Changes of state and energy transfer

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Changes of state and energy transfer — Free MYP4 Chemistry Practice Questions

1QuestionInterpreting heating and cooling graphsConcept Practice

3 marks~5 minCriterion A

The heating curve for water shows temperature (vertical axis) against time (horizontal axis). Five segments are labelled: A (solid ice warming), B (melting at

0^\circ\text{C}

), C (liquid water warming), D (boiling at

100^\circ\text{C}

), and E (steam warming).

Identify which segment represents the melting point of water. [1]

Explain why temperature remains constant during segment B, even though heat is continuously supplied. In your answer, refer to particle behaviour and how the energy is used. [2]

Solutions

2QuestionConstant temperature during phase changesAssessment Practice

6 marks~9 minCriterion D

A community thermal battery uses paraffin wax as a phase-change material (PCM) to store solar heat. The wax melts at a constant temperature of

47^\circ\text{C}

during the day, storing energy, and solidifies at night, releasing heat to replace natural gas for domestic heating. Paraffin wax is petroleum-derived; its production releases volatile organic compounds (VOCs) into the local atmosphere.

Explain why the temperature remains constant at

47^\circ\text{C}

while the paraffin wax is melting. [2]

Analyse the environmental trade-off between reduced natural gas combustion and VOC emissions from paraffin wax production. [2]

Evaluate whether the constant-temperature assumption for the PCM remains valid in a real thermal battery, considering heat-exchanger inefficiencies and impurities in the wax. [2]

Solutions

3QuestionEnergy transfer during state changes (heating/cooling curves)Assessment Practice

6 marks~9 minCriterion C

Data

Substance	Melting point ( $^\circ\text{C}$ )	Boiling point ( $^\circ\text{C}$ )
Water	0	100
Ethanol	−114	78
Mercury	−39	357
Methane	−182	−164

A heating curve shows the temperature of an unknown substance rising from

-50^\circ\text{C}

(solid) to

150^\circ\text{C}

(gas), with two plateaus: one at

0^\circ\text{C}

and one at

100^\circ\text{C}

.

Reference

Deduce the identity of the unknown substance, using the plateau temperatures and the reference data to support your answer. [2]

Explain the changes in kinetic energy and potential energy of the particles during the segment of the heating curve between the two plateaus. [2]

A second heating curve shows plateaus at

-39^\circ\text{C}

and

357^\circ\text{C}

. Analyse whether this substance would exist as a solid, liquid, or gas at room temperature (

25^\circ\text{C}

), and evaluate what this implies about the strength of its intermolecular forces compared with water. [2]

Solutions

4QuestionEnergy transfer during state changes (heating/cooling curves)Assessment Practice

5 marks~8 minCriterion B

A group of students heats separate 10 g samples of stearic acid from

20^\circ\text{C}

80^\circ\text{C}

at three different rates, recording temperature every 30 seconds.

Heating rate ( $^\circ\text{C min}^{-1}$ )	0.5	1.0	2.0
Plateau duration (min)	8.0	4.0	2.0

Deduce the relationship between heating rate and plateau duration. [1]

Justify a prediction for the plateau duration at a heating rate of

4.0^\circ\text{C min}^{-1}

. [2]

Analyse how the fixed latent heat of fusion of stearic acid accounts for the plateau durations observed at different heating rates. [2]

Solutions

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