Paper 2

1. A student investigated how the surface area of a piece of calcium carbonate affected the rate of reaction with hydrochloric acid.

a) What happens with the rate of reaction when the surface area increases?

Answer: increases


b) What are the units of rate of reaction?

Answer: g/s


c) Give a reason why the student observed bubbles.

Answer: A gas was made.


d) The figure below shows a sketch of how mass changes for the reaction between hydrochloric acid and small pieces of calcium carbonate. Sketch the line of best fit for the same reaction with larger calcium carbonate chips.


Answer: 


2. Chromatography is a separation technique that can be used to separate colours in washable ink pens. The figure below shows the set up for a chromatography experiment to separate the colours of the ink.


a) Explain why pencil is used to draw the start line and mark the solvent front.

Answer: Pencil is insoluble (will not run) and so will not interfere with the separation.


b) Explain why it is important that the solvent level is below the base line.

Answer: So that the colours dissolve into the solvent as it travels up the paper rather than dissolving directly into the solvent in the jar.


c) Explain why it is important to mark the solvent front as soon as the chromatogram is taken from the developing tank.

Answer: To mark where the solvent got to or to calculate the Rf value. If it is not done straight away, it might evaporate and you cannot see where it travelled to or it might carry on travelling upwards once out of the tank.


d) Suggest one improvement to this experiment that would give a more reproducible chromatogram.

Answer: Add a lid to reduce the evaporation rate of the solvent, which makes the separation clearer.


3. Ammonium chloride can reversibly decompose into ammonia and hydrogen chloride.

a) Give the symbol that shows this is a reversible reaction.

Answer: The symbol is "⇋".


b) State condition needed for the forward reaction to happen.

Answer: Heat is needed to decompose ammonium chloride.


c) Describe the observations for the forward reaction.

Answer: White solid changes to colourless gas.


4. Copper is a transition metal that can form compounds as a 2+ ion.

a) Give the formula of the copper(II) ion.

Answer: Cu2+


b) State the colour of a copper compound in a flame test.

Answer: green


c) Copper (II) sulfate will react with sodium hydroxide to make sodium sulfate and a copper(II) salt. Write a word equation for this reaction.

Answer: copper(II) sulfate + sodium hydroxide  →  sodium sulfate + copper(II) hydroxide


d) State the colour of the precipitate of the copper(II) compound made in this reaction.

Answer: blue


5. Flame tests are an analytical technique used to show that certain metal ions are present in a sample.

a) Describe how a flame test can be used to identify a sodium compound.

Answer: Use a clean nichrome (or platinum, inert) wire. Dip the wire in concentrated acid and burn the acid off in a blue flame. Put a sample of the chemical on the wire. Hold the sample into a blue flame and note the colour of the flame. If the flame is yellow (or orange) then sodium ions are present.


b) Define the term "cation".

Answer: positive ion


c) Explain why it would be difficult to use flame tests to determine the composition of a mixture of cations.

Answer: Some flame colours can be masked.


d) Explain why it is difficult to use a flame test to determine if a solution has lithium or calcium ions in it.

Answer: Lithium makes a crimson-red flame and calcium makes a red-orange flame. These look very similar in colour and it is difficult to tell them apart.


e) Explain why it is important to ensure that the wire used for the flame test is clean.

Answer: To prevent contamination and false positives.


6. When sodium is put into water it reacts to form hydrogen gas and one other product.

a) Write the word equation for this reaction.

Answer: sodium + water  →  hydrogen + sodium hydroxide


b) Describe the observations that you would make if you put sodium into water.

Answer: Metal melts into a ball, moves across the surface, gets smaller, appears to disappear. Head fizzing, see bubbles, effervescence.


c) Describe how the gas could be collected and tested to show it is hydrogen.

Answer: Fill a test tube with water and trap the metal under this tube. The gas pushes out the water, and so we collect it by displacement. When the test-tube is full, put a bung on it and remove the tube from the water. Then remove the bung and quickly hold a lighted splint into the gas. If you hear a pop the gas is hydrogen.


7. Crude oil is a finite resource found in rocks. Products of crude oil are often used as fuels.

a) Explain why crude oil is an example of a finite resource.

Answer: Crude oil cannot be replaced once it has been used.


b) Explain how crude oil is separated.

Answer: Crude oil is heated and evaporated until it forms a vapour. It then is put into a fractionating column that is cooler at the top than at the bottom. The vapour rises and condenses when it reaches its boiling point. Each fraction of crude oil has a different boiling range. Fractions have hydrocarbons with similar chain length. The longer the chain length, the greater the intermolecular forces and the higher the boiling point so longer or heavier hydrocarbon fractions are collected at the bottom of the fractionating column as they have high boiling points.


c) Methane (CH4 ) can be extracted from crude oil and used in camping gas. Write a balanced symbol equation for the complete combustion of methane.

Answer: CH4 + 2O2  →  CO2 + 2H2O


d) Large hydrocarbons can undergo a chemical reaction known as cracking to make smaller more useful hydrocarbons. Write a balanced symbol equation for the cracking of decane (C10H22 ) to form ethene and one other product.

Answer: C10H22  →  C2H4 + C8H18


e) Ethene can react with steam to make an alcohol. Draw the displayed formula of this alcohol.

Answer: 


8. The figure below shows the composition of gases in dry air.


a) Give the approximate fraction of dry air that is nitrogen.

Answer: An approximate value is 4/5


b) Describe how to test for the gas that makes up about 20% of dry air.

Answer: To test for oxygen, use a glowing splint that should re-light.


c) Describe how carbon dioxide decreased from its level in Earth’s early atmosphere to the present level.

Answer: The early atmosphere was mainly carbon dioxide. Then the percentage of carbon dioxide in the atmosphere decreased due to photosynthesis by algae and plants. Carbon dioxide also decreased due to the formation of sedimentary rocks and fossil fuels which captured (locked in) the carbon.


9. Flame emission spectroscopy is an instrumental method that can be used to analyse a substance.

a) Describe how flame emission spectroscopy can be used to identify metal ions and give their concentrations.

Answer: The sample is put into a flame. Light given out is passed through a spectroscope. The output is a line spectrum.


b) The flame emission spectroscope must be calibrated. The intensity of a flame is measured compared to known concentrations of the solutions. The figure below shows a graph of these data. Describe a line of best fit.


Answer: Straight line from (0,0) passing through data points.


10. Brine is a solution of sodium chloride. It can undergo electrolysis to make hydrogen, chlorine and sodium hydroxide.


a) Write a half equation for hydrogen being made at the cathode.

Answer: 2H+ + 2e-  →  H2


b) Describe a chemical test that can be used on the gas collected at the anode to show it is chlorine.

Answer: Damp blue litmus paper goes red then white (bleached).


c) Explain why the electrolysis of sodium chloride only happens if the chemical is molten or in solution.

Answer: Ions must be free to move.


d) Suggest what would be observed if universal indicator was added to the electrolyte.

Answer: At the start, the universal indicator would be green as the salt solution (brine) would be neutral. As the electrolysis happens, OH- ions (or sodium hydroxide) would be made. This will increase the pH and the solution will turn blue (purple).


11. Copper(II) chloride is widely used in industry as a catalyst.

a) Sketch a reaction profile of an exothermic reaction to show the effect of adding a catalyst.

Answer: 


b) Explain why copper makes a good catalyst.

Answer: It is a transition metal and has ions with different charges.


c) Write a balanced ionic equation for the formation of a blue precipitate when a solution of copper(II) chloride reacts with sodium hydroxide.

Answer: Cu2+(aq) + 2OH-(aq)  →  Cu(OH)2(s)


d) Write the ionic equation for the formation of a white precipitate when a solution of copper(II) chloride reacts with acidified silver nitrate.

Answer: Ag+(aq) + Cl-(aq)  →  AgCl(s)


e) The copper(II) chloride solution could be analysed using instrumental analysis. Describe the advantages of using instrumental analysis.

Answer: Sensitive – small amounts of chemical can be analysed. Informative – provide more information than traditional chemical tests such as concentration. Rapid – much faster than traditional chemical tests.


12. A student analysed an unknown compound, compound A. The compound gave a green flame in a flame test. When the student added hydrochloric acid followed by barium chloride solution to a solution of A, a white precipitate was produced.

a) Identify the positive ion in compound A.

Answer: copper ions (Cu+)


b) Identify the negative ion in compound A.

Answer: sulfate ions (SO42-)


c) Write the name and formula of compound A.

Answer: copper sulfate (CuSO4 )