Topic 5: Acids, Bases and salts. Acids contain ________ ions. Acids are solutions of pure compounds in water. The pure compounds are molecular but in water the molecules break up to form ions. They always give ______ ions. Example: Hydrochloric acid gives ___________ and __________ ions HCl (aq) _______ (aq) + ______(aq) The more the ______ ions there are in the solution, the more acidic it is. Examples of some common acids: Name Formula Hydrochloric acid Sulfuric acid Nitric acid Carbonic acid Ethanoic acid Methanoic acid Citric acid Strong or weak acid strong strong strong weak weak Weak weak HCl H2SO4 HNO3 H2CO3 CH3COOH HCOOH C6H8O7 Where it is found in the stomach in acid rain in acid rain in soft drinks in vinegar ants and nettles in lemons, oranges Strong and weak acids Different acids have different pH values. A typical _________ acid is hydrochloric acid. In this acid nearly all the acid molecules break up to form ions. Example: HCl(aq) _____(aq) Example: CH3COOH(aq) + _____(aq) _____________(aq) + __________(aq) The acid molecule splits up to form ions but at the same time they join again. Carbonic acid, ethanoic acid, methanoic acid and citric acid are all examples of ___________ acids. The basicity of an acid We have seen that it is characteristic of an acid to yield ________ ions in aqueous solution. The number of ________ ions produced per molecule of the acid is called its ___________. Page 1 of 19 Fill in the table below Acid Hydrochloric acid (HCl) Sulfuric acid (H2SO4) Phosphoric acid (H3PO4) Nitric acid (HNO3) Ions formed (H+ and Cl-) Basicity Monobasic Note: CH3COO-(aq) + CH3COOH(aq) H+(aq) The pH scale On this scale: An acidic solution has a pH number smaller than ________. An alkaline solution has a pH number greater than ________. A neutral solution has a pH number equal to _________ The stronger the acid the ________ the pH. The stronger the alkali the ________ the pH. The table below shows a list of indicators and their colours in acid and alkaline solutions. Indicator Phenolphthalein Methyl orange Red litmus Blue litmus Methyl red Colour in a neutral solution colourless orange red blue red Colour in an acidic solution colourless pink red red red Page 2 of 19 Colour in an alkaline solution Pink Yellow Blue Blue Yellow Reaction of the common metals with dilute acids Dilute hydrochloric acid Dilute sulfuric acid Reaction of Rapid effervescence in the cold to magnesium form hydrogen. The remaining with… solution is colourless. __________________________ Rapid effervescence in the cold to form hydrogen. The remaining solution is colourless. ____________________________ Reaction of Steady effervescence to liberate zinc with… hydrogen in the cold. The remaining solution is colourless. Usually slow effervescence to liberate hydrogen. The remaining solution is colourless. ____________________________ ___________________________ Reaction iron with.. of Fairly slow effervescence in the cold to give hydrogen. The remaining solution is pale green in colour. ____________________________ Fairly slow effervescence in the cold to give hydrogen. The remaining solution is pale green colour. ____________________________ The bubbles stop when the reaction is over. The unreacted magnesium is then removed by filtering. When magnesium is dropped in dilute sulfuric acid, hydrogen quickly bubbles off. The filtrate is heated to evaporate the water. A white solid is left behind. This solid is magnesium sulfate. Write a balanced equation for this reaction: ______________________________________ Note: sodium and potassium are too reactive to be used with any of these acids. These would explode. On the other hand copper, silver and gold are too unreactive and so they don’t react with these acids. When a metal takes the place of hydrogen in an acid, the compound that forms is called a salt. Therefore usually acids react with metals, forming hydrogen and a salt. Metal + Acid Salt + hydrogen The salts of sulfuric acid are always called ________________. The salts of hydrochloric acid are called __________________. The salts of nitric acid are called __________________. Page 3 of 19 Action of acids on insoluble bases In this method a soluble salt is produced from the reaction between an acid and an insoluble base. However no effervescence is observed as no gas is produced. Dilute acids will react with insoluble bases to give a salt and a water Insoluble base (metal oxide or hydroxide) + Acid Salt + Water Heat some dilute sulfuric acid in a beaker and add to it, a little at a time some black copper(II)oxide. Stir gently. Write down your observations: ________________________________________________ Chemical balanced equation is: ________________________________________________________________________ Many salts of metals can be similarly prepared using either an insoluble oxide or an insoluble hydroxide of the metal with the appropriate acid. Write balanced chemical equations: 1. zinc oxide + dilute sulfuric acid ____________________________________________________________________ 2. zinc hydroxide + dilute sulfuric acid ____________________________________________________________________ 3. lead (II)oxide + dilute nitric acid ____________________________________________________________________ 4. lead(II) hydroxide + dilute nitric acid ____________________________________________________________________ 5. magnesium oxide + dilute hydrochloric acid ____________________________________________________________________ 6. magnesium hydroxide + dilute hydrochloric acid ____________________________________________________________________ Actions of acids on carbonates Carbonate + Acid salt + water + carbon dioxide For example when copper (II) carbonate is added in excess to dilute nitric acid, effervescence is observed due to the production of _______________________. CuCO3(s) + 2 HNO3(aq) Cu(NO3)2(aq) + CO2(g) + H2O(l) Write balanced equation for the following reactions. 1. zinc carbonate + dilute sulfuric acid ___________________________________________________________________ 2. calcium carbonate + dilute hydrochloric acid ___________________________________________________________________ 3. lead (II) carbonate + dilute nitric acid ___________________________________________________________________ Page 4 of 19 Note: If the salt produced is insoluble in water, the salt will precipitate on the unchanged carbonate and stops the reaction. Example, if dilute sulphuric acid is added to calcium carbonate, there is rapid effervescence for a few seconds, but the reaction quickly stops. CaCO3(s) + H2SO4(aq) CaSO4(s) + CO2(g) + H2O(l) The very slightly soluble calcium sulfate precipitates on the calcium carbonate and the reaction stops. Action of acids on sulphites Sulphite + acid salt + water + sulfur dioxide When sodium sulphite is warmed with dilute sulphuric acid or dilute hydrochloric acid effervescence is observed and a gas _______________ is produced. Na2SO3(aq) + H2SO4(aq) Na2SO4(aq) + H2O(l) + SO2(g) Write a balanced equation for the reaction between sodium sulphite and dilute hydrochloric acid. ____________________________________________________________________ Bases and alkalis Definition of a base: _________________________________________________________________________ ________________________________________________________________________ Definition of an alkali: _________________________________________________________________________ ________________________________________________________________________ Page 5 of 19 Only a few alkalis are known. The common ones are: ට Sodium hydroxide (NaOH) also known as caustic soda ට Potassium hydroxide (KOH) also known as caustic potash ට Calcium hydroxide (Ca(OH)2 also known as lime water ට Ammonia solution (NH3 (aq)) Strong and weak alkalis NaOH(aq) Na+(aq) + OH-(aq) However ammonia solution is a ________ alkali because only some ammonia molecules form ions in solution. NH3(g) + NH4+(aq) + OH-(aq) H2O(l) Reaction of alkalis with acids All alkalis react with acids producing a ___________ and ____________. This is a solution of hydrochloric acid. It contains H+ ions and Clions. It will turn blue litmus _______. This is a solution of sodium hydroxide. It contain Na+ ions and OH- ions. It will turn red litmus to ______. When two solutions are mixed, H+ & OH- ions join to form water molecules. The result is a neutral solution of sodium chloride, containing Na+ & Clions. It has no effect on litmus. Write a balanced equation for this reaction. ________________________________________________ This reaction is called a __________________ reaction. The alkali has __________ the acid by removing its H+ ions, and turning them into water. Reaction of alkalis with ammonium salts. All the alkalis except ammonia will react with ammonium compounds. The products formed are salt, water and ammonia. Ammonium salt + alkali salt + water + Ammonia Write a balanced equation for the reaction between ammonium chloride and calcium hydroxide. _________________________________________________________________________ Page 6 of 19 Some useful bases Example: ට Magnesium hydroxide Mg(OH)2 is found in milk of magnesia. It neutralises excess stomach acid soothing indigestion. ට Some alkalis are used as bleaches and as alkaline cleaners. Many kitchen cleaners are alkalis because they contain ammonia (NH3) or sodium hydroxide (NaOH) which attack grease. ට Quicklime (calcium oxide – CaO) and slaked lime (calcium hydroxide – Ca(OH)2 are used to neutralise acid soils. Neutralisation An alkali can neutralise an acid and destroy its acidity. Let’s look at this reaction. HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l) Acid + Alkali Salt + water H+(aq) + OH-(aq) H2O(l) Neutralisation is the formation of water from hydroxide ions (OH- ions) and hydrogen ions (H+ ions). Normal salts and acid salts In this example the acid is hydrochloric acid (HCl). The hydrogen ion (H + ion) was replaced by a sodium ion (Na+ ion) so that sodium chloride, a salt was formed. When an acid can produce more than 1 hydrogen ion per molecule (example: H 2SO4 can produce 2H+ ions per molecule) the replacement of H + ions can occur in stages. Thus one of the H+ of sulfuric acid may be replaced by one Na+ ion to give Na+ HSO4- (sodium hydrogen sulfate), after which a second similar reaction may yield (Na +)2SO42- (sodium sulfate Na2SO4) H2SO4(aq) H+(aq) + HSO4-(aq) Give the reaction of: Sodium hydroxide + carbonic acid (H2CO3) Page 7 of 19 2H+(aq) + SO42-(aq) Examples: Acid Sulfuric acid H2SO4 Carbonic acid H2CO3 Acid Salt Sodium hydrogen sulfate NaHSO4 Sodium hydrogen carbonate NaHCO 3 Normal Salt Sodium sulfate Na2SO4 Sodium carbonate Na2CO3 Methods of preparing salts 1. Acid + metal Some dilute sulfuric acid is put in a beaker and excess zinc is added. The zinc begins to dissolve and effervescence is observed. Effervescence stops when all the acid has been used up. Some zinc is left unreacted. This is removed by ___________, which leaves an aqueous solution of ________________. The solution is heated to evaporate some of the water. Then it is left to cool. Crystals of ___________ start to form. Write a balanced equation for this reaction _________________________________________________________________________ Page 8 of 19 2. Acid + insoluble base Copper(II)oxide is an insoluble base. Although copper will not react with dilute sulfuric acid, copper(II)oxide will. Some copper(II)oxide is added to dilute sulfuric acid. On warming it dissolves and the solution turns blue. More is added until no more dissolves. The excess, unreacted copper(II)oxide is removed by ____________. This leaves a blue solution of _______________ in water. The solution is heated to evaporate some of the water. Then it is left to cool. Blue crystals of __________ start to form. Write a balanced equation for this reaction _________________________________________________________________________ 3. Acid + carbonate The same method (as in method 2) is used to prepare a salt from a carbonate. This is because most carbonates are insoluble in water. In this method dilute hydrochloric acid is added to excess copper(II)carbonate. Effervescence occurs while some unreacted copper(II)carbonate is left. This is removed by ____________ leaving a green solution of _____________. The solution is heated to evaporate some of the water. Then it is left to cool. Crystals of _____________ start to form. Write a balanced equation for this reaction. _________________________________________________________________________ 4. Precipitation from two aqueous solutions For example, insoluble barium sulphate is precipitated when solutions of barium chloride and magnesium sulphate are mixed. This is a solution of barium chloride BaCl2. It contains barium ions and chloride ions. This is a solution of magnesium sulphate, MgSO4. It contains magnesium ions and sulphate ions. When the two solutions are mixed, the barium ions and sulfate ions bond together, because they are strongly attracted to each other. Solid barium sulfate precipitates. Write a balanced equation for this reaction. _________________________________________________________________________ Page 9 of 19 Some useful salts ට Sodium hydrogen carbonate (NaHCO3) (baking powder) – Is used in the manufacture of baking powders. Under the action of heat it decomposes and gives off carbon dioxide which cause the cake to ‘rise’ and so be light. This is why it is commonly called ‘baking soda’ ට Sodium carbonate (Na2CO3.10H2O) – washing soda is used to soften hard water. ට Epsom salts (MgSO4.7H2O) – used in medicine as a laxative ට Gypsum and Plaster of Paris (CaSO4.2H2O) – Gypsum is chiefly employed for the manufacture of plaster of Paris. This is used for making casts for statues, wall plasters, and in sugery when you break a bone. Page 10 of 19 Hydrogen Laboratory Preparation Hydrogen can be produced by the reaction of a metal and a dilute acid Metal + Acid Salt + Hydrogen The apparatus is set up as shown below. Dilute hydrochloric acid was dropped on some pieces of zinc in a flat bottomed flask by means of a thistle funnel. Effervescence was observed, and the gas produced, that is ____________ was collected over water. __________________ which was formed was left in the flat bottomed flask. Write a balanced equation for this reaction. _________________________________________________________________________ Test for hydrogen The following method is a test to see if an unknown gas is hydrogen 1. A wooden splint is lit 2. The wooden splint is plunged into a test tube containing a small amount of the gas. 3. If the gas is hydrogen, the gas burns with a Squeaky pop. Other ways of producing hydrogen: Hydrogen is the product of other chemical reactions: 1. The reaction of reactive metals and water. Potassium, sodium and calcium all react with water to produce the ____________ and _________ gas. Write a balanced equation for the reaction of sodium with water: _________________________________________________________________________ Page 11 of 19 2. The reaction of metals and steam. The less reactive metals do not react with water but react with steam to produce the ___________ and __________ gas. Write a balanced equation for the reaction of aluminium and steam. _________________________________________________________________________ Hydrogen as a fuel Hydrogen combines with oxygen to form water. A mixture of the two gases will explode when lit. The reactions is: 2H2(g) + O2(g) 2 H2O(l) The hydrogen and oxygen are stored in tanks in the rockets in liquid form. Physical properties of hydrogen ට Hydrogen is an invisible gas, neutral to litmus, not toxic and if pure possesses no smell. ට Hydrogen is the lightest of all gases and so diffuses very rapidly. It is about 20 times lighter than air. ට It is almost insoluble in water. Chemical properties of hydrogen 1. Reaction with a metal When a compound is formed between an element and oxygen it is called an __________. When an element is formed between an element and reactive metals (example: NaH, CaH2, MgH2). Example: 2 Na(s) + H2(g) 2 NaH(s) Write a balanced equation for the reaction of magnesium with hydrogen _________________________________________________________________________ 2. Reaction with non metals Hydrogen can also react with non metals. Example: it reacts with chlorine to form _______________________ and with nitrogen to form __________ Write a balance equation for the reaction of hydrogen with chlorine. _________________________________________________________________________ Write a balanced equation for the reaction of hydrogen with nitrogen. _________________________________________________________________________ 3. Reduction of metallic oxides Hydrogen acts as a reducing agent, by removing oxygen. Copper(II)oxide is reduced to copper, by heating it in a stream of hydrogen. Hydrogen is oxidised to water. The reaction taking place is: CuO(s) + H2(g) Cu(s) + H2O(g) Page 12 of 19 Hydrides Hydrogen is unique since it can form H+ and H- ions and it can also share electrons with other atoms to form covalent compounds. Hydrogen forms ionic hydrides with metals. Example: NaH, CaH2 and MgH2 Hydrogen forms covalent hydrides with non-metals. Example: HCl and NH3 Questions 1. Alongside the names of various chemicals are shown their respective pH values in aqueous solution. Potassium hydroxide pH13, hydrogen bromide pH2, calcium hydroxide pH11, sodium chloride pH7, hydrogen chloride pH2, magnesium hydroxide pH10, citric acid pH4. Which of these substances is/are: a) strong acid ? b) weak acid ? c) strong alkali ? d) weak alkali ? e) neutral substance? In each case write a chemical equation to show the ion present in solution. 2.a) What makes a weak acid ? Give two examples b) What makes a strong alkali ? Give two examples 3. Write a chemical equation to represent the neutralisation of sulfuric acid with sodium hydroxide. 4. Write a chemical equation of hydrochloric acid with sodium hydroxide. 5. How can you test for the presence of water ? 6. Explain what this symbol means ? Give examples. 7. Write balanced chemical equations for the following soluble salt preparations: a) magnesium + sulfuric acid b) calcium carbonate + hydrochloric acid c) zinc oxide + hydrochloric acid d) potassium hydroxide + nitric acid Page 13 of 19 pH of solution: 1 (a) 14 Colour of universal indicator: (b) green (c) Description: strong acid (d) (e) 9) The following equation shows the reaction that takes place between an acid and an alkali: a) Name this type of reaction. b) Write a word equation that shows the reaction between nitric acid and potassium hydroxide. c) Write a symbol equation for the same reaction. 10) Complete the following equations: a) calcium carbonate + nitric acid b) sodium carbonate + sulphuric acid c) Ethanoic acid + sodium hydroxide Page 14 of 19