Energy changes Flashcards
How is energy moved around in chemical reactions?
chemicals store a different amount of energy .
If the products of a reaction store more energy than the original reactant, then they must have taken in the different in energy between the products and reactants from the surroundings during the reaction.
But if they store less, then the excess was transferred to the surroundings during the reaction.
What is an exothermic reaction?
a reaction which transfers energy to the surroundings usually by heating. Shown by a rise in the temperature
What is an example of exothermic reaction?
burning fuels( combustion) - gives out alot of energy.
Neutralisation reactions
oxidation reactions - adding sodium to water releases energy.
Uses of exothermic reactions?
some hand warmers use exothermic oxidation of iron in air to release energy.
self heating cans of hot chocolate and coffe also rely on exothermic reactions between chemicals in their bases.
What is an endothermic reaction?
reaction which takes in energy from the surroundings.Shows by a fall in the temperature
Examples of endothermic reactions?
reaction between citric acid and sodium hydrogencarbonate
thermal decomposition
Uses of endothermic reactions?
used in sports injury packs - the chemical reaction allows the pack to become instantly cooler without having to put it in the freezer.
What are reaction profiles?
diagrams that show the realtive energies of the reactiants and products in a reaction and how the energy changes over the course of the reaction.
For more on reaction profiles go to page 62
62
What happens during a chemical reaction?
old bonds are broken and new bonds are formed.
Energy must be supplied to break existing bonds - so bond breaking is an endothermic process
Energy is released when new bonds are formed - so bond formation is an exothermic process.
Bond Energy calculations PAGE 63
63 bottom
What is an electrochemical cell ?
a basic system made up of two different electrodes in contact with an electrolyte.
What is a fuel cell?
an electrical cell thats supplied with a feul and oxygen and uses energy from the reaction between them to produce electrical efficiently.
when the fuel enters the cell it becomes oxidised and sets up a potential difference within the cell.
More about cells and batteries on page 64
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