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Energy Transfers in Reactions - Peer Teaching 17/10/12

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Christopher Murtland

on 18 October 2012

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Transcript of Energy Transfers in Reactions - Peer Teaching 17/10/12

An endothermic reaction is one that has more bond breaking (energy required)

This means endothermic reactions take energy FROM the surroundings

You must keep supplying energy to continue these reactions However, as we know energy MUST always be conserved we can work out this difference

It is this difference in energy that tells us whether a reaction is exothermic or endothermic Energy Transfers
in Reactions Endothermic and
Exothermic Reactions Endothermic Exothermic The greatest conceptual leap for pupils will usually be the idea that to break bonds takes energy

Just remember; bonds are not full of energy that comes out when they break. They are a state of lower energy, laziness for the atoms

So it takes a good kick to get them moving about We can get
students to predict whether
a reaction is exothermic or endothermic An exothermic reaction is one that has more bond making (energy release)

This means endothermic reactions give energy OUT to the surroundings

Usually they release this through heat, (and sometimes light) Usually the energy to break and make different bonds are not equal Reactions In a reaction, reactants are turned into products

REACTANTS PRODUCTS For this to occur, atoms in the reactants are separated by a process of ‘bond breaking’ which takes energy to do These atoms then, join together to form the products in a process of ‘bond making’, which releases energy Energy Energy Energy Evolved Energy of Reactants
(Bond Breaking) Energy of Products
(Bond Making) Change in Energy of Surroundings Gives energy (heat) out to the surroundings Requires energy from the surroundings to keep reacting The burning of fossil fuels are examples of this:

Heat given off is used to heat water, which power steam-driven turbines, generating our electricity Photosynthesis is an example
of an endothermic reaction
(energy from sunlight) So is baking bread
(heat energy from
the oven) We do this with simple organic reactions using tables of mean bond enthalpies: Students can use the symbol equation and relevant structures to determine which bonds are broken and which are formed Bond enthalpy is the amount of energy required to BREAK a bond This gives a positive enthalpy value for the bonds broken, and a negative enthalpy for the bonds formed Adding them, a positive value means an ENDOTHERMIC reaction
and a negative one EXOTHERMIC Lets try an example Methane + Carbon Dioxide
Oxygen +Water The overall energy change from this reaction is :

Enthalpy of bond breaking plus the (negative) enthalpy of bond formation = -694kJ/mol 2644kJ/mol + -3338kJ/mol Why don’t you try working out whether this reaction is exo- or endothermic in your groups? Carbon + Carbon
Oxygen Dioxide Finished already? Why not try drawing the energy diagram? So it's EXOTHERMIC, woohoo!! -990kJ/mol Endothermic Step Exothermic Step by Christopher Murtland
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