CSEC Chemistry Paper 3 May/June 2005 - Free CXC Past Paper with Answers

Potassium dichromate ($\text{K}_2\text{Cr}_2\text{O}_7$) is a common reagent in a school's laboratory.

Describe ONE use of potassium dichromate as a laboratory reagent.

Your answer should include the test it is used for, the method of carrying out the test and any colour changes expected.

Calculate the oxidation number of chromium in potassium dichromate ($\text{K}_2\text{Cr}_2\text{O}_7$) and in chromium sulphate (${\text{Cr}}_2({\text{SO}}_{}$).

Based on your answer in (ii) above, deduce the type of reaction which occurs when $\text{K}_2\text{Cr}_2\text{O}_7$ is converted to ${\text{Cr}}_2({}_{}$.

In many Caribbean countries steel pans, in particular the tenor pans, are electroplated with chromium. Chrome-plating is believed to improve the pitch of the pan.

Describe how a tenor pan could be chrome-plated.

You should include in your answer:

• the electrolyte to be used
• the nature of the electrodes
• equations for the reactions occurring at EACH electrode
• precautions to be taken to ensure that the chrome layer does not peel off

Given that the mass of the chromium needed to plate a tenor pan is $0.52 kg$, calculate the length of time (in seconds) that a current of $20.0 A$ would have to flow through the electrolytic cell to plate the pan.

Use the following data in your calculation:

• Relative Atomic Mass: $Cr = 52$
• $1$ Faraday = $96,000$ coulombs.

Explain how the electronic configuration of the carbon atom, $\text{C}^{12}_6$, can account for the fact that the bonding in carbon compounds is covalent rather than ionic. You should use the bonding in methane to illustrate your answer.

Alkanes are relatively unreactive compounds. However, under certain conditions, they may undergo reactions with chlorine.

Write a balanced equation for the reaction between methane and chlorine. Include in your answer the reaction conditions and the type of reaction taking place.

By including certain functional groups such as the $\text{C}=\text{O}$, $-\text{OH}$ or $-\text{COOH}$ to the alkane structure, the reactivity greatly increases.

a) Identify an alkene and an acid from among the following compounds: $\text{C}_3\text{H}_8$, $\text{C}_3\text{H}_6\text{O}_2$, ,

Describe ONE chemical reaction in EACH case which can be used to illustrate the differences in REACTIVITY between a) an alkane and an alkene b) an alkane and a carboxylic acid.

In EACH case you should include a chemical equation in your answer.

Name the industrial process which is used to bring about conversion of the following types: $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{CH}_3 \longrightarrow \text{CH}_3\text{CH}_2\text{CH}_3 + \text{HC}=\text{CH}_2\text{CH}_3$ (long chain alkanes) (short chain alkanes) + alkenes

State ONE advantage of the process you named in (d) (i) above.

The electrochemical series is a convenient way for arranging metals. A metal's position in the electrochemical series can give information about its properties and uses.

Use the principles for placing metals in the electrochemical series to explain the following observations.

Copper, silver and gold are used to make coins but magnesium, sodium and calcium are not. Support your answer with a suitable ionic equation.

When a zinc rod is placed in a copper sulphate solution, after a while a brown deposit (precipitate) is produced, the zinc rod gets smaller and the blue colour of the solution fades. However, there is no visible change when a copper rod is placed in a zinc sulphate solution. Support your answer with a suitable equation.

Iron and aluminium are both extracted from ores mined from the earth. However, different methods are used to extract each metal from its ore. You should state in your answer the names of the methods of extraction used.

Sometimes a metal is most useful when it is pure. However, many metals are more useful when they contain a small percentage of one or more additional metals.

Suggest why copper wire might be more useful when it is pure but iron is almost never used in its pure form.

Discuss the role of EACH of the following in the making and baking of a dough:

• water
• gluten
• leavening agent (both yeast and baking powder)
• heat

Include in your answer THREE relevant chemical equations, with at least ONE equation EACH, from the action of yeast and baking powder.

Flour usually consists of between 65 – 80% carbohydrates (starches). Iodine can be used to test for the presence of starches.

There is no blue-black colouration when flour, which is stored in a hot, humid room for a long time, is tested with iodine.

Suggest a possible explanation for this. Include an appropriate chemical equation in your answer.

Carbohydrates are polymers of simple sugars. Using the symbol $\text{OH} - \text{OH}$ to represent a simple sugar, show how THREE of these units can be linked together to form part of the polymer chain found in carbohydrates present in flour.

Copy Table 1 in your answer booklet. In the blank spaces, fill in the purpose of steps 2, 3, 5, 6 and 7 in the canning process.

Nisin, an antibiotic, is often added after step 3 of the canning process. Suggest a possible reason for this.

You are required to select one of two metals, A or B, to coat steel cans for canning fruit juices. Metal A is just above hydrogen and metal B is just below hydrogen in the electrochemical series.

Which of these two metals would be more suitable? You should include in your answer: (i) A discussion on

• the need for coating the steel cans
• the rationale for selecting one metal over the other
• any possible limitations of your recommendations. (ii) Any relevant chemical equations

A discussion on

• the need for coating the steel cans
• the rationale for selecting one metal over the other
• any possible limitations of your recommendations.

Any relevant chemical equations