AQA GCSE Chemistry Past Paper PDF

# AQA GCSE Chemistry (Separate Science) Unit 5 Energy Changes Knowledge Organiser ## Exothermic and Endothermic Reactions When a chemical reaction takes place, energy is involved. Energy is transferred when chemical bonds are broken and when new bonds are made. * **Exothermic reactions** are thos...

# AQA GCSE Chemistry (Separate Science) Unit 5 Energy Changes Knowledge Organiser ## Exothermic and Endothermic Reactions When a chemical reaction takes place, energy is involved. Energy is transferred when chemical bonds are broken and when new bonds are made. * **Exothermic reactions** are those which involve the transfer of energy from the reacting chemicals to the surroundings. During a practical investigation, an exothermic reaction would show an increase in temperature as the reaction takes place. * Examples of exothermic reactions include combustion, respiration and neutralisation reactions. * Hand-warmers and self-heating cans are examples of everyday exothermic reactions. * **Endothermic reactions** are those which involve the transfer of energy from the surroundings to the reacting chemicals. During a practical investigation, an endothermic reaction would show a decrease in temperature as the reaction takes place. * Examples of endothermic reactions include the thermal decomposition of calcium carbonate. * Eating sherbet is an everyday example of an endothermic reaction. When the sherbet dissolves in the saliva in your mouth, it produces a cooling effect. Another example is instant ice packs that are used to treat sporting injuries. ## Reaction Profiles - Exothermic & Endothermic Energy level diagrams show us what is happening in a particular chemical reaction. The diagram shows us the difference in energy between the reactants and the products: * **Exothermic reaction:** * Reactants are at a **higher** energy level than the products. * Difference in energy is **released** to the surroundings and so the temperature of the surroundings **increases**. * **Endothermic reaction:** * Reactants are at a **lower** energy level than the products. * Difference in energy is **absorbed** from the surroundings and so the temperature of the surroundings **decreases**. ## Activation Energy * **Activation Energy** - the minimum amount of energy required for a chemical reaction to take place. * **Catalysts** increase the rate of a reaction. Catalysts provide an alternative pathway for a chemical reaction to take place by lowering the activation energy. ## Bond Making and Bond Breaking * In an **endothermic reaction**, energy is needed to **break chemical bonds**. The energy change (ΔH) in an endothermic reaction is **positive**. * In an **exothermic reaction**, energy is **released** when chemical bonds are made. The energy change (ΔH) in an exothermic reaction is **negative**. Bond energies are measured in kJ/mol. ## Calculations Using Bond Energies (Higher Tier Only) Bond energies are used to calculate the change in energy of a chemical reaction. **Calculate the change in energy for the reaction: 2H₂O₂ → 2H₂O +O₂** 1. **Write the symbol equation for the reaction.** 2H-O-O-H → 2H-O-H + O=O 2. **Work out the bonds that are breaking and the ones that are being made.** * **Breaking:** 2 (O-H) + 1 (O=O) = 2 * 464 + 146 = 1074 kJ/mol * **Forming:** 2 (H-O) + 1 (O=O) = 2 * 464 + 498 = 1426 kJ/mol 3. **Calculate the total bond energy change:** The energy change (ΔH) of a reaction is equal to the total energy of breaking bonds minus the total energy of making bonds. * ΔH = (Bonds Broken) - (Bonds Made) = 1074 kJ/mol - 1426 kJ/mol = -352 kJ/mol * **The reaction is exothermic as ΔH is negative.** ## AQA GCSE Chemistry (Separate Science) Unit 5 Energy Changes Knowledge Organiser - Required Practical **Aim:** To investigate the variables that affect temperature changes in reacting solutions, e.g. acid plus metals, acid plus carbonates, neutralisations and displacement of metals. **Equipment:** * Polystyrene cup * Measuring cylinder * Thermometer * 250cm³ Glass beaker * Measuring cylinder * Top pan balance **Method:** **Reaction between a metal and an acid.** 1. Gather the equipment. 2. Place the polystyrene cup inside the beaker. This will prevent the cup from falling over. 3. Using a measuring cylinder, measure out 30cm³ of the acid - use either hydrochloric acid or sulfuric acid. Pour this into the polystyrene cup. 4. Record the temperature of the acid using a thermometer. 5. Using a top pan balance, measure out an appropriate amount of the solid (for example, 10g) or use one strip of metal such as magnesium. 6. Add the solid to the acid and record the temperature. You may choose to record the temperature of the acid and metal every minute for 10 minutes. ## Chemical Cells A chemical cell converts **chemical energy** into **electrical energy**. More than one cell connected in series is called a **battery**. * There are two types of chemical cell: **rechargeable** and **non-rechargeable**. * **Non-rechargeable cells** will produce a voltage until the chemicals inside are used up. Once this occurs, the cell is no longer useful and can be recycled. * **Rechargeable cells and batteries** can be recharged multiple times. An electrical current is passed backwards through the cell, reversing the chemical reactions and the cell or battery can be used again to produce more electricity. Mobile phones contain rechargeable batteries. ## Voltage The voltage of a cell is affected by the combination of metals used inside it. The **bigger** the difference in the reactivity of the two metals, the **bigger** the voltage produced. For example, if the metals used inside the cell are magnesium and zinc, then the voltage produced will be small as the two metals are close together in the reactivity series. By comparison, if magnesium and copper are used, then the voltage produced will be larger as the metals are further apart in the reactivity series. ## Fuel Cells: Fuel cells work differently to chemical cells in that they need to be supplied with a fuel and oxygen. The constant supply of these two ingredients will allow a fuel cell to produce a voltage continuously. * **Inside the fuel cell, hydrogen is oxidised electrochemically; the fuel is not combusted.** This allows the reaction to take place at a lower temperature. * **Hydrogen-oxygen fuel cells** are an alternative to rechargeable barriers and cells as the only product that is produced is water. ## Ionic equations: * **Hydrogen + oxygen → water** 2H₂+ O₂→ 2H₂O * **At the cathode:** 2H₂ + 4OH^- → 4H₂O + 4e^- * **At the anode:** O₂ + 2H₂O + 4e^- → 4OH^- In the fuel cell, oxygen is being reduced (reduction is the gaining of electrons) whilst hydrogen is being oxidised (oxidation is the loss of electrons). **Oxidation and reduction happen simultaneously - this is called a redox reaction.**