Quantitative Chemistry AQA Chemistry topic 3
3.1 Conservation of Mass and Balanced Equations
Chemical Reactions A chemical reaction is when atoms are basically rearranged into something different. For example, consider burning methane: Methane + oxygen Carbon dioxide + water Notice that no atoms were made or destroyed in this reaction, they were simply rearranged. Therefore we can write the Law of the Conservation of Mass: Mass of products = mass of reactants
Balancing equations Consider the following reaction: Sodium + water sodium hydroxide + hydrogen Na Na + O + H H O H H H This equation doesn t balance there are 2 hydrogen atoms on the left hand side (the reactants and 3 on the right hand side (the products )
Balancing equations We need to balance the equation: Sodium + water sodium hydroxide + hydrogen Na H O H O Na H Na + + O H H O Na H H H Now the equation is balanced, and we can write it as: 2Na (s) + 2H 2 O (l) 2NaOH (aq) + H 2(g)
Some examples 2 2 Mg + O 2 Zn + 2 HCl 2 Fe + 3 Cl 2 NaOH + HCl CH 4 + 2 O 2 Ca + 2 H 2 O NaOH + H 2 SO 4 2 CH 3 OH + 3 O 2 2 MgO ZnCl 2 + H 2 2 FeCl 3 NaCl + H 2 O CO 2 + 2 H 2 O Ca(OH) 2 + H 2 Na 2 SO 4 + 2 H 2 O 2 CO 2 + 4 H 2 O
Atomic mass RELATIVE ATOMIC MASS, A r ( Mass number ) = number of protons + number of neutrons SYMBOL PROTON NUMBER = number of protons (obviously)
Some simple compounds Methane, CH 4 Ethyne, C 2 H 2 Water, H 2 O Sulphuric acid, H 2 SO 4 Carbon dioxide, CO 2 Key Hydrogen Oxygen Carbon Sulphur
Relative formula mass, M r The relative formula mass of a compound is the relative atomic masses of all the elements in the compound added together. E.g. water H 2 O: Relative atomic mass of O = 16 Relative atomic mass of H = 1 Therefore M r for water = 16 + (2x1) = 18 Work out M r for the following compounds: 1) HCl 2) NaOH 3) MgCl 2 4) H 2 SO 4 5) K 2 CO 3 H=1, Cl=35 so M r = 36 Na=23, O=16, H=1 so M r = 40 Mg=24, Cl=35 so M r = 24+(2x35) = 94 H=1, S=32, O=16 so M r = (2x1)+32+(4x16) = 98 K=39, C=12, O=16 so M r = (2x39)+12+(3x16) = 138
An example of Conservation of Mass Here s a classic experiment where magnesium is burned in a crucible: 2Mg + O 2 2MgO Mass of magnesium and crucible before burning = 78.25g Mass of magnesium and crucible after burning = 78.56g Mass is always conserved in any reaction, so where did this extra mass come from?
Example Questions For each of the following reactions, state whether or not the mass of the total system should go up or down and explain your answer: 1) Iron + oxygen iron oxide 2) Copper carbonate copper oxide + carbon dioxide For higher tier, write a balanced equation for each reaction, given that iron oxide is Fe 2 O 3 and copper carbonate is CaCO 3.
Example Questions 1) aluminium + oxygen aluminium oxide If you start with 27g of aluminium and 24g of oxygen, how much aluminium oxide will you make? 51g 2) magnesium + oxygen magnesium oxide If you start with 24g of magnesium and make 40g of magnesium oxide, how much oxygen was used? 16g 3) potassium oxide + water potassium hydroxide If you start with 9g of water and make 56g of potassium hydroxide, how much potassium oxide was used? 47g EXTENSION Write balanced symbol equations for above reactions
3.2 Moles (Higher only)
Recap Questions Here are carbon and oxygen on the Periodic Table and a picture of carbon dioxide: 1) What is the atomic mass of carbon? 2) What is the atomic mass of oxygen? 3) What is the molecular mass of carbon dioxide? Carbon dioxide, CO 2
A Mole in numbers Definition: A mole of a substance is the relative formula mass of that substance in grams, For example, 12g of carbon would be 1 mole of carbon......and 44g of carbon dioxide (CO 2 ) would be 1 mole etc... Q. How many moles are the following? 1. 23g of sodium 2. 48g of magnesium 3. 36g of carbon 4. 28g of iron 1 mol 2 mol 3 mol 0.5 mol
A Mole Definition: A mole of a substance ALWAYS contains the same number of molecules/ions/particles/atoms: Avogadro s Constant: 1 mole = 6.02x10 23 molecules Q. How many moles are the following? 1. How many molecules are in 2 moles of carbon? 2. What about 2 moles of magnesium? 3. How many molecules are in 46g of sodium? 4. How many molecules are in 23g of iron? 1.2x10 24 1.2x10 24 1.2x10 24 3.0x10 23
Molar Calculations No. of moles = Mass (g) Molar mass (g/mol) N = m M Some example questions: 1) Calculate the mass of 4 mol of lithium 2) Calculate the mass of 2 mol of sodium 3) Calculate the number of moles in 36g of carbon 4) Calculate the number of moles in 88g of carbon dioxide 5) Calculate the number of moles in 27g of water 28g 46g 3 mol 2 mol 1.5 mol
Calculating the mass of a product E.g. what mass of magnesium oxide is produced when 60g of magnesium is burned in air? Step 1: READ the equation: 2Mg + O 2 2MgO IGNORE the oxygen in step 2 the question doesn t ask for it Step 2: WORK OUT the relative formula masses (M r ): 2Mg = 2 x 24 = 48 2MgO = 2 x (24+16) = 80 Step 3: LEARN and APPLY the following 3 points: 1) 48g of Mg makes 80g of MgO 2) 1g of Mg makes 80/48 = 1.66g of MgO 3) 60g of Mg makes 1.66 x 60 = 100g of MgO
Calculating the mass of a product using moles Let s try this question again but using moles: Step 1: READ the equation: 2Mg + O 2 2MgO 2 moles of magnesium + 1 mole of oxygen forms 2 moles of magnesium oxide Step 2: WORK OUT the relative formula masses (M r ) of MgO: 2MgO = 2 x (24+16) = 80 Step 3: Apply these steps: 1) 60g of Mg is equal to 1.25 moles (60/48) 2) Therefore we will make 1.25 moles of magnesium oxide 3) Therefore we make 100g of MgO (1.25 moles)
1) When water is electrolysed it breaks down into hydrogen and oxygen: 204g produces 108g so 100g produces (108/204) x 100 = 52.9g of Al 2 O 3 2H 2 O 2H 2 + O 2 What mass of hydrogen is produced by the electrolysis of 6g of water? Work out M r : 2H 2 O = 2 x ((2x1)+16) = 36 2H 2 = 2x2 = 4 1. 36g of water produces 4g of hydrogen 2. So 1g of water produces 4/36 = 0.11g of hydrogen 3. 6g of water will produce (4/36) x 6 = 0.66g of hydrogen 2) What mass of calcium oxide is produced when 10g of calcium burns? 2Ca + O 2 2CaO M r : 2Ca = 2x40 = 80 2CaO = 2 x (40+16) = 112 80g produces 112g so 10g produces (112/80) x 10 = 14g of CaO 3) What mass of aluminium is produced from 100g of aluminium oxide? 2Al 2 O 3 4Al + 3O 2 M r : 2Al 2 O 3 = 2x((2x27)+(3x16)) = 204 4Al = 4x27 = 108
Another method Try using this equation: Mass of product IN GRAMMES Mass of reactant IN GRAMMES M r of product M r of reactant Q. When water is electrolysed it breaks down into hydrogen and oxygen: 2H 2 O 2H 2 + O 2 What mass of hydrogen is produced by the electrolysis of 6g of water? Mass of product IN GRAMMES 6g 4 36 So mass of product = (4/36) x 6g = 0.66g of hydrogen
Using Moles to balance equations Example question: 130g of zinc reacts with 146g of hydrochloric acid (HCl) to form 272g of zinc chloride (ZnCl 2 ) and some hydrogen (H 2 ). Answer the following: 1) How much hydrogen was produced? 4g 2) How many moles of each substance were reacted/produced? 1 of Zn, 2 of HCl, 1 of ZnCl 2, 1 of H 2 3) Write a balanced chemical equation for this reaction. Zn + 2HCl ZnCl 2 + H 2
Recap questions Carbon Methane, CH 4 Water, H 2 O Oxygen, O dioxide, CO 2 2 1) What are the molecular masses of these four compounds? Key Hydrogen 2) How many grams of each compound would you have if you had one mole of each one? 3) Which mole is the heaviest? Oxygen Carbon Sulphur
Limiting Reactants What if we don t get the ratios right? Example question: Consider the reaction you have when you burn methane: CH 4 + 2O 2 CO 2 + 2H 2 O A student burns 32g of methane in 72g of oxygen. Which reactant is used up completely? 32g of methane is 2 moles. 72g of oxygen is 2.25 moles of O 2. Therefore this reaction is limited by the 2 moles of methane the limiting reactant. Q. How much CO 2 would we expect to produce? 2 moles (88g)
A note about volume The two most commonly used units of volume in chemistry are the cm 3 and the dm 3 : 1cm 3 1dm 3 (= 1000cm 3 ) 1) Convert 1250cm 3 into dm 3 2) Convert 1cm 3 into dm 3 3) Convert 0.056dm 3 into cm 3 4) Convert 1.28dm 3 into cm 3
Concentration Concentration means how much of a chemical there is in a fixed volume and can be measured in g/dm 3 or mol/dm 3. A solution of low concentration ( dilute ) A solution of high concentration ( strong )
Questions on Concentration To calculate the concentration of a substance you could use one of these formulae: Conc. = Mass of substance (g) Volume of solvent (dm 3 ) Conc. = Amount of solute (mol) Volume of solvent (dm 3 ) Calculate, with units, the concentration of the following: 1) A solution of 10g salt in 1dm 3 of water 2) 2mol of hydrochloric acid in 500cm 3 of water 3) 10kg of salt in 200dm 3 of water 4) 0.5mol of sodium hydroxide in 100cm 3 of water
Converting concentrations To convert g/dm 3 into mol/dm 3 you can use the following formula: Concentration = Concentration (g/dm 3 ) (mol/dm 3 ) Molar mass Convert the following: 1) 0.5mol/dm 3 of sodium hydroxide into g/dm 3. 2) 2mol/dm 3 of HCl into g/dm 3. 3) 20g/dm 3 of NaCl into mol/dm 3. 4) 500g/dm 3 of CaCl 2 into mol/dm 3.
Changing a Concentration Wow that s a strong acid! How can I make it less concentrated? What would you do to change an acid of concentration 1 mol/dm 3 to an acid of concentration 0.1 mol/dm 3? Can you think of some examples where dilution is needed in food?
3.3 Yield and Atom Economy of Chemical Reactions (Chem only)
Problems with calculating mass Calculating the amount of a product or reactant may not always give you a reliable answer... 1) The reaction may not have completely 2) The reaction may have been 3) Some of the product may have been 4) Some of the reactants may have produced other The amount of product that is made is called the. This number can be compared to the maximum theoretical amount as a percentage, called the percentage yield. Words lost, yield, finished, reversible, products
Percentage Yield Percentage yield = Actual yield Predicted yield X 100% Some example questions: 1) The predicted yield of an experiment to make salt was 10g. If 7g was made what is the percentage yield? 70% 2) Dave is trying to make water. If he predicts to make 15g but only makes 2g what is the percentage yield? 13% 3) Sarah performs an experiment and has a percentage yield of 33%. If she made 50g what was she predicted to make? 150g
Percentage Yield (harder) Theoretical yield = the amount of product that should be made as calculated from the masses of atoms Actual yield = what was actually produced in a reaction Percentage yield = actual yield (in g) Example question: theoretical yield 65g of zinc reacts with 73g of hydrochloric acid and produces 102g of zinc chloride. What is the percentage yield? Zn + 2HCl ZnCl 2 + H 2 The theoretical yield is 136g (using Cl = 35.5) so the % yield is 75%
Atom Economy Percentage atom economy = Relative formula mass of useful product Total masses of products Calculate the atom economies of the following: 1) Converting ethanol into ethene (ethene is the useful bit): C 2 H 5 OH C 2 H 4 + H 2 0 61% 2) Making zinc chloride from zinc and hydrochloric acid: Zn + 2HCl ZnCl 2 + H 2 99%
3.4 Using Concentrations in mol/dm 3 (Chemistry only)
Revision about Moles Consider two liquids: 20cm 3 of 0.1mol/dm 3 of hydrochloric acid Now consider two gases: 20cm 3 of 0.1mol/dm 3 of sodium hydroxide These two beakers contain the same number of moles 20cm 3 of helium at room temperature and pressure 20cm 3 of argon at room temperature and pressure These two gases contain the same number of moles
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 (down to the next drop!) 5) Repeat until you get similar results
Titration Equations Q. 0.05dm 3 of HCl neutralises 0.1dm 3 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 + H 2 O Notice that 1 mole of HCl neutralises 1 mole of NaOH 2) Use this equation: No. of moles = concentration x volume a) So, the number of moles of NaOH is (0.5 x 0.1) = 0.05mol b) According to the equation, this will neutralise 0.05mol of HCl c) Therefore we have (0.05mol/0.05dm 3 ) = 1mol/dm 3 HCl
Titration Equations 1) 0.2dm 3 of HCl neutralises 0.1dm 3 of NaOH of concentration 0.5mol/dm 3. What is the concentration of the acid? HCl + NaOH NaCl + H 2 O 0.25mol/dm 3 2) H 2 SO 4 of concentration 0.4mol/dm 3 neutralises 0.1dm 3 of NaOH of concentration 0.2mol/dm 3. How much acid was used? H 2 SO 4 + 2NaOH Na 2 SO 4 + 2H 2 O 0.025mol/dm 3
3.5 Amounts of substances in relation to volumes
Calculating Volumes of Gases An important fact: 1 mole of a gas at room temperature (20 O C) and pressure (1 atm) occupies a volume of 24dm 3. 1) What is the volume of 2 moles of oxygen? 2) What is the volume of 0.25 moles of carbon dioxide? 3) How many moles would be in 8dm 3 of nitrogen? 4) How much volume would 80g of argon occupy? 5) A balloon contains 12dm 3 of carbon dioxide. What is the mass of this much CO 2? 48dm 3 6dm 3 1/3 mol 48dm 3 22g
Calculating the volume of a product REMEMBER THIS - At normal temperature and pressure the Relative Formula Mass (M r ) of a gas will occupy a volume of 24 litres e.g. 2g of H 2 has a volume of 24 litres 32g of O 2 has a volume of 24 litres 44g of CO 2 has a volume of 24 litres etc Q. When water is electrolysed it breaks down into hydrogen and oxygen: 2H 2 O 2H 2 + O 2 What VOLUME of hydrogen is produced by the electrolysis of 6g of water? From previous work Calculating the mass of a product we find that the MASS of hydrogen produced was 0.66g 2g of hydrogen (H 2 ) will occupy 24 litres (from the red box above), So 0.66g will occupy 0.66/2 x 24 = 8 litres
Example questions 1) What volume of hydrogen is produced when 18g of water is electrolysed? 2H 2 O 2H 2 + O 2 24dm 3 2) Marble chips are made of calcium carbonate (CaCO 3 ). What volume of carbon dioxide will be released when 500g of CaCO 3 is reacted with dilute hydrochloric acid? CaCO 3 + 2HCl CaCl 2 + H 2 O + CO 2 120dm 3 3) Magnesium will react with hydrochloric acid. What volume of hydrogen would be produced if you reacted 1g of magnesium with excess acid? Mg + 2HCl MgCl 2 + H 2 1dm 3