Subject: Chemistry | Level: GCSE | Exam Board: WJEC
Master the core concepts of Energy Changes in Chemistry, from identifying exothermic and endothermic reactions to mastering reaction profiles and calculating bond energies. This topic is essential for your GCSE exams, combining conceptual understanding with vital mathematical skills.
Revision Notes & Key Concepts
Revision Podcast Transcript
Hello and welcome to your GCSE Chemistry revision podcast. I'm so glad you're here, because today we're diving into one of the most important topics in your chemistry course — Energy Changes in Chemistry. Whether you're revising for your mocks or your final exams, this episode is going to give you everything you need to feel confident and exam-ready. So grab a pen, get comfortable, and let's get into it. By the end of this episode, you'll be able to distinguish between exothermic and endothermic reactions, draw and label reaction profiles perfectly, calculate energy changes using bond energies, and evaluate hydrogen fuel cells like a true examiner. We've got a lot to cover, so let's start right at the beginning. SECTION ONE: EXOTHERMIC AND ENDOTHERMIC REACTIONS Let's start with the big question — what actually happens to energy during a chemical reaction? Every chemical reaction involves energy. Bonds between atoms are broken in the reactants, and new bonds are formed in the products. Breaking bonds requires energy — you have to put energy in. Forming bonds releases energy — energy comes out. The overall energy change of a reaction depends on which of these processes dominates. In an exothermic reaction, more energy is released when bonds form in the products than is taken in to break bonds in the reactants. The result? Energy is transferred to the surroundings — and the surroundings get warmer. The temperature of the solution or the surroundings goes up. Think of burning wood, neutralisation reactions, or hand warmers. These are all exothermic. In an endothermic reaction, more energy is taken in to break bonds in the reactants than is released when bonds form in the products. Energy is taken from the surroundings — and the surroundings get cooler. The temperature goes down. Think of thermal decomposition of calcium carbonate, dissolving ammonium nitrate in water, or those cold packs athletes use on injuries. Here's the key exam tip: examiners are testing whether you understand the direction of energy transfer. In exothermic reactions, energy transfers from the chemical system to the surroundings. In endothermic reactions, energy transfers from the surroundings to the chemical system. A really common mistake candidates make is saying exothermic reactions give out heat to the reaction — no! The heat goes to the surroundings. Always be precise about this. Memory hook time: think exothermic — exit. Energy exits the reaction and goes into the surroundings. Endothermic — energy enters the reaction from the surroundings. Exit and Enter. Exothermic and Endothermic. Sorted. SECTION TWO: REACTION PROFILES Now let's talk about reaction profiles — also called energy level diagrams. These are absolutely guaranteed to come up in your exam, so you need to be able to draw them and interpret them. A reaction profile is a graph that shows how the energy of the reacting system changes as the reaction progresses. The x-axis is labelled Progress of Reaction. The y-axis is labelled Energy in kilojoules per mole. For an exothermic reaction profile: the reactants start at a higher energy level on the left. The curve rises up to a peak — this peak represents the transition state, the highest energy point in the reaction. Then the curve falls down to the products on the right, which are at a lower energy level than the reactants. The overall energy change, delta H, is negative — products are lower than reactants. For an endothermic reaction profile: the reactants start at a lower energy level. The curve rises to a peak, then falls to the products — but the products end up at a higher energy level than the reactants. Delta H is positive. Now, the activation energy. This is absolutely critical. The activation energy is the minimum energy that colliding particles must have for a reaction to occur. On a reaction profile, it is shown as the difference in energy between the reactants and the peak of the curve. It is always shown as an upward arrow from the reactants level to the peak — regardless of whether the reaction is exothermic or endothermic. This is where so many candidates lose marks. They draw the activation energy arrow from the products to the peak, or from the bottom of the graph to the peak. Wrong! It must go from the reactants energy level to the peak. Every single time. Let me give you a checklist for drawing a perfect reaction profile. Number one: draw and label your axes. Number two: draw the reactants as a horizontal line on the left. Number three: draw the curve rising to a peak, then falling to the products. Number four: draw the products as a horizontal line — higher than reactants for endothermic, lower for exothermic. Number five: draw a double-headed arrow from reactants to peak and label it activation energy. Number six: draw a double-headed arrow from reactants to products and label it delta H. If it points downward, delta H is negative — exothermic. If it points upward, delta H is positive — endothermic. Do all six of those things and you will earn every mark available. SECTION THREE: BOND ENERGY CALCULATIONS Right, now we're getting into the maths — and this is where candidates either pick up easy marks or throw them away. Let's make sure you're in the first group. The key principle is this: breaking bonds requires energy — it is endothermic. Forming bonds releases energy — it is exothermic. The overall energy change of a reaction is the difference between the energy needed to break bonds in the reactants and the energy released when bonds form in the products. The formula you must memorise is: Energy Change equals Energy In minus Energy Out. Or more precisely: Energy Change equals the sum of bond energies for bonds broken, minus the sum of bond energies for bonds formed. If the answer is negative — more energy was released forming bonds than was used breaking them — the reaction is exothermic. If the answer is positive — more energy was used breaking bonds than was released forming them — the reaction is endothermic. Let me walk you through an example. Consider the reaction: hydrogen plus chlorine gives two hydrogen chloride molecules. Step one: identify all the bonds broken in the reactants. We have one hydrogen-hydrogen bond and one chlorine-chlorine bond. The bond energy of hydrogen-hydrogen is 436 kilojoules per mole. The bond energy of chlorine-chlorine is 243 kilojoules per mole. Total energy in equals 436 plus 243, which equals 679 kilojoules per mole. Step two: identify all the bonds formed in the products. We have two hydrogen-chlorine bonds. The bond energy of each hydrogen-chlorine bond is 432 kilojoules per mole. Total energy out equals 2 times 432, which equals 864 kilojoules per mole. Step three: calculate the energy change. Energy change equals 679 minus 864, which equals negative 185 kilojoules per mole. The negative sign tells us this reaction is exothermic — energy is released to the surroundings. The most common mistake here is forgetting to multiply bond energies by the number of bonds. If you have two hydrogen-chlorine bonds, you must count both of them. Always go back to the balanced equation and count every single bond carefully. SECTION FOUR: ELECTROCHEMICAL CELLS AND FUEL CELLS Now let's move on to electrochemical cells and fuel cells. This section often appears as an evaluate question worth four to six marks, so you need to be able to argue both sides. First, chemical cells. A chemical cell produces a potential difference — a voltage — from a chemical reaction. Two different metals in an electrolyte solution will produce a voltage because one metal is more reactive than the other. The greater the difference in reactivity, the greater the potential difference. However, chemical cells eventually stop working when the reactants are used up. Now, fuel cells. A hydrogen-oxygen fuel cell is a device that uses hydrogen and oxygen to produce electricity through an electrochemical reaction. Hydrogen enters at the anode, where it is oxidised — it loses electrons. The electrons flow through the external circuit, producing an electric current. Hydrogen ions pass through the electrolyte membrane. At the cathode, oxygen reacts with the hydrogen ions and electrons to form water. The only direct product is water — which is why fuel cells are considered environmentally friendly. The overall equation is: two H two plus O two gives two H two O. For your evaluate questions, you need to know both advantages and disadvantages. Advantages of hydrogen fuel cells: they only produce water as a by-product, so they have no direct carbon dioxide emissions. They are more efficient than combustion engines. They can be continuously supplied with fuel, unlike batteries which need recharging. Disadvantages: hydrogen is currently produced mainly from natural gas, which does produce carbon dioxide — so the overall process may not be carbon neutral. Hydrogen is highly flammable and difficult to store and transport safely. The infrastructure for hydrogen refuelling stations is not yet widely available. Examiners want you to make a judgement at the end of an evaluate question. Don't just list pros and cons — conclude. For example: Overall, hydrogen fuel cells have significant potential as a clean energy source, but their environmental benefit depends on how the hydrogen is produced. Until renewable methods of producing hydrogen become widespread, their advantage over fossil fuels is limited. That kind of evaluative conclusion is what earns you the top marks. EXAM TIPS AND COMMON MISTAKES Let's now run through the key exam tips and the most common mistakes I see candidates make. Tip one: always state the direction of energy transfer. Don't just say exothermic means heat is released. Say: in an exothermic reaction, energy is transferred from the chemical system to the surroundings, causing the temperature of the surroundings to increase. Tip two: on reaction profiles, the activation energy arrow always goes from the reactants level to the peak. Not from the bottom of the graph. Not from the products. From the reactants to the peak. Tip three: in bond energy calculations, always show your working in three clear steps — bonds broken, bonds formed, energy change. Never just write the final answer. Examiners award marks for each step. Tip four: when asked to evaluate fuel cells, you must discuss both advantages and disadvantages and reach a conclusion. A one-sided answer will not access the top marks. Tip five: be careful with the command word explain. This requires you to say how or why something happens. Use the word because to link your cause and effect. QUICK-FIRE RECALL QUIZ Right, let's test what you've learned! Cover your notes and try to answer these questions out loud. Question one: What happens to the temperature of the surroundings in an exothermic reaction? The temperature increases, because energy is transferred from the reaction to the surroundings. Question two: On a reaction profile, what does the activation energy arrow show? The minimum energy needed for the reaction to occur — shown from the reactants energy level to the peak of the curve. Question three: In a bond energy calculation, if bonds broken equals 500 kilojoules per mole and bonds formed equals 650 kilojoules per mole, what is the energy change and is the reaction exothermic or endothermic? Energy change equals 500 minus 650, which equals negative 150 kilojoules per mole. Negative value means exothermic. Question four: Name one advantage and one disadvantage of hydrogen fuel cells. Advantage: only water is produced as a by-product. Disadvantage: hydrogen is currently mainly produced from fossil fuels, which releases carbon dioxide. Question five: What is the overall equation for the reaction in a hydrogen-oxygen fuel cell? Two H two plus O two gives two H two O. How did you do? If you got all five, brilliant — you're in great shape. If you missed any, go back and re-read those sections before your exam. SUMMARY AND SIGN-OFF Let's wrap up with the key points to take away from today's episode. One: exothermic reactions transfer energy to the surroundings — temperature increases. Endothermic reactions take energy from the surroundings — temperature decreases. Two: on reaction profiles, always label reactants, products, activation energy from reactants to peak, and the overall energy change delta H. Three: bond energy calculation formula — Energy Change equals Energy In, bonds broken, minus Energy Out, bonds formed. Negative answer means exothermic. Four: hydrogen fuel cells produce only water as a direct by-product, but the environmental impact depends on how the hydrogen is produced. Five: for evaluate questions, always give both sides and reach a conclusion. You've got this. Energy changes is a topic where careful, methodical working and precise language will earn you every mark. Practice drawing reaction profiles until you can do them in your sleep, and make sure your bond energy calculations always show three clear steps. Thank you so much for listening. Good luck in your exams — I know you're going to smash it. Until next time, keep revising and stay curious!
Key Terms & Definitions
- Exothermic Reaction
- A reaction that transfers energy to the surroundings, causing the temperature of the surroundings to increase.
- Endothermic Reaction
- A reaction that takes in energy from the surroundings, causing the temperature of the surroundings to decrease.
- Activation Energy
- The minimum amount of energy that particles must have to react when they collide.
- Bond Energy
- The amount of energy required to break one mole of a specific covalent bond.
- Reaction Profile
- A graph showing the relative energies of reactants and products, the activation energy, and the overall energy change of a reaction.
- Fuel Cell
- A chemical cell that is supplied with a fuel and oxygen, and uses energy from the reaction between them to produce a continuous electrical voltage.
Worked Examples
Worked Example
Question: Calculate the overall energy change for the reaction: $H_2 + Cl_2 \rightarrow 2HCl$. Bond energies (kJ/mol): H-H = 436, Cl-Cl = 243, H-Cl = 432. State whether the reaction is exothermic or endothermic. (4 marks)
Solution: Step 1: Calculate energy required to break bonds in reactants. Bonds broken: 1 x H-H (436) + 1 x Cl-Cl (243) Energy In = 436 + 243 = 679 kJ/mol Step 2: Calculate energy released forming bonds in products. Bonds formed: 2 x H-Cl (432) Energy Out = 2 x 432 = 864 kJ/mol Step 3: Calculate overall energy change. Energy Change = Energy In - Energy Out Energy Change = 679 - 864 = -185 kJ/mol Final answer: -185 kJ/mol. The reaction is exothermic.
Worked Example
Question: Evaluate the use of hydrogen fuel cells compared to rechargeable lithium-ion batteries to power electric cars. (6 marks)
Solution: Hydrogen fuel cells and rechargeable batteries both offer alternatives to fossil fuel engines, but have different pros and cons. A key advantage of hydrogen fuel cells is that they can be refuelled quickly (in minutes), similar to petrol cars, whereas lithium-ion batteries take much longer to recharge (often hours). Furthermore, fuel cells do not lose capacity over time, whereas batteries degrade and eventually need replacing. The only direct emission from a fuel cell is water, meaning no local pollutants. However, there are significant disadvantages. Hydrogen is highly flammable and must be stored under high pressure, making it difficult and dangerous to transport. The infrastructure for hydrogen refuelling is currently very limited compared to the growing network of electric charging points. Additionally, most hydrogen is currently manufactured from natural gas, a process that releases carbon dioxide, so it is not completely carbon-neutral. In conclusion, while hydrogen fuel cells offer superior range and refuelling times, lithium-ion batteries are currently more practical due to better infrastructure and safer energy storage.
Worked Example
Question: Explain why the reaction between methane and oxygen is exothermic, in terms of bond breaking and bond making. (3 marks)
Solution: The reaction is exothermic because energy is required to break the bonds in methane and oxygen, and energy is released when new bonds form in carbon dioxide and water. More energy is released when the new bonds are formed in the products than is required to break the bonds in the reactants.
Practice Questions
Question: A student investigates the temperature change when zinc reacts with copper sulfate solution. The initial temperature is 21.0 °C and the maximum temperature reached is 34.5 °C. State whether this reaction is exothermic or endothermic and explain your answer. (2 marks)
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Question: Sketch a fully labelled reaction profile for an endothermic reaction. (4 marks)
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Question: Hydrogen peroxide decomposes into water and oxygen. The overall energy change is -196 kJ/mol. Explain what this value tells you about the bond breaking and bond making processes in this reaction. (3 marks)
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Question: Explain why a polystyrene cup is used instead of a glass beaker when investigating temperature changes in chemical reactions. (2 marks)
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Question: Write the half-equation for the reaction that occurs at the cathode in a hydrogen fuel cell. (2 marks)
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