energy changes

• Energy is conserved in chemical reactions. The amount of energy in the universe at the end of a chemical reaction is the same as before the reaction takes place. If a reaction transfers energy to the surroundings the product molecules must have less energy than the reactants, by the amount transferred.
• An exothermic reaction is one that transfers energy to the surroundings so the temperature of the surroundings increases
• Exothermic reactions include combustion, many oxidation reactions and neutralisation.
• An endothermic reaction is one that takes in energy from the surroundings so the temperature of the surroundings decreases
• Endothermic reactions include thermal decompositions and the reaction of citric acid and sodium hydrogencarbonate. Some sports injury packs are based on endothermic reactions.
• (practical ) you can measure the amount of energy released by a chemical reaction in a solution by taking the temperature of the reagents, mixing them in a polystyrene cup and measuring the temperature of the solution at the end of the reaction
• reaction profiles show energy changes
• Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy. The minimum amount of energy that particles must have to react is called the activation energy.
• what is activation energy

  the minimum amount of energy that particles must have to react
• is the profile endothermic or exothermic

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• is the profile endothermic or exothermic?

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• name missing parts

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• Reaction profiles can be used to show the relative energies of reactants and products, the activation energy and the overall energy change of a reaction.
• During a chemical reaction: • energy must be supplied to break bonds in the reactants • energy is released when bonds in the products are formed.
• The energy needed to break bonds and the energy released when bonds are formed can be calculated from bond energies.
• The difference between the sum of the energy needed to break bonds in the reactants and the sum of the energy released when bonds in the products are formed is the overall energy change of the reaction.
• in an exothermic reaction, the energy released by forming new bonds is greater than the energy needed to break existing bonds.
• In an endothermic reaction, the energy needed to break existing bonds is greater than the energy released from forming new bonds.
• chemical reactions in a cell produce electricity
• The voltage produced by a cell is dependent upon a number of factors including the type of electrode and electrolyte
• A simple cell can be made by connecting two different metals in contact with an electrolyte
• Batteries consist of two or more cells connected together in series to provide a greater voltage.
• In non-rechargeable cells and batteries the chemical reactions stop when one of the reactants has been used up. Alkaline batteries are non-rechargeable.
• Rechargeable cells and batteries can be recharged because the chemical reactions are reversed when an external electrical current is supplied.
• Fuel cells are supplied by an external source of fuel (eg hydrogen) and oxygen or air. The fuel is oxidised electrochemically within the fuel cell to produce a potential difference.
• The overall reaction in a hydrogen fuel cell involves the oxidation of hydrogen to produce water.
• Hydrogen fuel cells offer a potential alternative to rechargeable cells and batteries.