February 2021 Measuring Energy Changes LO Understand energy

February 2021 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?

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

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

Exothermic and Endothermic • What happens to the energy in the system in exothermic reactions and endothermic reactions? H= products Progress of reaction products Energy / k. J) reactants H= + reactants Progress of reaction

Task • Draw the 2 graphs to show what happens to the amount of energy of the reactants as it proceeds for exothermic and endothermic reactions H=products Progress of reaction products Energy / k. J) reactants H=+ reactants Progress of reaction

Bond Breaking and Bond making • For a reaction to happen bonds have to be broken to then form new ones H H O Bond Breaking H C H H H O O O Bond Forming O O O H Energy in chemicals C H O Progress of reaction C O H H

BENDOMEX • Bendo = Breaking is endo • Mex = Making is exo

BENDOMEX reactants products H Bonds form Endo Bonds break Bond forming products Energy in chemicals reactants Exo H Bonds break • 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 •

Activity Copy the energy diagram Use your graph to help you explain why garages can store petrol safely but always have notices about not smoking near the petrol pumps. Energy / k. J) What is activation energy and why do even exothermic reactions require it? Activation. E nergy Petrol + oxygen Carbon dioxide + water Progress of reaction

Challenge Task Explain why you need to supply wood with heat before it will set fire

Task Make 2 sentences using the bullet points • A reaction • Exothermic • If the is • Endothermic 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?

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 activation reaction is called the ______ energy 2. In endothermic reactions bond more breaking requires ___ energy than is produced by bond forming. endothermic 3. Bond breaking is an _____ process.

Quick 5 • Write 1 to 5 in your books

Which of the following is an endothermic process? A. B. C. D. Burning wood Reacting an acid and alkali Reacting magnesium with acid Dissolving ammonium nitrate in water

Which of the following is true for an exothermic process? products Energy / k. J A. The reactants lose energy to the surroundings B. The reaction gets cold C. The energy diagram will be as shown D. The reaction will have a H that is positive (+). reactants Progress of reaction

Which of the following is true when chemical bonds are broken? A. The process is exothermic B. Energy is given out C. A physical change is occurring D. 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. X B. Y C. Z