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neurologyYou need to know

• An exothermic reaction transfers energy to the surroundings, so the temperature of the surroundings increases.
• An endothermic reaction takes in energy from the surroundings, so the temperature of the surroundings decreases.
• A reaction can be identified as exothermic if the measured temperature of the surroundings rises during the reaction.
• A reaction can be identified as endothermic if the measured temperature of the surroundings falls during the reaction.
• In an exothermic reaction profile, the reactants have more energy than the products, so the overall energy change is negative.
• In an endothermic reaction profile, the products have more energy than the reactants, so the overall energy change is positive.
• Activation energy is the minimum energy needed for reacting particles to react successfully.
• A reaction profile shows the relative energies of reactants and products, the activation energy, and the overall energy change as the reaction proceeds.
• Breaking chemical bonds requires energy, so bond breaking is endothermic.
• Making chemical bonds releases energy, so bond making is exothermic.
• The overall energy change of a reaction is the energy needed to break bonds minus the energy released when new bonds are made.
• If more energy is released by making bonds than is needed to break bonds, the reaction is exothermic.
• If more energy is needed to break bonds than is released by making bonds, the reaction is endothermic.
• Cells contain chemicals that react to produce electricity.
• A simple cell can be made by connecting two different metals in contact with an electrolyte.
• The potential difference produced by a cell depends on factors including the type of electrodes and the electrolyte used.
• Rechargeable cells and batteries can be recharged because an external electrical current reverses the chemical reactions inside them.
• A fuel cell is supplied continuously with an external fuel, such as hydrogen, and oxygen or air.
• The overall reaction in a hydrogen fuel cell is the oxidation of hydrogen to produce water.
• In an alkaline hydrogen fuel cell, the hydrogen electrode half equation is `2H_2 + 4OH^- \rightarrow 4H_2O + 4e^-` and the oxygen electrode half equation is `O_2 + 2H_2O + 4e^- \rightarrow 4OH^-`.

• Classify a reaction as exothermic or endothermic using temperature change data for the surroundings.
• Evaluate uses of exothermic and endothermic reactions by linking temperature change, energy transfer, usefulness, risks, and practical limitations.
• Draw a reaction profile with labelled reactants, products, activation energy, overall energy change, and a curved line showing the progress of the reaction.
• Use a reaction profile to identify whether a reaction is exothermic or endothermic from the relative energies of reactants and products.
• Calculate the energy needed to break bonds by multiplying each bond energy by the number of bonds broken.
• Calculate the energy released when bonds are made by multiplying each bond energy by the number of bonds formed.
• Calculate the overall energy change for a reaction using bond energies and state whether the reaction is exothermic or endothermic.
• Interpret cell data to compare the relative reactivity of metals and predict which metal combinations give a larger potential difference.
• Evaluate the use of cells, batteries, and hydrogen fuel cells by comparing factors such as voltage, rechargeability, fuel supply, emissions, cost, and practicality.