ESAT Chemistry: Acids, rates and energetics, set 2

• Answer all questions. No calculator.
• Each question has exactly one correct answer.

1. 11 mark

   Zinc metal is added to dilute sulfuric acid.

   Which set of products correctly results from this reaction?

   1. A Zinc sulfate, water and carbon dioxide.
   2. B Zinc sulfate and hydrogen gas.
   3. C Zinc oxide and sulfur dioxide.
   4. D Zinc sulfite and hydrogen.
2. 21 mark

   Sulfur trioxide, SO3, and calcium oxide, CaO, are each added, separately, to water.

   Which statement correctly describes the solutions formed?

   1. A Sulfur trioxide forms an alkaline solution and calcium oxide forms an acidic solution.
   2. B Both oxides form acidic solutions, since both are oxides.
   3. C Neither oxide reacts with water; oxides only react with acids or bases, never with water itself.
   4. D Sulfur trioxide forms an acidic solution and calcium oxide forms an alkaline solution.
3. 31 mark

   Sulfuric acid, H2SO4, is a diprotic acid: each molecule can donate up to two H+ ions.

   A solution contains 0.35 mol of sulfuric acid, and every acidic hydrogen atom is assumed to be donated. How many moles of H+ ions does this produce?

   1. A 0.7 mol
   2. B 0.35 mol
   3. C 1.05 mol
   4. D 0.175 mol
4. 41 mark

   The H+ ion concentration of a solution is increased by a factor of 10,000 (a factor of 10^4).

   By how many pH units does the pH change, and in which direction?

   1. A The pH increases by 4 units.
   2. B The pH decreases by 10,000 units.
   3. C The pH decreases by 4 units.
   4. D The pH increases by 10,000 units.
5. 51 mark

   Equal concentrations of hydrochloric acid, a strong acid, and ethanoic acid, a weak acid, are prepared separately.

   Which statement correctly compares the pH of the two solutions?

   1. A The two solutions have the same pH, since both contain the same concentration of dissolved acid.
   2. B The hydrochloric acid solution has a lower pH than the ethanoic acid solution, because hydrochloric acid fully ionises in water to release H+ ions, while only a small proportion of the ethanoic acid molecules ionise to release H+ ions.
   3. C The ethanoic acid solution has a lower pH than the hydrochloric acid solution, because ethanoic acid is the stronger of the two acids.
   4. D The two solutions cannot be prepared at equal concentration, since a weak acid can only be made up to a lower maximum concentration than a strong acid.
6. 61 mark

   Which one of the following reactions is a neutralisation reaction and is typically exothermic?

   1. A Potassium hydroxide solution reacting with dilute nitric acid to form potassium nitrate and water.
   2. B The thermal decomposition of calcium carbonate: CaCO3 -> CaO + CO2.
   3. C The electrolysis of molten lead bromide to form lead and bromine.
   4. D Magnesium burning in oxygen to form magnesium oxide.
7. 71 mark

   Nitrogen gas and hydrogen gas react together inside a sealed, rigid container to form ammonia gas.

   Which one of these changes would NOT increase the rate of this reaction?

   1. A Increasing the temperature of the gases.
   2. B Compressing the same amount of gas into a smaller container, increasing the pressure.
   3. C Adding a catalyst.
   4. D Transferring the same amounts of gas into a much larger container, lowering the pressure.
8. 81 mark

   Bromine water, which is orange, is added to an excess of aqueous sodium sulfite. Bromine reacts with sulfite ions, and every substance present in the mixture, once the reaction is complete, is colourless, with no gas and no precipitate formed at any point.

   Which method would best allow a student to measure the rate of this reaction over time?

   1. A Collecting any gas produced in a gas syringe and reading its volume at regular time intervals.
   2. B Timing how long it takes for a mark on paper beneath the reaction flask to be obscured by a forming precipitate.
   3. C Measuring the colour intensity of the mixture at regular time intervals using a colorimeter, since the orange colour fades as the reaction proceeds.
   4. D Weighing the reaction flask at regular time intervals, since the reaction involves a colour change.
9. 91 mark

   A student reacts excess marble chips with dilute hydrochloric acid twice, using the same mass of marble chips and the same concentration and volume of acid each time, but at two different temperatures. Both experiments are plotted as volume of carbon dioxide gas against time, on the same axes. Both curves rise and then level off at the same final volume, but Curve 1 rises more steeply at the start and levels off sooner than Curve 2.

   Which statement correctly interprets these two graphs?

   1. A Curve 1 corresponds to the higher-temperature experiment, since a higher temperature increases the rate of reaction without changing the total amount of gas produced.
   2. B Curve 1 corresponds to the lower-temperature experiment, since a lower temperature means less energy is wasted and so the reaction finishes faster.
   3. C Since both curves level off at the same final volume, the two experiments must have been carried out at the same temperature.
   4. D Curve 1 shows that more gas was produced overall in that experiment, since a steeper gradient always means a greater final volume.
10. 101 mark

    A student investigates a reaction catalysed by a solid catalyst. At the end of the reaction, the catalyst is filtered out, dried and weighed; its mass is unchanged from the start, and its chemical composition is unchanged too.

    Which statement correctly explains both this observation and how the catalyst increased the rate of the reaction, using collision theory?

    1. A The catalyst is chemically unchanged because it does not actually take part in the reaction at all; it increases the rate simply by raising the temperature of the reaction mixture.
    2. B The catalyst is chemically unchanged and not used up because it only provides an alternative reaction pathway with a lower activation energy, increasing the proportion of collisions that have enough energy to react, without being consumed by the reaction itself.
    3. C The catalyst's mass is unchanged because it reacts completely and is then instantly reformed in a separate step; it increases the rate by increasing the concentration of the reactants.
    4. D The catalyst's mass is unchanged because only a tiny, immeasurable amount of it is ever used up; it increases the rate by increasing the frequency of collisions between reacting particles.
11. 111 mark

    The combustion of methane gas is strongly exothermic overall, yet a spark or flame is needed to start the reaction; methane and oxygen can be mixed at room temperature without reacting at a noticeable rate.

    Which statement about the energy profile diagram for this reaction correctly explains this observation?

    1. A The reaction cannot really be exothermic, since an exothermic reaction should start without needing any external energy input at all.
    2. B The activation energy of this reaction must be very low, since once started, the reaction proceeds and releases energy very quickly.
    3. C The spark provides the overall energy released by the reaction, without which the reaction could not release any energy at all.
    4. D The reaction has a high activation energy, so very few room-temperature collisions between methane and oxygen particles have enough energy to react, even though the products are at a much lower energy than the reactants overall.
12. 121 mark

    An instant hand warmer sachet contains iron powder. When the sachet is opened and the iron is exposed to air, it reacts slowly with oxygen, and the sachet becomes noticeably warm to the touch.

    What does this observation show about the enthalpy change, delta H, for this reaction?

    1. A delta H is positive, and the reaction is exothermic, since energy has been released to the surroundings.
    2. B delta H is negative, and the reaction is endothermic, since heat has been felt leaving the sachet.
    3. C delta H is negative, and the reaction is exothermic, since energy has been released to the surroundings, raising their temperature.
    4. D delta H cannot be determined from this observation alone, since the sign of delta H depends only on the physical states of the reactants and products, not on any temperature change.
13. 131 mark

    A reversible reaction's forward direction is endothermic. A student claims: 'Since energy has to be put in to make the forward reaction happen, the reverse reaction must also need energy put in, because reversing a reaction does not create or destroy energy.'

    Is the student's reasoning correct?

    1. A No, because reversing an endothermic reaction gives an exothermic reaction: the energy absorbed going one way is exactly the energy released going the other way, so total energy is still conserved.
    2. B Yes, both directions require an input of energy, since energy cannot be created or destroyed.
    3. C No, because the reverse reaction is also endothermic, but requires a different, unrelated amount of energy input.
    4. D Yes, and additionally the size of the energy change must double for the reverse reaction, since it has to undo the entire forward change.
14. 141 mark

    In a calorimetry experiment, a reaction transfers 840 J of energy to a sample of water, raising its temperature by 10 degrees C. The specific heat capacity of water is 4.2 J per gram per degree C.

    Calculate the mass of water used in this experiment.

    1. A 84 g
    2. B 200 g
    3. C 0.05 g
    4. D 20 g
15. 151 mark

    Hydrogen gas reacts with chlorine gas to form hydrogen chloride gas: H2 + Cl2 -> 2HCl. The bond energy of the H-H bond is 436 kJ/mol, the bond energy of the Cl-Cl bond is 243 kJ/mol, and the bond energy of the H-Cl bond is 432 kJ/mol.

    Calculate the overall energy change for this reaction, and state whether it is exothermic or endothermic.

    1. A +185 kJ/mol; the reaction is endothermic.
    2. B -185 kJ/mol; the reaction is exothermic.
    3. C +247 kJ/mol; the reaction is endothermic.
    4. D +1111 kJ/mol; the reaction is endothermic.

Worked solutions

Every question below carries the reasoning, not just the answer. The official material for this test publishes a correct option letter and nothing else.

1. Question 1Answer: B

   1. A reactive metal reacts with a dilute acid to form a salt and hydrogen gas.
   2. Zinc is a reactive metal and sulfuric acid is a dilute acid here, so the products are a salt, zinc sulfate, and hydrogen gas: Zn + H2SO4 -> ZnSO4 + H2.
   3. This is different from a carbonate reacting with an acid, which instead produces a salt, water and carbon dioxide, since zinc is a metal, not a carbonate.
   4. So the correct products are zinc sulfate and hydrogen gas, option B.
   • Why not A: Confuses the reaction of an acid with a reactive metal with the reaction of an acid with a carbonate, which produces a salt, water and carbon dioxide instead of a salt and hydrogen.
   • Why not C: Invents a decomposition-type reaction with an oxide and sulfur dioxide, when zinc metal reacting with dilute sulfuric acid is a simple metal-acid reaction producing a salt and hydrogen, not an oxide.
   • Why not D: Confuses the naming of the salt formed from sulfuric acid: reacting a metal with sulfuric acid gives a sulfate salt (zinc sulfate), not a sulfite salt.

2. Question 2Answer: D

   1. Some oxides of non-metals react with water to form acidic solutions, and some oxides of metals react with water to form alkaline solutions.
   2. Sulfur trioxide is the oxide of a non-metal, sulfur, so it reacts with water to form sulfuric acid, an acidic solution: SO3 + H2O -> H2SO4.
   3. Calcium oxide is the oxide of a metal, calcium, so it reacts with water to form calcium hydroxide, an alkaline solution: CaO + H2O -> Ca(OH)2.
   4. So sulfur trioxide gives an acidic solution and calcium oxide gives an alkaline solution, option D.
   • Why not A: Swaps which oxide is which: it is the non-metal oxide, sulfur trioxide, that reacts with water to form an acidic solution, and the metal oxide, calcium oxide, that reacts with water to form an alkaline solution, not the other way round.
   • Why not B: Wrongly treats every oxide the same regardless of whether the parent element is a metal or a non-metal, when it is precisely this difference that determines whether the resulting solution is acidic or alkaline.
   • Why not C: Wrongly claims oxides never react with water directly; some oxides, including both of these, react directly with water without needing a separate acid or base present.

3. Question 3Answer: A

   1. A diprotic acid can donate up to two H+ ions from each molecule, so the moles of H+ ions produced is twice the moles of acid, provided every acidic hydrogen is donated.
   2. Here there is 0.35 mol of sulfuric acid.
   3. Moles of H+ = 0.35 x 2 = 0.7 mol.
   4. So this solution produces 0.7 mol of H+ ions, option A.
   • Why not B: Treats sulfuric acid as monoprotic, donating only one H+ per molecule, ignoring that it is diprotic.
   • Why not C: Treats sulfuric acid as triprotic, donating three H+ ions per molecule, as if it behaved like phosphoric acid instead.
   • Why not D: Halves the number of moles of acid instead of doubling it, as if each pair of acid molecules donated only one H+ ion between them.

4. Question 4Answer: C

   1. A change of 1 on the pH scale corresponds to a change in H+ ion concentration by a factor of 10.
   2. Here the H+ ion concentration increases by a factor of 10,000 = 10^4, so the pH changes by 4 units.
   3. Increasing the H+ ion concentration makes a solution more acidic, so the pH must decrease, not increase.
   4. So the pH decreases by 4 units, option C.
   • Why not A: Correctly finds a change of 4 pH units but gets the direction backwards: increasing the H+ ion concentration makes a solution more acidic, so its pH must fall, not rise.
   • Why not B: Mistakes the factor itself, 10,000, for the number of pH units by which the value changes, rather than converting that factor into a power of ten.
   • Why not D: Makes the same error as option B, using the factor of 10,000 directly as the number of pH units, and additionally gets the direction of the change backwards.

5. Question 5Answer: B

   1. Strength describes what proportion of an acid's molecules ionise in water to release H+ ions, while concentration describes how much of the acid substance is dissolved; the two are independent.
   2. Hydrochloric acid is a strong acid, so it fully ionises in water, releasing one H+ ion per molecule dissolved.
   3. Ethanoic acid is a weak acid, so at the same concentration only a small proportion of its molecules ionise to release H+ ions, giving a lower H+ concentration overall.
   4. A lower H+ concentration means a higher pH, so hydrochloric acid, with the higher H+ concentration, has the lower pH, option B.
   • Why not A: Confuses concentration (how much acid is dissolved) with strength (what proportion of it ionises), treating the two independent properties as the same thing.
   • Why not C: Swaps which of the two acids is the strong one: hydrochloric acid is the strong acid here, fully ionising, while ethanoic acid is the weak one that only partially ionises.
   • Why not D: Wrongly believes strength limits how concentrated a solution can be made; a weak acid can be dissolved at any concentration, including one equal to a strong acid's, it will simply ionise less fully at that concentration.

6. Question 6Answer: A

   1. A neutralisation reaction is a reaction between an acid and a base, and is often exothermic.
   2. Potassium hydroxide, a base, reacting with dilute nitric acid, an acid, to form a salt (potassium nitrate) and water is exactly this type of reaction, and it releases energy as it proceeds.
   3. The other three reactions are all real reaction types, decomposition, electrolysis and combustion, but none of them is a reaction between an acid and a base, so none of them counts as a neutralisation, even though combustion is also exothermic.
   4. So the neutralisation reaction here is potassium hydroxide with nitric acid, option A.
   • Why not B: Thermal decomposition of a carbonate is endothermic, not exothermic, and is not a reaction between an acid and a base, so it is not a neutralisation reaction either.
   • Why not C: Electrolysis requires a continuous input of electrical energy to proceed and is not a reaction between an acid and a base, so it is neither exothermic in this sense nor a neutralisation.
   • Why not D: Magnesium burning in oxygen is a genuinely exothermic reaction, but it is a combustion (oxidation) reaction between a metal and oxygen, not a reaction between an acid and a base, so it is not a neutralisation.

7. Question 7Answer: D

   1. For a reaction between gases, increasing the pressure increases the concentration of the gas particles in a given volume, increasing the frequency of collisions between them and so increasing the rate; the reverse is also true.
   2. Increasing the temperature and adding a catalyst both increase the rate for the usual collision-theory reasons, regardless of whether the reactants are gases.
   3. Transferring the same amounts of nitrogen and hydrogen gas into a much larger container lowers their concentration and the overall pressure, so collisions between the gas particles become less frequent, decreasing the rate.
   4. So the change that does NOT increase the rate here is moving the gases into a larger container, option D.
   • Why not A: Increasing temperature does increase the rate, by increasing both the frequency of collisions and the proportion of collisions with enough energy to react, so this is not a change that fails to increase the rate.
   • Why not B: Compressing the same amount of gas into a smaller container raises the pressure and the concentration of both gases, increasing the frequency of collisions between them, so this also increases the rate rather than leaving it unaffected.
   • Why not C: Adding a catalyst provides an alternative reaction pathway with a lower activation energy, increasing the proportion of successful collisions, so this too increases the rate rather than leaving it unaffected.

8. Question 8Answer: C

   1. The rate of a reaction can be tracked by measuring the loss of a reactant, the gain of a product, or a physical property that changes as the reaction proceeds, whichever is practical for the reaction in question.
   2. Here bromine water, which is orange, is decolourised as it reacts, with no gas and no precipitate formed at any point.
   3. A colorimeter measures how much light a solution absorbs, so it can track this fading colour directly and give a reading at regular time intervals as the orange colour disappears.
   4. So the appropriate method is a colorimeter, tracking the fading orange colour, option C.
   • Why not A: Assumes a gas is produced and could be collected, when this reaction between bromine water and sodium sulfite solution produces only colourless substances in solution, with no gas released.
   • Why not B: Assumes a precipitate forms and clouds the mixture, when this reaction produces no solid product at all, only colourless dissolved substances, so nothing ever obscures a mark beneath the flask.
   • Why not D: Confuses a colour change with a change in mass; nothing escapes an open flask here, gas, precipitate or otherwise, so the total mass would stay the same throughout regardless of how the colour changes.

9. Question 9Answer: A

   1. On a volume-time graph, the initial gradient of the curve shows the rate of reaction, while the final, flat volume shows the total amount of gas produced once the reaction is complete.
   2. A higher temperature increases the rate of reaction, giving a steeper initial gradient and causing the reaction to finish (level off) sooner, without changing the total amount of gas produced, since the same amount of the limiting reactant, the acid, is used each time.
   3. So the steeper curve that levels off sooner, Curve 1, must correspond to the higher-temperature experiment, option A.
   • Why not B: Reverses the effect of temperature on rate: a higher, not a lower, temperature is what increases the rate of reaction, giving a steeper initial gradient and an earlier levelling-off.
   • Why not C: Wrongly assumes the final volume of gas depends on temperature; here it depends only on the fixed amount of acid (the limiting reactant, since the marble is in excess), which is the same in both experiments, so the same final volume gives no information about temperature.
   • Why not D: Confuses the steepness of the curve, which shows the rate of reaction, with the final volume it reaches, which shows the total amount of gas produced; the question states both curves level off at the same final volume, so neither experiment produced more gas overall.

10. Question 10Answer: B

    1. A catalyst increases the rate of a reaction by providing an alternative reaction pathway, or mechanism, with a lower activation energy, increasing the proportion of collisions between reacting particles that have enough energy to react successfully.
    2. Because the catalyst itself is not consumed by the overall reaction, it is chemically unchanged and not used up at all by the end of the reaction, so recovering the same mass and composition is exactly what is expected.
    3. This is why a catalyst can be reused: it does not raise the temperature of the mixture, invent a reacted-then-reformed cycle, or increase how concentrated the reactants are; it simply opens a lower-energy route for the same collisions to succeed more often.
    4. So the correct explanation combines both facts: the catalyst is unchanged because it is not consumed, and it speeds up the reaction by lowering the activation energy via an alternative pathway, option B.
    • Why not A: Wrongly claims a catalyst plays no part in the reaction and instead acts by raising the temperature; a catalyst does take part, via an alternative reaction pathway, and it does not change the temperature of the mixture at all.
    • Why not C: Invents an unnecessary reacted-then-reformed mechanism and wrongly claims a catalyst increases reactant concentration; a catalyst is genuinely unchanged throughout, and it works by lowering the activation energy of the pathway, not by concentrating the reactants.
    • Why not D: Wrongly claims a catalyst is used up, even by a tiny amount; a catalyst is entirely unchanged and not consumed at all, and its effect on rate comes from lowering the activation energy, not merely from increasing how often particles collide.

11. Question 11Answer: D

    1. An energy profile diagram shows two separate features: the activation energy, the energy barrier between reactants and the peak of the curve, and the overall enthalpy change, the difference in energy between reactants and products.
    2. A reaction can have a large negative enthalpy change, releasing a lot of energy overall, while still having a high activation energy, meaning very few particles collide with enough energy to react without some additional energy input, such as a spark.
    3. Once the spark supplies enough energy for a few particles to react, the energy released by those reactions can then supply the activation energy for further collisions, allowing the reaction to continue without further external input.
    4. So methane combustion is genuinely exothermic overall but still needs a spark, because it has a high activation energy despite the large release of energy once it is under way, option D.
    • Why not A: Wrongly assumes an exothermic reaction cannot have an energy barrier at all; the overall enthalpy change (a large energy release, once reacting) and the activation energy needed to start reacting are two separate features of the same energy profile, so an exothermic reaction can still need a spark to begin.
    • Why not B: Confuses the activation energy, which governs how readily the reaction starts, with the rate once it is under way; a high activation energy is exactly why so few particles react at room temperature without a spark, even though the reaction proceeds rapidly and releases a lot of energy once enough particles have cleared that barrier.
    • Why not C: Overstates the spark's role: the spark supplies only enough energy to get an initial few collisions over the activation energy barrier; the much larger amount of energy released overall comes from the difference between the energy needed to break the reactants' bonds and the energy released forming the products' bonds, not from the spark itself.

12. Question 12Answer: C

    1. A reaction that releases energy to the surroundings is exothermic, and its enthalpy change, delta H, is negative.
    2. Here the iron reacting with oxygen makes the sachet feel warm, showing that energy has been released to the surroundings (the sachet and the air around it), raising their temperature.
    3. This is enough on its own to show the reaction is exothermic, with a negative delta H; no further information about physical states is needed to reach that conclusion.
    4. So this reaction has a negative delta H and is exothermic, option C.
    • Why not A: Correctly identifies the reaction as exothermic, since energy is released, but wrongly gives delta H a positive sign; an exothermic reaction always has a negative enthalpy change.
    • Why not B: Correctly gives delta H a negative sign, but wrongly labels this endothermic; a negative enthalpy change is the defining feature of an exothermic reaction, not an endothermic one.
    • Why not D: Wrongly claims the sign of delta H cannot be inferred from a temperature change, and wrongly claims it depends on physical states; a temperature rise in the surroundings is direct evidence that the reaction has released energy, giving it a negative delta H, regardless of the physical states involved.

13. Question 13Answer: A

    1. If a reversible reaction is endothermic in one direction, it must be exothermic in the reverse direction, and the size of the energy change is exactly the same in both directions, only the sign, and so the direction of energy transfer, is reversed.
    2. This is consistent with conservation of energy: the energy absorbed by the forward reaction is exactly the energy released by the reverse reaction, so no energy is created or destroyed overall.
    3. The student's reasoning wrongly assumes conservation of energy means both directions must absorb energy, when it actually requires the two directions to have exactly opposite signs of the same size.
    4. So the student is not correct: the reverse reaction is exothermic, releasing the same amount of energy the forward reaction absorbed, option A.
    • Why not B: Correctly invokes conservation of energy but draws the wrong conclusion from it: energy is conserved because the reverse reaction releases exactly the energy the forward reaction absorbed, not because both directions must absorb energy.
    • Why not C: Correctly rejects the student's conclusion but for the wrong reason, and wrongly claims the reverse reaction's energy change is unrelated in size to the forward reaction's, when it must be exactly the same size, just released rather than absorbed.
    • Why not D: Wrongly claims the size of the energy change must double on reversal; the reverse reaction releases exactly the same amount of energy that the forward reaction absorbed, not twice that amount.

14. Question 14Answer: D

    1. Specific heat capacity is defined as energy transferred divided by (mass x temperature change), so mass = energy transferred / (specific heat capacity x temperature change).
    2. Here energy transferred = 840 J, specific heat capacity = 4.2 J per gram per degree C, temperature change = 10 degrees C.
    3. Specific heat capacity x temperature change = 4.2 x 10 = 42, so mass = 840 / 42.
    4. 840 / 42 = 20, so the mass of water used is 20 g, option D.
    • Why not A: Omits the specific heat capacity from the calculation entirely, dividing the energy transferred by the temperature change alone (840 / 10) without also dividing by 4.2.
    • Why not B: Omits the temperature change from the calculation entirely, dividing the energy transferred by the specific heat capacity alone (840 / 4.2) without also dividing by the 10 degree C rise.
    • Why not C: Inverts the rearrangement of the calorimetry equation, computing (specific heat capacity x temperature change) / energy transferred instead of energy transferred / (specific heat capacity x temperature change).

15. Question 15Answer: B

    1. Bond breaking is endothermic, since it requires an input of energy, and bond formation is exothermic, since it releases energy.
    2. Bonds broken: one H-H bond and one Cl-Cl bond, so energy to break bonds = 436 + 243 = 679 kJ/mol.
    3. Bonds formed: two H-Cl bonds, so energy released forming bonds = 2 x 432 = 864 kJ/mol.
    4. Overall energy change = energy absorbed breaking bonds - energy released forming bonds = 679 - 864 = -185 kJ/mol, a negative value, so the reaction is exothermic overall, option B.
    • Why not A: Subtracts in the wrong order, energy released forming bonds minus energy absorbed breaking them (864 - 679 = +185), reversing the sign of the true energy change and so its exothermic/endothermic label too.
    • Why not C: Forgets that two H-Cl bonds are formed, not one, using only a single H-Cl bond energy (679 - 432 = 247) instead of doubling it to account for both H-Cl bonds in the product.
    • Why not D: Simply adds all three given bond energies together (436 + 243 + 432 = 1111) instead of separately totalling the energy needed to break the reactants' bonds and the energy released forming the products' bonds, then finding the difference between the two totals.