 436, Br–Br = 193 and H–Br = 366 kJ/mol, calculate ΔH and state whether the reaction is exothermic or endothermic.[3]

This question is about everyday applications of energy changes.

1. (a)Explain why a self-heating can warms its contents.[2]
2. (b)Explain why a self-chilling can cools its contents.[2]
3. (c)Name the minimum energy needed to start either reaction.[1]

(Higher) Ethene can be hydrogenated to ethane: C₂H₄ + H₂ → C₂H₆.

1. (a)Explain why bond breaking is endothermic.[2]
2. (b)Calculate ΔH using bond energies C=C 612, H–H 436, C–C 348 and C–H 412 kJ/mol.[3]
3. (c)State whether the reaction is exothermic or endothermic.[1]

In the temperature-change practical, 5 cm³ portions of sodium hydroxide are added to 25 cm³ of hydrochloric acid, giving these maximum temperature rises: 5 cm³ → 2.0 °C, 10 cm³ → 4.1 °C, 15 cm³ → 6.2 °C, 20 cm³ → 7.8 °C, 25 cm³ → 6.9 °C, 30 cm³ → 6.0 °C.

1. (a)Identify the volume of alkali giving the maximum temperature change.[1]
2. (b)Explain why the temperature change falls beyond that volume.[2]
3. (c)One run done in a glass beaker gives a smaller rise. Explain why.[1]
4. (d)Explain how fitting a lid improves the results.[2]

(Higher) An energy diagram for a reaction starts at 150 kJ/mol, rises to a maximum of 320 kJ/mol and finishes at 90 kJ/mol.

1. (a)Calculate ΔH and state what its sign shows.[2]
2. (b)Calculate the activation energy.[2]
3. (c)A catalyst lowers the peak to 270 kJ/mol. State the new activation energy and explain why ΔH is unchanged.[2]

Two hand warmers are compared: one lets iron filings rust inside a pouch; the other click-starts the crystallisation of a salt solution and can be reset by boiling.

1. (a)State the type of energy change both hand warmers rely on.[1]
2. (b)Explain why boiling resets the crystallisation hand warmer.[2]
3. (c)Name one variable that should be controlled when comparing how warm the two pouches get.[1]