What is on the axes of a reaction profile?

Energy on the vertical axis and progress of the reaction on the horizontal axis.

What is activation energy?

The minimum energy needed to start a reaction.

How is activation energy shown on a reaction profile?

As the height from the reactants up to the top of the peak.

What is the overall energy change?

The energy of the products minus the energy of the reactants.

Does breaking bonds take in or release energy?

It takes in energy (endothermic).

Does making bonds take in or release energy?

It releases energy (exothermic).

When is a reaction exothermic, in terms of bonds?

When more energy is released making bonds than is taken in breaking bonds.

When is a reaction endothermic, in terms of bonds?

When more energy is taken in breaking bonds than is released making bonds.

Why does butane need a spark to start burning?

The spark gives the reactants the activation energy needed.

Energy level diagrams and bond energyIB MYP Chemistry: Flashcards

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What does a reaction profile show?

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What does a reaction profile show?: How the energy of the substances changes as the reaction goes from reactants to products.
What is on the axes of a reaction profile?: Energy on the vertical axis and progress of the reaction on the horizontal axis.
Where are the products on the profile of an exothermic reaction?: Lower than the reactants.
Where are the products on the profile of an endothermic reaction?: Higher than the reactants.
What is activation energy?: The minimum energy needed to start a reaction.
How is activation energy shown on a reaction profile?: As the height from the reactants up to the top of the peak.
What is the overall energy change?: The energy of the products minus the energy of the reactants.
Does breaking bonds take in or release energy?: It takes in energy (endothermic).
Does making bonds take in or release energy?: It releases energy (exothermic).
When is a reaction exothermic, in terms of bonds?: When more energy is released making bonds than is taken in breaking bonds.
When is a reaction endothermic, in terms of bonds?: When more energy is taken in breaking bonds than is released making bonds.
Why does butane need a spark to start burning?: The spark gives the reactants the activation energy needed.
What is the sign of the overall energy change for an exothermic reaction?: Negative.

Exam questions on Energy level diagrams and bond energy

A camping stove burns butane gas. Butane does not catch fire by itself when it leaves the canister. The user has to press a button that makes a spark, and after that the flame keeps burning and releases a lot of heat.
Explain why a spark is needed to start the reaction even though the reaction releases energy overall.2 marks
Hydrogen gas reacts with chlorine gas to make hydrogen chloride: H₂ + Cl₂ → 2HCl. Breaking the bonds in one mole of hydrogen molecules and one mole of chlorine molecules takes in a total of 679 kJ. Making the bonds in two moles of hydrogen chloride releases a total of 864 kJ.
Use the data to work out the overall energy change for this reaction and state whether it is exothermic or endothermic.2 marks
Students at a school in Cairo want to find out whether the reaction between citric acid solution and sodium hydrogencarbonate powder is exothermic or endothermic. They plan to add 5 g of sodium hydrogencarbonate to 50 cm³ of citric acid solution in a polystyrene cup with a lid, stir the mixture and record the temperature.
Before the experiment the students made a hypothesis. Write a suitable hypothesis for the temperature change, giving a scientific reason.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).