Chemicals store certain amount of energy & different chemicals store different amounts. If products of reaction store more energy than original reactants, they must've taken in the difference in energy between the products and reactants from surroundings during reaction. If they store less, excess enegry was transferred to surroundings. Amount of energy doesn't change - moved around.
Exothermic reaction transfers energy to surroundings, usually by heating. Shown by rise in temp. Burning fuels - Combustion - gives out lots of energy. Neutralisation reactions. Oxidation reactions, e.g. adding Na to water releases energy. Everyday uses:
Some hand warmers use the exothermic oxidation of iron in air with salt solution catalyst to release enegry.
Self heating cans rely on exothermic reactions between chemicals in their bases.
Endothermic reaction takes in energy from surroundings. Shown by fall in temp. Include reaction between citric acid and sodium hydrogencarbonate. Thermal decomposition - heating calcium carbonate causes it to decompose into calcium oxide (quicklime) and CO2. Everyday uses:
Some sports injury packs - chemical reaction allows pack to become instantly cooler without having to put it in freezer.
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More Exothermic and Endothermic Reactions
You can measure amount of enegry released by a chemical reaction by taking temp. of reagents, mixing them in polystyrene cup and measuring temp. of solution at end. Problem with energy measurements is amount lost to surroundings. Can be reduced little by putting cup into beaker of cotton wool - more insulation, and putting lid on cup to reduce energy lost by evaporation.
Works for neutralisation reactions or reactions between metals and acids, or carbonates and acids.
Can also use to investigate effect variables have on amount of energy transferred e.g. mass or concentration of reactants. How to test effect of acid concentration on energy released in neutralisation between HCl and NaOH:
Put 25cm'3 of 0.25mol/dm'3 HCl and NaOH in separate beakers. Place beakers in water bath set to 25'C until both that temp. Add HCl followed by NaOH to polystyrene cup with lid. Take temp. of mixture every 30 secs, and record highest. Repeat steps 1-4 using 0.5mol/dm'3, then 1.0mol/dm'3 of HCl
Reaction profiles - diagrams that show relative energies of reactants and products in reaction, and how enegry changes over course of reaction.
Exothermic - Products at lower energy than reactants. Difference in height = overall energy change in reaction per mole (energy given out). Initial rise in energy represents energy needed to start reaction - activation energy - Ea - minimum amount of energy particles need to collide with each other and react. Greater Ea - more energy needed - has to be supplied e.g. heating reaction mixture.
Endothermic - products at higher energy than reactants. Difference in height - overall enegry change during reaction per mole (energy taken in)
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Bond Energies
During chemical reaction, old bonds broken and new ones formed. Energy must be supplied to break bonds - endothermic. Energy released when new bonds formed - exothermic.
In exothermic reactions the energy released by forming bonds is greater than the energy used to break them. Endothermic is vice versa.
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