8.2 Identification of Common Gases

A student investigated photosynthesis of algae by placing some algae in water with a light shining onto the algae.

The water contained a dissolved gas necessary for photosynthesis to occur.

Name the dissolved gas.

As photosynthesis occurred, gas bubbles were seen and the resulting gas was collected.

Describe a test that could be performed to identify the gas collected.

Give the results of the test.

The student increased the light intensity which was shining on the beaker containing algae and water.

How would the observations the student would make be different after increasing the light intensity?

Explain your answer.

A student investigated how the volume of gases produced changed with time during the electrolysis of sodium chloride solution.

Figure 1 shows the equipment the student used.

Figure 1

Why is hydrogen gas, not sodium, collected at the cathode?

Chlorine gas is collected at the anode.

Write the half equation for the reaction at the anode.

Describe a test for chlorine gas.

Give the result of the test if chlorine is present.

The student recorded the results shown in Table 1.

Table 1

The volume of chlorine collected at the anode is expected to be the same as the volume of hydrogen collected at the cathode.

Suggest why the volume of gas collected at each electrode is not the same.

Assume no gas has escaped from the apparatus.

A student investigated how the rate of reaction between magnesium and hydrochloric acid changed with temperature, shown in Figure 1.

The student measured the time for the magnesium to stop reacting.

Figure 1

The equation for the reaction is:

Mg(s) + 2HCl(aq)    MgCl₂(aq)  +  H₂(g)

Describe a test for hydrogen gas.

Give the result of the test if hydrogen is present and given the balanced symbol equation for the reaction that occurs if hydrogen is present.

State and explain what would happen to the rate of reaction between magnesium and hydrochloric acid as the temperature increased.

2.0 g of magnesium reacted with a solution containing 0.025 moles of hydrochloric acid.

Calculate the mass of hydrogen gas that you would expect to be produced in this reaction.

The student repeated the experiment but used a white powder, compound X, instead of magnesium.

A gas was produced during the reaction with hydrochloric acid. When the gas was bubbled through turned limewater, it turned cloudy.

The student carried out a further chemical test on compound X which confirmed the presence of potassium ions.

Give the formula of compound X.

A fungicide suitable for crops contains copper sulfate, CuSO₄, and ammonium carbonate, (NH₄)₂CO₃.         

A student tested a sample of the fungicide.

Hydrochloric acid was added and a gas was produced that turned limewater milky.

Complete the sentences. 

The gas produced was _________________________ which shows that _________________________ ions are in the mixture. 

The formula for the ion is: ________

Describe a method for testing the carbon dioxide gas produced from the reaction of ammonium carbonate, (NH₄)₂CO₃ and hydrochloric acid, HCl.

You may use a diagram.

Next, sodium hydroxide solution was added to a sample of the fungicide.

A gas was produced indicating that ammonia was produced.

Ammonia can react with chlorine gas to form hydrochloric acid and then ammonium chloride, a white powder. These are not the desirable products.

Describe how the gas in the reaction vessel could be tested in order to check for the presence of chlorine. 

Complete the sentence. 

The gas in the vessel could be tested using damp ____________________ which turns _____________________ if chlorine gas is present.

Chlorine gas also has a distinctive smell, like the smell of swimming pools and bleach.

Why should this not be used as a test to identify the gas?

Methane, CH₄ contains the elements carbon and hydrogen only.

A student wanted to find out which new substances are produced when methane is burned.

The student set up the apparatus shown below.

Name the liquid collected.

Name the gas which turns limewater milky.

When methane burns an exothermic reaction takes place.

What is meant by an exothermic reaction?

Hydrogen peroxide, H₂O₂, is often used as a bleach. It decomposes forming water and oxygen.

Write the balanced chemical equation for the decomposition of hydrogen peroxide.

Give a test for oxygen. 

Test: _________________________

 Result of test: _________________________

Describe the test for hydrogen. 

Test: _________________________ 

Result of test: _________________________

Two experiments were carried out to test if the mass of a manganese dioxide catalyst, MnO₂, affected the rate at which the hydrogen peroxide decomposed.

Complete Figure 1 to show how you could measure the volume of oxygen formed during the decomposition.

Figure 1

The same volume and concentration of hydrogen peroxide solution was used for both experiments.

What two other factors must be kept the same to make it a fair test?