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EdExcel Triple Science Unit C3

EdExcel Triple Science Unit C3. Chemistry in Action. N Smith St. Aidan’s. Topic 1 – Quantitative Analysis. Chemical Economics. Hi. We’re industrial scientists and we want to analyse this chemical. What tests could we do?. There are two main types of analysis:

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EdExcel Triple Science Unit C3

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  1. EdExcel Triple Science Unit C3 Chemistry in Action N Smith St. Aidan’s

  2. Topic 1 – Quantitative Analysis

  3. Chemical Economics Hi. We’re industrial scientists and we want to analyse this chemical. What tests could we do? There are two main types of analysis: • Qualitative – descriptions of what is present • Quantitative – analysis of how much of a chemical is present You need to use different tests for different ions – for example, if this chemical contains copper chloride then we’d need to verify by testing for copper ions and testing for chloride ions. Tests like these are important for industries like the water industry and medicine. Why?

  4. Testing using Precipitates CaCl2 + 2NaOH Ca(OH)2 + 2NaCl Some metal compounds form precipitates, i.e. an insoluble solid that is formed when sodium hydroxide is added to them. Consider calcium chloride: What precipitates are formed with the following metal compounds when they react with sodium hydroxide?

  5. Ammonium, nitrate, bromide and iodide ions Ammonium ions: Add sodium hydroxide, warm it and test the gas using damp litmus paper – ammonia gas turns damp litmus paper blue. Chloride, bromide and iodide ions: Add a few drops of dilute nitric acid followed by a few drops of silver nitrate solution. A white precipitate should be formed for chloride ions, a pale yellow precipitate should be formed for bromide ions and a darker yellow precipitate for iodide ions.

  6. Flame tests revision Compounds containing lithium, sodium, potassium, calcium and barium ions can be recognised by burning the compound and observing the colours produced: Lithium Red Sodium Yellow Potassium Lilac Calcium Brick red Barium Green

  7. Testing for carbonate ions Limewater Limewater turns milky/cloudy Calcium carbonate + hydrochloric acid calcium chloride + carbon dioxide + water

  8. Topic 2 – Quantitative Analysis

  9. Water Amazing facts about water: • 95% of your body mass is water (94% in women due to a higher body fat content) • Dinosaurs would have drunk the same water you do • Water dissolves more substances than any other liquid – most ionic substances are soluble and most covalent substances are insoluble • Around 75% of the world’s surface is made of water • To feel thirsty you need to lose around 1% of your body water • 5,000 children die every day due to not having clean drinking water • An average person in the West uses 200-300 litres of water every day

  10. Hard and Soft Water Advantages of hard water Disadvantages of hard water Definition: Hard water is water that contains lots of dissolved ions like magnesium and calcium that come from contact with rocks. When it reacts with soap it forms scum. Dissolved ions are good for your health More soap is needed to form lather They also help reduce the development of heart disease Can lead to deposits forming (e.g. limescale) so its more expensive There are two types of hard water – permanent and temporary. What is the difference between these two?

  11. Temporary hard water Temporary hard water is water that contains hydrogencarbonate ions (HCO3-). You can boil this water to soften it up and here’s what happens: Step 1 – The hydrogencarbonate ions decompose to produce carbonate ions Step 2 – The carbonate ions react with calcium and magnesium ions to form precipitates

  12. Measuring Water Hardness Hardness can be measured by adding soap and seeing how much soap it takes to form a lather: • Which sample is soft water? • Which sample is temporary hard water? • Which sample is permanent hard water?

  13. Removing hardness One way to remove permanent hardness: Ca2+ Pass the water through an “ion-exchange” column that contains a special resin to “swap” the calcium and magnesium ions for sodium or hydrogen ions. This is how some commercial water softeners work. Resin Ca2+ Na+ Resin Na+ Ca2+ Resin Na+

  14. The Mole Definition: A mole is a measure of the number of particles in a substance. 1 mole is 6x1023 particles. Molar Mass (g/mol) Molar mass is the mass of one mole of a substance and is equal to the relative atomic mass (in grams). For example: • Carbon has a relative atomic mass of 12, so 1mol of carbon atoms would have a mass of 12g • Aluminium has a relative atomic mass of 27, so 1mol of aluminium atoms would have a mass of ___g • Sodium hydroxide has a relative atomic mass of 40, so 2mols of NaOH would have a mass of ____g

  15. Molar Calculations m N = Mass (g) No. of moles = M Molar mass (g/mol) Some example questions: • Calculate the mass of 4mol of lithium • Calculate the mass of 2mol of sodium • Calculate the number of moles in 36g of carbon • Calculate the number of moles in 88g of carbon dioxide • Calculate the number of moles in 27g of water

  16. A note about volume… 1cm3 1dm3 (= 1000cm3) The two most commonly used units of volume in chemistry are the cm3 and the dm3: • Convert 1250cm3 into dm3 • Convert 1cm3 into dm3 • Convert 0.056dm3 into cm3 • Convert 1.28dm3 into cm3

  17. Concentration A solution of low concentration (“dilute”) A solution of high concentration (“strong”) Concentration means “how much of a chemical there is in a fixed volume” and can be measured in g/dm3 or mol/dm3.

  18. Questions on Concentration Conc. = Mass of substance (g) Volume of solvent (dm3) Conc. = Amount of solute (mol) Volume of solvent (dm3) To calculate the concentration of a substance you could use one of these formulae: Calculate, with units, the concentration of the following: • A solution of 10g salt in 1dm3 of water • 2mol of hydrochloric acid in 500cm3 of water • 10kg of salt in 200dm3 of water • 0.5mol of sodium hydroxide in 100cm3 of water

  19. Converting concentrations Concentration = Concentration (g/dm3) (mol/dm3) Molar mass To convert g/dm3 into mol/dm3 you can use the following formula: Convert the following: • 0.5mol/dm3 of sodium hydroxide into g/dm3. • 2mol/dm3 of HCl into g/dm3. • 20g/dm3 of NaCl into mol/dm3. • 500g/dm3 of CaCl2 into mol/dm3.

  20. Numbers of moles 20cm3 of 0.1mol/dm3 of hydrochloric acid 20cm3 of 0.1mol/dm3 of sodium hydroxide 20cm3 of helium at room temperature and pressure 20cm3 of argon at room temperature and pressure Consider two liquids: These two beakers contain the same number of moles Now consider two gases: These two gases contain the same number of moles

  21. Universal Indicator and the pH scale 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Strong acid Strong alkali Neutral Universal Indicator is a mixture of liquids that will produce a range of colours to show how strong the acid or alkali is: Stomach acid Lemon juice Water Soap Baking powder Oven cleaner An acid contains hydrogen ions, H+ An alkali contains hydroxide ions, OH-

  22. Neutralisation reactions Na H The sodium “replaces” the hydrogen from HCl Na Cl H2O General equation: H+(aq) + OH-(aq) H2O(l) Sodium chloride Water When acids and alkalis react together they will NEUTRALISE each other: Sodium hydroxide Hydrochloric acid OH Cl

  23. Common acids and alkalis 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

  24. 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

  25. Using the correct indicator 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Universal Indicator is a mixture of liquids that will produce a range of colours to show how strong the acid or alkali is: Strong acid Neutral Strong alkali Because of the gradual colour changes it’s not a very good indicator to use for titration. It’s better to use an indicator with a sudden colour change:

  26. Using the correct indicator Strong acid + strong alkali – use any indicator Strong acid + weak alkali – use methyl orange indicator Weak acid + weak alkali – use phenolphthalein indicator

  27. Titration Equations 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 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: • 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

  28. Titration Equations • 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

  29. Topic 3 – Electrolytic Processes

  30. Electrolytes An electrolyte is an ionic substance that has been melted or dissolved in water so that the ions are free to move: Cl- Na+ Cl- Na+ Cl- Na+ Cl- Na+ Cl- Na+ Cl- Na+

  31. Electrolysis Positive electrode (“anode”) ++++ ---- Negative electrode (“Cathode”) Electrolyte solution containing copper ions (cations) and chloride ions (anions) Cl- Cl- Cl- Cu2+ Cu2+ Cu2+

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

  33. Redox reactions 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 “Redox” reactions happen during electrolysis: These two processes are called REDOX REACTIONS OILRIG – Oxidation Is Loss of electrons Reduction Is Gain of electrons

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

  35. Electrolysis of molten sodium chloride Positive electrode (“anode”) ++++ ---- Negative electrode (“Cathode”) Cl- Cl- Cl- Na+ Na+ Na+ Sodium is a useful metal – it can be used in street lights and as a coolant for nuclear reactors. It can be made from the electrolysis of molten sodium chloride: Write half equations for these reactions and state where oxidation and reduction occur

  36. Electrolysis of Salt Water Chlorine gas (Cl2) Hydrogen gas (H2) Sodium chloride solution (salt water) NaCl(aq) Sodium hydroxide (NaOH(aq)) Positive electrode Negative electrode When solutions are electrolysed (instead of the molten compound) the results are different. Consider the electrolysis of salty water (a solution of sodium chloride):

  37. Electrolysis of Solutions ---- ++++ The electrolysis of sodium chloride solution brings different results to the electrolysis of molten sodium chloride because of the presence of hydrogen (H+) and hydroxide (OH-) ions. Two rules: • At the anode the “simplest” ion is discharged – e.g., in the electrolysis of a chloride the Cl- ion would be discharged (instead of the OH- ion) whereas in the electrolysis of a sulphate the OH- ion would be discharged (instead of the SO42- ion). The OH- ion will then form oxygen at the anode. • At the cathode a metal will be discharged if its less reactive then hydrogen. If it’s more reactive than hydrogen then hydrogen will be discharged.

  38. Products from electrolysis Given these two rules, complete the following table: Copper Chlorine Copper Oxygen Hydrogen Oxygen Lead Bromine

  39. Purifying Copper ++++ ---- Impure copper Pure copper Cu2+ Cu2+ Cu2+ At the positive electrode: Cu(s) Cu2+(aq) + 2e- At the negative electrode: Cu2+(aq) + 2e- Cu(s) Solution containing copper ions

  40. Electroplating ++++ ---- Silver electrode Object to be plated Ag+ Ag+ Ag+ Solution containing silver ions

  41. Topic 4 – Gases, Equilibria and Ammonia

  42. Revision about Volume 1cm3 1dm3 (= 1000cm3) The two most commonly used units of volume in chemistry are the cm3 and the dm3: • Convert 1250cm3 into dm3 • Convert 1cm3 into dm3 • Convert 0.056dm3 into cm3 • Convert 1.28dm3 into cm3

  43. Revision about Moles 20cm3 of 0.1mol/dm3 of hydrochloric acid 20cm3 of 0.1mol/dm3 of sodium hydroxide 20cm3 of helium at room temperature and pressure 20cm3 of argon at room temperature and pressure Consider two liquids: These two beakers contain the same number of moles Now consider two gases: These two gases contain the same number of moles

  44. Calculating Volumes of Gases An important fact: 1 mole of a gas at room temperature and pressure occupies a volume of 24dm3. • What is the volume of 2 moles of oxygen? • What is the volume of 0.25 moles of carbon dioxide? • How many moles would be in 8dm3 of nitrogen? • How much volume would 80g of argon occupy? • A balloon contains 12dm3 of carbon dioxide. What is the mass of this much CO2?

  45. Calculating the mass of a product IGNORE the oxygen in step 2 – the question doesn’t ask for it Step 1: READ the equation: 2Mg + O2 2MgO E.g. what mass of magnesium oxide is produced when 60g of magnesium is burned in air? Step 2: WORK OUT the relative formula masses (Mr): 2Mg = 2 x 24 = 48 2MgO = 2 x (24+16) = 80 Step 3: LEARN and APPLY the following 3 points: • 48g of Mg makes 80g of MgO • 1g of Mg makes 80/48 = 1.66g of MgO • 60g of Mg makes 1.66 x 60 = 100g of MgO

  46. When water is electrolysed it breaks down into hydrogen and oxygen: 2H2O 2H2 + O2 What mass of hydrogen is produced by the electrolysis of 6g of water? 2) What mass of calcium oxide is produced when 10g of calcium burns? 2Ca + O2 2CaO 3) What mass of aluminium is produced from 100g of aluminium oxide? 2Al2O3 4Al + 3O2 Work out Mr: 2H2O = 2 x ((2x1)+16) = 36 2H2 = 2x2 = 4 • 36g of water produces 4g of hydrogen • So 1g of water produces 4/36 = 0.11g of hydrogen • 6g of water will produce (4/36) x 6 = 0.66g of hydrogen • Mr: 2Ca = 2x40 = 80 2CaO = 2 x (40+16) = 112 • 80g produces 112g so 10g produces (112/80) x 10 =14g of CaO Mr: 2Al2O3 = 2x((2x27)+(3x16)) = 204 4Al = 4x27 = 108 204g produces 108g so 100g produces (108/204) x 100 =52.9g of Al2O3

  47. Calculating the volume of a product Q. When water is electrolysed it breaks down into hydrogen and oxygen: 2H2O 2H2 + O2 What VOLUME of hydrogen is produced by the electrolysis of 6g of water? REMEMBER THIS - At normal temperature and pressure the Relative Formula Mass (Mr) of a gas will occupy a volume of 24 litres e.g. 2g of H2 has a volume of 24 litres 32g of O2 has a volume of 24 litres 44g of CO2 has a volume of 24 litres etc • On the previous page we said that the MASS of hydrogen produced was 0.66g • 2g of hydrogen (H2) will occupy 24 litres (from the red box above), • So 0.66g will occupy 0.66/2 x 24 = 8 litres

  48. Example questions • What volume of hydrogen is produced when 18g of water is electrolysed? 2H2O 2H2 + O2 • Marble chips are made of calcium carbonate (CaCO3). What volume of carbon dioxide will be released when 500g of CaCO3 is reacted with dilute hydrochloric acid? CaCO3 + 2HCl CaCl2 + H2O + CO2 • Magnesium will react with hydrochloric acid. What volume of hydrogen would be produced if you reacted 1g of magnesium with excess acid? Mg + 2HCl MgCl2 + H2

  49. Reversible Reactions 20/12/2019 A + B C + D e.g. Ammonium chloride Ammonia + hydrogen chloride NH4Cl NH3 + HCl For example, consider copper sulphate: Hydrated copper sulphate (blue) + Heat Anhydrous copper sulphate (white) + Water CuSO4.5H2O CuSO4 + H2O Some chemical reactions are reversible. In other words, they can go in either direction: If a reaction is EXOTHERMIC in one direction what must it be in the opposite direction?

  50. Reversible Reactions 20/12/2019 A + B C + D Endothermic reactions Exothermic reactions Increased temperature: Increased temperature: A + B C + D A + B C + D Decreased temperature: Decreased temperature: A + B C + D A + B C + D When a reversible reaction occurs in a CLOSED SYSTEM (i.e. no reactants are added or taken away) an EQUILIBRIUM is achieved – in other words, the reaction goes at the same rate in both directions (a “dynamic equilibrium”): More products Less products Less products More products

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