OCR_CD2_Endo and Exo reactions

Chemical Applications

Assignment 3Endothermic and exothermic

reactions

OCR_CD2_Endo and Exo reactions

You need to understand that:

Chemical reactions either give out heat (exothermic) or, take in heat (endothermic).Keywords: energy, bonds,

Chemical changes produce heat changes

OCR_CD2_Endo and Exo reactions

Exothermic reactions increase the temperature.

• Examples include:– Burning reactions including the

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

Thermit reaction

Magnesium reacting with acid

OCR_CD2_Endo and Exo reactions

Secondly…

Endothermic reactions

OCR_CD2_Endo and Exo reactions © Boardworks Ltd 2003

Cools

Heatenergytakenin as the mixture returns back to room temp.

Starts 25°C Cools to 5°C

Ammoniumnitrate

Water

Returns to 25°C

Endothermic reactions cause a decrease in temperature.

OCR_CD2_Endo and Exo reactions

Activity:Activity:

• Calculate the Activation energy and the overall output energy for the following combustion reactions

• It’s best to start by drawing a diagram on the molecules reacting

Methane + Oxygen Carbon Dioxide + Water

CH4 + O2 CO2 + H2O

OCR_CD2_Endo and Exo reactions

Investigating exothermic and endothermic reactions

• The assignment for this unit requires a thorough description of exo- and endo- thermic reactions.

• Examples• Energy diagrams• Key phases: Activation energy, Energy

of reactants, energy of products etc.

OCR_CD2_Endo and Exo reactions

Assignment details…Assignment details…

Scenario … • We are a company making cold packs for sports

injuries. • They also make heat packs for expeditions.• We have asked a chemical company to send us

a list of reactions which produce a heat change to help us decide what to make our hot and cold packs from.

• We need to investigate which reactions would be best to use to make our hot and cold packs.

• You need to suggest reactants that would make effective heat / cold packs.

Exo- and Endothermic ReactionsExo- and Endothermic ReactionsReactants Initial

Temp ° CFinal

Temp ° CTemp

Change ° C

25cm³ Hydrochloric Acid25cm³ Sodium Hydroxide

25cm³ Hydrochloric Acid+ 2cm Magnesium Ribbon

25cm³ Sulphuric Acid+ 2cm Magnesium Ribbon

25cm³ Nitric Acid+ 2cm Magnesium Ribbon

25cm³ Water+ 1g Potassium Chloride

25cm³ Water+ 1g Ammonium Chloride

25cm³ Copper Sulphate+ 1.4g Iron filings

25cm³ Hydrochloric Acid +0.9g Zinc Granules

25cm³ Citric acid +1.2g Sodium Bicarbonate

25cm³ Sulphuric Acid+ 50cm³ Sodium Hydroxide

OCR_CD2_Endo and Exo reactions

Distinction theory: Assignment 3 BTEC Chemistry• Theory to help you:• Energy changes in reactions involve making and

breaking bonds.• Energy is absorbed by reactants to break their

bonds and then released by products when their bonds are made.

• Each bond between atoms has its own amount of energy needed to make or break the bond.

• If we know how much energy is needed to break the existing bonds and how much energy is released when making the new bonds then we can work out whether a reaction is exothermic or endothermic

Burning…Burning…

• When a hydrocarbon burns it joins with oxygen…

Methane + oxygen →

Oxygen

Hydrogen

Carbon

Burning…Burning…• When a hydrocarbon

burns it joins with oxygen…

Methane + oxygen →

The heat you supply with a matchSplits up the atoms…

This bit needs energy, lets say 10 Joules

Oxygen

Hydrogen

Carbon

• When a hydrocarbon burns it joins with oxygen…

Methane + oxygen → → Carbon dioxide + water

The atoms then recombine to make carbon dioxide and water

This bit needed 10 Joules…

When these atoms recombine, they release 15 Joules

Burning…Burning…Oxygen

Hydrogen

Carbon

Task…Task…

• Calculate the energy released when hydrogen is burnt in air to produce water.

Chemical Bond

Energy in Bond (Kj/mol)

H - H 436

O = O 498

H - O 464

Hydrogen + Oxygen Water

2H2 + O2 2H2O

Start by drawing a diagram of the atoms reacting...

OCR_CD2_Endo and Exo reactions

e.g.

2H2 + O2 2H2OChemical Bond Energy in Bond

Kj/mol

H - H 436

O = O 498

H - O 464

H

H

H

H+

O

O

To break these bonds takes (2 X 436) + (1 X 498) = 1370 KJ/ mol

H

H

H

H

O

O

H

H

H

H

OO

To make these bonds produces (4 X -464) = -1856 KJ/Mol

More energy is made creating the bonds than is used breaking the bonds so the reaction is exothermic

OCR_CD2_Endo and Exo reactions

Task…Task…

Calculate the energy released when methane burns in oxygen

Bond strengths…

Extension – repeat for propane

Chemical Bond

Energy in Bond Kj/mol

C - H 435

O = O 498

C=O 805

H - O 464

OCR_CD2_Endo and Exo reactions

BondEnergy (kJ/mol)

BondEnergy (kJ/mol)

H - H 436 N - N 160

C - H 413 N = O 631

N - H 393 N triple N 941

P - H 297 N - O 201

C - C 347 N - P 297

C - O 358 O - H 464

C - N 305 O - S 265

C - Cl 397 O - Cl 269

C = C 607 O - O 204

C = O 805 C - F 552

O = O 498 C - S 259

OCR_CD2_Endo and Exo reactions

Assignment

Long AQA Btec

Practical work completed

227-228 Show energy diagrams for exothermic and endothermic reactions. Appropriate terms are used (input energy / output energy / activation energy). Combustion of methane could be used to explain this.

Results from practical recorded on worksheet. Company name and logo on top of worksheet.

Best reactions for heat and cold are identified.

The most exothermic and the most endothermic reactions have both been identified

227-228 Use the most exothermic reaction to help you explain what an exothermic reaction is. Why is this the best reaction for a hot pack?

Use the most endothermic reaction to help you explain what an endothermic reaction is. Why is this the best reaction for a cold pack?

228-229 Explain how a chemical reaction involves breaking old bonds and making new bonds. Explain how these 2 stages of a reaction involve energy changes. Calculate energy changes for a named exo or endothermic reaction.

228-229 Write about why some reactions are exothermic and others are endothermic by talking about the energy changes involved when old bonds are broken and new bonds are made