13/11/2018 13/11/2018 Chemical Changes AQA 2016 Chemistry topic 4

4.1 Reactivity of Metals 13/11/2018

Metal Oxides Examples of metal oxides 13/11/2018 Examples of metal oxides A metal oxide is a compound formed when a metal reacts with oxygen. Here are two examples: Magnesium oxide – formed by burning magnesium Oxidation = “adding oxygen” Reduction = “reducing (taking away) oxygen” Iron oxide – formed by iron rusting

Reactions of metals with water 13/11/2018 When a metal reacts with water hydrogen is always given off. The other product will be either a metal hydroxide or a metal oxide. METAL + WATER METAL OXIDE + HYDROGEN METAL + WATER METAL HYDROXIDE + HYDROGEN Copy and complete the following reactions: Sodium + water Potassium + water Calcium + water Iron + steam

Words – gold, corroded, fizzes, disappear Adding acids to metals 13/11/2018 If an acid is added to a (fairly reactive) metal the metal will be quickly ________ by the acid. We can see a reaction happening because the mixture _________ and the metal eventually __________. Some metals, like ____, are so unreactive that nothing will happen. Words – gold, corroded, fizzes, disappear

Reactions of metals with acids 13/11/2018 METAL + ACID SALT + HYDROGEN e.g. magnesium + hydrochloric acid magnesium chloride + hydrogen Mg H Cl H Cl Copy and complete the following reactions: Calcium + hydrochloric acid Zinc + hydrochloric acid Iron + hydrochloric acid Lithium + sulphuric acid

Reactivity Practical Task: to place six metals in order of reactivity 13/11/2018 Task: to place six metals in order of reactivity Equipment: hydrochloric acid, calcium, copper, iron, aluminium, magnesium, zinc Conclusion: I think the order of reactivity is… Calcium Magnesium Aluminium Zinc Iron Copper The actual order of reactivity is:

The Reactivity Series Increasing reactivity 13/11/2018 Potassium Sodium Calcium Magnesium Aluminium Carbon Zinc Iron Lead Hydrogen Copper Silver Gold Increasing reactivity The Reactivity Series lists metals in order of reactivity: Note the two non-metals in here! The reactivity of these metals is related to their tendency to form positive ions.

What is the order of reactivity of these metals? 13/11/2018 What is the order of reactivity of these metals? Metal Reaction with dilute acid Reaction with water Reaction with oxygen A Some reaction Slow reaction Burns brightly B No reaction Reacts slowly C D Violent reaction E Reasonable reaction Reacts with steam only

Displacement reactions 13/11/2018 A displacement reaction is one where a MORE REACTIVE metal will DISPLACE a LESS REACTIVE metal from a compound. Mg Magnesium SO4 Cu Copper sulphate The magnesium DISPLACES the copper from copper sulphate SO4 Mg Magnesium sulphate Cu Copper Magnesium + copper sulphate magnesium sulphate + copper

Displacement Reactions 13/11/2018 Example 1 – Magnesium and copper sulphate O Cu S Copper sulphate + Mg O S Magnesium sulphate + Cu Copper Mg Magnesium Example 2 – Zinc and copper sulphate Zn O Cu S Zinc + Copper sulphate Copper Zinc sulphate

Some example reactions… 13/11/2018 Reaction Predicton (yes or no?) Observations Lead + copper sulphate Lead + iron sulphate Zinc + copper sulphate Zinc + iron sulphate Copper + copper sulphate Copper + iron sulphate Reaction No Reaction Reaction Reaction No Reaction No Reaction Extension work – write down the equations for these reactions

Extracting Metals Some definitions: 13/11/2018 Some definitions: A METAL ORE is a mineral or mixture of minerals from which it is “economically viable” to extract some metal. Most ores contain METAL OXIDES (e.g. rust = iron oxide). To “extract” a metal from a metal oxide we need to REDUCE the oxygen. This is called a REDUCTION reaction. To put it simply: Oxide Iron Iron ore “Reduce” the oxygen to make iron

Increasing reactivity How do we do it? 13/11/2018 Potassium Sodium Calcium Magnesium Aluminium Carbon Zinc Iron Tin Lead Copper Silver Gold Platinum Increasing reactivity Metals ABOVE CARBON, because of their high reactivity, are extracted by ELECTROLYSIS, which is very expensive! Metals BELOW CARBON are extracted by heating them with carbon in a BLAST FURNACE. This is a “displacement reaction” Carbon Oxide Iron These LOW REACTIVITY metals won’t need to be extracted because they are SO unreactive you’ll find them on their own, not in a metal oxide

Extracting metals What is an ore? 13/11/2018 What is an ore? In what form are metals usually found in the Earth? How do you get a metal out of a metal oxide? What is this type of reaction called? Type of metal Extraction process Examples High reactivity (i.e anything above carbon) Middle reactivity (i.e. anything below carbon) Low reactivity

Reduction and Oxidation 13/11/2018 Some examples of reduction: Aluminium + iron oxide aluminium oxide + iron heat 2Al(s) + Fe2O3(s) Al2O3(s) + 2Fe(s) heat Lead oxide + carbon lead + carbon dioxide heat 2PbO(s) + C(s) 2Pb(s) + CO2(s) heat An example of oxidation: Magnesium + oxygen magnesium oxide heat Mg(s) + O2(s) 2MgO(s) heat

What is a Redox Reaction? 13/11/2018 Basically, during a redox reaction electrons are either lost or gained: The Golden Rule: OILRIG Oxidation Is Loss (of electrons) Reduction Is Gain (of electrons) For example: Fe Fe2+ These reactions both involve the loss of electrons – they are Oxidation reactions 2Cl- Cl2 Fe2+ Fe These reactions both involve the gain of electrons – they are Reduction reactions Cl2 2Cl-

4.2 Reactions of Acids 13/11/2018

Reactions of metals with acids recap 13/11/2018 METAL + ACID SALT + HYDROGEN e.g. magnesium + hydrochloric acid magnesium chloride + hydrogen Mg H Cl H Cl Copy and complete the following reactions: Calcium + hydrochloric acid Zinc + hydrochloric acid Iron + hydrochloric acid Lithium + sulphuric acid

Redox reactions with acids and metals (higher only) 13/11/2018 Redox reactions with acids and metals (higher only) Let’s consider one of the reactions on the previous page in more detail: Zinc + hydrochloric acid Zinc chloride + hydrogen Zn + 2HCl ZnCl2 + H2 During this reaction, the zinc atoms lose electrons to form a 2+ ion, ready to bond with two ions of chlorine (Cl-). The hydrogen from the hydrochloric acid gains one electron to turn into a neutral diatomic atom. The Golden Rule: OILRIG Oxidation Is Loss (of electrons) Reduction Is Gain (of electrons) In other words, this is a REDOX reaction:

Common acids and alkalis 13/11/2018 Acids Alkalis Hydrochloric acid, HCl Sodium hydroxide, NaOH Nitric acid, HNO3 Potassium hydroxide, KOH Sulphuric acid, H2SO4 Magnesium hydroxide, Mg(OH)2 Calcium hydroxide, Ca(OH)2

Quiz on acids and alkalis 13/11/2018 Acid, alkali or both??? This a pH of less than 7 This could kill cells This is often used in cleaners or soap This would turn Universal Indicator red Sodium hydroxide is a common ____________ This would feel soapy on your skin This could be a corrosive This will turn universal indicator purple This would taste sour Lemon juice and vinegar are examples of weak _____

Neutralisation reactions 13/11/2018 A neutralisation reaction occurs when an acid reacts with an alkali. An alkali is a metal carbonate or metal hydroxide dissolved in water. ACID + ALKALI SALT + WATER Na Cl H O Copy and complete the following reactions: Sodium hydroxide + hydrochloric acid Calcium hydroxide + hydrochloric acid Sodium hydroxide + sulphuric acid Magnesium hydroxide + sulphuric acid

Adding acid to carbonates 13/11/2018 Carbonates are compounds containing carbon and oxygen. When an acid is added to a carbonate the carbonate starts to _______. A gas called ______ _______ is produced and the acid is neutralised. Carbonates used to be used as building materials but aren’t any more because acid rain would eventually ________ the building. Words – dissolve, fizz, carbon dioxide, oxygen

Reactions of metals carbonates with acid 13/11/2018 A metal carbonate is an alkaline compound containing a metal, carbon and oxygen. METAL CARBONATE + ACID SALT + CARBON DIOXIDE + WATER Mg H Cl C O Copy and complete the following reactions: Magnesium carbonate + hydrochloric acid Calcium carbonate + hydrochloric acid Sodium carbonate + sulphuric acid

Acids and metal hydroxides 13/11/2018 A neutralisation reaction occurs when an acid reacts with an alkali. An alkali is a metal oxide or metal hydroxide dissolved in water. ACID + ALKALI SALT + WATER Na Cl H O Copy and complete the following reactions: Sodium hydroxide + hydrochloric acid Calcium hydroxide + hydrochloric acid Sodium hydroxide + sulphuric acid Magnesium hydroxide + sulphuric acid

Reactions of metal oxides with acid 13/11/2018 A metal oxide is a compound containing a metal and oxide. They are sometimes called BASES. For example: Mg O Na Al Magnesium oxide Sodium oxide Aluminium oxide METAL OXIDE + ACID SALT + WATER Mg O H Cl Copy and complete the following reactions: Magnesium oxide + hydrochloric acid Calcium oxide + hydrochloric acid Sodium oxide + sulphuric acid

Making salts 13/11/2018 Whenever an acid and alkali or base neutralise each other we are left with a salt, like a chloride or a sulphate, and water. Complete the following table: Hydrochloric acid Sulphuric acid Nitric acid Sodium hydroxide Sodium chloride + water Potassium hydroxide Potassium sulphate + water Calcium hydroxide Calcium nitrate + water

Required Practical 1 - Soluble Salts 13/11/2018 Method: Neutralise an acid using a solid insoluble substance, such as a metal, metal oxide, metal hydroxide or metal carbonate. Keep adding the solid until an excess is left. Filter off the excess to leave you with a salt solution. Crystallise the solution to leave the salt.

Universal Indicator and the pH scale 13/11/2018 Universal Indicator is a mixture of liquids that will produce a range of colours to show how strong the acid or alkali is: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Stomach acid Lemon juice Water Soap Baking powder Oven cleaner Strong alkali Strong acid Neutral

Acids and Alkalis 13/11/2018 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Acids produce hydrogen ions when they dissolve in water: H Cl + - Alkalis produce hydroxide ions: H O Na - +

Different indicators 13/11/2018 Litmus paper, used to simply indicate if something is acidic or alkaline Universal indicator paper and pH meters, used to detect a pH value

Neutralisation reactions 13/11/2018 When acids and alkalis react together they will NEUTRALISE each other: Sodium hydroxide Hydrochloric acid Na H OH Cl The sodium “replaces” the hydrogen from HCl Cl Na Sodium chloride H2O Water General equation: Acid + alkali or base Salt + water

Neutralisation reactions 13/11/2018 The basic equation for any neutralisation reaction is: H+(aq) + OH-(aq) H2O(l) Write word and chemical equations for the following reactions: Hydrochloric acid + sodium hydroxide Hydrochloric acid + potassium hydroxide Nitric acid + potassium hydroxide Sulphuric acid + calcium hydroxide Nitric acid + copper oxide, CuO Sulphuric acid + calcium carbonate, Ca(CO)3

Req. Practical 2 (Chem only) - Titration 13/11/2018 Req. Practical 2 (Chem only) - Titration 1) Fill a burette with sodium hydroxide solution of known concentration 2) Accurately measure out 25cm3 of acid and place it in the conical flask 3) Add phenolphthalein indicator to the flask 4) Slowly add the alkali until the mixture in the flask turns pink 5) Repeat until you get similar results

Titration Equations (higher only) 13/11/2018 Q. 0.05dm3 of HCl neutralises 0.1dm3 of NaOH of concentration 0.5mol dm-3. What is the concentration of the acid? The key steps: 1) Look at the equation to compare the numbers of moles: HCl + NaOH NaCl + H2O Notice that 1 mole of HCl neutralises 1 mole of NaOH 2) Use this equation: No. of moles = concentration x volume So, the number of moles of NaOH is (0.5 x 0.1) = 0.05mol According to the equation, this will neutralise 0.05mol of HCl Therefore we have (0.05mol/0.05dm3) = 1mol dm-3 HCl

Titration Equations 13/11/2018 0.2dm3 of HCl neutralises 0.1dm3 of NaOH of concentration 0.5mol dm-3. What is the concentration of the acid? H2SO4 of concentration 0.4mol dm-3 neutralises 0.1dm3 of NaOH of concentration 0.2mol dm-3. How much acid was used? HCl + NaOH NaCl + H2O H2SO4 + 2NaOH Na2SO4 + 2H2O

Universal Indicator and the pH scale 13/11/2018 Recall the pH scale: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Stomach acid Lemon juice Water Soap Baking powder Oven cleaner Strong alkali Strong acid Neutral A “strong” acid is one that is complete ionised in aqueous solutions. Examples are hydrochloric acid and sulphuric acid. A weak acid is therefore only partly ionised in aqueous solutions. Examples are ethanoic, citric and carbonic acids.

More Information on Acids 13/11/2018 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Fact 1: In the pH scale, if the pH decreases by one unit (e.g. 6 to 5) then the concentration of hydrogen ions has increased by a factor of 10. Q. If hydrochloric acid has a pH of 1 and carbonic acid has a pH of 4, how many more hydrogen ions are in the HCl? Conc. H2SO4 Fact 2: “strong” and “weak” refer to the degree if ionisation whereas “dilute” and “concentrated” refer to how much acid there is in a given volume.

4.3 Electrolysis 13/11/2018

Testing for hydrogen 13/11/2018 “POP”

Testing for Oxygen 13/11/2018 Oxygen will relight a glowing splint

Testing for Chlorine Chlorine “bleaches” damp indicator paper 13/11/2018 Chlorine “bleaches” damp indicator paper

Electrolytes 13/11/2018 An electrolyte is an ionic substance that has been melted or dissolved in water so that the ions are free to move and conduct electricity: Cl- Na+ Cl- Na+ Cl- Na+ Cl- Na+ Cl- Na+ Cl- Na+

Electrolysis ++++ ---- 13/11/2018 Passing an electric current through an electrolyte will cause the ions to move: Positive electrode (“anode”) ++++ ---- Negative electrode (“Cathode”) Cu2+ Cl- Electrolyte solution containing copper ions and chloride ions Cu2+ Cl- Cu2+ Cl-

Electrolysis 13/11/2018 During electrolysis the substance being broken down is called the “electrolyte”. = chloride ion = copper ion When we electrolysed copper chloride the _____ chloride ions moved to the ______ electrode and the ______ copper ions moved to the ______ electrode – OPPOSITES ATTRACT!!!

Electrolysis half equations 13/11/2018 We need to be able to write “half equations” to show what happens during electrolysis (e.g. for copper chloride): At the negative electrode the positive ions GAIN electrons to become neutral copper ATOMS. The half equation is: Cu2+ + e- Cu 2 At the positive electrode the negative ions LOSE electrons to become neutral chlorine MOLECULES. The half equation is: Cl- - e- Cl2 2 2

Extracting Metals recap 13/11/2018 Potassium Sodium Calcium Magnesium Aluminium Carbon Zinc Iron Tin Lead Copper Silver Gold Platinum Increasing reactivity Metals ABOVE CARBON, because of their high reactivity, are extracted by ELECTROLYSIS, which is very expensive as you need to melt the ore! Displaced by carbon in a blast furnace Don’t need to be extracted

Words – melting point, replaced, negative, bauxite, reactive, move Extracting Aluminium 13/11/2018 Aluminium has to be extracted from its ore (called ________) by electrolysis. This is because aluminium is very ___________. The ore is mixed with cryolite to lower its ________ ________. The ore is then melted so that the ions can ______. The positively charged aluminium ions gather at the ___________ electrode. Oxygen forms at the positive electrode and causes it to wear away, which means that they have to be __________ frequently. Words – melting point, replaced, negative, bauxite, reactive, move

Required Practical 3 - Electrolysis of Aqueous Solutions 13/11/2018 Required Practical 3 - Electrolysis of Aqueous Solutions The ions discharged by electrolysis depend on the relative reactivity of the elements involved: ++++ ---- At the positive electrode (anode), oxygen is produced UNLESS the solution contains halide ions, in which case a halogen is produced. At the negative electrode (cathode), hydrogen is produced IF the metal is more reactive than hydrogen. These patterns happen because in the aqueous solution water molecules break down, producing hydrogen ions and hydroxide ions that are then discharged.

Products from electrolysis 13/11/2018 Given these two rules, complete the following table: Electrolyte Product at cathode Product at anode Copper chloride solution Copper sulfate solution Sodium sulfate solution Molten lead bromide Copper Chlorine Copper Oxygen Hydrogen Oxygen Lead Bromine

Redox reactions “Redox” reactions happen during electrolysis: 13/11/2018 “Redox” reactions happen during electrolysis: At the positive electrode (anode) the negative ions LOSE electrons to become neutral – this is OXIDATION At the negative electrode (cathode) the positive ions GAIN electrons to become neutral – this is REDUCTION These two processes are called REDOX REACTIONS OILRIG – Oxidation Is Loss of electrons Reduction Is Gain of electrons

Electrolysis equations 13/11/2018 Try writing half equations for the electrolysis of copper sulfate… At the negative electrode (cathode): Cu2+ + 2e- Cu At the positive electrode (anode): 4OH- - 4e- O2 + 2H2O