OCR Gateway Additional Science C3: Chemical Economics W Richards The Weald School
Elements These atoms are ALL iron – there’s nothing else in here If a solid, liquid or gas is made up of only one type of atom we say it is an element. For example, consider a tripod made up of iron:
Compounds Sodium chloride (salt) Methane Glucose Compounds are formed when two or more elements are chemically combined. Some examples:
Chemical formulae Na Cl K I O O O The chemical formulae of a molecule or compound is simply a way of showing the ratio of atoms in it. For example… = sodium chloride (NaCl) = potassium iodide (KI) = potassium nitrate (KNO3) K N
Some simple compounds… Methane, CH4 Key Hydrogen Oxygen Carbon Sulphur Sulphuric acid, H2SO4 Carbon dioxide, CO2 Water, H2O Ethyne, C2H2
Chemical formulae • Try drawing these: • Water H2O • Carbon dioxide CO2 • Calcium sulphate CaSO4 • Magnesium hydroxide Mg(OH)2
Naming compounds • This happens with the following elements: • H2 • N2 • O2 • F2 • Cl2 • Br2 These elements always go around in pairs (diatomic molecules). For example, hydrogen looks like this: Rule 1– If two identical elements combine then the name doesn’t change
Naming compounds Rule 2 – When two elements join and one is a halogen, oxygen or sulphur the name ends with ____ide e.g. Magnesium + oxygen magnesium oxide • Sodium + chlorine • Magnesium + fluorine • Lithium + iodine • Chlorine + copper • Oxygen + iron • KBr • LiCl • CaO • MgS • KF
Naming compounds Rule 3 – When three or more elements combine and two of them are hydrogen and oxygen the name ends with hydroxide e.g. Sodium + hydrogen + oxygen Sodium hydroxide • Potassium + hydrogen + oxygen • Lithium + hydrogen + oxygen • Calcium + hydrogen + oxygen • Mg(OH)2
Naming compounds Rule 4 – When three or more elements combine and one of them is oxygen the ending is _____ate e.g. Copper + sulphur + oxygen Copper sulphate • Calcium + carbon + oxygen • Potassium + carbon + oxygen • Calcium + sulphur + oxygen • Magnesium + chlorine + oxygen • Calcium + oxygen + nitrogen • AgNO3 • H2SO4 • K2CO3
Balancing equations Sodium + water sodium hydroxide + hydrogen O Na Na H H H H H O Consider the following reaction: + + 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 Sodium + water sodium hydroxide + hydrogen O O Na Na Na Na H H H H H H H H O O 2Na(s) + 2H2O(l) 2NaOH(aq) + H2(g) We need to balance the equation: + + Now the equation is balanced, and we can write it as:
Some examples Mg + O2 Zn + HCl Fe + Cl2 NaOH + HCl CH4 + O2 Ca + H2O NaOH + H2SO4 CH3OH + O2 MgO ZnCl2 + H2 FeCl3 NaCl + H2O CO2 + H2O Ca(OH)2 + H2 Na2SO4 + H2O CO2 + H2O 2 2 2 3 2 2 2 2 3 2 2 2 2 2 4
Simple formulae to learn “Covalent” formulae “Ionic” formulae H2O CO2 NH3 H2 O2 N2 SO2 Water Carbon dioxide Ammonia Hydrogen Oxygen Nitrogen Sulphur dioxide NaCl CaCl2 MgO HCl H2SO4 HNO3 NaOH Ca(OH)2 CaCO3 Al2O3 Fe2O3 Sodium chloride Calcium chloride Magnesium oxide Hydrochloric acid Sulphuric acid Nitric acid Sodium hydroxide Calcium hydroxide Calcium carbonate Aluminium oxide Iron oxide
Bonding revision Cl Cl Hi. My name’s Johnny Chlorine. I’m in Group 7, so I have 7 electrons in my outer shell I’d quite like to have a full outer shell. To do this I need to GAIN an electron. Who can help me?
Bonding Cl + - Cl Na Na Here comes my friend, Sophie Sodium Hey Johnny. I’m in Group 1 so I have one electron in my outer shell. I’m quite happy to get rid of it. Do you want it? Okay Now we’ve both got full outer shells and we’ve both gained a charge. We’re both called IONS and we’ve formed an IONIC bond.
Covalent Bonding Cl H Cl H Here comes one of my friends, Harry Hydrogen Hey Johnny. I’ve only got one electron but it’s really close to my nucleus so I don’t want to lose it. Fancy sharing? Now we’re both really stable. We’ve formed a covalent bond, a bond that often forms between non-metals.
Rates of Reaction introduction Which of these reactions would be classed as “a fast rate of reaction” and which would be slow?
Measuring the Rate of Reaction Two common methods:
Rate of reaction graph v1 Reaction takes a long time here Reaction is quicker here Time taken for reaction to complete Temperature/ concentration
Rate of reaction graph v2 Slower rate of reaction here due to reactants being used up – “limiting reactant” Fast rate of reaction here Slower reaction Rate of reaction = amount of product formed/reactant used up time Amount of product formed/ amount of reactant used up Time Common units are g/s, g/min, cm3/s or cm3/min
Rate of reaction graph Q. What if less reactants were used? Amount of product formed/ reactant used up Time
Rates of Reaction Oh no! Here comes another one and it’s got more energy… No effect! It didn’t have enough energy! Here comes another one. Look at how slow it’s going… Hi. I’m Mike Marble. I’m about to have some acid poured onto me. Let’s see what happens… It missed! Here comes an acid particle…
Rates of Reaction Chemical reactions occur when different atoms or molecules _____ with enough energy (the “________ Energy): • Basically, the more collisions we get the _______ the reaction goes. The rate at which the reaction happens depends on four things: • The _______ of the reactants, • Their concentration • Their surface area • The ______ the reactants are under Words – activation, quicker, pressure, temperature, collide
Catalysts • Task • Research and find out about two uses of catalysts in industry, including: • Why they are used • The disadvantages of each catalyst
Catalyst Summary Catalysts are used to ____ __ a reaction to increase the rate at which a product is made or to make a process ________. They are not normally ___ __ in a reaction and they are reaction-specific (i.e. different reactions need _________ catalysts). Words – different, speed up, used up, cheaper
Rate of reaction graph - catalysts With catalyst Without catalyst Amount of product formed/ reactant used up Time
An example question Consider the following graph showing the mass lost during a reaction between marble chips of different surface areas and acid: • How much mass had been lost for the large chips in the first 200s? • How much extra mass had been lost by the small chips (compared to large) after 400s? • What mass of large chips would you predict to have left after 500s? 0.8g 0.2g 94.2g
Dust Explosions The aftermath of an explosion at the CTA Acoustics manufacturing plant in Kentucky – the factory had a build up of resin dust in the air (source: US Chemical Safety Board) An exploding custard truck (source: Daily Telegraph)
Atomic mass RELATIVE ATOMIC MASS, Ar (“Mass number”) = number of protons + number of neutrons 4 He SYMBOL 2 PROTON NUMBER = number of protons (obviously)
Relative formula mass, Mr Relative atomic mass of O = 16 Relative atomic mass of H = 1 The relative formula mass of a compound is the relative atomic masses of all the elements in the compound added together. E.g. water H2O: Therefore Mr for water = 16 + (2x1) = 18 Work out Mr for the following compounds: • HCl • NaOH • MgCl2 • H2SO4 • K2CO3 H=1, Cl=35 so Mr = 36 Na=23, O=16, H=1 so Mr = 40 Mg=24, Cl=35 so Mr = 24+(2x35) = 94 H=1, S=32, O=16 so Mr = (2x1)+32+(4x16) = 98 K=39, C=12, O=16 so Mr = (2x39)+12+(3x16) = 138
Conservation of Mass 2Mg + O2 2MgO Mass of magnesium and crucible after burning = 78.56g Mass of magnesium and crucible before burning = 78.25g Here’s a classic experiment where magnesium is burned in a crucible: Mass is always conserved in any reaction, so where did this extra mass come from?
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
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
Another method Q. 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? Mass of product IN GRAMMES Mr of product Mass of reactant IN GRAMMES Mr of reactant Try using this equation: Mass of product IN GRAMMES 4 6g 36 So mass of product = (4/36) x 6g = 0.66g of hydrogen
Problems with calculating masses Calculating the amount of a product may not always give you a reliable answer... • The reaction may not have completely _______ • The reaction may have been _______ • Some of the product may have been ____ • 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 (in g) theoretical yield 65g of zinc reacts with 73g of hydrochloric acid and produces 102g of zinc chloride. What is the percentage yield? Zn + 2HCl ZnCl2 + H2 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 Example question: The theoretical yield is 136g (using Cl = 35.5) so the % yield is 75%
Percentage yield Actual yield Percentage yield = X 100% Predicted yield • Some example questions: • The predicted yield of an experiment to make salt was 10g. If 7g was made what is the percentage yield? • Dave is trying to make water. If he predicts to make 15g but only makes 2g what is the percentage yield? • Sarah performs an experiment and has a percentage yield of 33%. If she made 50g what was she predicted to make? 70% 13% 150g
Atom Economy Relative formula mass of useful product Percentage atom economy = Total masses of products • Converting ethanol into ethene (ethene is the useful bit): • C2H5OH C2H4 + H20 • Making zinc chloride from zinc and hydrochloric acid: • Zn + 2HCl ZnCl2 + H2 Calculate the atom economies of the following: 61% 99%
Chemical Economics Hi. We’re industrial scientists and we want to make lots of chemicals and sell them to make money. What problems would we face? • Possible problems with making chemicals: • Reactions often produce chemicals that aren’t commercially useful or that can’t be sold • To reduce the costs we want to make sure that no reactants are wasted Therefore we need reactions and processes that give us a high percentage yield and high atom economy.
Endothermic and exothermic reactions 07/06/2014 Energy Energy Step 1: Energy must be SUPPLIED to break bonds: Step 2: Energy is RELEASED when new bonds are made: A reaction is EXOTHERMIC if more energy is RELEASED then SUPPLIED. If more energy is SUPPLIED then is RELEASED then the reaction is ENDOTHERMIC
Common examples of these reactions Burning Photosynthesis Cooling packs Hand warmer packs Are these reactions exothermic or endothermic?
Example reactions 07/06/2014
Energy from fuels 07/06/2014 Copper calorimeter Water Spirit burner Fuel
Experimental values Step 1: Calculate the energy gained by the water: Energy gained by water = mass of water x 4.18 J/gC0 x change in temperature Step 2: Divide this value by the mass of the alcohol used to find out the energy gained by the water per gram of alcohol Energy gained per gram = (answer to Step 1) / mass of alcohol burned