Chemical reactions can be exothermic or endothermic, depending on the direction of energy transfer between the reacting chemicals and the surroundings.
(a)Which one of these is an example of an exothermic reaction?(1)
A) photosynthesis
B) thermal decomposition of calcium carbonate
C) combustion of methane
D) dissolving ammonium nitrate in water
(b)State the direction of energy transfer between the reacting chemicals and the surroundings during an endothermic reaction.(1)
(c)Give one everyday example of an exothermic reaction and one everyday example of an endothermic reaction, other than combustion.(2)
(Total for Question 1 is 4 marks)
2
Self-heating cans of coffee use an exothermic reaction (e.g. between calcium oxide and water) to warm the drink. Instant cold packs used to treat sports injuries use an endothermic reaction (e.g. ammonium nitrate dissolving in water).
(a)Explain, in terms of energy transfer, how the reaction in a self-heating can warms the drink.(2)
(b)Explain, in terms of energy transfer, why the cold pack feels cold when it is activated.(2)
(Total for Question 2 is 4 marks)
3
In the required practical to investigate temperature changes in reacting solutions, a student uses dilute hydrochloric acid. The bottle is labelled with a hazard symbol showing a black exclamation mark on an orange background.
(a)Name the hazard indicated by this hazard symbol.(1)
(b)State one safety precaution the student should take when using dilute hydrochloric acid in this practical.(1)
(c)Explain why it is important for the student to check hazard information before starting the practical.(1)
(Total for Question 3 is 3 marks)
4
The diagram shows a reaction profile for an exothermic reaction.
(a)State, giving a reason from the diagram, whether the reaction is exothermic or endothermic.(2)
(b)Calculate the overall energy change for this reaction.(2)
(c)State the activation energy shown on the diagram.(1)
(Total for Question 4 is 5 marks)
5
A different reaction has the following reaction profile.
(a)State whether this reaction is exothermic or endothermic.(1)
(b)Calculate the activation energy for this reaction.(2)
(c)A catalyst is added to this reaction. State the effect, if any, that the catalyst has on the activation energy and on the overall energy change.(1)
(Total for Question 5 is 4 marks)
6
A student investigates how the concentration of hydrochloric acid affects the temperature change when it reacts with an excess of sodium hydroxide solution (a neutralisation reaction).
(a)Identify the independent variable in this investigation.(1)
(b)Identify the dependent variable in this investigation.(1)
(c)State two variables that should be controlled to make this a fair test.(2)
(Total for Question 6 is 4 marks)
7
The table shows a student's results for the temperature change when hydrochloric acid reacts with sodium hydroxide solution.
(a)Identify the anomalous result.(1)
(b)Suggest one possible reason for this anomalous result.(1)
(c)Calculate the mean temperature change, excluding the anomalous result.(2)
(d)Suggest one improvement to the method that would reduce heat loss to the surroundings.(1)
(Total for Question 7 is 5 marks)
8
A student plots a graph of temperature (y-axis) against time (x-axis) for a reaction between magnesium and dilute sulfuric acid. The temperature starts at 20 C, rises steadily and reaches a peak of 34 C at 50 seconds, then gradually falls as the mixture cools.
(a)State the maximum temperature reached during the reaction.(1)
(b)Calculate the temperature change during the reaction.(1)
(c)Explain why the temperature decreases after the peak at 50 seconds.(1)
(d)Suggest one way the student could get a more accurate value for the maximum temperature change.(1)
(Total for Question 8 is 4 marks)
9
Explain, in terms of energy transfers, why breaking bonds is endothermic and forming new bonds is exothermic, and how these two energy transfers determine whether a reaction is exothermic or endothermic overall.
(Total for Question 9 is 3 marks)
10
A simple chemical cell can be made by placing a zinc electrode and a copper electrode into an electrolyte (e.g. dilute sulfuric acid) and connecting them with wires and a voltmeter.
(a)Explain how this cell produces a voltage (potential difference).(2)
(b)Predict, giving a reason, how the voltage would change if the zinc electrode were replaced with a magnesium electrode.(2)
(Total for Question 10 is 4 marks)
11
Batteries contain chemical cells connected together. Some batteries are non-rechargeable and some are rechargeable.
(a)State one difference between how a non-rechargeable battery and a rechargeable battery behave once fully discharged.(2)
(b)Give one advantage and one disadvantage of using rechargeable batteries instead of non-rechargeable batteries.(2)
(Total for Question 11 is 4 marks)
12
Hydrogen fuel cells are used to generate electricity in some vehicles. The overall reaction in a hydrogen fuel cell is between hydrogen and oxygen, producing water.
(a)Write the balanced symbol equation for the overall reaction in a hydrogen fuel cell.(2)
(b)State one energy transfer that takes place inside a hydrogen fuel cell.(1)
(c)Give one advantage and one disadvantage of using hydrogen fuel cells instead of rechargeable batteries to power a vehicle.(2)
(Total for Question 12 is 5 marks)
13
In a hydrogen fuel cell, hydrogen is oxidised at the negative electrode.
(a)Write the half equation for the reaction that occurs at the negative electrode of a hydrogen fuel cell.(2)
(b)State whether this half equation represents oxidation or reduction, giving a reason.(1)
(Total for Question 13 is 3 marks)
14
Hydrogen reacts with chlorine to form hydrogen chloride: H2 + Cl2 -> 2HCl. Bond energies: H-H = 436 kJ/mol, Cl-Cl = 242 kJ/mol, H-Cl = 431 kJ/mol.
(a)Calculate the energy needed to break all the bonds in the reactants.(1)
(b)Calculate the 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)
(Total for Question 14 is 4 marks)
15
A car manufacturer is deciding whether to develop its new electric car using a rechargeable lithium-ion battery or a hydrogen fuel cell. Lithium-ion battery: range per charge = 300 km; time to refill/recharge = 35 minutes (rapid charger) or 8 hours (home charger); charging infrastructure = widespread (many public and home chargers); CO2 released while driving = none. Hydrogen fuel cell: range per fill = 500 km; time to refuel = 5 minutes; refuelling infrastructure = very limited (few hydrogen stations); CO2 released while driving = none (only water produced). Evaluate the advantages and disadvantages of using a rechargeable lithium-ion battery compared with a hydrogen fuel cell to power the car. Use the information given and your own knowledge of both technologies.
(Total for Question 15 is 6 marks)
16
Methane burns in oxygen: CH4 + 2O2 -> CO2 + 2H2O. Bond energies: C-H = 413 kJ/mol, O=O = 498 kJ/mol, C=O = 805 kJ/mol, O-H = 463 kJ/mol.
(a)Calculate the total energy needed to break all the bonds in the reactants.(2)
(b)Calculate the total energy released when the new bonds form in the products.(2)
(c)Calculate the overall energy change for the combustion of methane.(1)
(Total for Question 16 is 5 marks)
17
A reversible reaction has a forward activation energy of 95 kJ/mol. Taking the energy level of the reactants as 0 kJ, the overall energy change for the forward reaction is -55 kJ/mol.
(a)State the energy level of the products, relative to the reactants.(1)
(b)Calculate the activation energy for the reverse reaction.(2)
(c)State the effect that adding a catalyst would have on the activation energy calculated in part (b).(1)
(Total for Question 17 is 4 marks)
18
Hydrogen reacts with fluorine: H2 + F2 -> 2HF. The overall energy change for this reaction is -540 kJ/mol. Bond energies: H-H = 436 kJ/mol, F-F = 158 kJ/mol.
(a)Calculate the total energy needed to break the bonds in the reactants.(1)
(b)Show that the mean bond energy of the H-F bond is 567 kJ/mol.(3)
(Total for Question 18 is 4 marks)
19
A student adds a catalyst to a reversible reaction taking place in a sealed container.
(a)Explain, in terms of bonds, why adding a catalyst does not change the overall energy change of the reaction.(2)
(b)Describe how the shape of the reaction profile changes when a catalyst is used, compared with the uncatalysed reaction.(2)
(Total for Question 19 is 4 marks)
Mark scheme · C5 Energy Changes
Question 1
(a) B1 C - combustion of methane cao
(a) Answer: C
(b) B1 energy transferred from the surroundings to the chemicals (energy taken in) oe
(b) Answer: Energy is transferred from the surroundings to the reacting chemicals.
(c) B1 exothermic example, e.g. hand warmer / neutralisation / respiration / setting of an exothermic adhesive oe
(c) B1 endothermic example, e.g. sports cold pack / thermal decomposition of a metal carbonate / citric acid with sodium hydrogencarbonate oe
(c) Answer: Exothermic: a self-heating hand warmer (or neutralisation, respiration). Endothermic: a sports cold pack (or thermal decomposition of a metal carbonate).
Question 2
(a) M1 the reaction is exothermic oe
(a) A1 energy is transferred to the surroundings (the drink), increasing its temperature oe
(a) Answer: The reaction is exothermic, so it transfers energy to the surroundings (the drink), causing its temperature to increase.
(b) M1 the dissolving process is endothermic oe
(b) A1 energy is transferred from the surroundings (skin/injury) to the chemicals, decreasing the surroundings' temperature oe
(b) Answer: The dissolving process is endothermic, so it takes in energy from the surroundings (the skin and injury), lowering their temperature and making the pack feel cold.
(c) B1 so appropriate precautions can be taken to reduce the risk of harm/injury oe
(c) Answer: So that the correct precautions can be taken to reduce the risk of harm or injury while carrying out the practical.
Question 4
(a) M1 exothermic
(a) A1 because the products (160 kJ) have less energy than the reactants (240 kJ) oe
(a) Answer: Exothermic, because the products have less energy (160 kJ) than the reactants (240 kJ).
(b) M1 160 - 240 oe
(b) A1 -80 kJ (accept 80 kJ released) cao
(b) Answer: -80 kJ (80 kJ released per mole of reaction)
(c) B1 60 kJ (300 - 240) cao
(c) Answer: 60 kJ
Question 5
(a) B1 endothermic
(a) Answer: Endothermic
(b) M1 260 - 150 oe
(b) A1 110 kJ cao
(b) Answer: 110 kJ
(c) B1 activation energy decreases; overall energy change is unaffected oe
(c) Answer: The activation energy decreases (the catalyst provides a pathway with a lower activation energy); the overall energy change is unaffected.
Question 6
(a) B1 concentration of the hydrochloric acid
(a) Answer: The concentration of the hydrochloric acid.
(b) B1 the (maximum) temperature change of the reaction mixture
(b) Answer: The temperature change (temperature rise) of the reaction mixture.
(c) B1 volume of sodium hydroxide solution used oe
(c) B1 starting temperature of the solutions / same insulated equipment used / same concentration of sodium hydroxide oe (any second valid control variable)
(c) Answer: For example: the volume of sodium hydroxide solution used, and the starting temperature of both solutions (also accept: same insulated container, same concentration of sodium hydroxide).
(b) Answer: For example, the thermometer may have been misread, or the solutions may not have mixed/reacted fully before the temperature was recorded.
(c) M1 (6.2 + 6.4) / 2, excluding 11.9 oe
(c) A1 6.3 C cao
(c) Answer: 6.3 C
(d) B1 use a lid on the insulated container / use a polystyrene cup with a lid oe
(d) Answer: Use an insulated container with a lid (e.g. a polystyrene cup with a lid and a hole for the thermometer).
Question 8
(a) B1 34 C cao
(a) Answer: 34 C
(b) B1 14 C (34 - 20) cao
(b) Answer: 14 C
(c) B1 the reaction has finished (no more energy is being released) and the mixture loses energy (heat) to the cooler surroundings oe
(c) Answer: The reaction has finished, so no more energy is being released; the warm mixture then loses energy (heat) to the cooler surroundings, so it cools down.
(d) B1 extrapolate the cooling part of the graph back to the time the reactants were mixed oe; or use a data logger for continuous, more frequent readings
(d) Answer: Extrapolate the falling (cooling) part of the graph back to the time the reactants were first mixed, to estimate the true peak temperature (correcting for heat already lost before 50 s).
Question 9
B1 breaking bonds requires energy to be supplied/taken in from the surroundings, so it is endothermic oe
B1 forming (making) new bonds releases/transfers energy to the surroundings, so it is exothermic oe
B1 whether the overall reaction is exothermic or endothermic depends on the balance/difference between the energy needed to break bonds and the energy released forming bonds oe
Answer: Breaking bonds takes in energy from the surroundings (endothermic); forming new bonds releases energy to the surroundings (exothermic). The overall reaction is exothermic if more energy is released forming bonds than is needed to break them, and endothermic if more energy is needed to break bonds than is released forming them.
Question 10
(a) M1 chemical reactions occur at the two different metal electrodes oe
(a) A1 this produces a difference in electrical potential between the electrodes, driving electrons through the external circuit oe
(a) Answer: Chemical reactions take place at each electrode; because the two metals are different, they react to different extents, producing a difference in electrical potential between them, which drives electrons through the external circuit (wire) - a voltage.
(b) M1 voltage increases oe
(b) A1 because magnesium is more reactive than zinc (further from copper in the reactivity series), giving a bigger difference in reactivity between the two metals oe
(b) Answer: The voltage would increase, because magnesium is more reactive than zinc, so it is further away from copper in the reactivity series; a bigger difference in reactivity between the two electrodes produces a bigger voltage.
Question 11
(a) M1 a non-rechargeable battery cannot be reused because its chemical reaction cannot easily be reversed oe
(a) A1 a rechargeable battery's chemical reaction can be reversed by supplying electrical energy (charging), so it can be reused many times oe
(a) Answer: A non-rechargeable battery's chemical reaction cannot be reversed, so once discharged it must be thrown away/replaced. A rechargeable battery's reaction can be reversed by supplying electrical energy (charging), so it can be reused many times.
(b) B1 advantage, e.g. cheaper in the long term / less waste produced / fewer batteries need to be manufactured oe
(b) B1 disadvantage, e.g. takes time to recharge / more expensive to buy initially / voltage falls as it discharges / limited number of charge cycles oe
(b) Answer: Advantage: cheaper overall and less waste, since the battery can be reused many times instead of being thrown away. Disadvantage: it takes time to recharge (and may be more expensive to buy initially).
Question 12
(a) M1 correct formulae: H2, O2, H2O
(a) A1 correctly balanced: 2H2 + O2 -> 2H2O cao
(a) Answer: 2H2 + O2 -> 2H2O
(b) B1 chemical energy (store) is transferred to electrical energy (and some to thermal energy) oe
(b) Answer: Chemical energy (stored in the hydrogen and oxygen) is transferred to electrical energy (with some wasted as thermal energy).
(c) B1 advantage, e.g. only water is produced at the point of use / can be refuelled much faster than batteries can be recharged / greater range per fill oe
(c) B1 disadvantage, e.g. hydrogen is difficult/expensive to store and transport / few hydrogen refuelling stations exist / producing hydrogen may itself release CO2 oe
(c) Answer: Advantage: only water is produced at the point of use, and the vehicle can be refuelled in a few minutes. Disadvantage: hydrogen is difficult and expensive to store safely, and there are very few hydrogen refuelling stations available.
(a) Answer: 2H2 -> 4H+ + 4e- (or the simplest form H2 -> 2H+ + 2e-)
(b) B1 oxidation, because electrons are lost oe
(b) Answer: Oxidation, because hydrogen atoms lose electrons to form H+ ions.
Question 14
(a) B1 678 kJ/mol (436 + 242) cao
(a) Answer: 678 kJ/mol
(b) B1 862 kJ/mol (2 x 431) cao
(b) Answer: 862 kJ/mol
(c) M1 678 - 862, ft from a and b oe
(c) A1 -184 kJ/mol; exothermic, because more energy is released forming bonds than is needed to break them cao
(c) Answer: -184 kJ/mol; exothermic
Question 15
Level 1 (1-2): Basic, isolated statements are made about the battery or the fuel cell (or both), with little or no linkage or comparison between them. No reasoned conclusion is given.
Level 2 (3-4): Some relevant comparisons are made between the battery and the fuel cell, using at least two factors (e.g. range, refuelling/recharging time, infrastructure). A conclusion is given but with limited justification.
Level 3 (5-6): A detailed and logically structured evaluation compares the battery and the fuel cell across several factors (e.g. range, refuelling/recharging time, infrastructure, wider environmental impact), using the given information and own knowledge. A justified conclusion is reached.
Indicative content:
Battery: produces no CO2 while driving, but has a shorter range (300 km) than the fuel cell (500 km).
Battery: rapid recharging still takes much longer (35 minutes) than refuelling a fuel cell (5 minutes); home charging takes even longer (8 hours).
Battery: charging infrastructure is already widespread, making it more practical to use now.
Fuel cell: produces no CO2 at the point of use (only water), and can be refuelled almost as quickly as a petrol car.
Fuel cell: has a longer range than the battery, useful for long journeys.
Fuel cell: hydrogen refuelling infrastructure is currently very limited, making the car impractical for most drivers.
Wider issues: producing hydrogen (e.g. from natural gas) may release CO2 unless made using renewable electricity (electrolysis); mining lithium and disposing of batteries also has environmental impacts.
A valid conclusion should weigh these points, e.g. the battery is currently more practical because of existing charging infrastructure, even though the fuel cell has a longer range and faster refuelling.
Question 16
(a) M1 (4 x 413) + (2 x 498) oe
(a) A1 2648 kJ/mol cao
(a) Answer: 2648 kJ/mol
(b) M1 (2 x 805) + (4 x 463) oe
(b) A1 3462 kJ/mol cao
(b) Answer: 3462 kJ/mol
(c) B1 -814 kJ/mol (2648 - 3462), ft from a and b cao
(c) Answer: -814 kJ/mol
Question 17
(a) B1 -55 kJ (i.e. 55 kJ below the reactants) cao
(a) Answer: -55 kJ (55 kJ below the reactants)
(b) M1 peak energy (95) - products energy (-55), ft from a oe
(b) A1 150 kJ/mol cao
(b) Answer: 150 kJ/mol
(c) B1 it would decrease (lower) the activation energy oe
(c) Answer: It would decrease (lower) the activation energy for the reverse reaction, as the catalyst provides an alternative reaction pathway.
Question 18
(a) B1 594 kJ/mol (436 + 158) cao
(a) Answer: 594 kJ/mol
(b) M1 sets up overall energy change = energy to break bonds - energy released forming bonds: -540 = 594 - (2 x H-F) oe
(b) M1 rearranges to 2 x H-F = 594 + 540 (= 1134), ft from a oe
(b) A1 H-F = 567 kJ/mol cso (answer given; working must be shown)
(b) Answer: 567 kJ/mol (as given)
Question 19
(a) M1 the catalyst does not change the reactants or the products, so exactly the same bonds are broken and formed oe
(a) A1 the energy needed to break bonds and the energy released forming bonds are therefore unchanged, so the overall energy change (their difference) is unchanged oe
(a) Answer: The catalyst does not change the identity of the reactants or products, so the same bonds are broken and formed either way. As the energy to break bonds and the energy released forming bonds are unchanged, the overall energy change (their difference) stays the same.
(b) M1 the peak (activation energy) of the curve is lower with the catalyst oe
(b) A1 the energy levels of the reactants and products (start and end of the curve) stay the same oe
(b) Answer: The curve reaches a lower peak (a lower activation energy) when the catalyst is used, but the energy levels of the reactants and the products remain exactly the same as without the catalyst.