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09 December 2018 Measuring Energy Changes
LO: Understand energy changes of reactions What happens when you strike a match? Why don’t matches spontaneously catch on fire in the box? What does rubbing the match n the box supply the match with?
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Learning Outcomes Grade 4– Draw simple reaction profiles (energy level diagrams) for exothermic and endothermic reactions. Great: Grade 6– Explain how to use reaction profiles to identify reactions as exothermic or endothermic Fantastic :Grade 7 - Explain that the activation energy is the energy needed for a reaction to occur Awesome Grade 9: Describe endothermic and exothermic reactions in terms of bond breaking and bond making
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Task Measure the temperature change and therefore the energy transferred of 2 reactions Burning candle Citric acid and sodium hydrogen carbonate Comment on whether the reaction is endothermic or exothermic and the transfer of energy between system and surroundings E = mcΔT
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Exothermic and Endothermic
What happens to the energy in the system in exothermic reactions and endothermic reactions? exothermic endothermic products Energy / kJ) Progress of reaction reactants H= - H= + products Energy / kJ Progress of reaction reactants
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Task exothermic endothermic
Draw the 2 graphs to show what happens to the amount of energy of the reactants as it proceeds for exothermic and endothermic reactions exothermic endothermic products Energy / kJ) Progress of reaction reactants H=- H=+ products Energy / kJ Progress of reaction reactants
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Bond Breaking and Bond making
For a reaction to happen bonds have to be broken to then form new ones Progress of reaction Energy in chemicals O C H Bond Breaking Bond Forming O H C O C H H
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BENDOMEX Bendo = Breaking is endo Mex = Making is exo
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BENDOMEX Where the energy from bond forming exceeds that needed for bond breaking the reaction is exothermic. Where the energy for bond breaking exceeds that from bond forming the reaction is endothermic. Energy in chemicals reactants products H Bonds break Bond forming Bonds form Exo Endo
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Copy the energy diagram
Activity Copy the energy diagram ActivationEnergy What is activation energy and why do even exothermic reactions require it? Petrol + oxygen Energy / kJ) Use your graph to help you explain why garages can store petrol safely but always have notices about not smoking near the petrol pumps. Carbon dioxide + water Progress of reaction
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Explain why you need to supply wood with heat before it will set fire
Challenge Task Explain why you need to supply wood with heat before it will set fire
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Task Make 2 sentences using the bullet points
A reaction is Exothermic Endothermic If the energy taken to break the bonds Is less Is more Than the energy given out to make the bonds How could you include activation energy in these sentences?
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The energy needed to start off a reaction is called the ______ energy
Copy the summary using the words from the box to fill in the gaps: more endothermic activation The energy needed to start off a reaction is called the ______ energy In endothermic reactions bond breaking requires ___ energy than is produced by bond forming. Bond breaking is an _________ process. activation more endothermic
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Quick 5 Write 1 to 5 in your books
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Which of the following is an endothermic process?
Burning wood Reacting an acid and alkali Reacting magnesium with acid Dissolving ammonium nitrate in water
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Which of the following is true for an exothermic process?
The reactants lose energy to the surroundings The reaction gets cold The energy diagram will be as shown The reaction will have a H that is positive (+). products Energy / kJ Progress of reaction reactants
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Which of the following is true when chemical bonds are broken?
The process is exothermic Energy is given out A physical change is occurring The reaction will have a H that is positive(+).
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Which of the following is true for an exothermic reaction?
Bond breaking involves a bigger energy change than bond making Bond making involves a bigger energy change than bond breaking Bond making involves the same energy change as bond breaking
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Which of the following is the activation energy:
X Y Z
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