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Energy changes – exothermic and endothermic reactions

An exothermic reaction gives out heat energy. The surroundings(including any thermometer) get hotter. This is because chemical

energy is being converted into heat energy during the reaction. Energyis taken in to break bonds in the reactants, but a greater amount ofenergy is given out when new bonds in the products are made.e.g. combustion/burning e.g. of fuels

respiration (supplies energy for the body)many oxidation reactions (e.g. rusting)

neutralisation reactions

An endothermic reaction requires energy to be supplied to get it to go.The heat energy is taken in, and converted to chemical energy, so thesurroundings (including any thermometer) gets colder. More energy isrequired to break the necessary bonds in the reactants than is given

out when the new bonds in the products are formed.e.g. photosynthesis

- doesn’t go without light energy supplied thermal decompositions

- don’t go unless heated continuously 

traditional lime kiln

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Practical uses of exothermic reactions:

non-reusable

reusable

self-heatingcans

hand

warmers

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Practical uses of endothermic reactions:

Product Description

“Instant Ice Packs are activated by

squeezing an internal compartment within

the pack. This starts a chemical reaction

that instantly makes the pack cold. The

Instant Ice Pack stays cold for up to 30minutes, making it ideal for the early

treatment of sports injuries.“ 

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Some reactions are irreversible – once the reaction has taken place, it can’t beundone.

e.g. a cooked egg can’t turn back into a raw onee.g. the waste gases propelling a rocket can’t turn back

into rocket fuel

We use a single arrow in the equations for these reactions:

e.g. CaCO3  CaO + CO2

limestone quicklime 

Other reactions are reversible. They can proceed in either direction – reactants turning into products, or products turning into reactants.

We use a double arrow in equations for these reactions: e.g.

N2 + 3 H2  2 NH3

nitrogen hydrogen ammonia

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Some examples:Ammonia reacts with hydrogen chloride (both colourless gases)to form white clouds of ammonium chloride.

Ammonium chloride is a white crystalline solid, but if you sniffit you can smell ammonia distinctly. This is because ammoniumchloride is reacting to make ammonia and hydrogen chloride.

NH3(g) + HCl(g)  NH4Cl(s)

Cobalt chloride crystals are blue, but turn pink when theybecome damp. Drying the crystals with a hair-dryer turnsthem blue again. This can be used as a test for thepresence of water.

CoCl2.2H2O + 4 H2O   CoCl2.6H2O

wet

dry

dishes of ammonia andhydrogen chloride,opened side by side

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The well-known blue crystals of copper sulphate contain waterbonded into the structure of the crystals. We call this hydratedcopper sulphate.

If heated continuously, blue copper sulphate crystalsturn into a white powder, giving off steam in theprocess. The white powder is called anhydrous coppersulphate. The solid contains no water. The reaction isendothermic – heat is required to keep it going.

The reaction is also reversible. White anhydrous coppersulphate turns into blue hydrated copper sulphate if water isadded. This is the reverse reaction. It is also exothermic – theheat being produced can be seen as steam and heard as ahissing and spitting when the water is added.

Anhydrous copper sulphate is usedas a test for water. The steam shows

the reaction is exothermic.