EXOTHERMIC AND ENDOTHERMIC REACTIONS Reaction profiles

Objectives To draw reaction profiles for exothermic and endothermic reactions To understand the meaning of activation energy

SIMPLE INTRODUCTION Exothermic means “ Heat out” Heat is given out to the surroundings Endothermic means “Heat in” Heat is taken in from the surroundings All reactions are either exothermic or endothermic

SOME REACTIONS ARE OBVIOUSLY EXOTHERMIC e.g. methane gas burning is obviously exothermic e.g. The reaction between magnesium and copper oxide is obviously an exothermic reaction HOWEVER it isn’t quite as simple as this All reactions are in fact a mixture of exo and endo parts If we say a reaction is exothermic, then what this really means is that the exo part of the reaction is greater than the endo part of the reaction

To put it simply: When we say a reaction is exothermic what we are really saying is that the overall reaction is exothermic Similarly: When we say a reaction is endothermic what we are really saying is that the overall reaction is endothermic

This clearly needs some extra explanation Imagine an example using money instead of energy Imagine you are investing money into an investment fund in a bank We will show the results of the investment over time on a graph On the graph paying in money shown by  direction payback money shown by  direction

“paid in” money shown by  direction “ “payback money” shown by  direction Money in fund Time Would this be a good investment?

This is a good investment because you have more “payback” than “pay in” Money in fund Paid back Paid in Time

Take an actual example. How much money would you make on this deal? Answer = £2,000 Money in fund £5,000 £7,000 The profit is the difference between the two arrows Time

A MEMBER OF STAFF ACTUALLY MADE AN INVESTMENT WHICH COULD BE REPRESENTED BY THS GRAPH Money in fund £3,500 £5,000 How did he feel about this? Time

Overall £5,000 paid in and £3,500 paid back So overall loss, or drop of £1,500

What if you made more than one investment at the same time? Imagine you invest £ 5,000 and £4,000 Imagine the paybacks are £2,000 and £8,000 How do you feel – was this a good deal? OR Total investment = £5,000 + £4,000 = £9,000 Total payback = £2,000 + £8,000 = £10,000 So

Back to Chemistry- Energy instead of Money In chemistry the “paying in” part of reactions is breaking bonds between atoms. e.g. breaking up hydrogen molecules H H H  H  2 separate atoms 1 molecule  Breaking bonds always involves a “paying in” of energy and is represented by an  direction on the energy graph. We call this an investment of energy

In chemistry the “paying back” part of reactions is making bonds between atoms. e.g. the making of a hydrogen molecule from two hydrogen atoms  H  H H H 2 separate atoms  1 molecule This is shown by a  direction on an energy graph

Let’s now apply this to an actual example Magnesium reacts with iron oxide to make magnesium oxide and iron Mg + FeO = MgO + Fe Mg  Fe O  Mg O  Fe This bond must break This bond must be made This involves “pay in” of energy This involves a “pay back” of energy

On an energy graph this will look like: Mg + FeO = MgO + Fe Energy “PAY IN” to break the FeO bond Energy “PAY BACK” on making the MgO bond Energy Time More “PAY BACK” energy than “PAY IN” energy – this is a GOOD energy deal. A “GOOD” energy deal is Exothermic

An Exothermic reaction Mg + FeO = MgO + Fe This is the ENDO’ part of the reaction This is the EXO’ part of the reaction Energy Time We say this is an exothermic reaction (overall) since the exothermic part is greater than the endothermic part

Working out the overall energy change 400kJ/mole 330kJ/mole 70kJ/mole The overall energy change is given by the difference between the lengths of the two arrows The overall energy change is 70kJ/mole In exam questions, you will be given energy values so that you can work out the lengths of the arrows We say that ∆H = - 70kJ/mole

What sort of reaction would have this energy graph and why? Time This sort of reaction would be ……………………… because the energy ………………. in to …………………… bonds is …………… than the energy ………… back from ……………………… bonds. The ………………………………………. part of the reaction is greater than the ……………………………………. part.

The activation energy Ea is the amount of energy the reactant particles need when they collide with each other in order to react.

Exothermic reactions - a summary Activation energy Energy Reactants Products Progress of reaction In exothermic reactions the energy of the reactants is greater than the energy of the products

Endothermic reactions - a summary Energy Activation energy Products Reactants Progress of reaction In endothermic reactions the energy of the products is greater than the energy of the reactants The activation energy is the minimum energy required to break the bonds and start the reaction off

Exothermic reactions and catalysts A.E. without cat Energy A.E. with catalyst Reactants Products Progress of reaction A catalyst lowers the activation energy. This helps the bonds to break more easily so the reaction works faster at that temperature. NOTE that the overall energy change is the SAME

How would a catalyst change the energy graph of an endothermic reactions? A. E. no cat Energy Products A.E. with catalyst Reactants Progress of reaction Again the reaction would go faster because the activation energy has been lowered Again note that the overall energy change is the same