C3bL4  Endothermic and Exothermic Reactions

Key Words

Activation energy - energy needed to get a reaction going.

Bond - the "joint" between two atoms in a molecule.

Bond energy - the energy required to break a bond.

Catalyst - a substance that speeds up the rate of reaction, but is not used up in the reaction.

Endothermic - a reaction which requires energy to drive the reaction.

Energy - ability to do a job of work.

Energy profile diagram - a way of showing the energy changes as a picture.

Exothermic - reaction gives out heat.

Reaction - chemical interaction between substances in which a new substance is formed.

Test Yourself

Homework
Physics GCSE
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Grade E

All chemical reactions involve an energy change of some sort.  In all chemical reactions, bonds are broken and remade.  It takes energy to break the bonds, but energy is given out when the bonds are remade.

Some reactions give out heat.  They are exothermic.  The total energy given out by the remaking of bonds is more than the energy needed to break the bonds.

Sometimes the energy given out by remaking bonds is less than the energy needed to break the bonds.  The reaction takes in heat; it is endothermic.

We can show the energy changes as a picture, an energy profile diagram.

For an exothermic reaction, we see:

For an endothermic reaction we get:

Note that there is an activation energy in exothermic reactions.  If you mix petrol and air, the vapour will not burn until a heat source has been applied.

 

Grade C - A* (Higher level only)

The bond energy is the amount of energy needed to break one mole of bonds.  Some bond energies are shown in the table:

 

Bond Bond energy (kJ mol-1)

C-H

413
O=O 496
O-H 463
C=O 743

Suppose we want to break 1 mole of O-H bonds, we need to put in 463 000 J (463 kJ).  When they reform, we get 463 kJ back as heat.   Let us look at how bonds are rearranged in this reaction:

methane + oxygen  ® carbon dioxide + water

CH4(g) + 2O2(g) ® CO2(g) + 2H2O(g)

Let's do an energy audit on what happens in the reaction:

1.  Look at the reactants:

  • There are 4 C-H bonds.  Each one has a bond energy of 413 kJ mol-1.  The total energy in a mole of these bonds is 4 × 413 = 1652 kJ

  • There are 2 O=O bonds.  The total energy in a mole of these bonds is 2 × 496 = 992 kJ

  • Total energy needed to break these bonds is 1652 + 992 = 2644 kJ

2.  Now look at the products:

  • There are 2 C=O bonds.  The total energy per mole is 2 × 743 = 1486 kJ

  • There are 4 O-H bonds. The total energy per mole is 4 × 463  = 1852 J

  • The total energy given out is 1486 + 1852 = 3338 kJ.

3. The amount of energy given out in this exothermic reaction is given by:

energy out = bond energy of the products - bond energy of the reactants

energy = 3338 - 2644 =  694 kJ mol-1

Chemists have a convention that says that energy out is negative, so strictly speaking, we should write -694 kJ mol-1.

We can sum this up as an energy profile diagram:

You can see that there is quite a big hump to cross before the reaction happens.  We don't actually have to put this amount of energy in to set methane burning.  A spark will have enough energy to break the bonds of a few (million) molecules.  As they give out heat, more molecules will be activated, and the reaction propagates.  The way the reaction happens does not affect the total energy per mole.

The effect of a catalyst is to lower the energy hill (lower the activation energy), making the rate of reaction faster:

  A catalyst does NOT:

  • increase the yield of a reaction;

  • increase the energy given out in the reaction.

Grade A

There are no notes at this level