Post on 20-Jan-2023
Task for the week commencing: 20/04/2020 due 24/04/2020
Print or write the question and answer on a document that you can submit. Answers should be in your own words (not copied andpasted). Attempt to write an answer without using any other resources (textbook, exercise book or internet).
Assessment
The symbol equation below shows the reaction when methane burns in oxygen.
CH4 + 2O2 → CO2 + 2H2O
An energy level diagram for this reaction is shown below.
2. Explain the significance of x, y and z on the energy level diagram in terms of the energy transfers which occur when these chemical bonds are broken and formed.
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(Total 9 marks)
1. Which chemical bonds are broken and which are formed during this reaction?
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KS4 SCIENCE SUCCESS CRITERIA1. Read the notes in your exercises book the night before
each science lesson.
2. Review the topic being studied on BBC Bitesize for 10 minutes the night before each lesson.
3. On Saturday - Use a science text book/ BBC Bitesize and make your own revision cards for the topic being studied.
4. On Sunday – Read and learn the information on each revision card for the topic being studied.
What is a reaction profile?DO NOW
Define activation energy.
Copy the reaction profile for endothermic and expthermicreactions.
CHEMISTRY TOPIC 5 – ENERGY CHANGES
Reaction Profiles
Learning Objectives
Use reaction profiles to identify
reactions as exothermic or endothermic.
Draw simple reaction profiles
(energy level diagrams) for
exothermic and endothermic.
BRIDGES – Making Connections
CHEMISTRY TOPIC 5 – ENERGY CHANGES
Reaction Profiles
Learning Objectives
Use reaction profiles to identify
reactions as exothermic or endothermic.
Draw simple reaction profiles
(energy level diagrams) for
exothermic and endothermic.
BRIDGES – Making Connections
Main•Exothermic means “Heat out” Heat is given out to the surroundings•Endothermic means “Heat in” Heat is taken in from the surroundings•All reactions are either exothermic or endothermic.
SOME REACTIONS ARE OBVIOUSLY EXOTHERMICe.g. methane gas burning is exothermic as there is a flame that is hot.e.g. The reaction between magnesium and copper oxide is an exothermic reaction because there is a temperature rise.HOWEVER it isn’t quite as simple as this.All reactions are in fact a mixture of exo and endo processes.If we say a reaction is exothermic, then what this really means is that the exopart of the reaction is greater than the endo part of the reactionWhen we say a reaction is exothermic what we are really saying is that the overall
reaction is exothermic.When we say a reaction is endothermic what we are really saying is that the overall
reaction is endothermic.
Extra explanationImagine an example using money instead of energy.Imagine you are investing money into an investment fund in a bank.We will show the results of the investment over time on a graph.
On the graph: paying in money shown by direction & payback money shown by direction
Money in fund
Time
Would this be a good investment?
Money
in fund
Time
Paid in
Paid back
This is a good investment because you have more “payback” than “pay in”
Money
in fund
Time
£5,000 £7,000
Take an actual example. How much money would you make on this deal?
Answer = £2,000
The profit is the difference between the two arrows
Money
in fund
Time
£5,000 £3,500
A MEMBER OF STAFF ACTUALLY MADE AN INVESTMENT WHICH COULD BE REPRESENTED BY THS GRAPH.
How did he feel about this?
Overall £5,000 paid in and £3,500 paid back
So overall loss, or drop of £1,500
What if you made more than oneinvestment at the same time?Imagine you invest £ 5,000 and £4,000Imagine the paybacks are £2,000 and £8,000
How do you feel – was this a good deal?
OR
Total investment = £5,000 + £4,000 = £9,000Total payback = £2,000 + £8,000 = £10,000
So
Back to Chemistry- Energy instead of Money
In chemistry the “paying in” part of reactions is breaking bonds between atoms. e.g. breaking up hydrogen molecules
Breaking bonds always involves a “paying in” of energy and is represented by an direction on the energy graph. We call this an investment of energy
H H H H
1 molecule 2 separate atoms
In chemistry the “paying back” part of reactions is making bonds between atoms. e.g. the making of a hydrogen molecule from two hydrogen atoms
H H H H
1 molecule2 separate atoms
This is shown by a direction on an energy graph
Let’s now apply this to an actual example
Magnesium reacts with iron oxide to make magnesium oxide and iron
Mg + FeO = MgO + Fe
Mg Fe O FeOMg
This bond must break This bond must be madeThis involves “pay in”of energy
This involves a “pay back”of energy
On an energy graph this will look like:
Energy
Time
Energy “PAY IN” to break the FeO bond
Energy “PAY BACK” on making the MgO bond
Mg + FeO = MgO + Fe
More “PAY BACK” energy than “PAY IN” energy – this is a GOOD energy deal.A “GOOD” energy deal is Exothermic
Energy
Time
Mg + FeO = MgO + Fe
This is the ENDO’ part of the reaction
This is the EXO’ part of the reaction
We say this is an exothermic reaction (overall) since the exothermic part is greater than the endothermic part
Working out the overall energy change
The overall energy change is given by the difference between the lengths of the two arrows
The overall energy change is In exam questions, you will be given energy values so that you can work out the lengths of the arrows
330kJ/mole 400kJ/mole
70kJ/mole
70kJ/mole
Energy
Time
What sort of reaction would have this energy graph and why?
This sort of reaction would be ……………………… because the energy ………………. for …………………… bonds is …………… than the energy ………………… from ……………………… bonds.
The ………………………………………. part of the reaction is greater than the ……………………………………. part.
Assessment
endothermic absorbedbreaking greater released making
endothermicexothermic
Exothermic reactions - a summary
Energy
Progress of reaction
ReactantsProducts
Activation energy
In exothermic reactions the energy of the reactants is greater than the energy of the products
The activation energy is the minimum energy required to break the bonds and start the reaction off
Endothermic reactions - a summary
Energy
Progress of reaction
Reactants
ProductsActivation energy
In endothermic reactions the energy of the products is greater than the energy of the reactants
The activation energy is the minimum energy required to break the bonds and start the reaction off
CHEMISTRY TOPIC 5 – ENERGY CHANGES
Reaction Profiles
Learning Objectives
Use reaction profiles to identify
reactions as exothermic or endothermic.
Draw simple reaction profiles
(energy level diagrams) for
exothermic and endothermic.
BRIDGES – Making Connections
Exothermic reactions and catalysts
Energy
Progress of reaction
ReactantsProducts
A.E. without cat
A.E. with catalyst
A catalyst lowers the activation energy. This helps the bonds to break more easily so the reaction works faster at that temperature.NOTE that the overall energy change is the SAME
Add notes to your diagram.
How would a catalyst change the energy graph of an endothermic reactions?
Energy
Progress of reaction
Reactants
Products
A. E. no cat
A.E. with catalyst
Again the reaction would go faster because the activation energy has been lowered
Again note that the overall energy change is the same
Add notes to your diagram.
CHEMISTRY TOPIC 5 – ENERGY CHANGES
Reaction Profiles
Learning Objectives
Use reaction profiles to identify
reactions as exothermic or endothermic.
Draw simple reaction profiles
(energy level diagrams) for
exothermic and endothermic.
BRIDGES – Making Connections
Mark off areas of the topic cob=vered at the end of each lesson.
Required Practical - Distinguish between exothermic and endothermic reactions on the basis of the temperature change of the surroundings.
Required Practical - Evaluate uses and applications of exothermic and endothermic reactions given appropriate information.
Draw simple reaction profiles (energy level diagrams) for exothermic and endothermic reactions showing the relative energies of reactants and products, the activation energy and the overall energy change, with a curved arrow to show the energy as the reaction proceeds.Use reaction profiles to identify reactions as exothermic or endothermic.Explain that the activation energy is the energy needed for a reaction to occur.(HT) Be able to calculate the energy transferred in chemical reactions using bond energies supplied.