Energy Changes

What you'll learn

Energy changes occur in all chemical reactions, and understanding whether reactions release or absorb energy is fundamental to GCSE Chemistry. This guide covers exothermic and endothermic reactions, practical applications, reaction profiles, and bond energy calculations. You'll learn to interpret energy level diagrams, calculate energy changes using bond energies, and apply this knowledge to authentic exam questions.

Key terms and definitions

Exothermic reaction — a chemical reaction that transfers energy to the surroundings, usually as heat, causing the temperature of the surroundings to increase

Endothermic reaction — a chemical reaction that takes in energy from the surroundings, usually as heat, causing the temperature of the surroundings to decrease

Activation energy — the minimum energy that colliding particles must have for a reaction to occur

Reaction profile — an energy level diagram showing the relative energies of reactants and products, and the activation energy of a reaction

Bond energy — the energy required to break one mole of a particular covalent bond in the gaseous state

Overall energy change — the difference between the energy required to break bonds in the reactants and the energy released when bonds form in the products

Core concepts

Exothermic reactions

Exothermic reactions transfer energy to the surroundings. The energy released usually comes from the formation of new chemical bonds in the products.

Common examples of exothermic reactions:

• Combustion reactions (burning fuels like methane, petrol, wood)
• Oxidation reactions (rusting of iron, respiration)
• Neutralisation reactions (acid + alkali → salt + water)
• Many displacement reactions (reactive metal + less reactive metal compound)

Observable changes in exothermic reactions:

• Temperature of surroundings increases
• Energy is given out (released)
• Products have less energy than reactants
• ΔH is negative (when you progress to A-level)

Practical applications:

• Self-heating cans for coffee or soup use exothermic reactions between calcium oxide and water
• Hand warmers contain iron powder that oxidises exothermically when exposed to air
• Combustion of fuels in power stations, vehicles, and heating systems

Endothermic reactions

Endothermic reactions take in energy from the surroundings. Energy is required to break bonds in the reactants, and less energy is released when new bonds form in the products.

Common examples of endothermic reactions:

• Thermal decomposition reactions (breaking down calcium carbonate by heating)
• Photosynthesis (plants absorb light energy to convert carbon dioxide and water into glucose)
• Dissolving certain ionic compounds (ammonium nitrate dissolving in water)
• Electrolysis processes

Observable changes in endothermic reactions:

• Temperature of surroundings decreases
• Energy is taken in (absorbed)
• Products have more energy than reactants
• Continuous energy input often required (e.g. heating)

Practical applications:

• Sports injury cold packs contain ammonium nitrate and water in separate compartments; when mixed, the endothermic dissolving process cools the pack
• Sherbet sweets feel cold on the tongue due to endothermic dissolving

Reaction profiles

Reaction profiles are diagrams that show the energy changes during a chemical reaction. They display the relative energies of reactants and products, and importantly, the activation energy.

Exothermic reaction profile:

• Reactants start at a higher energy level
• Energy increases to reach activation energy peak
• Products finish at a lower energy level
• Overall energy change is downward (energy released)
• The difference between reactants and products represents energy transferred to surroundings

Endothermic reaction profile:

• Reactants start at a lower energy level
• Energy increases to reach activation energy peak
• Products finish at a higher energy level
• Overall energy change is upward (energy absorbed)
• The difference between reactants and products represents energy absorbed from surroundings

Key features to label:

• Reactants energy level
• Products energy level
• Activation energy (from reactants to peak)
• Overall energy change (difference between reactants and products)
• Progress of reaction (x-axis)
• Energy (y-axis)

The activation energy is always present in both exothermic and endothermic reactions. It represents the energy barrier that must be overcome for reactants to be converted into products. Catalysts work by providing an alternative reaction pathway with a lower activation energy, shown as a lower peak on the reaction profile.

Bond energy calculations

Chemical reactions involve breaking bonds in reactants (requires energy) and making bonds in products (releases energy). The overall energy change depends on the difference between these two processes.

Energy required to break bonds:

• Breaking bonds is always endothermic (requires energy input)
• Different bonds have different bond energies
• Bond energy values are given in kJ/mol
• You must break ALL bonds in the reactant molecules

Energy released when making bonds:

• Making bonds is always exothermic (releases energy)
• The same bond always releases the same amount of energy when formed
• You must account for ALL bonds formed in the product molecules

Calculating overall energy change:

The overall energy change for a reaction is calculated using:

Overall energy change = Energy required to break bonds − Energy released when making bonds

Determining if a reaction is exothermic or endothermic:

• If more energy is released making bonds than is required to break bonds: overall energy change is negative, reaction is exothermic
• If more energy is required to break bonds than is released making bonds: overall energy change is positive, reaction is endothermic

Important considerations:

• Bond energy calculations give approximate values because bond energies are averages
• All substances must be in the gaseous state for bond energy calculations to be accurate
• You need to count the number of each type of bond carefully
• Double bonds (C=O, O=O) count as one bond but have higher bond energies than single bonds

Measuring energy changes practically

Simple calorimetry experiments:

Temperature changes in reactions can be measured using simple apparatus:

• Polystyrene cup (insulator to reduce heat loss)
• Thermometer or temperature probe
• Known volumes and concentrations of reactants
• Lid to reduce energy transfer to surroundings

Method for measuring temperature change:

1. Measure starting temperature of reactants
2. Mix reactants in insulated container
3. Stir mixture continuously
4. Record maximum or minimum temperature reached
5. Calculate temperature change (ΔT)

Suitable reactions for school laboratory:

• Neutralisation: hydrochloric acid + sodium hydroxide solution
• Displacement: copper sulfate solution + zinc powder
• Dissolving: various salts dissolving in water

Limitations:

• Energy is lost to surroundings (even with insulation)
• Not all solutions have the same specific heat capacity
• Measured values are often lower than theoretical values for exothermic reactions
• Thermometers have limited precision

Using energy changes

Understanding energy changes helps explain many everyday phenomena and industrial processes.

Fuel comparison:

Different fuels release different amounts of energy when burned. Comparing fuels involves:

• Measuring temperature rise when burning a fuel
• Using equal masses or volumes of different fuels
• Controlling variables (same volume of water, same apparatus)
• Calculating energy transferred to water

Choosing appropriate fuels:

• High energy output per gram (efficiency)
• Availability and cost
• Safety and ease of storage
• Environmental impact (products of combustion)

Industrial applications:

• Haber process (making ammonia) is exothermic; heat must be removed
• Cracking of hydrocarbons is endothermic; continuous heating required
• Blast furnace uses exothermic reactions to maintain high temperatures

Worked examples

Example 1: Identifying energy changes

Question: A student adds magnesium ribbon to hydrochloric acid in a beaker. The temperature of the beaker increases from 20°C to 35°C.

(a) State whether this reaction is exothermic or endothermic. [1 mark]

(b) Explain your answer. [2 marks]

Mark scheme answer:

(a) Exothermic [1]

(b) The temperature increased [1], which means energy was transferred to the surroundings / released to the solution [1]

Example 2: Bond energy calculation

Question: Hydrogen reacts with chlorine to form hydrogen chloride:

H₂ + Cl₂ → 2HCl

Use the bond energies in the table to calculate the overall energy change for this reaction.

Bond	Bond energy (kJ/mol)
H-H	436
Cl-Cl	243
H-Cl	432

[4 marks]

Mark scheme answer:

Bonds broken:

• 1 × H-H = 436 kJ
• 1 × Cl-Cl = 243 kJ
• Total energy required = 436 + 243 = 679 kJ [1]

Bonds made:

• 2 × H-Cl = 2 × 432 = 864 kJ [1]

Overall energy change = 679 − 864 [1] = −185 kJ [1]

(The negative value indicates the reaction is exothermic)

Example 3: Reaction profile interpretation

Question: The diagram shows a reaction profile for a chemical reaction.

[Diagram shows reactants at 50 kJ, peak at 100 kJ, products at 30 kJ]

(a) State the activation energy for this reaction. [1 mark]

(b) Calculate the overall energy change. [1 mark]

(c) Is this reaction exothermic or endothermic? Explain your answer. [2 marks]

Mark scheme answer:

(a) 50 kJ [1] (Activation energy = 100 − 50, from reactants to peak)

(b) −20 kJ / 20 kJ released [1] (Overall energy change = 30 − 50, products − reactants)

(c) Exothermic [1] because the products have less energy than the reactants / energy is released / overall energy change is negative [1]

Common mistakes and how to avoid them

• Confusing which process requires or releases energy: Remember that breaking bonds ALWAYS requires energy (endothermic), while making bonds ALWAYS releases energy (exothermic). The overall reaction type depends on which process involves more energy.

• Incorrectly counting bonds in molecules: In bond energy calculations, count each individual bond carefully. For example, CO₂ has two C=O double bonds (not one), and H₂O has two O-H bonds. Draw out the structural formula if unsure.

• Mixing up activation energy and overall energy change: Activation energy is measured from reactants to the peak of the reaction profile. Overall energy change is the difference between products and reactants. They are completely different values.

• Reversing the bond energy calculation: The formula is energy IN (breaking bonds) minus energy OUT (making bonds). Don't subtract the wrong way around, or you'll get the sign reversed and incorrectly identify whether the reaction is exothermic or endothermic.

• Stating temperature goes up/down instead of explaining energy transfer: In explanation questions, don't just describe the temperature change — explain that energy is transferred to or from the surroundings. Use precise terminology like "energy is released to the surroundings" rather than vague statements.

• Forgetting that bond energies are averages: Bond energy calculations give approximate values because bond energies vary slightly depending on the molecule. Don't be concerned if your calculated value differs slightly from experimental data.

Exam technique for "Energy Changes"

• Command word awareness: "State" requires a simple answer (exothermic/endothermic). "Explain" requires a reason linked to energy transfer. "Calculate" requires working shown with units. "Describe" needs a sequence of observations or changes without necessarily explaining why.

• Show your working in calculations: For bond energy questions, clearly separate "bonds broken" and "bonds made" sections. Write out each bond calculation even if some are repetitive (e.g., 4 × C-H = 4 × 413 = 1652 kJ). This allows partial credit if you make an arithmetic error.

• Use data from tables accurately: Check you're using the correct bond energy value from the table. If the question gives C-H as 413 kJ/mol, use exactly that value, not a rounded number you remember from class.

• Link temperature changes to energy transfer explicitly: Don't assume the examiner will infer the connection. Write "the temperature increased because energy was released to the surroundings" rather than just "the temperature increased."

Quick revision summary

Energy changes occur in all reactions. Exothermic reactions transfer energy to surroundings (temperature rises); endothermic reactions absorb energy from surroundings (temperature falls). Reaction profiles show activation energy and overall energy change. Bond energy calculations involve energy required to break bonds (endothermic) minus energy released making bonds (exothermic). Overall energy change determines reaction type: negative values indicate exothermic reactions, positive values indicate endothermic reactions. Understanding energy changes explains fuel efficiency, industrial processes, and everyday phenomena.