Energy change during chemical rcts

Energy Change During Chemical Reactions

Endothermic and Exothermic Changes

Exothermic & EndothermicExothermic & EndothermicExothermic & EndothermicExothermic & Endothermic

Exothermic & Endothermic

1. An exothermic reaction is one which releases heat energy to the surroundings

2. The temperature of the surroundings increases

Exothermic reactionsExothermic reactionsExothermic reactionsExothermic reactions

Exothermic reactions

© Boardworks Ltd 2003

Exothermic Reactions

• Examples include:– Burning reactions including the combustion

of fuels.– Detonation of explosives.– Reaction of acids with metals.

Thermit reaction

Magnesium reacting with acid

Exothermic reactions increase in temperature.


Say whether these processes are exothermic.1. Charcoal burning2. A candle burning.3. A kettle boiling4. Ice melting5. A firework exploding

yes

yes

yes

no

no

You have to put heat inin for boiling and melting.

You get heat outout from all the other processes

Activity


• Magnesium + Hydrochloric acid

Gets hot

25o C 45o Cmagnesium

Hydrochloricacid

Heatenergygivenout



45o C

• Almost immediately the hot reaction products start to lose heat to the surroundings and eventually they return to room temperature.

25o CChemical energy becomes heat energy.

The reaction mixture gets hotter.

Eventually this heat is lost to the surroundings.

It follows that reaction products have less chemical energy than the reactants had to start with.



Ene

rgy

/ kJ

)

Progress of reaction (time)

Energy Level Diagram for an Exothermic Reaction

reactantsReactants have more chemical energy.

Some of this is lost as heat which spreads out into the room.

productsProducts now have less chemical energy than reactants.


Ene

rgy

/ kJ

Progress of reaction

reactants

products

H=negative

Energy Level Diagram for an Exothermic Reaction 2.

H is howmuch energyis given out

H is negativebecause the products have less energy than the reactants.

Energy Level Diagram for an Exothermic Reaction


Exothermic reactions give out energy. There is a temperature rise and H is negative.

Exothermic Reaction - Definition

products

Ene

rgy

/ kJ

)


reactants

H is negative


Activity

1. An endothermic reaction is on which takes in heat energy from the surroundings

2. The temperature of the surroundings decreases

Endothermic reactionsEndothermic reactionsEndothermic reactionsEndothermic reactions

Endothermic reactions


Endothermic Reactions

• Endothermic chemical reactions are relatively rare.

• A few reactions that give off gases are highly endothermic - get very cold.

• Dissolving salts in water is another process that is often endothermic.

Endothermic reactions cause a decrease in temperature.


Cools

Heatenergytakenin as the mixture returns back to room temp.

Starts 25°C Cools to 5°C

Ammoniumnitrate

Water

Endothermic reactions cause a decrease in temperature.

Returns to 25°C



25o C

• The cold reaction products start to gain heat from the surroundings and eventually return to room temperature.

5o C The reactants gain energy.25o C

This comes from the substances used in the reaction and the reaction gets cold.

Eventually heat is absorbed from the surroundings and the mixture returns to room temperature.

Overall the chemicals have gained energy.



products

Ene

rgy

/ kJ

)


reactants

H=+

Energy Level Diagram for an Endothermic Process

This is positivebecause the products have more energy than the reactants.

This is howmuch energyis taken in


Endothermic reactions take in energy. There is a temperature drop and H is positive.

Endothermic Reaction Definition

H=+

products

Ene

rgy

/ kJ


reactants



Are these endothermic or exothermic?1. A red glow spread throughout the mixture and

the temperature rose. 2. The mixture bubbled vigorously but the

temperature dropped 150C.3. Hydrazine and hydrogen peroxide react so

explosively and powerfully that they are used to power rockets into space.

4. The decaying grass in the compost maker was considerably above the outside temperature.

exo

endo

exo

exo

Activity

Endothermic and exothermic Endothermic and exothermic reactionsreactions

Step 1: Energy must be SUPPLIED to break bonds:

Step 2: Energy is RELEASED when new bonds are made:

A reaction is EXOTHERMIC if more energy is RELEASED then SUPPLIED. If more energy is SUPPLIED then is

RELEASED then the reaction is ENDOTHERMIC

Energy level diagramsEnergy level diagrams

Energy level

Reaction progress

Activation energy

Energy given out

by reactionUsing a catalyst might lower the

activation energy

Exothermic vs endothermic:Exothermic vs endothermic:

EXOTHERMIC – more energy is given out than is taken in (e.g. burning, respiration)

ENDOTHERMIC – energy is taken in but not necessarily given out (e.g. photosynthesis)

Burning MethaneBurning MethaneCH4 + 2O2 2H2O + CO2

To burn methane you have to break all of these bonds:

And then you have to make these ones:

Burning MethaneBurning MethaneCH4 + 2O2 2H2O + CO2

Methane

Carbon dioxide Water

Oxygen

Bond energiesBond energies

C-H = 435 Kj

O=O = 497 Kj

Total for breaking bonds = 4x435 + 2x497 = 2734 KJ/mol

H-O = 464 KjC=O = 803 Kj

Total for making bonds = 2x803 + 4x464 = 3462 KJ/mol

Total energy change = 2734-3462 = -728 KJ/mol

Drawing this on an energy Drawing this on an energy diagram:diagram:

2734 Kj3462 Kj

More energy is given out (3462) than is given in (2734) – the reaction is EXOTHERMIC. The total (“nett”) energy change is –728 Kj. An endothermic reaction would have a positive energy change.

-728 Kj

But…But…

… you don’t need to know about bond energies or how to calculate

energy changes for the exam!


Copy the summary using the words from the box to fill in the gaps:

endothermic lose positive

exothermic common

1. Exothermic reactions are _____.

2. Reactions that get cold are called _____.

3. Bond forming is an _______ process.

4. Endothermic reactions have a _____ H.

5. In exothermic reactions the chemicals ___ chemical energy. continued

common

endothermic

exothermic

positive

lose

Activity


Copy the summary using the words from the box to fill in the gaps:

more endothermic activation

1. The energy needed to start off a reaction is called the ______ energy

2. In endothermic reactions bond breaking requires ___ energy than is produced by bond forming.

3. Bond breaking is an _________ process.

activation

more

endothermic

Activity


Which of the following is an endothermic process?

A. Burning woodB. Reacting an acid and alkaliC. Reacting magnesium with acidD. Dissolving ammonium nitrate in water


Which of the following is true for an exothermic process?

products

Ene

rgy

/ kJ


reactants

A. The reactants lose energy to the surroundings

B. The reaction gets coldC. The energy diagram will

be as shownD. The reaction will have a

H that is positive (+).


Which of the following is true when chemical bonds are broken?

A. The process is exothermicB. Energy is given outC. A physical change is occurringD. The reaction will have a H that is

positive(+).


Which of the following is true for an exothermic reaction?

A. Bond breaking involves a bigger energy change than bond making

B. Bond making involves a bigger energy change than bond breaking

C. Bond making involves the same energy change as bond breaking


Which of the following is the activation energy:

A. XB. YC. Z


That’s all folks!

Technology

Energy change during chemical rcts