Energy Changes

Exothermic and Endothermic Reactions

- energy is conserved in chemical reactions, it is neither created nor destroyed

- a reaction in which energy is transferred from the reacting substances to their surroundings is called an exothermic reaction

- a reactioon in which energy is transferred to the reacting substances from their surroundings is called an endothermic reaction

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Using Energy Transfers from Reactions

- exothermic changes can be used in handwarmers and self-heating cans, crystallisation of a supersaturated solution is used in reusable warmers however, disposable, one-off warmers heat up the surroundings for longer 

- endothermic changes can be used in instant cold packs for sports injuries 

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Reaction Profiles

- can show the relative difference in the energy of reactants and products on reaction profiles

- bond breaking is endothermic, whereas bond making is exothermic

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Bond Energy Calculations

- in chemical reactions, energy must be supplied to break the bonds between atoms in the reactants

- when new bonds are formed between atoms in a chemical reaction, energy is released 

- in an exothermic reaction, the energy released when new bonds are formed is greater than the energy absorbed when bonds are broken

- in an endothermic reaction, the energy released when new bonds are formed is less than the energy absorbed when bonds are broken

- can calculate the overall energy change in a chemical reaction using bond energies

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Chemical Cells and Batteries

- metals tend to lose electrons and form positive ions

- when 2 metals are dipped in a salt solution and joined by a wire, the more reactive metal will donate electrons to the less reactive metal, this forms a simple electrical cell

- the greater the difference in reactivity between the 2 metals, the higher the voltage produced by the cell

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Fuel Cells

- much of the world relies on fossil fuels however, they are non- renewable and they cause pollution

- hydrogen is one alternative fuel, it can be burned in combustion engines or used in fuel cells to power vehicles 

- hydrogen gas is oxidised and provides a source of electrons in the hydrogen fuel cell, in which the only waste product is water

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