A hydrogen fuel cell is a type of chemical cell. Unlike a battery, it does not store its reactants inside the cell itself.
(a)State how the fuel (hydrogen) and oxygen are supplied to a hydrogen fuel cell.(1)
(b)Contrast this with how the reactants are held in a non-rechargeable battery.(1)
(c)State what happens to the electricity a hydrogen fuel cell can produce if the supply of hydrogen and oxygen is maintained continuously.(1)
(Total for Question 1 is 3 marks)
2
A simple chemical cell can be made by placing two different metal electrodes into an electrolyte and connecting them with wires and a voltmeter, as shown in Figure 1. A student, Owusu, sets up a cell using a magnesium electrode and a silver electrode dipped into dilute sulfuric acid.
(a)State two components, other than the two metal electrodes, that are needed to complete this simple chemical cell.(2)
(b)Explain why this cell produces a voltage between the magnesium and silver electrodes.(2)
(c)Predict, giving a reason based on the reactivity series, whether replacing the silver electrode with a copper electrode (keeping the magnesium electrode and electrolyte the same) would increase or decrease the voltage produced.(2)
(Total for Question 2 is 6 marks)
3
In a simple chemical cell, the more reactive metal electrode loses electrons more readily than the less reactive metal electrode.
(a)State which electrode, the more reactive metal or the less reactive metal, acts as the negative electrode in a simple cell.(1)
(b)State whether conventional current in the external circuit flows in the same direction as electron flow, or the opposite direction.(1)
(c)Explain why the less reactive metal electrode becomes the positive electrode of the cell.(2)
(Total for Question 3 is 4 marks)
4
The overall reaction taking place inside a hydrogen fuel cell is between hydrogen and oxygen, producing water.
(a)Write the balanced symbol equation, including state symbols, for the overall reaction in a hydrogen fuel cell.(2)
(b)State the energy transfer that takes place inside a hydrogen fuel cell, and identify, in terms of electron transfer, what happens to hydrogen during this reaction.(2)
(Total for Question 4 is 4 marks)
5
A non-rechargeable battery is described as 'flat' once it can no longer produce a useful voltage, whereas a rechargeable battery can be restored by connecting it to a charger.
(a)Explain, in terms of the chemicals inside it, why a non-rechargeable battery goes flat.(2)
(b)Explain how connecting a rechargeable battery to a charger restores its ability to produce a voltage.(2)
(Total for Question 5 is 4 marks)
6
Chemical cells and batteries are used to store chemical energy and release it as electrical energy when needed.
(a)State what is meant by a 'chemical cell'.(1)
(b)State what is meant by a 'battery'.(1)
(c)State what is meant by an 'electrode'.(1)
(d)State one similarity between how a battery and a hydrogen fuel cell each produce electricity.(1)
(Total for Question 6 is 4 marks)
7
Fatima investigates how the voltage produced by a simple chemical cell depends on the pair of metal electrodes used. She sets up cells using different pairs of metal electrodes dipped into dilute sulfuric acid, connects a voltmeter, and repeats each measurement three times. Her results are shown in Figure 2.
(a)Identify the independent variable and the dependent variable in this investigation.(2)
(b)State two variables Fatima should control to make this a fair test.(2)
(c)Identify the anomalous result in the 'zinc and copper' row of the table, and calculate the mean voltage for this pair of metals, excluding the anomalous result.(3)
(d)Describe the pattern shown by Fatima's results, linking it to the reactivity series.(2)
(e)Suggest one improvement Fatima could make to increase confidence in her conclusion.(1)
(f)The bottle of dilute sulfuric acid used is labelled with a hazard symbol showing a black exclamation mark on an orange background. Name the hazard indicated by this symbol, and state one safety precaution Fatima should take.(2)
(g)Fatima now wants to investigate how the concentration of the sulfuric acid electrolyte affects the voltage produced by a zinc and copper cell. Describe how she could adapt her method to investigate this, name one variable she would need to keep constant, and state how she should present her results to identify any pattern.(4)
(Total for Question 7 is 16 marks)
8
A torch can be powered either by non-rechargeable alkaline batteries or by a rechargeable lithium-ion battery pack. Evaluate the use of a rechargeable lithium-ion battery pack compared with non-rechargeable alkaline batteries, for powering a torch that is used regularly over several years. In your answer, consider how each type of battery works, and their relative cost, convenience and environmental impact.
(Total for Question 8 is 6 marks)
9
Higher tier only. When a chemical reaction takes place, the covalent bonds in the reactants are broken and new covalent bonds are formed to make the products. The bond energy of a covalent bond is the energy needed to break one mole of that bond (measured in kJ per mole).
(a)State whether breaking a covalent bond is an exothermic or endothermic process, and state whether forming a new covalent bond is an exothermic or endothermic process.(2)
(b)A reaction is found to be exothermic overall. State what this tells you about the relative amounts of energy needed to break the bonds in the reactants and released forming the bonds in the products.(2)
(Total for Question 9 is 4 marks)
10
Higher tier only. Hydrogen reacts with bromine to form hydrogen bromide: H2 + Br2 -> 2HBr. Bond energies: H-H = 436 kJ/mol, Br-Br = 193 kJ/mol, H-Br = 366 kJ/mol.
(a)Calculate the total energy needed to break all the bonds in the reactants.(1)
(b)Calculate the total energy released when the new bonds form in the products.(1)
(c)Calculate the overall energy change for this reaction, and state whether the reaction is exothermic or endothermic.(2)
(d)Explain, using your answers above, why this reaction is exothermic.(1)
(Total for Question 10 is 5 marks)
11
Some vehicle manufacturers are developing cars powered by hydrogen fuel cells, as an alternative to both petrol/diesel engines and rechargeable battery-electric vehicles.
(a)Give one advantage of a hydrogen fuel-cell vehicle compared with a petrol vehicle.(1)
(b)Give one advantage of a hydrogen fuel-cell vehicle compared with a rechargeable battery-electric vehicle.(1)
(c)Suggest two reasons why hydrogen fuel-cell vehicles are not yet widely used on UK roads, despite these advantages.(2)
(d)Hydrogen used in fuel cells is often produced industrially from natural gas (a fossil fuel), rather than by the electrolysis of water using renewable electricity. Explain why this affects how 'clean' hydrogen fuel-cell vehicles really are overall.(2)
(Total for Question 11 is 6 marks)
12
Higher tier only. Hydrogen reacts with oxygen (including inside a hydrogen fuel cell) according to the equation 2H2 + O2 -> 2H2O. Bond energies: H-H = 436 kJ/mol, O=O = 498 kJ/mol, O-H = 463 kJ/mol.
(a)Calculate the total energy needed to break all the bonds in the reactants (2 mol of H2 and 1 mol of O2).(2)
(b)Each mole of H2O contains two O-H bonds. Calculate the total energy released when the new bonds form in 2 mol of H2O.(2)
(c)Calculate the overall energy change for this reaction (per 2 mol of hydrogen gas), and state whether the reaction is exothermic or endothermic.(2)
(d)State one reason why this large exothermic energy change makes hydrogen a useful fuel for a hydrogen fuel cell.(1)
(Total for Question 12 is 7 marks)
13
The reaction 2H2 + O2 -> 2H2O releases 482 kJ of energy for every 2 mol of hydrogen gas that reacts completely.
(a)Calculate the energy released, in kJ, when 1 mol of hydrogen gas reacts completely.(1)
(b)A hydrogen fuel-cell vehicle's tank contains 6 mol of hydrogen gas. Calculate the total chemical energy that could be released if all of this hydrogen reacts completely.(2)
(c)The fuel cell transfers only 60% of this chemical energy usefully into electrical energy; the rest is wasted, mostly as thermal energy. Calculate the useful electrical energy output, in kJ, for the 6 mol of hydrogen in part b.(2)
(Total for Question 13 is 5 marks)
14
Higher tier only. Nitrogen reacts with hydrogen in the Haber process to produce ammonia: N2 + 3H2 -> 2NH3. Bond energies: N-N (triple bond, in N2) = 945 kJ/mol, H-H = 436 kJ/mol, N-H = 391 kJ/mol.
(a)Calculate the total energy needed to break all the bonds in the reactants (1 mol of N2 and 3 mol of H2).(2)
(b)Each mole of NH3 contains three N-H bonds. Calculate the total energy released forming the bonds in 2 mol of NH3.(2)
(c)Calculate the overall energy change for this reaction, and state whether it is exothermic or endothermic.(2)
(Total for Question 14 is 6 marks)
15
Higher tier only. Inside a car engine, the high temperatures and pressures allow nitrogen and oxygen from the air to react together: N2 + O2 -> 2NO. Bond energies: N-N (triple bond, in N2) = 945 kJ/mol, O=O = 498 kJ/mol, N=O = 630 kJ/mol.
(a)Calculate the overall energy change for this reaction.(3)
(b)State whether this reaction is exothermic or endothermic, and use this to explain why nitrogen monoxide only forms in significant amounts at the high temperatures inside a car engine, rather than at room temperature.(2)
(Total for Question 15 is 5 marks)
16
Higher tier only. Inside a hydrogen fuel cell with an alkaline electrolyte, hydrogen is oxidised at the negative electrode and oxygen is reduced at the positive electrode.
(a)Write the half equation for the reaction at the negative electrode.(2)
(b)Write the half equation for the reaction at the positive electrode.(2)
(c)Show how adding these two half equations together, cancelling the electrons and any other species common to both sides, gives the overall equation for the reaction in a hydrogen fuel cell.(2)
(Total for Question 16 is 6 marks)
17
Higher tier only. Hydrogen reacts with iodine: H2 + I2 -> 2HI. This reaction has an overall energy change of +52 kJ/mol. Bond energies: H-H = 436 kJ/mol, I-I = 152 kJ/mol.
(a)Calculate the total energy needed to break the bonds in the reactants.(1)
(b)The overall energy change of a reaction is equal to the energy needed to break the bonds in the reactants minus the energy released forming the bonds in the products. Use this, and your answer to part a, to calculate the bond energy of the H-I bond.(3)
(c)Explain, referring to your calculations, why this reaction is described as endothermic.(1)
(Total for Question 17 is 5 marks)
18
Higher tier only. Propene reacts with hydrogen, in the presence of a nickel catalyst, to form propane: C3H6 + H2 -> C3H8. Figure 3 shows the carbon skeleton of propene and propane, the number of hydrogen atoms bonded to each carbon atom, and the hydrogen molecule, H2.
(a)Using Figure 3, state the total number of each of the following bond types present in one molecule of propene (C3H6) and one molecule of hydrogen (H2), combined: C-H bonds, C-C bonds, C=C bonds and H-H bonds.(3)
(b)Calculate the total energy needed to break all of these bonds, using the bond energies C-H = 413 kJ/mol, C-C = 347 kJ/mol, C=C = 612 kJ/mol and H-H = 436 kJ/mol.(2)
(c)Using Figure 3, state the number of C-H bonds and C-C bonds in one molecule of propane (C3H8), and calculate the total energy released when all of these bonds are formed.(2)
(d)Calculate the overall energy change for this reaction, and state whether it is exothermic or endothermic.(2)
(Total for Question 18 is 9 marks)
Mark scheme · C5b Bond Energies, Cells and Fuel Cells
Question 1
(a) B1 they are supplied continuously from external tanks/an external supply, rather than being stored inside the cell oe
(a) Answer: Hydrogen and oxygen are supplied continuously to the cell from external tanks, rather than being stored inside the cell itself.
(b) B1 in a non-rechargeable battery, all of the reactants needed are sealed inside the battery from the start oe
(b) Answer: In a non-rechargeable battery, all the reactants are sealed inside the battery from the moment it is made.
(c) B1 the fuel cell will keep on producing electricity (a voltage) for as long as hydrogen and oxygen keep being supplied; it does not go flat like a battery oe
(c) Answer: It will keep producing electricity for as long as hydrogen and oxygen keep being supplied - unlike a battery, it does not go flat.
Question 2
(a) B1 an electrolyte (a solution that can conduct electricity/allow ions to move) oe
(a) B1 a wire (external circuit) connecting the electrodes, together with a device such as a voltmeter to measure/use the voltage produced oe
(a) Answer: An electrolyte (here, dilute sulfuric acid) and a wire (external circuit), together with a voltmeter to measure the voltage produced.
(b) M1 the two metals have different tendencies to lose electrons (react to different extents) oe
(b) A1 this produces a difference in electrical potential between the two electrodes, which drives electrons around the external circuit when it is completed oe
(b) Answer: Magnesium and silver have different tendencies to lose electrons, so a difference in electrical potential (a voltage) is set up between the two electrodes, which drives electrons around the external circuit.
(c) M1 copper is more reactive than silver (silver is the least reactive of the two) oe
(c) A1 so the difference in reactivity between magnesium and copper is smaller than between magnesium and silver, so the voltage would decrease oe
(c) Answer: The voltage would decrease, because copper is more reactive than silver, so the difference in reactivity between magnesium and copper is smaller than between magnesium and silver.
Question 3
(a) B1 the more reactive metal (electrode) oe
(a) Answer: The more reactive metal is the negative electrode.
(b) B1 the opposite direction (conventional current flows from positive to negative through the external circuit; electrons flow from negative to positive) oe
(b) Answer: Conventional current flows in the opposite direction to electron flow: from the positive electrode to the negative electrode through the external circuit, while electrons flow from negative to positive.
(c) M1 the less reactive metal loses electrons less readily than the more reactive metal oe
(c) A1 so electrons flow towards it through the external circuit (relative to the more reactive electrode), making it the positive terminal oe
(c) Answer: The less reactive metal loses electrons less readily than the more reactive metal, so electrons flow towards it through the external circuit, making it the positive electrode.
Question 4
(a) M1 correct formulae shown: H2, O2, H2O oe
(a) A1 correctly balanced with state symbols: 2H2(g) + O2(g) -> 2H2O(l) cao
(a) Answer: 2H2(g) + O2(g) -> 2H2O(l)
(b) B1 chemical energy (stored in the hydrogen and oxygen) is transferred to electrical energy, with some energy wasted as thermal energy oe
(b) B1 hydrogen is oxidised (it loses electrons) oe
(b) Answer: Chemical energy is transferred to electrical energy (with some wasted as thermal energy). Hydrogen is oxidised, losing electrons, during this reaction.
Question 5
(a) M1 one (or more) of the reactants inside the cell has been completely used up oe
(a) A1 so the chemical reaction that produces the voltage can no longer take place oe
(a) Answer: One of the reactants inside the battery has been completely used up, so the chemical reaction that produces the voltage can no longer take place.
(b) M1 an external electrical current/supply from the charger reverses the chemical reaction inside the battery oe
(b) A1 this regenerates (re-forms) the original reactants, so the battery can produce a voltage again oe
(b) Answer: The charger passes an external electrical current through the battery, reversing the chemical reaction and regenerating the original reactants, so the battery can produce a voltage again.
Question 6
(a) B1 a single unit that uses a chemical reaction between two electrodes and an electrolyte to produce a voltage (a source of electrical energy) oe
(a) Answer: A chemical cell is a single unit that uses a chemical reaction to produce a voltage.
(b) B1 two or more chemical cells connected together (in series) oe
(b) Answer: A battery is two or more chemical cells connected together.
(c) B1 a conductor (often a metal) that allows electric current to pass into or out of an electrolyte/cell oe
(c) Answer: An electrode is a conductor, often a metal, that allows electric current to pass into or out of a cell.
(d) B1 both convert chemical energy (from a chemical/redox reaction) into electrical energy oe
(d) Answer: Both a battery and a hydrogen fuel cell convert chemical energy into electrical energy through a chemical reaction.
Question 7
(a) B1 independent variable = the pair of metals used (the metal combination) oe
(a) B1 dependent variable = the voltage produced (the voltmeter reading) oe
(a) Answer: Independent variable: the pair of metals used. Dependent variable: the voltage produced (voltmeter reading).
(b) B1 the concentration of the dilute sulfuric acid (electrolyte) oe
(b) B1 the volume of electrolyte used and/or the size (surface area) of the electrodes and/or the distance apart of the electrodes (any second valid control) oe
(b) Answer: For example: the concentration of the dilute sulfuric acid used, and the size (surface area) of the electrodes.
(c) B1 1.65 V cao
(c) M1 (1.08 + 1.10) / 2 oe
(c) A1 1.09 V cao
(c) Answer: Anomalous result: 1.65 V. Mean voltage (excluding the anomaly) = (1.08 + 1.10) / 2 = 1.09 V.
(d) M1 the greater the difference in reactivity between the two metals used, the greater the voltage produced oe
(d) A1 e.g. magnesium and silver, which are furthest apart in the reactivity series, give the highest voltage (about 3.2 V); iron and copper, which are closest together, give the lowest voltage (about 0.8 V) oe
(d) Answer: The greater the difference in reactivity between the two metals, the greater the voltage produced: magnesium and silver (furthest apart in reactivity) give the highest voltage, while iron and copper (closest together) give the lowest voltage.
(e) B1 repeat each measurement more times (and/or test additional pairs of metals with a range of reactivity differences) oe
(e) Answer: Repeat each measurement more times, and test additional pairs of metals covering a wider range of reactivity differences.
(f) Answer: The symbol indicates the acid is an irritant. Fatima should wear eye protection (safety goggles) throughout the investigation.
(g) B1 vary the concentration of the sulfuric acid electrolyte in a series of steps (e.g. by diluting a stock solution), using the same zinc and copper electrodes each time oe
(g) B1 measure the voltage produced at each concentration, repeating each concentration to check for anomalies oe
(g) B1 keep constant, e.g. the volume of electrolyte used, the size/surface area of the electrodes, or the distance apart of the electrodes oe
(g) B1 plot a graph of voltage (y-axis) against concentration (x-axis) to identify the pattern/trend oe
(g) Answer: Vary the concentration of the sulfuric acid in steps (e.g. by dilution), keeping the same zinc and copper electrodes, and measure (repeating) the voltage at each concentration. Keep the volume of electrolyte and the size/distance apart of the electrodes constant. Plot a graph of voltage against concentration to identify the pattern.
Question 8
Level 1 (1-2): Simple, isolated statements are made, such as identifying that one type is rechargeable and one is not, with little or no linked explanation of cost, convenience or environmental impact.
Level 2 (3-4): A logical evaluation is given, covering some relevant factors (e.g. how the two chemistries differ, and one of cost, convenience or environmental impact), with some supporting detail, but coverage is not balanced or complete.
Level 3 (5-6): A balanced, well-linked evaluation is given, correctly explaining the difference between the reversible reaction in a rechargeable cell and the irreversible reaction in a non-rechargeable cell, and comparing cost, convenience and environmental impact, leading to a justified overall conclusion.
Indicative content:
A non-rechargeable alkaline battery uses a chemical reaction that cannot easily be reversed; once a reactant is used up, the battery must be thrown away and replaced.
A rechargeable lithium-ion battery uses a reversible reaction; connecting it to a charger passes an electrical current through it that reverses the reaction, regenerating the original reactants so it can be reused many times.
Buying replacement alkaline batteries repeatedly over several years is likely to cost more in total than the higher one-off cost of a rechargeable battery pack and charger.
Rechargeable batteries are convenient in that new batteries do not need to be bought, but less convenient in that they take time to recharge and the torch may be unusable (or need a charged spare) while charging.
Single-use alkaline batteries create ongoing waste, since large numbers must be manufactured and disposed of over the torch's lifetime, using up raw materials and energy.
Lithium-ion batteries reduce the number of batteries thrown away, but contain reactive and potentially hazardous materials, and have a finite number of charge cycles before they must eventually be replaced and recycled.
Overall, for a torch used regularly over several years, a rechargeable lithium-ion battery pack is likely to be the better financial and environmental choice, though a non-rechargeable battery may be more convenient for occasional or emergency use where recharging is not possible.
Question 9
(a) B1 breaking a bond is endothermic (energy must be supplied) oe
(a) B1 forming a bond is exothermic (energy is released) oe
(a) Answer: Breaking a covalent bond is endothermic (energy must be supplied). Forming a new covalent bond is exothermic (energy is released).
(b) M1 more energy is released when the new bonds in the products are formed than is needed to break the bonds in the reactants oe
(b) A1 so overall, energy is transferred to the surroundings (the overall energy change is negative) oe
(b) Answer: More energy is released forming the new bonds in the products than is needed to break the bonds in the reactants, so overall energy is transferred to the surroundings (the energy change is negative).
Question 10
(a) B1 629 kJ/mol (436 + 193) cao
(a) Answer: 629 kJ/mol
(b) B1 732 kJ/mol (2 x 366) cao
(b) Answer: 732 kJ/mol
(c) M1 629 - 732, ft from a and b
(c) A1 -103 kJ/mol; exothermic cao
(c) Answer: -103 kJ/mol; exothermic
(d) B1 because more energy is released forming the H-Br bonds (732 kJ/mol) than is needed to break the H-H and Br-Br bonds in the reactants (629 kJ/mol) oe
(d) Answer: More energy is released forming the H-Br bonds (732 kJ/mol) than is needed to break the H-H and Br-Br bonds (629 kJ/mol), so overall energy is released.
Question 11
(a) B1 no carbon dioxide (or other polluting exhaust gases) is produced at the point of use - only water is produced oe
(a) Answer: No carbon dioxide or other polluting gases are produced at the point of use - only water.
(b) B1 refuelling with hydrogen takes only a few minutes, much faster than recharging a battery oe
(b) Answer: Refuelling with hydrogen takes only a few minutes, much faster than recharging a battery.
(c) B1 there are very few hydrogen refuelling stations, so drivers cannot rely on being able to refuel easily oe
(c) B1 hydrogen is difficult and expensive to store and transport safely, since it is flammable and has a very low density (it must be compressed or cooled) oe
(c) Answer: There are very few hydrogen refuelling stations in the UK, and hydrogen is difficult, expensive and potentially hazardous to store and transport, since it is flammable and has a very low density.
(d) M1 producing hydrogen from natural gas releases carbon dioxide during production, even though the fuel cell itself only produces water oe
(d) A1 so the overall environmental impact of a hydrogen fuel-cell vehicle depends on how the hydrogen was produced, not just on how it is used in the vehicle oe
(d) Answer: Producing hydrogen from natural gas releases carbon dioxide, even though the fuel cell itself only produces water, so the true environmental benefit depends on how the hydrogen was produced, not just how it is used.
(b) M1 2 mol H2O contains 4 O-H bonds in total, so 4 x 463 oe
(b) A1 1852 kJ/mol cao
(b) Answer: 1852 kJ/mol
(c) M1 1370 - 1852, ft from a and b
(c) A1 -482 kJ/mol; exothermic cao
(c) Answer: -482 kJ/mol; exothermic
(d) B1 a large amount of energy is released (and transferred electrically) for every mole of hydrogen used, giving the fuel cell a high energy output per mole of fuel oe
(d) Answer: A large amount of energy is released per mole of hydrogen used, giving the fuel cell a high energy output for the fuel it consumes.
Question 13
(a) B1 241 kJ (482 / 2) cao
(a) Answer: 241 kJ
(b) M1 6 x 241 oe
(b) A1 1446 kJ cao
(b) Answer: 1446 kJ
(c) M1 1446 x 0.6 oe
(c) A1 867.6 kJ cao (accept 868 kJ)
(c) Answer: 867.6 kJ
Question 14
(a) M1 945 + (3 x 436) oe
(a) A1 2253 kJ/mol cao
(a) Answer: 2253 kJ/mol
(b) M1 2 mol NH3 contains 6 N-H bonds in total, so 6 x 391 oe
(b) A1 2346 kJ/mol cao
(b) Answer: 2346 kJ/mol
(c) M1 2253 - 2346, ft from a and b
(c) A1 -93 kJ/mol; exothermic cao
(c) Answer: -93 kJ/mol; exothermic
Question 15
(a) M1 bonds broken = 945 + 498 = 1443 oe
(a) M1 bonds made = 2 x 630 = 1260 oe
(a) A1 +183 kJ/mol cao
(a) Answer: +183 kJ/mol
(b) M1 endothermic (positive energy change), ft from a
(b) A1 at the high temperatures inside a car engine, particles collide more frequently and with more energy, so more collisions have enough energy to overcome the activation energy needed for this endothermic reaction oe
(b) Answer: Endothermic. At the high temperatures inside a car engine, particles collide more often and with more energy, so enough collisions have sufficient energy to overcome the activation energy needed - this does not happen at room temperature.
(c) M1 adds the two half equations and cancels the 4e-, the 4OH- and 2 of the 4 mol H2O common to both sides oe
(c) A1 2H2 + O2 -> 2H2O cao
(c) Answer: 2H2 + O2 -> 2H2O
Question 17
(a) B1 588 kJ/mol (436 + 152) cao
(a) Answer: 588 kJ/mol
(b) M1 rearranges to find energy released forming bonds: 588 - 52 = 536 kJ/mol oe
(b) M1 recognises 536 kJ/mol corresponds to 2 mol of H-I bonds (in 2 mol HI), so divides by 2 oe
(b) A1 268 kJ/mol cao
(b) Answer: 268 kJ/mol
(c) B1 because the energy needed to break the bonds in the reactants (588 kJ/mol) is greater than the energy released forming the bonds in the products (536 kJ/mol), so overall energy is absorbed from the surroundings oe
(c) Answer: The energy needed to break the bonds in the reactants (588 kJ/mol) is greater than the energy released forming the bonds in the products (536 kJ/mol), so overall energy is absorbed from the surroundings - the reaction is endothermic.
Question 18
(a) B1 6 C-H bonds (2 + 1 + 3, from the hydrogen atom counts shown) oe
(a) B1 1 C-C bond and 1 C=C bond oe
(a) B1 1 H-H bond (from the H2 molecule) oe
(a) Answer: 6 C-H bonds, 1 C-C bond, 1 C=C bond and 1 H-H bond.