Energy Changes (Exothermic, Endothermic, Profiles)

Quick-Fire Definitions

Exothermic: A reaction that transfers energy to the surroundings (temperature increases).
Endothermic: A reaction that takes in energy from the surroundings (temperature decreases).
Activation energy (Eₐ): The minimum energy needed for a reaction to start - shown by the "hump" on a reaction profile.
Bond energy: The energy needed to break one mole of a particular covalent bond (in kJ/mol).
Catalyst: A substance that speeds up a reaction by providing an alternative pathway with lower activation energy. Not used up.
Fuel cell: An electrochemical cell that converts the chemical energy of a fuel (e.g. Hydrogen) directly into electrical energy.

Exothermic & Endothermic Reactions

Exothermic Reactions

An exothermic reaction transfers energy to the surroundings, usually by heating. The temperature of the surroundings increases.

Combustion: burning fuels (CH₄ + 2O₂ → CO₂ + 2H₂O)
Neutralisation: acid + alkali → salt + water
Oxidation: metals reacting with oxygen

Everyday uses: self-heating cans, hand warmers.

Endothermic Reactions

An endothermic reaction takes in energy from the surroundings, usually by heating. The temperature of the surroundings decreases.

Thermal decomposition: CaCO₃ → CaO + CO₂
Dissolving ammonium nitrate: caused by breaking ionic bonds.
Citric acid + sodium hydrogencarbonate

Everyday uses: instant cold packs for sports injuries.

All reactions involve both bond breaking (endothermic - requires energy) and bond making (exothermic - releases energy). Whether the overall reaction is exothermic or endothermic depends on which process transfers more energy.

Required Practical: Temperature Changes Required Practical

Investigate the variables that affect temperature changes in reacting solutions, e.g. Acid + alkali neutralisation.

Measure a fixed volume of dilute acid (e.g. 25 cm³ of HCl) into a polystyrene cup using a measuring cylinder.
Record the starting temperature of the acid using a thermometer.
Add a measured volume of alkali (e.g. 25 cm³ of NaOH) and stir.
Record the highest temperature reached.
Calculate the temperature change: ΔT = final temperature − initial temperature.
Repeat with different concentrations of alkali to investigate how concentration affects the temperature change.

The polystyrene cup minimises heat loss to the surroundings, providing a more precise measurement of the maximum temperature reached.

A polystyrene cup is used because it is a good insulator - it minimises heat loss to the surroundings, making the temperature measurement more accurate.

Interpreting temperature data

A student adds NaOH to HCl in a polystyrene cup. The temperature rises from 21°C to 28°C. Is this exothermic or endothermic?

Step 1: ΔT = 28 − 21 = +7°C.

Step 2: The temperature of the surroundings (solution) increased.

Step 3: Energy was transferred to the surroundings → this is an exothermic reaction.

A common mistake is confusing the direction of energy transfer. If the temperature goes up, the reaction is exothermic (energy is released into the solution). If the temperature goes down, the reaction is endothermic (energy is taken from the solution).

Reaction Profiles

Reaction profiles are energy diagrams showing the energy of reactants and products.

Reaction profiles showing the energy changes. Notice how adding a catalyst (purple dotted line) lowers the activation energy (Eₐ) without changing the overall energy change (ΔH).

Exothermic Profile

Products are at a lower energy level than reactants. The energy difference is released to the surroundings. The "hump" represents the activation energy (Eₐ) - the minimum energy needed to start the reaction.

Endothermic Profile

Products are at a higher energy level than reactants. The energy difference is taken in from the surroundings.

Effect of Catalysts on Profiles

A catalyst provides an alternative reaction pathway with a lower activation energy. On the profile, the "hump" is smaller, but the overall energy change (ΔH) remains the same.

When drawing reaction profiles, always label: reactants, products, activation energy (Eₐ), and the overall energy change. The x-axis is "Progress of reaction" and the y-axis is "Energy".

Bond Energy Calculations (HT)

Chemical reactions involve breaking old bonds (endothermic) and making new bonds (exothermic).

Overall energy = Energy to break bonds − Energy released making bonds

If more energy is released making bonds than is needed to break bonds → exothermic (negative value).
If more energy is needed to break bonds than is released making bonds → endothermic (positive value).

Worked Example 1: H₂ + Cl₂ → 2HCl (exothermic)

Bond energies: H–H = 436 kJ/mol, Cl–Cl = 242 kJ/mol, H–Cl = 431 kJ/mol

Breaking: 1 × H–H + 1 × Cl–Cl = 436 + 242 = 678 kJ

Making: 2 × H–Cl = 2 × 431 = 862 kJ

Overall: 678 − 862 = −184 kJ/mol (exothermic)

Worked Example 2: N₂ + O₂ → 2NO (endothermic)

Bond energies: N≡N = 941 kJ/mol, O=O = 498 kJ/mol, N=O = 587 kJ/mol

Breaking: 1 × N≡N + 1 × O=O = 941 + 498 = 1439 kJ

Making: 2 × N=O = 2 × 587 = 1174 kJ

Overall: 1439 − 1174 = +265 kJ/mol (endothermic - more energy needed to break bonds than is released making them)

Worked Example 3: Combustion of methane CH₄ + 2O₂ → CO₂ + 2H₂O

Bond energies: C–H = 413, O=O = 498, C=O = 805, O–H = 464 kJ/mol

Breaking: 4 × C–H + 2 × O=O = (4 × 413) + (2 × 498) = 1652 + 996 = 2648 kJ

Making: 2 × C=O + 4 × O–H = (2 × 805) + (4 × 464) = 1610 + 1856 = 3466 kJ

Overall: 2648 − 3466 = −818 kJ/mol (exothermic - combustion always releases energy)

A negative answer = exothermic (energy released). A positive answer = endothermic (energy absorbed). Always show your working clearly in the exam. The methane combustion calculation is one of the most commonly set exam questions.

Chemical & Fuel Cells Chemistry Only

Chemical Cells (Batteries)

A chemical cell produces a voltage when two different metals are dipped into an electrolyte and connected by a wire. Electrons flow from the more reactive metal to the less reactive metal through the external circuit.

The greater the difference in reactivity between the two metals, the greater the voltage.

Predicting voltage from reactivity

Three cells are set up using these metal pairs in the same electrolyte: (A) Zn and Cu, (B) Mg and Cu, (C) Fe and Cu. Which gives the highest voltage?

Step 1: The reactivity order is Mg > Zn > Fe > Cu.

Step 2: The biggest gap in reactivity gives the highest voltage.

Answer: Cell B (Mg and Cu) gives the highest voltage because magnesium and copper are furthest apart in the reactivity series.

Non-rechargeable cells go flat when the reactants are used up. Rechargeable cells can be recharged because the chemical reactions are reversible - applying an external electrical current reverses the reaction.

Hydrogen Fuel Cells

A hydrogen fuel cell is an electrochemical device that converts the chemical energy in hydrogen directly into electrical energy. Unlike a battery, it does not store energy internally. Instead, it requires a continuous external supply of hydrogen fuel and oxygen (from the air).

The only product is water, making hydrogen fuel cells a clean energy source at the point of use.

A hydrogen fuel cell showing the 5-step process. Hydrogen enters the anode (1), is oxidised to release electrons and H⁺ ions (2), the H⁺ ions pass through the electrolyte membrane (3), oxygen is reduced at the cathode to form water (4), and oxygen is supplied from the air (5). The electron flow through the external circuit generates electricity.

How a Hydrogen Fuel Cell Works

A fuel cell works through a series of electrochemical steps:

Hydrogen gas enters the anode (the negative electrode) on the left side of the cell.
At the anode, hydrogen molecules are oxidised: each H₂ molecule loses its electrons to form H⁺ ions (protons).
The electrons travel through the external circuit from anode to cathode, generating an electric current that can power a device (motor, lamp, etc.).
The H⁺ ions pass through the electrolyte (a proton exchange membrane) to reach the cathode.
At the cathode (the positive electrode), oxygen is reduced: it combines with the H⁺ ions and the electrons to form water.

The overall reaction is simply the oxidation of hydrogen:

2H₂ + O₂ → 2H₂O + electrical energy

The key principle: chemical energy is converted directly into electrical energy, without combustion. This is more efficient than burning the hydrogen as a fuel because there is no heat loss from a flame.

Fuel Cell Half-Equations Higher Tier

At the anode (negative electrode), hydrogen is oxidised:

2H₂(g) → 4H⁺(aq) + 4e⁻

Hydrogen atoms lose electrons (oxidation is loss, "OIL"). The electrons flow through the external circuit.

At the cathode (positive electrode), oxygen is reduced:

O₂(g) + 4H⁺(aq) + 4e⁻ → 2H₂O(l)

Oxygen atoms gain electrons (reduction is gain, "RIG"). The H⁺ ions combine with oxygen and electrons to form water.

Remember OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons). In the fuel cell, hydrogen is oxidised at the anode and oxygen is reduced at the cathode. The overall equation is 2H₂ + O₂ → 2H₂O.

Advantages of Hydrogen Fuel Cells

Clean at point of use: the only product is water. No CO₂, no soot, no pollutant gases.
No recharging needed: as long as hydrogen and oxygen are supplied, the cell produces electricity continuously. It does not "go flat" like a battery.
High efficiency: fuel cells convert chemical energy directly to electrical energy without combustion, so less energy is wasted as heat.
Compact and lightweight: no heavy moving parts, making them suitable for vehicles and portable devices.
Reliable: no moving parts means fewer mechanical failures.

Disadvantages of Hydrogen Fuel Cells

Hydrogen storage: H₂ is a gas at room temperature and must be stored under very high pressure or at extremely low temperatures (liquefied). This is expensive and requires specialist tanks.
Hydrogen production: most hydrogen is currently made from natural gas (a fossil fuel) via steam reforming, which releases CO₂. For truly zero-carbon hydrogen, it must be produced by electrolysis of water using renewable electricity.
Expensive catalysts: fuel cells require platinum as a catalyst at the electrodes, which is rare and costly.
Limited infrastructure: there are very few hydrogen filling stations compared to petrol stations, so widespread adoption is slow.
Hydrogen is flammable: it is highly explosive when mixed with air, which raises safety concerns during storage and transport.

Fuel Cells vs. Rechargeable Batteries

This is a very common exam comparison. Make sure you understand the key differences:

Feature	Hydrogen Fuel Cell	Rechargeable Battery
Energy source	External fuel supply (H₂ + O₂)	Chemical energy stored internally
Running time	Runs as long as fuel is supplied	Runs until chemicals are used up, then needs recharging
Waste products	Water only	None during use (but toxic materials in manufacture and disposal)
Recharging	Not needed: just refuel with H₂	Must be plugged in to reverse the chemical reaction
Portability	Needs fuel tank and supply	Self-contained, fully portable
Environmental	Clean if H₂ from renewable sources	Requires mining of lithium and cobalt; recycling is difficult

Evaluate the use of hydrogen fuel cells to power cars compared to petrol engines.

For fuel cells: Hydrogen fuel cells produce only water as a waste product, so they cause no air pollution or CO₂ emissions at point of use. They are more efficient than petrol engines because they convert chemical energy directly to electrical energy.

Against fuel cells: Most hydrogen is currently produced from fossil fuels, so CO₂ is still released during production. Hydrogen is expensive to store (high pressure tanks), and there are very few hydrogen refuelling stations. Platinum catalysts are also expensive.

Conclusion: Hydrogen fuel cells are a promising technology for reducing vehicle emissions, but their environmental benefit depends on how the hydrogen is produced. Widespread adoption requires investment in renewable hydrogen production and refuelling infrastructure.

A 6-mark "evaluate" question on fuel cells is common in AQA exams. Structure your answer with advantages, disadvantages, and a conclusion that weighs both sides. Always mention that the hydrogen source matters: if made from fossil fuels, the overall process is not truly carbon-neutral.

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Topic 4: Chemical Changes

Reactivity series, extraction of metals, acids, bases and electrolysis.

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Factors affecting rate, collision theory, catalysts and reversible reactions.

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