210 likes | 403 Views
Week 3, Lesson 3. Chapter 13 – Introducing Acids & Bases. Acids. Acids are commonly used in our homes. Many foods contain acids to enhance flavour or as a preservative. The sour taste of some food substances is due to the presence of acid.
E N D
Week 3, Lesson 3 Chapter 13 – Introducing Acids & Bases
Acids • Acids are commonly used in our homes. • Many foods contain acids to enhance flavour or as a preservative. • The sour taste of some food substances is due to the presence of acid. • In industry solutions of acids are used extensively to produce a wide range of products, such as fertilisers, drugs, explosives and plastics. • Acids are also used to clean metal surfaces before use.
Acids cont… • The three most commonly used acids in the lab are hydrochloric acid, sulfuric acid and nitric acid. • Acids can cause severe problems if misused. • Eg, acid rain, acid contamination.
Properties of Acids • Change the colour of some indicators. • Tend to be corrosive • Taste sour • React with bases • Have a relatively low pH.
Bases • Acids react with bases. • In solution, bases sometimes taste bitter and have a slippery feel. • They react with some plant extracts to counteract the effect of acids. • If a base is continually added to a solution of an acid, the properties of the acid slowly disappear. • The same goes for if an acid is added to a basic solution. • The acid and base are said to have neutralised each other. • Bases are effective cleaners because they react with fats and oils to produce water-soluble soaps.
Properties of Bases • Turn litmus blue • Have a slippery feel • Are caustic • React with acids • Have a relatively high pH
Safety with Acids and Bases • Acids and bases should be treated with caution and you should avoid these solutions coming in contact with your skin and eyes. • You should always wear safety goggles and lab coats. • Concentrated sulfuric acid is a viscous liquid and accidents can happen if water is added to the acid in order to produce dilute solutions. • The ionisation/hydrolysis of sulfuric acid is an energy-releasing reaction. • A common method of neutralising an acid spill is to add sodium hydrogen carbonate powder.
Indicators • One of the characteristic properties of acids and bases is their ability to change to colour of certain plant extracts. • Litmus is a dye obtained from lichen. • In the presence of acid, litmus turns red. • Such plant extracts are called indicators. • Indicators are often extracted from plant dyes and are themselves acids or bases. • They change from one colour in acid to another in basic solution. • Common indicators include methyl orange, phenolpthalein and litmus. • Universal indicator is a mixture of many indicators and changes through a range of colours to easily establish the pH of a solution. • The indicators that undergo a single colour change are also used for many analyses. • pH meters are often used in laboratories for more accurate determination of solution acidity and are not affected by coloured solutions.
Reactions Involving Acids and Bases • Acids react with many metals particularly those found in the main groups of the periodic table, although they also react with several transition metals. • These reactions typically produce a solution of a metal salt and hydrogen gas. • Salts are compounds usually made up of a metal cation and a non-metal anion. • The salt produced will depend on the acid used in the reaction. • Acids also react with many compounds such as metal hydroxides and metal carbonates. • A salt is again produced with each of these, together with water. • In the case of metal carbonates, carbon dioxide is also formed.
General Reaction Types involving Acids • There are numerous ways in which acids and bases react. • It is possible to group some reactions together on the basis of the reactants involved and the products formed. • The following are six of the more common reaction types.
Acid + Reactive Metal Salt + Hydrogen • Reactive metals include Ca, Mg, K but not Cu, Ag or Au. • When dilute acids are added to main group metals, and some transition metals, bubbles of hydrogen gas are released, and a salt is formed. • For example, the reaction between dilute hydrochloric acid and zinc metal can be represented by the equation: 2HCl(aq) + Zn(s) ZnCl2(aq) + H2(g)
Cont… • This reaction can also be represented by an ionic equation. • In an aqueous solution the hydrochloric acid is ionised and the zinc chloride is dissociated. • The equation can therefore be written as: 2H+(aq) + 2Cl-(aq) + Zn(s) Zn2+(aq) + 2Cl-(aq) +H2(g) However the chloride is the only spectator ion. The ionic equation is therefore: 2H+(aq) + Zn(s) Zn2+(aq) + H2(g)
Acid + Metal Hydroxide Salt + Water • Metal hydroxides include NaOH, Ca(OH)2 and Mg(OH)2. • The hydroxide ions from metal hydroxides readily react with the H+ ion from acid. • The products are a salt and water. • For example, the reaction between solutions of sulfuric acid and sodium hydroxide can be represented as: H2SO4(aq) + 2NaOH(aq) Na2SO4(aq) + H2O(l)
Cont… • The sulfuric acid is ionised in solution and both sodium hydroxide and sodium sulfate are ionic, and therefore dissociate. • Water however is a covalent molecular substance that does not ionise to a significant extent. • So the equation becomes: 2H+(aq) + SO42-(aq) + 2Na+(aq) + 2OH-(aq) 2Na+(aq) + SO42-(aq) + 2H2O(l) The ionic equation is therefore: H+(aq) + OH-(aq) H2O(l)
Acid + Metal Oxide Salt + Water • Metal oxides include Na2O, MgO, CaO and ZnO. • Metal oxides are usually basic since they contain the oxide ion. • Water soluble oxides tend to form the hydroxide ion. • The reaction between dilute nitric acid and solid calcium oxide can be represented by the equation: 2HNO3(aq) + CaO(s) Ca(NO3)2(aq) + H2O(l)
Cont… • The calcium oxide is solid, so the ions are not dissociated. The nitric acid is ionised in solution and therefore dissociates. • The equation becomes: 2H+(aq) + 2NO3-(aq) + CaO(s) Ca2+(aq) + 2NO3-(aq) + H2O(l) The nitrate ions are the spectator ions so the ionic equation is: 2H+(aq) + CaO(s) Ca2+(aq) + H2O(l)
Acid + Metal Carbonate Salt + Water + Carbon Dioxide • Metal carbonates include Na2CO3, MgCO3 and CaCO3. • Acids reacting with metal carbonates produce carbon dioxide gas with a salt and water. • For example the reaction between a solution of nitric acid and solid magnesium carbonate can be represented by the equation: 2HNO3(aq) + MgCO3(s) Mg(NO3)2(aq) + H2O(l) + CO2(g) The nitrate ions are the spectator ions so the ionic equation is: 2H+(aq) + MgCO3(s) Mg2+(aq) + H2O(l) + CO2(g)
Acid + Metal Hydrogen Carbonate Salt + Water + Carbon Dioxide • Metal hydrogen carbonates include NaHCO3, KHCO3 and Ca(HCO3)2. • Acids added to metal hydrogen carbonates (aka bicarbonates) produce a salt, water and carbon dioxide. • For example, the reaction between solutions of hydrochloric acid and sodium hydrogen carbonate can be represented by the equation: HCl(aq) + NaHCO3(aq) NaCl(aq) + H2O(l) + CO2(g) The sodium and chloride are spectator ions in this reaction so the ionic equation is: H+(aq) + HCO3-(aq) H2O(l) + CO2(g)
Acidic Oxide (non-metal oxide) + Base Salt + Water • Acidic oxides include SO2, SO3, P4O10 and CO2. • When oxides are added to water they form acidic solution. • The reaction of these with bases produce a salt and water. • For example the reaction between carbon dioxide and a solution of calcium hydroxide can be represented by the equation: CO2(aq) + Ca(OH)2(aq) CaCO3(s) + H2O(l)
Cont… • The table below lists some common acidic oxides and the anions they produce in the reaction.