heat energy changes in chemical reactions

exothermic : releases heat e.g precipitaion, neutralisation and displacement reactions.

endothermic : takes in heat e.g precipitaion reactions.

precipitaion : aqueous solutions mixed together that form a solid.

neutralisation : changing a solutions ph to 7

displacement reactions : a more reactive element replaces a less reactive element.

combustion reactions (burning) are exothermic as the heat felt is thermal energy being released.

reaction profiles: graph which models the energy changes which take place in a chemical reaction.

when a salt dissolves in water, the change is exothermic or endothermic, depending on the salt.the temperature increases as calcium chloride dissolves, but decreases when ammonium chloride dissolves.

this occurs beacuse...

when more energy is given out making the bonds than is breaking them,the reaction is exothermic (releases energy).

when less energy is given out making the bonds than taken in breaking the bonds, the reaction is endothermic (absorbs energy)

during a chemical reaction, bonds in the reactants break and new bonds are made to form the products:

energy is transfered to the reactants to break their bonds-this means bond breaking is endothermic

energy is transfered to the surroundings when making bonds, which means bond making is exothermic.

reactions with high activation energies take longer to start, whereas reactions with low activation energies are faster to start.

mole: 1 mole of any substance contains 6.02 *10^23 atoms of that particular substance.

bond energy: amount of energy needed to break one mole of any particular covalent bond. measured in mol^-1

step 1 : find the balanced equation.

step 2: calculate energy in (bonds broken) from the products.a table of bond energies will be given in exams.

step 3: calculate energy out (bonds made) from the reactants.

step 4: energy change = energy in - energy out. if the number is negative, the reaction is exothermic.if the number is positive, the reaction is endothermic.

tip: drawing the bonds with help!

for example...

H-----O-----H   (H2O)