A reaction has an overall energy change of +150 kJ mol⁻¹. Is it exothermic or endothermic, and why?

It is endothermic, because the overall energy change is positive.

For H₂ + Cl₂ → 2HCl the bond energies are H–H 436, Cl–Cl 243 and H–Cl 432 kJ mol⁻¹. Calculate the overall energy change.

Breaking takes 436 + 243 = 679 kJ mol⁻¹ and making releases 2 × 432 = 864 kJ mol⁻¹, so the change is 679 − 864 = −185 kJ mol⁻¹.

For H₂ + Br₂ → 2HBr the bond energies are H–H 436, Br–Br 193 and H–Br 366 kJ mol⁻¹. Calculate the overall energy change.

Breaking takes 436 + 193 = 629 kJ mol⁻¹ and making releases 2 × 366 = 732 kJ mol⁻¹, so the change is −103 kJ mol⁻¹.

For N₂ + O₂ → 2NO the bond energies are N≡N 945, O=O 498 and N=O 631 kJ mol⁻¹. Calculate the overall energy change and state the type of reaction.

Breaking takes 945 + 498 = 1443 kJ mol⁻¹ and making releases 2 × 631 = 1262 kJ mol⁻¹, giving +181 kJ mol⁻¹, so the reaction is endothermic.

Edexcel GCSE Chemistry 1CH0

Key Concepts in Chemistry

Topic overview Formulae, word equations and balanced equations Hazard symbols and assessing risk The changing model of the atom Protons, neutrons and electrons The nucleus, mass number and atomic mass Elements and isotopes Calculating particle numbers and relative atomic mass Mendeleev's periodic table How the modern periodic table is arranged Electronic configuration and the periodic table How ionic bonds form Ions in ionic compounds Formulae of ionic compounds and ionic equations Naming compounds and the ionic lattice Covalent bonds and molecules Dot and cross diagrams for simple molecules Classifying substances and the properties of ionic compounds Properties of simple molecular substances, and forms of carbon Graphite and diamond Fullerenes and graphene Polymers and models of bonding The properties of metals Relative formula mass and percentage composition Empirical and molecular formulae Conservation of mass in closed and open systems Reacting masses from balanced equations Concentration in grams per cubic decimetre Moles and the Avogadro constant Limiting reactants and stoichiometry

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Topic overview Acids, alkalis and the pH scale Indicators: litmus, methyl orange and phenolphthalein Hydrogen ion concentration and the pH scale Investigating pH change when neutralising an acid Dilute and concentrated, weak and strong acids Bases and alkalis Reactions of acids, and testing for hydrogen and carbon dioxide Neutralisation and its ionic equation Preparing soluble salts from insoluble and soluble reactants Preparing pure, dry copper sulfate crystals Carrying out an acid-alkali titration Solubility rules Predicting precipitates and preparing an insoluble salt Electrolytes and what electrolysis is Movement of ions and predicting the products of molten electrolysis Electrolysis with inert electrodes Writing half equations Oxidation and reduction at the electrodes Electrolysis of copper sulfate solution Investigating the electrolysis of copper sulfate

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Topic overview Transition metals and their properties Corrosion and electroplating Preventing rusting Alloys and alloy steels How the uses of metals relate to their properties Concentration in mol dm-3 and titration calculations Carrying out an accurate titration Percentage yield Atom economy and choosing a reaction pathway Molar volume of a gas Avogadro's law and gas volumes The Haber process Conditions used in industrial reactions Fertilisers and ammonium salts Preparing ammonium sulfate in the lab and in industry Chemical cells and fuel cells Evaluating fuel cells

Groups in the Periodic Table

Topic overview The groups of the periodic table, and the alkali metals Reactions of the alkali metals with water The pattern in alkali metal reactivity and why it happens Physical properties of the halogens The test for chlorine, and halogens reacting with metals Hydrogen halides, and explaining halogen reactivity Displacement reactions of the halogens Group 0: the noble gases

Rates of Reaction and Energy Changes

Topic overview Investigating the rate of a reaction Measuring rate and interpreting rate graphs Collision theory and factors affecting rate Catalysts and enzymes Exothermic and endothermic changes Bond breaking, bond making and the overall energy change Bond energies, activation energy and reaction profiles

Fuels and Earth Science

Topic overview Hydrocarbons and crude oil Fractional distillation and the fractions Properties of the fractions and homologous series Complete and incomplete combustion Carbon monoxide and soot Pollutants and acid rain Hydrogen and fossil fuels compared Cracking and why it is necessary The Earth's early atmosphere How the oceans formed and how oxygen appeared The test for oxygen and the greenhouse gases Today's atmosphere and evidence for climate change

Chemical Analysis, Organic Chemistry and Materials

Topic overview Why an ion test must be unique, and flame tests Tests for metal ions and for ammonia Tests for carbonate, sulfate and halide ions Identifying the ions in an unknown salt Instrumental methods and flame photometry Alkanes and why they are saturated Alkenes and why they are unsaturated Reactions of alkenes, and combustion What a polymer is, and how poly(ethene) forms Other addition polymers, and monomers from polymers Uses of polymers, and natural polymers including DNA Polyesters and condensation polymerisation Problems with polymers and recycling Alcohols and the -OH functional group Comparing the energy released by alcohol fuels Making ethanol by fermentation and distillation Carboxylic acids and the -COOH functional group Oxidising alcohols to carboxylic acids, and homologous series The size of nanoparticles Uses and risks of nanoparticles Choosing a material for a use

Rates of Reaction and Energy Changes

Bond energies, activation energy and reaction profiles

3 things to know
Paper 2

Overview

Working out the overall energy change from bond energies for HCl, HBr and NO, what activation energy is and why a spark is needed, and reading reaction profiles including a catalysed route.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

What is activation energy, and how does it explain a spark igniting methane, the effect of heating, and fast reactions at room temperature?

Key term

State how the overall energy change of a reaction is worked out from bond energies.

Numbers

Breaking the bonds in a reaction takes in 1500 kJ mol⁻¹ and making the new bonds releases 1350 kJ mol⁻¹. Calculate the overall energy change.

Questions from this subtopic

Each one with the model answer underneath.

1
State how the overall energy change of a reaction is worked out from bond energies.
Model answer
The total energy needed to break the bonds in the reactants is found, and the total energy released in making the bonds in the products is subtracted from it.
2
Breaking the bonds in a reaction takes in 1500 kJ mol⁻¹ and making the new bonds releases 1350 kJ mol⁻¹. Calculate the overall energy change.
Model answer
1500 − 1350 = +150 kJ mol⁻¹.
3
A reaction has an overall energy change of +150 kJ mol⁻¹. Is it exothermic or endothermic, and why?
Model answer
It is endothermic, because the overall energy change is positive.
4
For H₂ + Cl₂ → 2HCl the bond energies are H–H 436, Cl–Cl 243 and H–Cl 432 kJ mol⁻¹. Calculate the overall energy change.
Model answer
Breaking takes 436 + 243 = 679 kJ mol⁻¹ and making releases 2 × 432 = 864 kJ mol⁻¹, so the change is 679 − 864 = −185 kJ mol⁻¹.
5
For H₂ + Br₂ → 2HBr the bond energies are H–H 436, Br–Br 193 and H–Br 366 kJ mol⁻¹. Calculate the overall energy change.
Model answer
Breaking takes 436 + 193 = 629 kJ mol⁻¹ and making releases 2 × 366 = 732 kJ mol⁻¹, so the change is −103 kJ mol⁻¹.
6
For N₂ + O₂ → 2NO the bond energies are N≡N 945, O=O 498 and N=O 631 kJ mol⁻¹. Calculate the overall energy change and state the type of reaction.
Model answer
Breaking takes 945 + 498 = 1443 kJ mol⁻¹ and making releases 2 × 631 = 1262 kJ mol⁻¹, giving +181 kJ mol⁻¹, so the reaction is endothermic.

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