Energy level diagrams and bond energyIB MYP Chemistry: Subtopic test

1. 1
   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.

   (a)

   What name is given to the minimum energy needed to start a reaction?

   [1 mark]

   • ABond energy
   • BOverall energy change
   • CSpecific heat capacity
   • DActivation energy

   (b)

   On a reaction profile for the burning of butane, how do the energy levels of the products and reactants compare?

   [1 mark]

   • AThe products are higher in energy than the reactants
   • BThe products and reactants are at the same energy
   • CThe products are lower in energy than the reactants
   • DThe products have no energy

   (c)

   Explain why a spark is needed to start the reaction even though the reaction releases energy overall.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   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.

   (a)

   Which statement about bond breaking is correct?

   [1 mark]

   • AIt releases energy to the surroundings
   • BIt takes in energy from the surroundings
   • CIt does not involve any energy change
   • DIt always releases more energy than bond making

   (b)

   Which statement explains why this reaction is exothermic?

   [1 mark]

   • AMore energy is released making new bonds than is taken in breaking old bonds
   • BMore energy is taken in breaking old bonds than is released making new bonds
   • CThe same amount of energy is taken in and released
   • DNo bonds are broken in the reaction

   (c)

   Use the data to work out the overall energy change for this reaction and state whether it is exothermic or endothermic.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   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.

   (a)

   Before the experiment the students made a hypothesis. Write a suitable hypothesis for the temperature change, giving a scientific reason.

   [3 marks]

   (b)

   The temperature of the mixture fell from 21.0 °C to 12.5 °C. Explain what this shows about the reaction in terms of bonds and a reaction profile, and suggest one way to improve the reliability of the results.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A city bus company in Chile is deciding whether to replace its diesel buses with buses that burn hydrogen. Hydrogen reacts with oxygen to form water: 2H₂ + O₂ → 2H₂O. The reaction releases a large amount of energy for each gram of hydrogen, but it needs a spark to start. At present most hydrogen is made from natural gas, a process that releases carbon dioxide. Hydrogen has to be stored under high pressure and there are very few refuelling stations in the country.

   (a)

   Explain, in terms of bond breaking and bond making and a reaction profile, why burning hydrogen releases energy overall but still needs a spark to start.

   [6 marks]

   (b)

   Discuss the benefits and drawbacks of using hydrogen buses, and evaluate whether the company should adopt them.

   [6 marks]

   Total for question 4: 12 marks

End of questions