JC-Learn Science Notes Acids and Bases 2 1 P a g e
Acids and Bases 2 The two most common laboratory acids are hydrochloric acid (HCl) and sulfuric acid (H2SO4). The two most common laboratory bases are sodium hydroxide (NaOH) and calcium hydroxide (Ca(OH)2). Sodium hydroxide is commonly called caustic soda and is used to clear blocked drains. It is found that the properties of an acid are neutralised by a base. Therefore, this type of reaction is called a neutralisation reaction. This is because the substance formed is often neutral. Example: Toothpaste is basic substance. It neutralises the acids that damage our teeth. When an acid and a base neutralize each other, a salt is formed; for example, sodium chloride. In general, a neutralisation reaction may be represented as acid + base = salt + water. A salt is formed when the hydrogen in an acid is replaced by a metal. For example, if the H in HCl is replaced by a metal like sodium, we form the salt sodium chloride. Titration The general procedure is that the acid (in a burette) is slowly added to base (in conical flask). The acid is added until the indicator starts to change colour. A burette is used because it is an accurate way of measuring liquid volumes. The method of adding one solution from a burette to another solution in order to find out how much of the two solutions will just react with each other is called titration. Mandatory Experiment *To titrate hydrochloric acid against sodium hydroxide and prepare a sample of sodium chloride 2 P a g e
Apparatus required: burette, conical flask, retort stand and clamp, pipette, pipette filler, white tile, funnel, wash bottle, dropper, evaporating dish, Bunsen burner, tripod, wire gauze, beakers, dilute hydrochloric acid, dilute sodium hydroxide, methyl orange indicator Hyrochloric acid + sodium hydroxide HCl + NaOH NaCl + H2O Method 1. Set up apparatus as shown in diagram 2. Add the hydrochloric acid from the pipette slowly into the conical flask with sodium hydroxide. Once the indicator in the conical flask changes colour, stop the titration and note the volume of the acid added. 3. Repeat experiment without using the indicator 4. The sodium chloride solution is put into an evaporating basin and then this is put under Bunsen burner. 5. It is evaporated almost to dryness. The solution is left to cool. 6. Crystals of sodium chloride are formed. Result White crystals of sodium chloride are formed Conclusion Hydrochloric acid and sodium hydroxide react to form sodium chloride 3 P a g e
Fossil Fuels A fuel is any substance that burns in oxygen to produce heat. A common example of a fuel that is a gas is natural gas. Natural gas is mainly methane. The most commonly use fuels are coal, oil and natural gas. Coal came from compressed plants and rock from millions of years ago. Oil and natural gas came from decayed remains of sea creatures. Fossil fuels are fuels that were formed from the remains of plants and animals that lived millions of years ago. Fossil fuels contain hydrocarbons hydrocarbons are compounds consisting of hydrogen and carbon only. Burning hydrocarbons When hydrocarbons are burned, carbon dioxide and water are always formed. A candle (which contains hydrocarbons) is burned and this lets off water and carbon dioxide. The liquid formed is tested with blue cobalt chloride paper and it turns pink, showing it is water. The gas formed turns limewater milky, showing it is carbon dioxide. Fossil fuel + oxygen carbon dioxide + water 4 P a g e
Eg : methane + oxygen carbon dioxide + water Acid Rain Rain is naturally slightly acidic. The carbon dioxide in air reacts with rain to form carbonic acid, with a ph of 5.5. Rainwater with a ph of less than 5.5 is called acid rain, and this contains dilute sulfuric acid and nitric acid. 1. Sulfuric acid Sulfur dioxide comes from burning fossil fuels (coal and oil). Sulfur dioxide is very soluble in water and it dissolves in water to form sulfuric acid. 2. Nitric acid nitric acid comes from the emission from exhausts of motorcars. Cars engines take in nitrogen and oxygen from air. Car exhausts emit oxides of nitrogen. These oxides of nitrogen dissolve in water vapour in clouds. This forms nitric acid. Problems of acid rain: Acid rain destroys lakes it washes aluminium salts into lakes, which interferes with operation of fish gills and they die as a result. Acid rain harms trees damages foliage and removes minerals from soil. Tree roots should absorb these minerals. Therefore, many trees die. Acid rain attacks stone and iron Many buildings suffer from acid rain because limestone reacts with acid rain. Solving acid rain problems: Remove sulfur dioxide from emissions of power stations. Reduce sulfur content of fuels like oil and gas. Burn less fossil fuels and use other forms of energy, eg. wind energy. 5 P a g e