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Question 1 Report
This experiment is used by a metal-finishing company to decide the temperature of an acid cleaning bath. Equal strips of magnesium are placed in hydrochloric acid solution at different temperatures. The students record the time until each strip has disappeared. Fig. 1 represents the collision model for a reaction between magnesium and acid particles. In box A, the collision does not have enough energy. In box B, the collision has enough energy to lead to reaction.
(a) State the gas formed when magnesium reacts with hydrochloric acid. [1]
(b) Give the balanced chemical equation for this reaction. [2]
(c) Use the collision model to explain why increasing temperature decreases the time taken for the magnesium to disappear. [3]
(d) Name one variable, other than temperature, that must be controlled in this experiment. [1]
(e) Calculate the mean rate of reaction if a magnesium strip of mass 0.36 g disappears in 90 s. Give the answer in g/s. [2]
(f) Suggest why the company should not use a very high temperature, even if it gives a fast rate reaction. [1]
(a) Magnesium reacting with hydrochloric acid produces hydrogen gas. [1]
(b) The balanced equation is:
\[\mathrm{Mg+2HCl\rightarrow MgCl_2+H_2}\]
[2]
(c) At higher temperature, particles have more kinetic energy. They move faster and collide more often. Also, a greater proportion of collisions have enough energy to react, so magnesium disappears in less time. [3]
(d) One variable that must be controlled is the concentration of hydrochloric acid. The volume of acid, mass or surface area of magnesium, and type of magnesium strip are also acceptable controls. [1]
(e) Mean rate is mass used divided by time:
\[\frac{0.36\text{ g}}{90\text{ s}}=0.004\text{ g s}^{-1}\]
[2]
(f) A very high temperature increases energy costs. It may also make hot acid more hazardous, and hydrogen, which is flammable, would be produced more rapidly. [1]
(a) Magnesium reacting with hydrochloric acid produces hydrogen gas. [1]
(b) The balanced equation is:
\[\mathrm{Mg+2HCl\rightarrow MgCl_2+H_2}\]
[2]
(c) At higher temperature, particles have more kinetic energy. They move faster and collide more often. Also, a greater proportion of collisions have enough energy to react, so magnesium disappears in less time. [3]
(d) One variable that must be controlled is the concentration of hydrochloric acid. The volume of acid, mass or surface area of magnesium, and type of magnesium strip are also acceptable controls. [1]
(e) Mean rate is mass used divided by time:
\[\frac{0.36\text{ g}}{90\text{ s}}=0.004\text{ g s}^{-1}\]
[2]
(f) A very high temperature increases energy costs. It may also make hot acid more hazardous, and hydrogen, which is flammable, would be produced more rapidly. [1]
Question 2 Report
The table shows results from a harbour safety inspection of fuel canisters used on small boats. Each canister contains a liquefied hydrocarbon gas. A technician released equal volumes of gas into separate chambers and supplied the same amount of oxygen. Carbon monoxide was measured after ignition. The canisters must not be used in cabins unless there is good ventilation because carbon monoxide is poisonous.
| condition in chamber | appearance of flame | carbon monoxide concentration / ppm |
|---|---|---|
| excess oxygen | blue | 0 |
| limited oxygen | yellow | 180 |
| very limited oxygen | yellow, smoky | 620 |
(a) State the name given to a reaction in which a hydrocarbon burns with excess oxygen. [1]
(b) Give the two substances formed when butane undergoes this reaction. [2]
(c) Explain why carbon monoxide is detected when the oxygen supply is limited. [2]
(d) Use Table 1 to describe the change in carbon monoxide concentration as less oxygen is available. [2]
(e) Suggest one action a boat owner could take to reduce the risk from carbon monoxide when using a gas stove. [1]
(a) Burning a hydrocarbon with excess oxygen is called complete combustion. [1]
(b) Butane undergoing complete combustion forms carbon dioxide and water. [2]
(c) With limited oxygen, there is insufficient oxygen for complete combustion. Carbon is then oxidised to carbon monoxide rather than carbon dioxide. [2]
(d) Carbon monoxide concentration increases as oxygen availability decreases: it rises from 0 ppm in excess oxygen to 180 ppm in limited oxygen, then to 620 ppm in very limited oxygen. [2]
(e) The owner could ventilate the cabin by opening vents or windows. Servicing the stove or fitting a carbon monoxide alarm would also reduce risk. [1]
(a) Burning a hydrocarbon with excess oxygen is called complete combustion. [1]
(b) Butane undergoing complete combustion forms carbon dioxide and water. [2]
(c) With limited oxygen, there is insufficient oxygen for complete combustion. Carbon is then oxidised to carbon monoxide rather than carbon dioxide. [2]
(d) Carbon monoxide concentration increases as oxygen availability decreases: it rises from 0 ppm in excess oxygen to 180 ppm in limited oxygen, then to 620 ppm in very limited oxygen. [2]
(e) The owner could ventilate the cabin by opening vents or windows. Servicing the stove or fitting a carbon monoxide alarm would also reduce risk. [1]
Question 3 Report
The diagram shows a section of the material chosen to insulate a signal cable at a tidal-energy station. The cable may be splashed with a sodium chloride solution, so the coating must not allow water to reach the metal. Fig. 1 represents one repeat unit of the polymer. The bonds continuing beyond the brackets show that many units are joined together.
Fig. 1
(a) Name the monomer used to make this polymer. [1]
(b) Draw the displayed formula of this monomer. [2]
(c) State the type of reaction that changes the monomer into the polymer. [1]
(d) Explain why this polymer coating helps to prevent corrosion of the metal conductor when the cable is in contact with salt water. [3]
(a) The monomer is chloroethene. The polymer repeat unit has the arrangement \(\mathrm{-CH_2-CHCl-}\), so the monomer must have had a double bond between these carbon atoms. [1]
(b) The displayed formula of chloroethene is:
This shows \(\mathrm{CH_2=CHCl}\) with all bonds. [2]
(c) The reaction is addition polymerisation. [1]
(d) The polymer forms a continuous barrier around the metal conductor. This prevents water reaching the metal and also prevents sodium chloride solution, including its ions, reaching it. Without water and salt solution at the conductor, the conditions required for corrosion are removed. [3]
(a) The monomer is chloroethene. The polymer repeat unit has the arrangement \(\mathrm{-CH_2-CHCl-}\), so the monomer must have had a double bond between these carbon atoms. [1]
(b) The displayed formula of chloroethene is:
This shows \(\mathrm{CH_2=CHCl}\) with all bonds. [2]
(c) The reaction is addition polymerisation. [1]
(d) The polymer forms a continuous barrier around the metal conductor. This prevents water reaching the metal and also prevents sodium chloride solution, including its ions, reaching it. Without water and salt solution at the conductor, the conditions required for corrosion are removed. [3]
Question 4 Report
Fig. 1 shows an energy profile drawn for a reversible reaction used to make an organic chemical. Route P is the reaction without a catalyst. Route Q is the same reaction when a catalyst is used. A student must decide whether adding a catalyst is a useful way to obtain a larger equilibrium yield.
The products have less energy than the reactants.
(a) Which route, P or Q, represents the catalysed reaction? [1]
(b) State the effect of a catalyst on activation energy. [1]
(c) Use Fig. 1 to state whether the forward reaction is exothermic or endothermic. [1]
(d) Explain why a catalyst makes equilibrium reached sooner. [3]
(e) State the effect of adding a catalyst on the position of equilibrium and on the equilibrium yield. [2]
(f) Give one reason why a catalyst is valuable in an industrial reaction even when it does not increase yield. [2]
(g) Name one condition that could change the position of equilibrium for a reaction involving gases. [2]
(a) Route Q is the catalysed route because its peak is lower than route P, showing a lower activation energy. [1]
(b) A catalyst lowers the activation energy. [1]
(c) The forward reaction is exothermic because the products have less energy than the reactants. [1]
(d) A lower activation energy means that more particles have sufficient energy to react when they collide. The forward reaction is therefore faster. The catalyst also speeds up the reverse reaction, so the forward and reverse rates become equal sooner and equilibrium is reached sooner. [3]
(e) A catalyst causes no change in the position of equilibrium and no change in equilibrium yield. It speeds both directions equally rather than favouring one direction. [2]
(f) A catalyst is valuable because it makes the reaction faster. This can allow a lower operating temperature, reducing energy costs, or increase production per hour. [2]
(g) Temperature or pressure can change the position of equilibrium in a reaction involving gases. [2]
(a) Route Q is the catalysed route because its peak is lower than route P, showing a lower activation energy. [1]
(b) A catalyst lowers the activation energy. [1]
(c) The forward reaction is exothermic because the products have less energy than the reactants. [1]
(d) A lower activation energy means that more particles have sufficient energy to react when they collide. The forward reaction is therefore faster. The catalyst also speeds up the reverse reaction, so the forward and reverse rates become equal sooner and equilibrium is reached sooner. [3]
(e) A catalyst causes no change in the position of equilibrium and no change in equilibrium yield. It speeds both directions equally rather than favouring one direction. [2]
(f) A catalyst is valuable because it makes the reaction faster. This can allow a lower operating temperature, reducing energy costs, or increase production per hour. [2]
(g) Temperature or pressure can change the position of equilibrium in a reaction involving gases. [2]
Question 5 Report
A field kitchen uses propane cylinders to heat water for an expedition. Before choosing a cylinder, a student burns a measured mass of propane beneath a metal pan containing water. The displayed formula in Fig. 1 is for propane. Table 1 gives results from one trial. Some energy is transferred to the pan and the air rather than to the water.
| measurement | value |
|---|---|
| mass of water | 500 g |
| initial temperature of water | 18.0 degrees C |
| final temperature of water | 46.0 degrees C |
| mass of propane burned | 1.20 g |
(a) Give the molecular formula of propane. [1]
(b) Name the two products formed when propane burns completely in excess oxygen. [2]
(c) Complete the balanced equation for this reaction.
C3H8 + 5O2 → 3CO2 + .............. [2]
(d) Calculate the temperature rise of the water. [1]
(e) Calculate the energy transferred to the water, in J. Use: energy = mass of water × 4.2 × temperature rise. [3]
(f) Explain why the energy calculated in part (e) is less than the total energy released by the propane reaction. [2]
(a) Propane has molecular formula C3H8. [1]
(b) Complete combustion in excess oxygen produces carbon dioxide and water. [2]
(c) \[\mathrm{C_3H_8+5O_2\rightarrow3CO_2+4H_2O}\]
The missing product is 4H2O. [2]
(d) \[46.0-18.0=28.0\degree\mathrm{C}\]
The temperature rise is 28.0 °C. [1]
(e) \[\text{energy}=500\text{ g}\times4.2\times28.0\degree\mathrm{C}=58800\text{ J}\]
The energy transferred to the water is 58 800 J. [3]
(f) Not all the energy from combustion heats the water: some heats the metal pan, and some is transferred to the surrounding air. Energy transfer from flame to pan and water is therefore incomplete. [2]
(a) Propane has molecular formula C3H8. [1]
(b) Complete combustion in excess oxygen produces carbon dioxide and water. [2]
(c) \[\mathrm{C_3H_8+5O_2\rightarrow3CO_2+4H_2O}\]
The missing product is 4H2O. [2]
(d) \[46.0-18.0=28.0\degree\mathrm{C}\]
The temperature rise is 28.0 °C. [1]
(e) \[\text{energy}=500\text{ g}\times4.2\times28.0\degree\mathrm{C}=58800\text{ J}\]
The energy transferred to the water is 58 800 J. [3]
(f) Not all the energy from combustion heats the water: some heats the metal pan, and some is transferred to the surrounding air. Energy transfer from flame to pan and water is therefore incomplete. [2]
Question 6 Report
Fig. 1 shows the results from a student testing four metals with sulfate solutions. Each test tube was kept at the same temperature and contained the same volume of solution. A reaction is shown by a metal coating or a change in the solution. The student uses the results to compare the reactivity of magnesium, zinc, iron and copper.
(a) State which test tube shows no displacement reaction. [1]
(b) Give two observations expected in test tube B. [2]
(c) Complete the equation for the reaction in test tube B.
Zn + FeSO4 → ........................................ [2]
(d) Use Fig. 1 to give the four metals in order of decreasing reactivity. [3]
(a) The test tube containing copper and magnesium sulfate shows no displacement reaction. Copper is less reactive than magnesium, so it cannot displace magnesium from its sulfate solution. [1]
(b) In the tube containing zinc and iron sulfate, an iron metal coating or deposit forms on the zinc. The solution becomes pale green as iron sulfate is formed from the initially colourless zinc sulfate solution. [2]
(c)
\[\mathrm{Zn+FeSO_4\rightarrow ZnSO_4+Fe}\]
Zinc is more reactive than iron, so zinc displaces iron from iron sulfate. [2]
(d) The metals in decreasing reactivity are:
magnesium > zinc > iron > copper
Each reaction shows that the added metal is more reactive than the metal ion it displaces; the lack of reaction with magnesium sulfate places copper below magnesium. [3]
(a) The test tube containing copper and magnesium sulfate shows no displacement reaction. Copper is less reactive than magnesium, so it cannot displace magnesium from its sulfate solution. [1]
(b) In the tube containing zinc and iron sulfate, an iron metal coating or deposit forms on the zinc. The solution becomes pale green as iron sulfate is formed from the initially colourless zinc sulfate solution. [2]
(c)
\[\mathrm{Zn+FeSO_4\rightarrow ZnSO_4+Fe}\]
Zinc is more reactive than iron, so zinc displaces iron from iron sulfate. [2]
(d) The metals in decreasing reactivity are:
magnesium > zinc > iron > copper
Each reaction shows that the added metal is more reactive than the metal ion it displaces; the lack of reaction with magnesium sulfate places copper below magnesium. [3]
Question 7 Report
The diagram shows the cooling record for molten magnesium chloride before it is fed into an electrolysis cell with electrodes. A technician needs the substance to be liquid before electrolysis can take place. The sample is allowed to cool in a dry container, and its temperature is measured every few minutes. Section QR is horizontal because a solid-liquid change is taking place.
(a) State the state of magnesium chloride before point Q and name its state after 12 min. [2]
(b) Identify the section where the substance changes state, give its temperature, and name the change. [3]
(c) Explain why the temperature stays constant between Q and R even though energy is being removed. [4]
(d) Calculate the mean rate of temperature decrease between 8 min and 12 min. State two differences between the particles in a solid and those in a liquid, and state which state can flow. [5]
(a) Before point Q, magnesium chloride is a liquid. After 12 min, it is a solid. [2]
(b) The change of state occurs on section QR, at 714 degrees C. Since the sample is cooling from liquid to solid, the change is freezing or solidification. [3]
(c) Energy is still removed between Q and R. It is used to form stronger attractions between particles as they become arranged in a solid. Therefore the temperature does not decrease until the change of state is complete. [4]
(d) Between 8 min and 12 min:
\[\text{temperature decrease}=714-650=64\text{ degrees C}\]
\[\text{time}=12-8=4\text{ min}\]
\[\text{mean rate}=\frac{64}{4}=16\text{ degrees C min}^{-1}\]
Solid particles are in fixed positions in a regular arrangement. Liquid particles are still close together but have an irregular arrangement and can move past each other. The liquid state can flow. [5]
(a) Before point Q, magnesium chloride is a liquid. After 12 min, it is a solid. [2]
(b) The change of state occurs on section QR, at 714 degrees C. Since the sample is cooling from liquid to solid, the change is freezing or solidification. [3]
(c) Energy is still removed between Q and R. It is used to form stronger attractions between particles as they become arranged in a solid. Therefore the temperature does not decrease until the change of state is complete. [4]
(d) Between 8 min and 12 min:
\[\text{temperature decrease}=714-650=64\text{ degrees C}\]
\[\text{time}=12-8=4\text{ min}\]
\[\text{mean rate}=\frac{64}{4}=16\text{ degrees C min}^{-1}\]
Solid particles are in fixed positions in a regular arrangement. Liquid particles are still close together but have an irregular arrangement and can move past each other. The liquid state can flow. [5]
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