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neurologyYou need to know

• An exothermic reaction transfers energy to the surroundings, causing the temperature of the surroundings to increase.
• An endothermic reaction takes in energy from the surroundings, causing the temperature of the surroundings to decrease.
• Combustion, neutralisation and many oxidation reactions are common examples of exothermic reactions.
• Thermal decomposition and some reactions between solids, such as barium hydroxide with ammonium chloride, are examples of endothermic reactions.
• A reaction can be identified as exothermic if the measured temperature of the surroundings increases during the reaction.
• A reaction can be identified as endothermic if the measured temperature of the surroundings decreases during the reaction.
• In an exothermic reaction profile, the products are at a lower energy level than the reactants.
• In an endothermic reaction profile, the products are at a higher energy level than the reactants.
• Activation energy is the minimum energy needed for reacting particles to collide successfully and start a chemical reaction.
• A reaction profile shows the relative energies of reactants and products, the activation energy, and the overall energy change as the reaction proceeds.
• The overall energy change is the difference between the energy of the reactants and the energy of the products.
• In an exothermic reaction, the overall energy change is negative because energy is transferred from the reaction to the surroundings.
• In an endothermic reaction, the overall energy change is positive because energy is taken in from the surroundings.
• Energy is needed to break chemical bonds, so bond breaking is an endothermic process.
• Energy is released when chemical bonds are made, so bond making is an exothermic process.
• A reaction is exothermic overall when the energy released by making bonds is greater than the energy taken in to break bonds.
• A reaction is endothermic overall when the energy taken in to break bonds is greater than the energy released by making bonds.
• The overall energy change can be calculated using `\text{energy change} = \text{energy to break bonds} - \text{energy released making bonds}`.
• Self-heating cans and hand warmers use exothermic reactions because they transfer thermal energy to the surroundings.
• Instant cold packs use endothermic reactions because they take in thermal energy from the surroundings.

• Identify a reaction as exothermic or endothermic from temperature data by linking a temperature increase to exothermic change and a temperature decrease to endothermic change.
• Draw an exothermic reaction profile with reactants above products, a curved pathway, the activation energy labelled from the reactant level to the peak, and the overall energy change shown downwards.
• Draw an endothermic reaction profile with products above reactants, a curved pathway, the activation energy labelled from the reactant level to the peak, and the overall energy change shown upwards.
• Interpret a reaction profile by comparing the energy levels of reactants and products to decide whether the reaction is exothermic or endothermic.
• Calculate the total energy needed to break bonds by multiplying each bond energy by the number of that bond broken and then adding the values.
• Calculate the total energy released when making bonds by multiplying each bond energy by the number of that bond made and then adding the values.
• Calculate the overall energy change for a reaction using `\Delta H = \text{energy to break bonds} - \text{energy released making bonds}` and state whether the result is exothermic or endothermic.
• Evaluate an application of an exothermic or endothermic reaction by using data about energy transfer, temperature change, cost, safety or practicality to justify a conclusion.