Quick Notes

Quick Notes

Chemistry Chapter 3: Thermochemistry
3.1 Heat Change in Reactions
3.1 Heat Change in Reactions
CHEMISTRY • Form 5 • Chapter 3: Thermochemistry

3.1Heat Change in Reactions

Chemical reactions always involve a change in energy, which usually occurs in the form of heat energy. Based on whether heat is released or absorbed, chemical reactions are classified into exothermic reactions and endothermic reactions.

Learning Objectives

  • Deduce exothermic and endothermic reactions through activities.
  • Interpret and construct energy level diagrams.
  • Relate heat changes in chemical reactions to the breaking and formation of chemical bonds.

Comparison of Energy Level Diagrams and Bond Energy Profiles

A side-by-side visual reference showing energy levels of reactants versus products, the direction of the ΔH arrow, and how bond breaking versus bond forming determines the net sign of ΔH.

Brief Explanation

Understand the key ideas behind heat changes in chemical reactions.

Thermochemistry deals with heat changes during chemical reactions. In an exothermic reaction, chemical potential energy stored in reactants is converted and released as heat energy to the surroundings, causing the surrounding temperature to rise and giving ΔH a negative value. In an endothermic reaction, heat energy is absorbed from the surroundings into the chemical system, leading to a drop in the temperature of the surroundings and giving ΔH a positive value.

Exothermic Reactions

Chemical reactions that release heat to the surroundings. The surrounding temperature rises and the reaction container feels hot.

Examples: Respiration, oxidation of metals, neutralisation reaction, combustion of fuel, production of ammonia, and dissolving sodium hydroxide in water.

Endothermic Reactions

Chemical reactions that absorb heat from the surroundings. The surrounding temperature drops and the reaction container feels cold.

Examples: Photosynthesis, decomposition of metal carbonates when heated, decomposition of metal nitrates when heated, and dissolving ammonium salts in water.

Feature Exothermic Reaction Endothermic Reaction
Heat flow Heat is released to the surroundings. Heat is absorbed from the surroundings.
Surrounding temperature Increases Decreases
Container feel Hot Cold
Sign of ΔH Negative (−ΔH) Positive (+ΔH)
Energy content Hproducts < Hreactants Hproducts > Hreactants
Heat of Reaction (ΔH): The heat change of one mole of reactant that reacts or one mole of product that is formed. Its unit is kJ mol−1.

Formula: ΔH = Hproducts − Hreactants, where Δ is the Greek letter delta denoting difference or change.

Laboratory Activity 3A

Aim: To study the types of reactions based on heat change and changes in thermometer readings when substances dissolve in water.

Materials: Solid sodium hydroxide (NaOH), solid anhydrous calcium chloride (CaCl2), solid ammonium nitrate (NH4NO3), solid sodium thiosulphate (Na2S2O3), distilled water.

Apparatus: Polystyrene cups with lids, spatula, measuring cylinder, and thermometer.

Safety Caution: Sodium hydroxide corrodes; handle with care.

Procedure

  1. Measure 20 cm3 of distilled water and pour it into a polystyrene cup.
  2. Put a thermometer in the polystyrene cup, leave it for two minutes, and record the initial temperature of the distilled water.
  3. Add one spatula of solid sodium hydroxide, NaOH, into the polystyrene cup.
  4. Stir the mixture carefully with the thermometer.
  5. Record the highest or lowest temperature of the mixture.
  6. Repeat steps 1 to 5 using solid NH4NO3, solid Na2S2O3, and solid anhydrous CaCl2.

Energy Change During Bond Breaking and Formation

Chemical reactions involve breaking existing bonds in reactants and forming new bonds in products.

  • Bond breaking: Absorbs heat energy (endothermic process).
  • Bond formation: Releases heat energy (exothermic process).

In an exothermic reaction, the heat energy released during the formation of bonds in the products is greater than the heat energy absorbed to break bonds in the reactants.

In an endothermic reaction, the heat energy absorbed to break bonds in the reactants is greater than the heat energy released during the formation of bonds in the products.

Example Calculation

H2(g) + Cl2(g) → 2HCl(g)

Bond energies: H–H = 436 kJ mol−1, Cl–Cl = 243 kJ mol−1, H–Cl = 432 kJ mol−1.

  • Heat absorbed to break bonds: 436 + 243 = 679 kJ.
  • Heat released during bond formation: 2 × 432 = 864 kJ.
  • Net heat of reaction: ΔH = 679 − 864 = −185 kJ mol−1 (exothermic).
Steps to Construct an Energy Level Diagram:
  1. Draw an arrow pointing upwards.
  2. Label the vertical axis as "Energy".
  3. Draw two horizontal energy levels.
  4. Write the reactant and product formulas at their correct energy levels.
  5. Draw a vertical arrow from the reactant energy level to the product energy level.
  6. Write the numerical value and sign of ΔH.

Try to Answer First

Answer mentally, then check the explanation.

1

Why does the temperature of the reaction mixture rise during an exothermic reaction?

Answer: The reaction releases heat energy into the surroundings, which causes the temperature of the surroundings to increase.
2

In an endothermic reaction, which has a higher total energy content: the reactants or the products?

Answer: The products have a higher total energy content than the reactants.
3

What is the relationship between the heat absorbed for bond breaking and the heat released during bond formation in an endothermic reaction?

Answer: The heat energy absorbed to break the bonds in the reactants is greater than the heat energy released during the formation of the bonds in the products.

Common Mistakes

Avoid these errors when working with heat changes.

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Forgetting to write the sign (+ or −) for the value of ΔH; in thermochemistry, the sign is mandatory.
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Drawing the vertical ΔH arrow in the wrong direction; the arrow must always start from the reactant level and point toward the product level.
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Confusing bond breaking with bond forming; students often incorrectly assume bond breaking releases energy. Bond breaking always absorbs energy, while bond formation releases energy.
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Omitting the "Energy" label on the vertical axis or forgetting units (kJ mol−1) when constructing energy level diagrams.

Concept Misunderstandings

Compare each misconception with the correct fact.

Misconception

A cold container means energy was lost or destroyed.

Correct Fact

Energy is conserved. The container feels cold because heat is absorbed from the surroundings into the reacting particles to form products with higher energy content.

Misconception

Exothermic reactions do not require any bond breaking.

Correct Fact

All chemical reactions involve both bond breaking (which absorbs heat) and bond formation (which releases heat). A reaction is exothermic because the heat released during bond formation is greater than the heat absorbed to break bonds.

Summary

Key facts to remember.

  • Exothermic Reactions: Release heat to surroundings; surrounding temperature increases; container feels hot; ΔH is negative; Hproducts < Hreactants; energy released forming bonds > energy absorbed breaking bonds.
  • Endothermic Reactions: Absorb heat from surroundings; surrounding temperature decreases; container feels cold; ΔH is positive; Hproducts > Hreactants; energy absorbed breaking bonds > energy released forming bonds.
  • Energy Level Diagram: Uses an upward axis labelled "Energy", two distinct horizontal levels for reactants and products, and an arrow from reactants to products indicating the value and sign of ΔH.

Short Activity

Complete all parts before checking your score.

A. Objective Quiz

1

Which of the following processes is an endothermic reaction?

2

The thermochemical equation for a reaction is:
Pb(NO3)2(aq) + Na2SO4(aq) → PbSO4(s) + 2NaNO3(aq)   ΔH = −50 kJ mol−1
Which statement is correct about this reaction?

B. Fill in the Blanks

3

During a chemical reaction, heat energy is absorbed to ______ existing chemical bonds in the reactants.

4

The unit used for heat of reaction, ΔH, is ______.

C. Match the Observation or Process with the Reaction Type

Drag each choice on desktop. On mobile, tap a choice and then tap the matching box.

Observation / Process

Dissolving ammonium salts in water Dissolving sodium hydroxide in water Container temperature drops Container temperature rises

Reaction Type

1Endothermic reaction
2Exothermic reaction
3Characteristic of endothermic change
4Characteristic of exothermic change
Complete every answer before checking.
 

Keywords

Tap or hover over a term to read its meaning.

Exothermic reaction Endothermic reaction Heat of reaction (ΔH) Energy level diagram Bond breaking Bond formation