The energy level diagram for this reaction is shown below.

Transcription

1 Q. Methanol can be made when methane reacts with oxygen. (a) The energy level diagram for this reaction is shown below. (i) What is the energy change represented by A? () (ii) Use the energy level diagram to explain how it shows that this reaction is exothermic. () (b) A student did an experiment to find the energy released when methanol burns in air. The student: weighed a spirit burner containing methanol set up the equipment as shown in the diagram recorded the initial temperature Page of 4

2 lit the spirit burner put out the flame when the temperature of the water had risen by about 0 C stirred the water and recorded the highest temperature of the water reweighed the spirit burner containing the methanol. The student repeated the experiment and recorded his results. Experiment Experiment Experiment 3 Initial mass of spirit burner and methanol in g Final mass of spirit burner and methanol in g Initial temperature in C Highest temperature in C Temperature change in C Use the diagram and the information in the table to answer the questions. (i) The main error in this experiment is energy loss. Suggest one way that the equipment could be changed to reduce energy loss. () (ii) The temperature change in Experiment is greater than the temperature change in Experiment and Experiment 3. Explain why. () (iii) Suggest one reason why the student repeated the experiment. () Page of 4

3 (iv) Use the temperature change in Experiments and 3 to calculate how much energy is released when g of methanol burns. The equation that you need to use is: Energy released in joules = 00 x 4. x mean temperature change Show clearly how you work out your answer. Burning g of methanol releases... J () (Total 9 marks) Q. Hydrogen could be the fuel used in all cars. One advantage is that when hydrogen reacts with oxygen only water is produced. The chemical equation for this reaction is: H + O H O This equation can be written showing the structural formulae. H H + O O H O H (a) Use the bond energies in the table to calculate the energy change for this reaction. Bond Bond energy in kj H H 436 O = O 498 O H Energy change =... kj (3) Page 3 of 4

4 (b) Suggest why the bond energy of O = O is higher than the bond energies of both H H and O H () (c) In terms of bond energies, explain why hydrogen can be used as a fuel () (Total 6 marks) Q3. Read the information about energy changes and then answer the questions. A student did an experiment to find the energy change when hydrochloric acid reacts with sodium hydroxide. The equation which represents the reaction is: HCl + NaOH NaCl + H O The student used the apparatus shown in the diagram. The student placed 50 cm 3 of hydrochloric acid in a glass beaker and measured the temperature. The student then quickly added 50 cm3 of sodium hydroxide solution and stirred the mixture with the thermometer. The highest temperature was recorded. The student repeated the experiment, and calculated the temperature change each time. Page 4 of 4

5 (a) The biggest error in this experiment is heat loss. Suggest how the apparatus could be modified to reduce heat loss. () (b) Suggest why it is important to stir the chemicals thoroughly. () (c) Which one of these experiments was probably carried out on a different day to the others? Explain your answer. () (d) Suggest why experiment 4 should not be used to calculate the average temperature change. () (e) Calculate the average temperature change from the first three experiments. Answer =... C () (f) Use the following equation to calculate the energy change for this reaction. energy change in joules = average temperature change Answer =... J () Page 5 of 4

6 (g) Which one of these energy level diagrams, A or B, represents the energy change for this reaction? Explain why. () (Total 7 marks) Q4. Energy can be measured in kilojoules (kj) or in kilocalories (kcal). The table shows some information about different varieties of crisps. Variety of crisps Energy in kj per 5 g packet Cheese 548 Salted 550 Chicken 545 Steak 540 (a) Arrange the different varieties of crisps in order of increasing energy. The first one has been done for you. Steak () (b) One variety not given in the table is broccoli. On the label it states that the energy per 5 g packet is 30 kcal. kcal = 4. kj Calculate the energy of 5 g of broccoli crisps in kj. Energy =... kj () (Total marks) Page 6 of 4

7 Q5. You will find the information on the Data Sheet helpful when answering this question. This equation shows the reaction between ethene and oxygen. C H 4 + 3O CO + H O The structural formulae in the equation below show the bonds in each molecule involved. Use the three stages shown at (a), (b) and (c) below to calculate the nett energy transfer when the formula mass ( mole) of ethene reacts with oxygen. (a) Write down the bonds broken and the bonds formed during the reaction. (Some have already been done for you.) () (b) Calculate the total energy changes involved in breaking and in forming all of these bonds. (Some have already been done for you.) (4) Page 7 of 4

8 (c) Describe, as fully as you can, what the figures in (b) tell you about the overall reaction. () (Total 8 marks) Q6. Methane and oxygen react together to produce carbon dioxide and water. The methane gas will not burn in oxygen until a flame is applied, but once lit it continues to burn. (a) Explain why energy must be supplied to start the reaction but it continues by itself once started. (4) Page 8 of 4

9 (b) Sketch an energy level diagram for the reaction and indicate on the diagram the nett energy released. (3) (Total 7 marks) Page 9 of 4

10 M. (a) (i) activation energy or energy needed to start the reaction (ii) the reaction is exothermic because the energy level / value of products is less than the energy level / value of reactants allow the reaction is exothermic because arrow B goes down or methanol is below methane and oxygen or arrow C is bigger than arrow A for mark allow energy level of products is lower unqualified or the energy level of reactants is higher unqualified for mark (b) (i) use a lid / cover over the calorimeter or any mention of how the calorimeter could be safely enclosed / insulated (ii) a greater mass of methanol was burned in Experiment therefore the temperature change was greater because more energy was transferred / released (iii) any one from: to improve or check repeatability / quality of results to make it easier to spot an anomalous measurement to be able to calculate an average mean value (iv) 880 (J) for correct answer if answer is incorrect allow one mark for used as the mean temperature change [9] M. (a) reactants H-H + O = O = 370 (kj) products 4 O-H = 856 (kj) energy change = 486(kJ) Page 0 of 4

11 (b) O=O has a double (covalent) bond or O-H and H-H only have single (covalent) bonds (c) hydrogen can be used as a fuel because when it reacts with oxygen more energy is released in bond making than used in bond breaking therefore the reaction releases energy or the reaction is exothermic [6] M3. (a) eg plastic (beaker) / insulation / lid / cover or any mention of enclosed any sensible modification to reduce heat loss ignore prevent draughts ignore references to gas loss (b) all the substances react or all (the substances) react fully / completely or heat evolved quickly or distribute heat accept to mix them so they react is insufficient for the mark accept increase chances of (successful) collisions / collision rate increase do not accept rate of reaction increase / make reaction faster (c) experiment and different / higher / initial / starting temperature accept experiment and the room is hotter / at higher temperature do not accept temperature change / results higher (d) temperature change does not fit pattern accept anomalous / odd or it is the lowest or it is lower than the others or it is different to the others results are different is insufficient Page of 4

12 (e) 7 / 7.0 (f) ( ) = 940 ecf from (e) (g) diagram A and reaction exothermic / heat evolved / Δ H is negative / temperature rises accept energy is lost (to the surroundings) [7] M4. (a) chicken, cheese, salted (b) 546 allow 545, 548, 550 [] ## (a) Bonds broken Bonds formed number type number type 3 [O=O] 4 [O H] each for mark (b) Total energy change Total energy change in breaking bonds in forming bonds = = 856 each for mark Total = 3758 Total = 5076 each for mark 4 (c) net energy transfer = 38 this energy is released in the reaction/it is an exothermic reaction each for mark [N.B. credit e.c.f. (a) (b) and (b) (c)] [8] Page of 4

13 M6. (a) idea that existing bonds must first be broken for mark (credit molecules / atoms more likely to react when they collide) energy is released when new bonds form gains mark but more energy is released when new bonds form gains marks or overall reaction exothermic this breaks more bonds so the reaction continues for mark max 4 (b) reactant level higher than product level (names of reactants and products not required) indication that activation energy required (i.e. the hump ) any correct indication of nett energy change (i.e. between product and reactant levels even if other marks not gained) for mark each 3 [7] Page 3 of 4

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