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Energy changes in reactions — KCSE Chemistry

KCSE Chemistry · 64 practice questions · 4 syllabus objectives · 4 revision lessons

27 easy26 medium11 hard

Last updated · Aligned to the KNEC KCSE syllabus

What You'll Learn

Key learning outcomes for this topic, aligned to the KNEC KCSE syllabus.

Define exothermic and endothermic reactions and represent them using energy profile diagrams

Define enthalpy change (ΔH) and calculate it from experimental data using q = mcΔT

Apply Hess's law to calculate enthalpy changes for reactions that cannot be measured directly

Energy changes in reactions

Revision Notes

Concise lesson notes for Energy changes in reactions, written to the KCSE Chemistry marking standard. Read the first lesson free below.

Exothermic and Endothermic Reactions

In chemistry, reactions can be classified based on energy changes. Exothermic reactions release energy, usually in the form of heat, to the surroundings. This results in an increase in temperature. Common examples include combustion and respiration. In contrast, endothermic reactions absorb energy from the surroundings, leading to a decrease in temperature. Photosynthesis is a classic example of an endothermic reaction.

Energy profile diagrams visually represent these reactions:

  • Exothermic Reaction Diagram: Starts at a higher energy level, with a downward slope indicating energy release.
  • Endothermic Reaction Diagram: Begins at a lower energy level, with an upward slope showing energy absorption.

Understanding these definitions and diagrams is crucial for analyzing chemical reactions effectively.

Key points to remember

  • Exothermic reactions release energy and increase temperature.
  • Endothermic reactions absorb energy and decrease temperature.
  • Energy profile diagrams illustrate energy changes during reactions.
  • Exothermic diagrams show a downward slope; endothermic show an upward slope.
  • Examples include combustion (exothermic) and photosynthesis (endothermic).

Worked example

Define exothermic and endothermic reactions.

  • Exothermic reactions: Release energy, e.g., combustion.
  • Endothermic reactions: Absorb energy, e.g., photosynthesis.

Draw energy profile diagrams for both.

  • Exothermic: Starts high, slopes down.
  • Endothermic: Starts low, slopes up.

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More lessons in this topic

Lesson 2: Understanding Enthalpy Change (ΔH)

Objective: Define enthalpy change (ΔH) and calculate it from experimental data using q = mcΔT

Enthalpy change (ΔH) is defined as the heat change at constant pressure during a chemical reaction. It indicates whether a reaction is exothermic (releases heat, ΔH < 0) or endothermic (absorbs heat, ΔH > 0). To calculate ΔH from experimental data, we use the formula:

q = mcΔT
where:

  • q = heat energy (in joules)
  • m = mass of the substance (in grams)
  • c = specific heat capacity (in J/g°C)
  • ΔT = change in temperature (final temperature - initial temperature)

For example, if 100g of water is heated from 20°C to 80°C, and the specific heat capacity of water is 4.18 J/g°C, we can calculate the enthalpy change as follows:

  1. Calculate ΔT:
    ΔT = 80°C - 20°C = 60°C
  2. Substitute values into the formula:
    q = (100g)(4.18 J/g°C)(60°C) = 25080 J
    Thus, ΔH = +25080 J, indicating an endothermic process.
  • Enthalpy change (ΔH) is heat change at constant pressure.
  • ΔH < 0 indicates an exothermic reaction.
  • ΔH > 0 indicates an endothermic reaction.
  • Use q = mcΔT to calculate enthalpy change.
  • Include units in calculations for accuracy.

Calculate the enthalpy change when 200g of a substance absorbs heat, raising its temperature from 25°C to 75°C, with a specific heat capacity of 2.5 J/g°C.
Answer:
ΔT = 75°C - 25°C = 50°C
q = (200g)(2.5 J/g°C)(50°C) = 25000 J, thus ΔH = +25000 J.

Lesson 3: Applying Hess's Law for Enthalpy Changes

Objective: Apply Hess's law to calculate enthalpy changes for reactions that cannot be measured directly

Hess's law states that the total enthalpy change in a reaction is the sum of the enthalpy changes for individual steps, regardless of the pathway taken. This principle allows us to calculate enthalpy changes for reactions that cannot be measured directly.

Steps to apply Hess's Law:

  1. Identify the target reaction for which you need to find the enthalpy change.
  2. Write the reactions for which you know the enthalpy changes.
  3. Manipulate these reactions (reverse or multiply) to match the target reaction.
  4. Sum the enthalpy changes from the manipulated reactions to find the overall change.

Example: Given the following reactions:

  • A → B, ΔH = +100 kJ
  • B → C, ΔH = -50 kJ
  • C → D, ΔH = +30 kJ

To find ΔH for A → D:

  • Combine the reactions:
    • A → B (100 kJ)
    • B → C (-50 kJ)
    • C → D (30 kJ)

Total ΔH = 100 - 50 + 30 = +80 kJ. Thus, the enthalpy change for A → D is +80 kJ.

  • Hess's law allows calculation of indirect enthalpy changes.
  • Total enthalpy change is the sum of individual steps.
  • Manipulate known reactions to match the target reaction.

Calculate the enthalpy change for the reaction: A → D using given reactions.

  1. A → B, ΔH = +100 kJ
  2. B → C, ΔH = -50 kJ
  3. C → D, ΔH = +30 kJ

Total ΔH = 100 - 50 + 30 = +80 kJ.

Lesson 4: Understanding Energy Changes in Reactions

Objective: Energy changes in reactions

In chemical reactions, energy changes are crucial to understanding how reactions occur. Energy changes can be classified into two main types: exothermic and endothermic reactions.

  • Exothermic reactions release energy to the surroundings, often in the form of heat. A common example is the combustion of fuels.
  • Endothermic reactions absorb energy from the surroundings, leading to a decrease in temperature. An example is photosynthesis in plants.

The enthalpy change (ΔH) is a key concept here, representing the heat content change during a reaction. If ΔH is negative, the reaction is exothermic; if positive, it is endothermic.

To illustrate, consider the reaction of hydrogen with oxygen to form water:

  • 2H₂ + O₂ → 2H₂O
  • This reaction releases energy, resulting in a negative ΔH, indicating it is exothermic.

Understanding these energy changes is essential for predicting reaction behavior and applications in real life.

  • Exothermic reactions release energy, causing temperature increase.
  • Endothermic reactions absorb energy, leading to temperature decrease.
  • Enthalpy change (ΔH) indicates heat content change during reactions.
  • Negative ΔH signifies an exothermic reaction.
  • Positive ΔH signifies an endothermic reaction.

Explain the energy change in the reaction:
C + O₂ → CO₂.

  • This reaction is exothermic as it releases energy.
  • The enthalpy change (ΔH) is negative, indicating heat release.

Sample Questions

Read 3 questions and answers free. Sign up to access all 64 questions with full KNEC-style marking schemes and a personalised study plan.

1
easySHORT ANSWER4 marks

In the study of chemical reactions, understanding the energy dynamics involved is crucial. This includes defining key concepts related to the formation of substances under specific conditions and representing these processes accurately. (a) Define the standard enthalpy of formation (ΔH°f). (2 marks) (b) State the standard conditions of temperature and pressure. (1 mark) (c) Write the thermochemical equation for the formation of water (H₂O). (1 mark)

Answer & marking scheme

Part (a) — 4 marks
The standard enthalpy of formation is the enthalpy change when one mole of a compound is formed from its constituent elements (1 mk)
with all substances in their standard states (most stable form) under standard conditions (1 mk)
2
easySHORT ANSWER3 marks

In the study of chemical processes, understanding the thermal effects associated with the dissolution of substances is crucial. Consider the scenario where a specific substance is dissolved in water within an insulated container, and analyze the resulting energy changes. (a) State the observation when ammonium nitrate (NH₄NO₃) is dissolved in water in an insulated container. (1 mark) (b) Is the dissolving process exothermic or endothermic? Explain. (2 marks)

Answer & marking scheme

Part (a) — 3 marks
the temperature of the solution rises / the container feels hot (1 mk)
3
easySHORT ANSWER4 marks

In the study of chemical reactions, understanding the energy dynamics involved is crucial. This includes defining key concepts related to the formation of substances under specific conditions and representing these processes accurately. (a) Define the standard enthalpy of formation (ΔH°f). (2 marks) (b) State the standard conditions of temperature and pressure. (1 mark) (c) Write the thermochemical equation for the formation of carbon dioxide (CO₂). (1 mark)

Answer & marking scheme

Part (a) — 4 marks
The standard enthalpy of formation is the enthalpy change when one mole of a compound is formed from its constituent elements (1 mk)
with all substances in their standard states (most stable form) under standard conditions (1 mk)
4

In the study of chemical processes, understanding the thermal effects associated with the dissolution of substances is crucial. Consider the scenario where a specific substance is dissolved in water within an insulated container, and analyze the resulting energy changes. (a) State the observation when sodium hydroxide (NaOH) pellets is dissolved in water in an insulated container. (1 mark) (b) Is the dissolving process exothermic or endothermic? Explain. (2 marks)

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Frequently asked questions

What does the KCSE Chemistry topic "Energy changes in reactions" cover?

Energy changes in reactions covers Define exothermic and endothermic reactions and represent them using energy profile diagrams; Define enthalpy change (ΔH) and calculate it from experimental data using q = mcΔT; Apply Hess's law to calculate enthalpy changes for reactions that cannot be measured directly, and more, all aligned to the official KNEC KCSE Chemistry syllabus.

How many practice questions are available for Energy changes in reactions?

HighMarks has 64 Energy changes in reactions practice questions for KCSE Chemistry, each with a full marking scheme. The first 3 are free; sign up to access the rest, plus all KCSE mock exams and past papers.

Are these aligned with the KNEC KCSE syllabus?

Yes. Every objective on this page is taken directly from the official KNEC KCSE Chemistry syllabus. Practice questions match the KCSE exam format and are graded against the standard KNEC marking scheme.

How should I revise Energy changes in reactions for the KCSE exam?

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