Chemical Energetics 1:Chemical Energetics 1:

AllAll chemical reactions involve an chemical reactions involve an energy change.energy change.

The transfer of energy, usually heat, into or out of the reaction mixture’.

For example:

When petrol burns heat is given out.When ammonium nitrate dissolves in water heat is taken in.

Chemical Energetics 2:Chemical Energetics 2:

Instead of writing ‘energy change’ all the Instead of writing ‘energy change’ all the time chemists use the symbols:time chemists use the symbols:

HHPronounced ‘delta aitch’.=greek letter ‘delta’ meaning change.H=heat.So, H means ‘heat change’.

Temperature Changes Temperature Changes 11

Some reactions cause the temperature of Some reactions cause the temperature of the reaction mixture to the reaction mixture to increase.increase.

This type of reaction is called This type of reaction is called exothermic.exothermic.

Heat energy is given out by the reaction Heat energy is given out by the reaction mixture hence the surroundings increase mixture hence the surroundings increase in temperature.in temperature.

Temperature Changes Temperature Changes 22

Some reactions cause the temperature of Some reactions cause the temperature of the reaction mixture to the reaction mixture to decrease.decrease.

This type of reaction is called This type of reaction is called endothermic.endothermic.

Heat energy is taken in by the reaction Heat energy is taken in by the reaction mixture hence the surroundings decrease mixture hence the surroundings decrease in temperature.in temperature.

Representing Energy Changes:Representing Energy Changes:The energy changes in a chemical reaction can The energy changes in a chemical reaction can be conveniently represented usingbe conveniently represented using

energy level diagramsenergy level diagrams

Energy level diagrams make it easier to decide Energy level diagrams make it easier to decide whether a reaction is exothermic (gives out heat whether a reaction is exothermic (gives out heat and gets hotter) or endothermic (takes in heat and gets hotter) or endothermic (takes in heat and gets cooler).and gets cooler).

See next slide for some examples.

Energy Level DiagramsEnergy Level DiagramsThis energy level diagram shows This energy level diagram shows a reaction where a reaction where H is H is negativenegative (an exothermic reaction).(an exothermic reaction).

This energy level diagram shows a reaction where H is positive (an endothermic reaction).

Reactants

Products

H negative, exothermic.

en

erg

y

Reactants

Products

H positive, endothermic.

en

erg

y

Making and Breaking Bonds:Making and Breaking Bonds:Making chemical bonds is Making chemical bonds is alwaysalways exothermic. exothermic.

Breaking chemical bonds is alwaysalways endothermic.

All chemical reactions involve bond ‘rearrangements’. Bonds are broken and new ones are formed.

Chemical reactions are in two stages:

1. Breaking bonds, an endothermic process.2. Making new bonds, an exothermic process.

Reaction Profiles:Reaction Profiles:These are a more detailed version of the energy These are a more detailed version of the energy level diagrams previously looked at.level diagrams previously looked at.

They show the endothermic bond breaking and They show the endothermic bond breaking and the exothermic bond making stages of a reaction.the exothermic bond making stages of a reaction.

Reaction profiles can be used to do numerical calculations on the energy changes that happen in chemical reactions.

The next slide shows the energy profile for the reaction between hydrogen and chlorine to form hydrogen chloride.

Reaction Profile for:Reaction Profile for:HH22

+ Cl+ Cl2 2 2HCl 2HCl

energy

H-H, Cl-Cl

Reactants

H, H, Cl, Cl (Atoms)

H-Cl, H-ClProducts

Energy taken in to break bonds.

Energy given out when bonds are made.

Overall energy change, H

Calculating Energy Changes: Calculating Energy Changes: The energy change for a reaction can be calculated The energy change for a reaction can be calculated using ‘bond energies’.using ‘bond energies’.‘Bond energy’ is the amount of energy required to break a bond. This value is always endothermicalways endothermic, ie. has a positive value.

When a bond is formed the energy given out is exactly equal to, but of opposite sign, to the energy required to break the bond. Bond making is always an always an exothermicexothermic process.

.

An Example An Example Calculation:Calculation:Consider the reaction between hydrogen and chlorine:Consider the reaction between hydrogen and chlorine:

H2 + Cl2 2HCl to see the reaction profile.

In this reaction one H-H bond and one Cl-Cl bond is broken and two H-Cl bonds are formed.

The H-H bond energy is 436kJ/molThe Cl-Cl bond energy is 242kJ/molSo, the energy needed to break these bonds is 436 + 242 = 678kJ

The H-Cl bond energy is 431kJSo the energy given out when these bonds areformed is 2 x -431 = -862kJ

Overall change = 678 – 862 = -184kJ, an exothermic reaction. Next slide shows the reaction profile with these values added.

Reaction Profile for:Reaction Profile for:HH22

+ Cl+ Cl2 2 2HCl 2HCl

energy

H-H, Cl-Cl

Reactants

H, H, Cl, Cl (Atoms)

H-Cl, H-ClProducts

Energy out = -862kJ

Overall energy change,H = -184kJ

Energy in= +678kJ

Chemical reactions take place in two Chemical reactions take place in two stages:stages:

EndothermicEndothermic reactions get cold, they take in heat. reactions get cold, they take in heat.

ExothermicExothermic reactions get hot, they give out heat. reactions get hot, they give out heat.

Summary

Bond breaking is endothermic, bond making is exothermic.

The energy changes in a chemical reaction can be represented using energy level diagrams and reaction profile diagrams.

1. An endothermic bond breaking stage.2. An exothermic bond making stage.