ExamLearn.ie. The Air & Oxygen

Transcription

1 ExamLearn.ie The Air & Oxygen

2 The Air & Oxygen The air is a mixture of gases, which forms a blanket around the earth. Another name for the air is the atmosphere. *To investigate the percentage of oxygen in air (using night light) Apparatus required: basin, night-light, graduated cylinder, matches, water 1. Light a night light and float it on surface of water in basin 2. Put graduated cylinder over burning the night light burns. The water rises up inside the graduated cylinder. The water is pushed up by pressure of atmosphere. 3. Allow apparatus to cool. Measure volume of gas left in graduated cylinder. 4. By subtraction, calculate percentage of oxygen in air Oxygen occupies approximately one fifth of air.

3 A more accurate method of carrying out the experiment is shown here: *To measure the percentage of oxygen in air (using gas syringes) Apparatus required: two gas syringes, two syringe holders, two retort stands, silica glass tube and connectors, Bunsen burner, copper turnings 1. Set up apparatus shown in the diagram 2. Fill one syringe with 100cm 3 of air. The other syringe should read 0cm 3. Pass all the air from the left-hand syringe over the heated copper into the right-hand syringe. It will be observed that brown copper begins to turn grey-black. This is because the copper is reacting is reacting with the oxygen in air to form copper oxide. 4. Push the air to and from several times over the copper so that all the oxygen in the air reacts with the copper. 5. Allow the apparatus to cool. Push all the gas into left-hand syringe and measure the volume of air in it.

4 6. The total volume of air in syringe has decreased. This decrease must be equal to the amount of oxygen in the air. Results Volume of air in syringe before heating = 100 cm 3 Volume of gas left after heating and cooling = 21cm 3 Percentage of oxygen in air is 21%. Composition of air: Nitrogen 78% Oxygen 21% Inert (unreactive) gases + carbon dioxide 1% *To show that carbon dioxide is present in air Limewater tests for the presence of carbon dioxide as it turns milky white when carbon dioxide is passed through. Apparatus required: gas-washing bottle, two-holed rubber stopper, glass tubing, vacuum pump, and limewater 1. Set up apparatus as shown in diagram. Take care to connect vacuum pump to shorter piece of glass tubing. 2. Turn on tap to operate vacuum pump. Allow air to be drawn through limewater for half an hour. Result Limewater turns milky. This shows us that carbon dioxide is present in air.

5 *To show that water vapour is present in air Apparatus required: boiling tube, retort stand, rubber stopper, beaker, salt, ice, cobalt chloride paper 1. Set up apparatus with test tube of salt and ice mix hanging from retort stand. Make sure outside of tube is dry before experiment. 2. Observe droplets of liquid condense on outside of tube. Collect this liquid and test with cobalt chloride paper. This is blue, but turns pink in presence of water. Result The liquid turns blue cobalt chloride paper pink. The liquid that condensed on the outside of tube is water. Therefore, water vapour is present in air. Oxygen Preparation and Properties The easiest method of preparing oxygen is by decomposing hydrogen peroxide. This is a colourless liquid that slowly

6 decomposes to give off oxygen and form water. The chemical formula for it is H 2 O 2. Word equation: hydrogen peroxide water + oxygen Chemical equation: 2H 2 O 2 2H 2 O 2 + O 2 A compound called manganese dioxide is used to make hydrogen peroxide decompose quicker. The chemical formula for it is MnO 2. It is called a catalyst a catalyst is a substance that changes the speed of a chemical reaction but is not used up in the reaction itself. *(a) To prepare a sample of oxygen *(b) To examine the properties of oxygen Apparatus required: dropping funnel, Buchner flask, delivery tubing, trough, gas jars, gas jar covers, beehive shelf, wooden splints, deflagrating spoon, hydrogen peroxide, manganese dioxide, water, blue and red litmus paper, charcoal, limewater, magnesium ribbon (a) to prepare oxygen sample: 1. Set up apparatus as shown 2. Allow hydrogen peroxide from dropping funnel to fall on manganese dioxide so that oxygen is produced quickly. The oxygen is collected in gas jar, which is filled with water and placed on top of beehive shelf. 3. Collect five gas jars of the gas

7 (b) To examine properties of oxygen 1. Litmus and oxygen: Note that thegas is colourless and odourless. Put pieces of moist red litmus paper and blue litmus paper into jar of gas. Note that there is no colour change, so oxygen is a neutral gas. 2. Wooden splint and oxygen: Light wooden splint and place into jar of oxygen. Note how it burns more vigorously. Take out burning splint and shake so it is just a glow. Put glowing splint back into jar and notice how is bursts into flames gain. Oxygen rekindles a glowing splint. 3. Carbon and Oxygen: Heat some carbon (charcoal) on deflagrating spoon. When it is glowing, put it in jar of oxygen. The carbon continues to burn brightly, as this reaction is occurring carbon +oxygen = carbon dioxide. Add limewater to the jar and notice how it turns milky white, showing carbon dioxide is now present, as it was formed when carbon was burned in oxygen. 4. Carbon dioxide and water: Burn more carbon in another jar of oxygen. Add water and blue litmus paper. Shake gas jar and notice how blue litmus paper turns red. This shows how carbon dioxide dissolves in water to form an acidic substance. This acidic substance is called carbonic acid. 5. Magnesium and oxygen: Burn magnesium metal in air, note how easily it burns with bright flame. Now burn it in jar of oxygen. Note how it burns a lot more vigorously in oxygen now. When magnesium stops burning, note the white powder formed in the gas jar. This is called magnesium oxide. 6. Magnesium oxide and water: Add water and litmus paper to gas jar of magnesium oxide. Note that the red colour of litmus paper turns blue. This shows a base has been formed. This base is magnesium hydroxide. Uses of oxygen Breathing oxygen stored in cylinders is used to support breathing eg. cylinders of oxygen for mountain climbing. Space rockets no oxygen in outer space, so rockets carry their own oxygen