P1 5 Energy Changes

• heat energy is released - temp of surroundings increases • reactants at a higher energy level than products • energy released through the formation of bonds is greater than the energy needed to break bonds • combustion, oxidation, neutralisation • self-heating cans, hand warmers

• heat energy is taken in - temp of surroundings decreases • reactants at a lower energy level than products • the energy required to break bonds (backwards) is greater than the energy released when new bonds are formed (forwards) • thermal decomposition, citric acid+ hydrogen carbonate • cold packs

• measure a volume of two reactants (25cm3) • place one liquid in a polystyrene cup, then place an insulating lid on top with a thermometer through it • record the starting temp of the solution • add the second solution and record the temp • wait 30secs and record the temp again • repeat until there is no change in temp -> if the temp increases the reaction is exothermic -> insulating lid+ polystyrene limit dissipation of energy to surroundings, put inside another cup to increase accuracy -> other variables that affect temp change: concentration of a reactant, reactivity of metal being used

• collision theory: reactant particles must collide with each other -> providing particles have sufficient energy to start a reaction • a collision that produces a reaction is a successful collision • activation energy - minimum amount of energy needed for a collision to be successful • activation energy is diff for different reactions

• when a bond is formed energy is released (exothermic, - kJ/mol) • to break a bond energy needs to be supplied (endothermic, + kJ/mol) • bond energies are different for different bonds • energy required to make a bond = energy required to break the same bond

energy to form bonds - energy to break bonds total bond energy of reactants - total bond energy of products measured in kJ/mol