energy changes in reactions
types of thermal reactions
exothermic and endothermic
exothermic reaction
give thermic energy to their surroundings
endothermic reaction
reaction takes in thermic energy from surroundings
energy in endo/exothermic reactions
exothermic reactions products have less energy than the reactants -endothermic reactions products have more energy than reactants
bond making and breaking as reactions
bond breaking is an endothermic process -bond making is an exothermic process
activation energy
minimum energy colliding particles must have to react
enthalpy change of reaction
overall energy change of a reaction
where does activation energy come from
-surroundings -some exothermic reactions need heat to start -some reactions only need a spark and others witha high activation energy need heating throughout reaction
key ideas in calculating enthalpy changes
-bonds in reactants must be broken :takes energy -new bonds are made creating products:gives out energy
how to calculate enthalpy changes
change for reaction =energy in for bond breaking -energy out from bond making
bond energy
energy needed to break bonds or released when the same bonds form
hydrogen-oxygen fuel cell
uses hydrogen and oxygen to produce electricity the only chemical produced is water which is harmless
advantages of hydrogen oxygen fuel cell
-harmless -hydrogen can be produced through electrolysis of water which is a plentiful resource
disadvantages of hydrogen oxygen fuel cell
hydrogen must be made into a liquid to be practical to use -hydrogen stations are not widely avalible -hydrogen is mostly obtained through fossil fuels
advantages of petrol
widely available -relativley cheap
disadvantages of petrol
burning petrol produces -carbon dioxide -oxides of nitrogen -carbon monoxide -unburnt volatile gases