- Basic Facts and Types of Reaction
- Combustion and Fermentation
- Smelting and Electrolysis
- Corrosion and Pollution
- Rate of Reaction
Basic Facts and Types of Reaction
New substances are produced (the products) when a chemical reaction takes place between reacting substances (the reactants). The chemical reaction is a permanent change and can be expressed by a word equation. However, the equation gives no indication of the speed of the reaction.
In a chemical reaction, the mass does not change (the total mass of the products is the same as the total mass of the reactants) but there is always a change in energy. This is usually heat so the temperature in a reaction changes. A visible change may occur, such as a gas given off; a solid made; or a colour change.
A reaction which produces energy is said to be exothermic (e.g. combustion). A reaction which takes in energy is called an endothermic reaction (e.g. photosynthesis).
exothermic: sodium hydroxide (NaHO) + hydrochloric acid (HCl) --> sodium chloride (NaCl) + water (H2O)
endothermic: calcium nitrate (Ca(NO3)2) + sodium carbonate (2NaCO3) --> calcium carbonate (CaCO3) + sodium nitrate (2NaNO3)
Many reactions involve oxygen and hydrogen. Oxidation takes place when oxygen is added or hydrogen is removed. Reduction is the opposite of oxidation (i.e. oxygen is removed or hydrogen is added). For example
magnesium (2Mg) + oxygen (O2) --> magnesium oxide (2MgO) is oxidation (magnesium is oxidized - oxygen is added)
ethene (C2H4) + hydrogen (H2) --> ethane (C2H6) is reduction (ethene is reduced - hydrogen is added)
Very often oxidation and reduction take place together in a redox reaction. For example, lead(II) oxide heated in a stream of dry hydrogen:
lead(II) oxide (PbO) + hydrogen (H2) --> lead (Pb) + water (H2O)
Lead(II) oxide is reduced to lead since oxygen is lost. Hydrogen is oxidized as oxygen is added. Hydrogen is called the reducing agent as it brings about the reduction of lead(II) oxide. Lead(II) oxide is called the oxidising agent as it brings about the oxidation of hydrogen.
A decomposition reaction is when a substance splits (decompose) into two or more sunstances. Some substances decompose slowly on standing - hydrogen peroxide (H2O2) decomposes slowly at room temperature:
hydrogen peroxide (2H2O2)--> water (2H2O) + oxygen (O2)
This reaction can be speeded up with a catalyst such as manganese(IV) oxide. This is an example of catalytic decomposition.
Thermal decomposition is decomposition on heating. Examples are:
calcium carbonate --(heat)--> calcium oxide + carbon dioxide
copper(II) sulphate crystals --(heatr)--> anhydrous copper(II) sulphate + water
Combustion and Fermentation
A combustion reaction is a reaction where a substance (e.g. fuel) reacts with oxygen to release energy (heat & light). When a combustion occurs oxides (e.g. CO2) are formed. That is, combustion reactions are oxidation reactions such as burning carbon in oxygen or buring magnesium in oxygen:
carbon + oxygen --> carbon dioxide
magnesium + oxygen --> magnesium oxide
When hydrocarbons burn in oxygen, different products are possible dependent on the supply of oxygen. If burnt in excess air or oxygen, CO2 and water are produced. However, if burnt in a limited amount of air/oxygen, carbon monoxide (CO) and water are produced. CO is very poisonopus as it combines with haemoglobin in the blood to form carboxy-haemoglobin which prevents oxygen being transported around the body and leads to death. Combustion needs oxygen and heat to continue the reaction.
Fermentation is an important industrial process which converts sugar or starch solution into ethanol and carbon dioxide gas. Enzymes in yeast act on the solution at room temperature and fermentation continues until about 10% of ethanol is present after which the enxymes are then poisoned by the ethnaol. Fermentation is used to prepare wine from grape juice or beer from hops.
Smelting and Electrolysis
Smelting is the extraction of metals from their ores. It uses heat and a chemical reducing agent (i.e. a reduction reaction where oxygen is removed) to decompose the ore, driving off other elements as gases or slag and just leave the metal behind. The reducing agent is commonly a source of carbon such as coke (or in earlier times charcoal). The carbon (or carbon monooxide derived from it) removes oxygen from the ore, leaving behind elemental metal. As most ores are impure, flux (a chemical cleaning agent) such as limestone to react with the impurities to form slag (calcium silicate). Because the reduction agent is carbon, only metals below carbon in the reactivity series can be extracted by smelting.
Metals above carbon in the reactivity series are too reactive to be decomposed by heat and 'displaced' by carbon. These metals and other substances that cannot be decomposed by heating (such as lead(II) bromide) may be decomposed with electricity. This is known as electrolysis, which can only be done if the substance is molten or in solution. The metal element of the substance will always deposit onto the cathode (negative electrode) and the other element either produce at the anode (positive electrode). Electrolysis of
molten lead(II) bromide: lead produced at the cathode and bromine produced at the anode
copper(II) sulphate solution: copper deposited as a red/brown solid on the cathode and bubles of oxygen produced at the anode
Corrosion and Pollution
Metals have a tendency to corrode. Corrosion is an oxidation process where metal reacts with oxygen and water in the air:
metal + oxygen --> metal oxide
The higher the metal in the reactive series, the more reactive it is and the more quickly it will corrode. Postassium and sodium corrode very quickly and they need to be stored in paraffin oil to prevent then coming into contact with air.
Corrosion of iron and steel is called rusting. Both air and water have to be present for rusting to occur. In fact it is oxygen in the air which is necessary for rusting. Other substances such as carbon dioxide, sulphur dioixide and salt speed up rusting. Rusting can be reduced by:
- Oiling or greasing
- Painting (iron railings)
- Coating with plastic (e.g. washing-up racks)
- Galvanising (coating with zinc)
- Sacrificial protection where a more reactive metal over iron will react to form a hard oxide rather than the iron turning to rust
The most common source of pollution is the burning of fossil fuels, mainly in vehicles and power stations. The combustion of fossil fuels produce CO2 which is a greenhouse gas and sulphur dioxide which contributes to acid rain.
Both methane and CO2 are greenhouse gases which trap the heat inside the atmosphere and prevent it being radiated back into space. This keeps the Earth warmer than it would be without these gases and causes global warming. Sulphur dioxide in the atmosphere causes acid rain which destroy vegetation on land and kills fish in lakes as well as demaging buildings.
Rate of Reaction and Enzymes
A reaction which takes place quickly is called a fast reaction and is finished in a short time. The rate of reaction can be speed up by:
- Increase the surface area of the solid - small lumps of a chemical have a much larger surface area than a single large lump of the same mass
- Increase the concentration of the reactants - double the concentration often double the rate of reaction (i.e. halve the time taken for the reaction). In reactions involving gases, the concentration can be increased by increasing the pressure
- Increase the temperature -
- Use a catalyst - substance which alters the rate of chenical reaction without being used up. For example, vanadium(V) oxide is the catalyst to speed up the contact process to produce sulphuric acid (sulphur dioxide + oxygen --> sulphur trioxide). Sometimes a catalyst is used to slow down a reaction, such as additives added to food to prevent it going bad.
Enzymes are biological catalysts that control chemical reactions in living things. Enxymes are proteins and they have specific properties:
- A particular enzyme will only catalyse particular reactions, not all reactions
- Work only over a limited temperature range (in the human body, enzymes will work at around the normal body temperature of 37°C
- When enzymes are heated they are chemically changed and then stop working
Examples of enzymes are:
- amylase in saliva which breaks down large starch molecules into small glucose molecules
- enzymes in yeast which convert sugar into ethanol during fermentation
- enzymes in biological washing powders which remove stains in cold water