CHEMISTRY FORM TWO STUDY NOTES TOPIC 1: OXYGEN & TOPIC 2: HYDROGEN

Saturday, August 12, 2017 0
TOPIC 1: OXYGEN
Oxygen exists in air to an extent of 21% by volume (or 23% by weight). It is the most abundant element on earth, accounting for ½ the total mass of the earth’s crust. Oxygen is mainly found in combined states as oxides, hydroxides, silicates, sulphates, carbonates, water, etc. Its ease of combination with other elements to form compounds shows that oxygen is a very reactive element.
Oxygen can be prepared in the laboratory from either hydrogen peroxide solution or potassium chlorate salt.
A Sample of Oxygen Gas in the Laboratory
Prepare a sample of oxygen gas in the laboratory
(i) Laboratory preparation of oxygen from hydrogen peroxide solution
The most common method for the preparation of oxygen in the laboratory is by decomposition of hydrogen peroxide solution. The gas is prepared by catalysing the decomposition of hydrogen peroxide with manganese (IV) oxide. At room temperature hydrogen peroxide decomposes (breaks down) very slowly. It decomposes to water and oxygen.
To speed up the decomposition process, and hence collect substantial amount of oxygen gas within a short time, black manganese (IV) oxide is added as a catalyst.
A catalyst is a substance that, although present in small quantities, will alter the rate of a chemical reaction but will remain chemically unchanged at the end of the reaction.
Preparation method
Hydrogen peroxide (20 vol.) is added drop by drop to manganese (IV) oxide, which catalyses the decomposition of the peroxide. Oxygen is collected over water as shown in figure bellow. The gas is collected by downward displacement of water because it is only slightly soluble in water.

Apparatus for laboratory preparation of oxygen from hydrogen peroxide solution
(ii) Laboratory preparation of oxygen from potassium chlorate
Oxygen can also be prepared by thermal decomposition of potassium chlorate. When this compound is heated, it decomposes slowly into potassium chloride and oxygen:
Preparation method
A grinded mixture of potassium chlorate and manganese (IV) oxide, at a ratio of 4:1, is placed in hard glass tube and fitted up as shown in figure bellow. The mixture is then heated and oxygen gas is readily given off. The gas is collected over water. Oxygen has almost the same density as air, so it cannot be collected by the upward displacement of air. It is possible to collect it by downward displacement of water as shown in the figure because it is only slightly soluble in water.

Apparatus for laboratory preparation of oxygen from potassium chlorate
Test for oxygen
Oxygen rekindles a glowing splint of wood. No gases behave like this except dinitrogen oxide, NO2, from which oxygen can be distinguished by the following properties:
1. Oxygen has no smell but dinitrogen oxide has a sweet, sickly smell.
2. When heated with nitrogen monoxide, oxygen produces brown fumes of nitrogen dioxide.
Dinitrogen oxide has no effect on nitrogen monoxide.
Simple Experiments to Demonstrate Properties of Oxygen Gas
Perform simple experiments to demonstrate properties of oxygen gas
1. Action of oxygen on metals
The manner in which oxygen reacts with metals is summarized in the list below.
Reaction with specific metals
Sodium
When burnt in excess of oxygen, sodium burns with an intense yellow flame to give sodium peroxide.
The product is a yellow solid which dissolves in water to give an alkaline solution.
Calcium
The metal burns in air with a red flame giving a white solid of calcium oxide:
Magnesium
Magnesium burns with a brilliant white flame, leaving a white ash of magnesium oxide:
Iron
Iron burns in air with a shower of sparks leaving a brown-black solid of triiron tetraoxide:
Copper
Copper burns in a stream of oxygen to give a black solid of copper (II) oxide:
In general, metals react with oxygen to form basic oxides.
Action of oxygen on non-metals
Carbon
Red-hot carbon combines vigorously with oxygen to form carbon dioxide, giving no residue:
Sulphur
Sulphur burns with a blue flame giving misty white fumes of sulphur dioxide:
Phosphorus
Phosphorus bursts into flame in air or oxygen, without being heated (that is why it is stored under water). A white solid, phosphorus pentoxide is formed.
Properties of Oxygen
Explain properties of oxygen
Physical properties
  1. It is a clear, colourless gas with no smell.
  2. It is a neutral gas (it is neither basic nor acidic in character)
  3. It is slightly soluble in water (100 cm3 of water at room temperature dissolves about 4 cm3 of oxygen).
  4. It has almost the same density as water although slightly denser than air. 5. It boils at -183ºC and freezes at -218ºC.
Chemical properties
  1. Oxygen supports combustion
  2. It is a very strong oxidizing agent.
  3. Oxygen is very reactive. It reacts vigorously with a great many metals and non-metals to form basic and acidic oxides respectively. Metal + Oxygen gives metallic oxide (most of these are basic in character). Non-metals + Oxygen gives non–metallic oxide (most of these are acidic in character).

 TOPIC 2: HYDROGEN
Hydrogen is the lightest of all the elements. There is very little hydrogen in the earth's atmosphere. Hydrogen is so light that its molecules are not held by the earth's gravity and they diffuse into space. Overall, it is the most common element in the universe. It is probable that is forms about 90% of the total mass of the universe. It is believed that the sun composes almost of hydrogen and helium. Hydrogen occurs naturally in air as hydrogen gas. It also occurs in combined state in water, acids, petroleum, and natural gas and in almost all organic substances (proteins, carbohydrates, fats, etc.).
The Preparation of Hydrogen Gas in a Laboratory
Explain the preparation of hydrogen gas in a laboratory
Hydrogen is most commonly prepared in the laboratory by the action of dilute mineral acids on certain metals. The most convenient way to prepare hydrogen in the laboratory is by addition of dilute hydrochloric acid on zinc granules. Zinc and hydrochloric acid are chosen because they produce the gas at a steady rate.
The gas may be collected by downward displacement of water. But when the gas is required free from moisture it is passed through water to remove first, any hydrogen chloride gas and then through concentrated sulphuric acid to remove moisture before being collected by upward delivery. The gas is prepared by upward delivery method because it is lighter than air and is soluble in water.
Method of preparation
Set up the apparatus as shown in figure bellow. Into a flat-bottomed flask, put some pieces of zinc and add dilute hydrochloric acid by means of a thistle funnel. There is effervescence, and a gas is given off which is collected over water. Zinc chloride, which is formed, dissolves to form zinc chloride solution.

Preparation of hydrogen by the action of dilute hydrochloric acid on zinc metal
Test for hydrogen
A mixture of hydrogen and air explodes with a 'pop' sound when a flame is applied.
The Properties of Hydrogen
Explain the properties of hydrogen
Physical properties
Includes
  1. It is a colourless, tasteless and odourless gas.
  2. It is almost insoluble in water (2 volumes of hydrogen gas dissolve in 100 volumes of water at 8ºC).
  3. It is the lightest of all gases. It is about 20 times lighter than air (one litre of hydrogen at 0ºC and 760 mmHg pressure weighs 0.0899 grams)
  4. It condenses at -254ºC to a colourless liquid (and liquid hydrogen freezes at -259 ºC to form colourless crystals).
  5. It is neutral to litmus. 6. It does not support combustion.
Chemical properties
1. It combines easily with other chemical substances at high temperatures. For example, it combines with oxygen to form water. A mixture of the two gases will not react at room temperature. At higher temperatures, or when a flame is applied, the mixture will explode. When hydrogen and oxygen explode, the product is water.
Water is just the common name for the substance "hydrogen oxide".
2. Hydrogen acts as a reducing agent, by removing oxygen from some compounds. For example, copper (II) oxide is reduced to copper by heating it in a stream of hydrogen. The hydrogen is oxidized to water.
3. It is neither acidic nor basic, so it a neutral gas.
4. A mixture of hydrogen and oxygen explodes when lit.
An experiment on reduction of copper (II) oxide (CuO) using hydrogen
Aim: To investigate the effect of hydrogen on copper (II) oxide
Procedure
  1. Put about 5 g of copper (II) oxide in a Pyrex test tube and set up the apparatus as shown in figure bellow. Observe and note the colour of copper (II) oxide before the start of the experiment. What colour is it?
  2. By means of a thistle funnel, add hydrochloric acid in a bottle containing zinc metal to generate hydrogen gas. Pass the gas through a U-tube containing a solid drying agent, calcium chloride.
  3. Place a dry cobalt (II) chloride paper near the mouth of a test tube as shown in figure bellow.
  4. Allow the hydrogen gas to pass through the apparatus for some time in order to displace all the air before lighting it.
  5. Heat the copper (II) oxide strongly until no further changes in colour of the cobalt (II) chloride paper takes place. You may repeat the experiment using lead (II) oxide and compare the results.

Reduction of copper (II) oxide with hydrogen gas
Questions
  1. What happens to the copper (II) oxide during the experiment?
  2. (a) What happens to cobalt (II) chloride paper? (b)Why is it used? (c) What other substance can serve the same purpose as cobalt (II) chloride paper?
  3. Enough time should be allowed for all the air in the test tube to be replaced by hydrogen before lighting the gas. What is bad about lighting a mixture of air and hydrogen?
  4. What do you think can cause the size of the hydrogen flame to deteriorate?
  5. (a) What element did hydrogen take from the copper (II) oxide? (b) Can hydrogen take the same element from any metal oxide?
Answers
1. Black copper (II) oxide is reduced by hydrogen to brown copper metal.
2. (a) Cobalt (II) chloride paper changes its colour from blue to pink.
(b) The paper is used to indicate that water has been formed in the reaction between copper (II) oxide and hydrogen. This water turns the colour of the paper from blue to pink.
(c) The other substance that can be used instead of cobalt (II) chloride paper is white anhydrous copper (II) sulphate, which turns blue when in contact with water.
3. Enough time should be allowed for hydrogen to replace the air in the test-tube because a mixture of hydrogen and oxygen in the tube is explosive when lit.
4. The size of the hydrogen flame deteriorates with time as hydrogen supply grows smaller following complete displacement of hydrogen of the hydrochloric acid with zinc. Deterioration can also be caused by use of excess copper (II) oxide or strong heating, meaning that most hydrogen is used in the reduction of the oxide.
5. (a) The element taken by hydrogen from copper (II) oxide is oxygen. In this experiment, hydrogen reduces copper (II) oxide to copper, while hydrogen itself is oxidized to water:
(b) No. Hydrogen can only reduce those metals that are below it in the electrochemical (activity) series.
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