Energy Changes You will learn about: Exothermic & Endothermic Reactions Energy level diagrams ...

Energy ChangesEnergy Changes

You will learn about:You will learn about: Exothermic & Endothermic ReactionsExothermic & Endothermic Reactions Energy level diagramsEnergy level diagrams Bond Making and BreakingBond Making and Breaking Calculating enthalpy changeCalculating enthalpy change Activation energyActivation energy Energy profile diagramsEnergy profile diagrams PhotographyPhotography PhotosynthesisPhotosynthesis

Exothermic & Exothermic & Endothermic ReactionsEndothermic Reactions

ExoExo:out, :out, ThermicThermic: Heat energy: Heat energy Exothermic reactionExothermic reaction: one in which heat : one in which heat

energy is energy is releasedreleased and there is an and there is an increaseincrease in temperature of the mixture. in temperature of the mixture.

EndoEndo:in:in Endothermic reactionEndothermic reaction: one in which heat : one in which heat

energy is energy is absorbedabsorbed and there is a and there is a decreasedecrease in the temperature of the in the temperature of the mixture. mixture.

Endothermic reactionEndothermic reaction

Dissolution of potassium nitrate in waterDissolution of potassium nitrate in water

- eg. of endothermic reactioneg. of endothermic reaction

- heat energy is absorbed- heat energy is absorbed => decrease in the temperature of the => decrease in the temperature of the

mixturemixture

Exothermic reactionExothermic reaction

Dissolution of calcium oxide in waterDissolution of calcium oxide in water

eg. of exothermic reactioneg. of exothermic reaction

- heat energy is released - heat energy is released => increase in temperature of the mixture=> increase in temperature of the mixture

Enthalpy ChangeEnthalpy Change

Heat energy given out or taken in, during a Heat energy given out or taken in, during a reaction is measured in units called Joules (J). reaction is measured in units called Joules (J).

Large amounts are measured in kiloJoules Large amounts are measured in kiloJoules (kJ). (kJ).

The symbol for the heat of reaction (heat The symbol for the heat of reaction (heat energy change or enthalpy change) is energy change or enthalpy change) is HH. .

Numerically,Numerically, HH = Change in heat energy content = Total = Change in heat energy content = Total energy content of products – total energy energy content of products – total energy content of reactants content of reactants


If If Total energy content of products > Total energy Total energy content of products > Total energy content of reactantscontent of reactants H > 0H > 0 Reaction is endothermic Reaction is endothermic

IfIfTotal energy content of products < Total energy Total energy content of products < Total energy content of reactantscontent of reactants H < 0H < 0Reaction is exothermic Reaction is exothermic

The sign of The sign of H H will tell whether the reaction is will tell whether the reaction is exothermic or endothermic!exothermic or endothermic!

ExamplesExamples

Example 1:Example 1:

When 1 mole of hydrochloric acid reacts When 1 mole of hydrochloric acid reacts with one mole of sodium hydroxide, 57.3 with one mole of sodium hydroxide, 57.3 kJ of heat is produced. Draw an energy kJ of heat is produced. Draw an energy level diagram to represent this reaction.level diagram to represent this reaction.

ExamplesExamples

Example 2:Example 2:

When 1 mole of hydrogen reacts with 1 When 1 mole of hydrogen reacts with 1 mole of iodine to give 2 moles of mole of iodine to give 2 moles of hydrogen iodide, 52 kJ of heat is hydrogen iodide, 52 kJ of heat is absorbed. Draw an energy level diagram absorbed. Draw an energy level diagram to represent this reaction.to represent this reaction.

Bond Making and Bond Making and BreakingBreaking When 2 atoms joined together by chemical When 2 atoms joined together by chemical

bond, heat energy is given out. bond, heat energy is given out. Bond Bond making is a exothermic change. (making is a exothermic change. (H = -ve)H = -ve)

Energy is needed to break chemical bond. Energy is needed to break chemical bond. Bond breaking is a endothermic change. Bond breaking is a endothermic change. ((H=+ve).H=+ve).

Energy for making and breaking a covalent Energy for making and breaking a covalent bond is the bond is the samesame. This is the . This is the bond energybond energy. .

Enthalpy Change in a Enthalpy Change in a ReactionReaction

Old bonds are broken in a reaction Old bonds are broken in a reaction (endothermic) while new bonds are formed (endothermic) while new bonds are formed (exothermic). (exothermic).

The Enthalpy Change, The Enthalpy Change, H, (overall heat H, (overall heat change) of the reaction = [heat absorbed to change) of the reaction = [heat absorbed to break old bonds in reactants] - [heat given break old bonds in reactants] - [heat given out to form new bonds in products]out to form new bonds in products]

The The signsign of of H will tell whether reaction is H will tell whether reaction is exothermic or endothermic.exothermic or endothermic.


If the energy absorbed for bond breaking is If the energy absorbed for bond breaking is less than that released during bond forming, less than that released during bond forming, the reaction is exothermic. the reaction is exothermic.

If the energy absorbed for bond breaking is If the energy absorbed for bond breaking is more than that released during bond more than that released during bond forming, the reaction is endothermic. forming, the reaction is endothermic.

QUIZZESQUIZZES

1. Classify each of the following changes 1. Classify each of the following changes as either endothermic, exothermic or no as either endothermic, exothermic or no heat exchange. heat exchange.

ChangeChange Type of heat changeType of heat change

Forming a covalent bondForming a covalent bond

Mixing hydrogen and oxygenMixing hydrogen and oxygen

Reacting hydrogen and oxygen togetherReacting hydrogen and oxygen together

Forming nitrogen atoms from nitrogen Forming nitrogen atoms from nitrogen moleculesmolecules

Forming bromine vapour from liquid bromineForming bromine vapour from liquid bromine

Exothermic

No heat change

Exothermic

Endothermic

Endothermic

2) Fe +2) Fe + 2HCl 2HCl FeCl FeCl22 + H + H22; ; H=-156kJH=-156kJ

(i) How do you know from the equation (i) How do you know from the equation above that the reaction is exothermic? above that the reaction is exothermic?

(ii) How much heat is produced in the (ii) How much heat is produced in the reaction of 1 mole of HCl?reaction of 1 mole of HCl?

The negative sign in front of H.

Amount of heat produced by 1 mole HCl = 156/2=78kJ

Worked exampleWorked exampleMethod 1:Method 1: E.g. 2HE.g. 2H22 + O + O22 2H 2H22OO Heat released when new bonds are formed Heat released when new bonds are formed

= bond energies of 4 O-H bonds = bond energies of 4 O-H bonds

= 4x 463kJ = 1852kJ= 4x 463kJ = 1852kJ Heat absorbed when old bonds are broken Heat absorbed when old bonds are broken

= bond energies of 2 H-H bonds + 1 O=O bond = (2x436) = bond energies of 2 H-H bonds + 1 O=O bond = (2x436) + (1x496) = 1368kJ+ (1x496) = 1368kJ

Enthalpy Change, Enthalpy Change, H of reaction H of reaction

= 1368 -1852 = -484kJ (the -ve =>exothermic= 1368 -1852 = -484kJ (the -ve =>exothermic))

Method 2:Method 2:

E.g. 2HE.g. 2H22 + O + O22 2H 2H22OO Heat involved in bond making of 4 O-H bonds Heat involved in bond making of 4 O-H bonds

= 4x (-463)kJ = -1852kJ= 4x (-463)kJ = -1852kJ Heat involved in bond breaking of Heat involved in bond breaking of

2 H-H bonds + 1 O=O bond 2 H-H bonds + 1 O=O bond

= 2x(+436) + 1x(+496) = +1368kJ= 2x(+436) + 1x(+496) = +1368kJ Enthalpy Change, Enthalpy Change, H of reaction H of reaction

= +1368 -1852 = -484kJ (-ve =>exothermic= +1368 -1852 = -484kJ (-ve =>exothermic))

Activation EnergyActivation Energy

For a chemical reaction to occur, the reactant For a chemical reaction to occur, the reactant particles must collideparticles must collide

Most particles do not have enough energy Most particles do not have enough energy when they collide to start a reaction. They just when they collide to start a reaction. They just bounce apart and do not react.bounce apart and do not react.


Some particles collide with enough Some particles collide with enough energy to start a reaction.energy to start a reaction.


The minimum energy that molecules The minimum energy that molecules must possess in order for a chemical must possess in order for a chemical reaction to occur is called the activation reaction to occur is called the activation energy, Ea. energy, Ea.

A reaction that takes place easily at room A reaction that takes place easily at room temperature has a fairly low activation temperature has a fairly low activation energy. energy.

A reaction that requires a higher A reaction that requires a higher temperature to occur has a higher temperature to occur has a higher activation energy.activation energy.


Activation energy is the energy barrier that the Activation energy is the energy barrier that the

colliding molecules must overcome in order for colliding molecules must overcome in order for

a reaction to occura reaction to occur


Consider the reaction of hydrogen with Consider the reaction of hydrogen with oxygen: 2Hoxygen: 2H22(g) + O(g) + O22(g) (g) → 2H→ 2H22O (l)O (l)

The reaction takes place in two steps:The reaction takes place in two steps:1. A burning match provides the necessary 1. A burning match provides the necessary activation energy. This energy gives the activation energy. This energy gives the reactant molecules enough energy to break reactant molecules enough energy to break their covalent bonds when they collide, their covalent bonds when they collide, forming hydrogen and oxygen atoms.forming hydrogen and oxygen atoms.2. Once the reaction starts, the hydrogen and 2. Once the reaction starts, the hydrogen and oxygen atoms combine to form water. As new oxygen atoms combine to form water. As new bonds are formed, energy is given out.bonds are formed, energy is given out.

Activation energy – Activation energy – exothermic rxnexothermic rxn

If a reaction is exothermic, enough energy is If a reaction is exothermic, enough energy is given out in the reaction of a few particles to given out in the reaction of a few particles to provide the activation energy for the remaining provide the activation energy for the remaining particles. So the reaction keeps going.particles. So the reaction keeps going.

This is the case for the reaction between This is the case for the reaction between hydrogen and oxygen; when the first hydrogen hydrogen and oxygen; when the first hydrogen molecule reacts, the heat give out enables molecule reacts, the heat give out enables other molecules to react.other molecules to react.

Activation energy- Activation energy- endothermic rxnendothermic rxn

If the reaction is endothermic, insufficient If the reaction is endothermic, insufficient energy is given out when bonds are energy is given out when bonds are made to provide the activation energy made to provide the activation energy needed for the reaction to continue.needed for the reaction to continue.

Thus, heat energy must be continually Thus, heat energy must be continually added to provide sufficient activation added to provide sufficient activation energy.energy.

Energy profile diagramsEnergy profile diagrams

An energy profile diagram is a way of An energy profile diagram is a way of representing the energy changes that representing the energy changes that occur during a chemical change. The occur during a chemical change. The energy difference between the products energy difference between the products and reactants represents the heat of and reactants represents the heat of reaction.reaction.

It includes the energy barrier, the It includes the energy barrier, the activation energy.activation energy.

Energy Profile for exothermic rxnEnergy Profile for exothermic rxn

Exothermic reactionExothermic reaction

Energy profile for endothermic rxnEnergy profile for endothermic rxn

Endothermic reactionsEndothermic reactions

Examples of exothermic Examples of exothermic changeschanges

Changes of state (condensation and freezing)Changes of state (condensation and freezing) Dissolving of some salts in waterDissolving of some salts in water

eg. Naeg. Na22COCO33(s) (s) → → NaNa22COCO33(aq) + heat (aq) + heat Combustion reactionsCombustion reactions

All combustion reactions are exothermic.All combustion reactions are exothermic.

eg. 2Heg. 2H22(g) + O(g) + O22(g) (g) → 2H→ 2H22O(l) + heatO(l) + heat Neutralisation reactionsNeutralisation reactions

HH++(aq) + OH(aq) + OH--(aq) (aq) → H→ H22O(l) + heatO(l) + heat Metal displacement reactionsMetal displacement reactions

Zn(s) + CuZn(s) + Cu2+2+ (aq) (aq) → Zn→ Zn2+2+ (aq) + Cu(s) + heat (aq) + Cu(s) + heat

Examples of endothermic Examples of endothermic changeschanges

Changes of state (melting & boiling)Changes of state (melting & boiling) Dissolving of some salts in waterDissolving of some salts in water

eg. NHeg. NH44Cl(s) + heat Cl(s) + heat → NH→ NH44Cl(aq)Cl(aq) Decomposition of compoundsDecomposition of compounds

eg. CuCOeg. CuCO33(s) + heat → CuO(s) + CO(s) + heat → CuO(s) + CO22(g)(g) Reactions between acids and hydrogencarbonatesReactions between acids and hydrogencarbonates

eg. when some headache tablets (mixture of solid eg. when some headache tablets (mixture of solid acids such as citric acid and sodium acids such as citric acid and sodium hydrogencarbonate) are added to water, heat hydrogencarbonate) are added to water, heat energy is absorbed.energy is absorbed.

Formation of nitrogen oxide in a car engineFormation of nitrogen oxide in a car engineNN22(g) + O(g) + O22(g) → 2NO(g) (g) → 2NO(g)

Combustion of fuelsCombustion of fuels

What are fuels?What are fuels? Fuels are substances that can burn Fuels are substances that can burn

easily in air to give out energy.easily in air to give out energy. The most commonly used fuels are fossil The most commonly used fuels are fossil

fuels such as coal, petroleum and natural fuels such as coal, petroleum and natural gas. They are formed from decayed plant gas. They are formed from decayed plant and animals that lived millions of years and animals that lived millions of years ago.ago.


What happens when fuels burn?What happens when fuels burn? Coal is mainly carbon.Coal is mainly carbon.

C(s) + OC(s) + O22(g) (g) → CO→ CO22(g) + heat energy(g) + heat energy Most fuels contain carbon and hydrogen. When these Most fuels contain carbon and hydrogen. When these

fuels burn, carbon dioxide, water and heat energy are fuels burn, carbon dioxide, water and heat energy are produced. The equation for the combustion of natural produced. The equation for the combustion of natural gas, which contains mainly methane isgas, which contains mainly methane is

CHCH44(g) + 2O(g) + 2O22(g) → CO(g) → CO22(g) + 2H(g) + 2H22O(g) + heat O(g) + heat energy energy

All combustion reactions are exothermic processes.All combustion reactions are exothermic processes.


Covalent bondCovalent bond Bond energy kJ/mol of bondsBond energy kJ/mol of bonds

H-HH-H 436436

C-CC-C 348348

C=CC=C 612612

C-HC-H 412412

C-OC-O 323323

C=OC=O 743743

O-OO-O 138138

O=OO=O 496496

O-HO-H 463463

• We can calculate the heat of combustion of fuels given the bond energy values.

Table: Bond energy values


Calculate the enthalpy change for the Calculate the enthalpy change for the complete combustion of butene with complete combustion of butene with the following structure.the following structure.

H C

H

C

H

C

H

H

HC

H

H

Calculation of heat of combustionCalculation of heat of combustion

C4H8 + 6O2 →4CO2 + 4H2O

Heat involved in breaking 8C-H, 2C-C, 1C=C & 6O=O bonds

= 8(+412) + 2(+348) +612 + 6(+496)

= +7580 kJ

Heat involved in making 8C=O & 8O-H Heat involved in making 8C=O & 8O-H bondsbonds

= 8(-743) + 8(-463)= 8(-743) + 8(-463)

= -9648 kJ= -9648 kJ Enthalpy change = +7580 – 9648Enthalpy change = +7580 – 9648

= -2068 kJ= -2068 kJ

Calculation of heat of combustionCalculation of heat of combustion

Group work – Alternative Group work – Alternative FuelsFuels

Research on the following, highlighting one of theResearch on the following, highlighting one of thefuels and do a powerpoint presentation, includingfuels and do a powerpoint presentation, includingthe chemistry behind the fuel, the social-economicthe chemistry behind the fuel, the social-economicissues involved, and show examples where thisissues involved, and show examples where thisfuel is being used. Do a comparative study on the fuel is being used. Do a comparative study on the alternative energy that is being employed in alternative energy that is being employed in Singapore and another country.Singapore and another country. 1. Hydrogen fuel & other types of fuel cells1. Hydrogen fuel & other types of fuel cells 2. Biofuels2. Biofuels 3. Nuclear energy & solar energy3. Nuclear energy & solar energy

Light and Chemical Light and Chemical ReactionsReactions

Reactions in which light energy is Reactions in which light energy is absorbed are endothermic. absorbed are endothermic.

PhotographyPhotography Photosynthesis Photosynthesis

PhotographyPhotography

Photographic film: coated with crystals of Photographic film: coated with crystals of silver bromide or silver chloride. silver bromide or silver chloride.

Light hit film electrons transferred from the Light hit film electrons transferred from the chloride or bromide ions to the silver ions.chloride or bromide ions to the silver ions.

Silver metals formed the dark areas on Silver metals formed the dark areas on film. film.

ThThisis is an example of is an example of redoxredox reaction. reaction. 2AgBr2AgBr 2Ag + Br 2Ag + Br22 H = +199kJH = +199kJ

PhotosynthesisPhotosynthesis

Chlorophyll absorbs light energy used to Chlorophyll absorbs light energy used to make sugars from carbon dioxide and make sugars from carbon dioxide and water. water.

6CO6CO22 + 6H + 6H22O O C C66HH1212OO66 + 6O + 6O22

H = +2816kJH = +2816kJ

Group work – Alternative Group work – Alternative FuelsFuelsResearch on the following, highlighting one of theResearch on the following, highlighting one of thefuels and do a powerpoint presentation, includingfuels and do a powerpoint presentation, includingthe chemistry behind the fuel, the social-the chemistry behind the fuel, the social-economic, environmental issues involved, and economic, environmental issues involved, and show examples where this fuel is being used. The show examples where this fuel is being used. The pros and cons as well. Do a comparative study on pros and cons as well. Do a comparative study on the alternative energy that is being employed in the alternative energy that is being employed in Singapore and China.Singapore and China. 1. Hydrogen fuel & other types of fuel cells1. Hydrogen fuel & other types of fuel cells 2. Solar energy2. Solar energy 3. Biofuels3. Biofuels 4. Nuclear energy4. Nuclear energy

Energy Changes You will learn about: Exothermic & Endothermic Reactions Energy level diagrams ...

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