Compounds PDF

Compounds  A compound is a pure substance formed when two or more elements combine chemically.  A pure substance is a material that has a constant composition (is homogeneous) and has consistent properties throughout.  Water is a simple compound formed from the elements hydrogen and oxyge...

Compounds  A compound is a pure substance formed when two or more elements combine chemically.  A pure substance is a material that has a constant composition (is homogeneous) and has consistent properties throughout.  Water is a simple compound formed from the elements hydrogen and oxygen. Hydrogen + Oxygen → Water (pure element) (pure element) (pure compound) Cont.  The element hydrogen reacting with the element oxygen to produce the compound water. Cont.  A model of water showing 2 H atoms and one O atom.  Models such as this can be built to show what a compound looks like: Cont.  Water molecules contain two atoms of hydrogen and one atom of oxygen, and water has the chemical formula H2O.  If there is only one atom of an element in the molecule, no number is required in the formula, only its symbol is given, as in the case of oxygen in the water molecule H2O. Cont. Stop and think: What is the formula for the molecule shown in the diagram which contain carbon (black sphere) and hydrogen (white spheres)? Cont.  Elements other than hydrogen will also react with oxygen to form compounds called oxides.  For example, magnesium reacts violently with oxygen gas to form the white powder magnesium oxide. This reaction is accompanied by a release of energy as new chemical bonds are formed. Cont.  Magnesium burns brightly in oxygen to produce magnesium oxide. Cont.  When a new substance is formed during a chemical reaction, a chemical change has taken place. magnesium + oxygen → magnesium oxide  When substances such as hydrogen and magnesium combine with oxygen in this way, they are said to have been oxidised, and this process is known as oxidation.  Reduction is the opposite of oxidation. In this process oxygen is removed rather than added. NB: Oxidation is gain of oxygen. Reduction is loss of oxygen. Cont.  Chemical processes that involves reduction and oxidation are called redox reactions.  For example, to extract iron from iron(III) oxide, the oxygen has to be removed (reduction).  The reduction of iron(III) oxide can be done in a blast furnace using carbon monoxide.  The iron(III) oxide loses oxygen to the carbon monoxide and is reduced to iron. iron(III) oxide + carbon monoxide → iron + carbon dioxide Cont.  Carbon monoxide is the reducing agent.  A reducing agent is a substance that reduces another substance during a redox reaction.  In the reaction, carbon monoxide is oxidised to carbon dioxide by the iron(III) oxide.  In this process, the iron(III) oxide is the oxidising agent.  An oxidising agent is a substance which oxidises another substance during a redox reaction. Cont. NB:  Redox reactions involve simultaneous oxidation and reduction.  An oxidising agent is a substance that oxidises another substance and is itself reduced.  A reducing agent is a substance that reduces another substance and is itself oxidised. Practice Questions 1) Zinc is extracted from its ore zinc blende in a furnace by a redox reaction. What does the term ‘redox reaction’ mean? 2) Identify the oxidising and reducing agents in each of the following reactions: a) copper(II) oxide + hydrogen → copper + water b) tin(II) oxide + carbon → tin + carbon dioxide c) PbO(s) + H2(g) → Pb(s) + H2O(l).