OCR_CD2_Endo and Exo reactions Chemical Applications Assignment 3 Endothermic and exothermic...
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Transcript of OCR_CD2_Endo and Exo reactions Chemical Applications Assignment 3 Endothermic and exothermic...
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 © 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
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