Monitoring and Controlling Chemical Reactions 09/11/2018 09/11/2018 Monitoring and Controlling Chemical Reactions OCR Gateway 2016 Chemistry topic 5 W Richards The Weald School

C5.1 Monitoring Chemical Reactions 09/11/2018 C5.1 Monitoring Chemical Reactions

Recap of the “Mole” (HT only) 09/11/2018 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 (CO2) would be 1 mole etc... Q. How many moles are the following? 23g of sodium 48g of magnesium 36g of carbon 28g of iron 1 mol 2 mol 3 mol 0.5 mol

Concentration (HT only) 09/11/2018 Concentration means “how much of a chemical there is in a fixed volume” and can be measured in g/dm3 or mol/dm3. A solution of low concentration (“dilute”) A solution of high concentration (“strong”)

Titration 09/11/2018 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 (HT only) 09/11/2018 Q. 0.05dm3 of HCl neutralises 0.1dm3 of NaOH of concentration 0.5mol/dm3. 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/dm3 HCl

Titration Equations 09/11/2018 0.2dm3 of HCl neutralises 0.1dm3 of NaOH of concentration 0.5mol/dm3. What is the concentration of the acid? H2SO4 of concentration 0.4mol/dm3 neutralises 0.1dm3 of NaOH of concentration 0.2mol/dm3. How much acid was used? HCl + NaOH NaCl + H2O 0.25mol/dm3 H2SO4 + 2NaOH Na2SO4 + 2H2O 0.025mol/dm3

Molar Volume (HT only) 09/11/2018 An important fact: 1 mole of a gas at room temperature (20OC) and pressure (1 atm) 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? 48dm3 6dm3 1/3 mol 48dm3 22g

Questions on Concentration (HT only) 09/11/2018 To calculate the concentration of a substance you could use one of these formulae: Conc. = Mass of substance (g) Volume of solvent (dm3) Conc. = Amount of solute (mol) 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

Converting concentrations (HT only) 09/11/2018 To convert g/dm3 into mol/dm3 you can use the following formula: Concentration = Concentration (g/dm3) (mol/dm3) Molar mass 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.

Calculating the mass of a product (chem only) 09/11/2018 E.g. what mass of magnesium oxide is produced when 60g of magnesium is burned in air? IGNORE the oxygen in step 2 – the question doesn’t ask for it Step 1: READ the equation: 2Mg + O2 2MgO 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

Calculating the mass of a product using moles (chem only) 09/11/2018 Calculating the mass of a product using moles (chem only) Let’s try this question again but using moles: Step 1: READ the equation: 2Mg + O2 2MgO “2 moles of magnesium + 1 mole of oxygen forms 2 moles of magnesium oxide” Step 2: WORK OUT the relative formula masses (Mr) of MgO: 2MgO = 2 x (24+16) = 80 Step 3: Apply these steps: 60g of Mg is equal to 1.25 moles (60/48) Therefore we will make 1.25 moles of magnesium oxide Therefore we make 100g of MgO (1.25 moles)

Mr: 2Al2O3 = 2x((2x27)+(3x16)) = 204 4Al = 4x27 = 108 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? 09/11/2018 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 2) What mass of calcium oxide is produced when 10g of calcium burns? 2Ca + O2 2CaO Mr: 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? 2Al2O3 4Al + 3O2 Mr: 2Al2O3 = 2x((2x27)+(3x16)) = 204 4Al = 4x27 = 108 204g produces 108g so 100g produces (108/204) x 100 = 52.9g of Al2O3

So mass of product = (4/36) x 6g = 0.66g of hydrogen Another method 09/11/2018 Try using this equation: Mass of product IN GRAMMES Mass of reactant IN GRAMMES Mr of product Mr of reactant 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 4 6g 36 So mass of product = (4/36) x 6g = 0.66g of hydrogen

Percentage Yield Percentage yield = Actual yield Predicted yield 09/11/2018 Percentage yield = Actual yield Predicted yield X 100% 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

Percentage Yield (harder) 09/11/2018 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) theoretical yield Example question: 65g of zinc reacts with 73g of hydrochloric acid and produces 102g of zinc chloride. What is the percentage yield? Zn + 2HCl ZnCl2 + H2 The theoretical yield is 136g (using Cl = 35.5) so the % yield is 75%

Total masses of products Atom Economy 09/11/2018 Percentage atom economy = Relative formula mass of useful product Total masses of products Calculate the atom economies of the following: Converting ethanol into ethene (ethene is the useful bit): C2H5OH C2H4 + H20 Making zinc chloride from zinc and hydrochloric acid: Zn + 2HCl ZnCl2 + H2 61% 99%

Choosing a Reaction Pathway (HT only) 09/11/2018 Hi. We’re trying to make some chemicals and need to decide which reaction pathway to use. What factors do we need to consider when deciding which reaction pathway to use? What is the atom economy of each pathway? What is the % yield of each pathway? What rate does each reaction happen at? What is the equilibrium position of each pathway? How useful are the by-products of each reaction?

C5.2 Controlling Reactions 09/11/2018

Measuring the Rate of Reaction 09/11/2018 Two common methods: Table of results: Time/s Mass or reactants/g OR volume of gas formed/cm3

Measuring the Rate of Reaction 09/11/2018 Another common method using the formation of a precipitate: Table of results: Concentration of acid or sodium thiosulfate Time taken for cross to disappear

Amount of gas formed/cm3 Example Results 09/11/2018 Time/s Amount of gas formed/cm3 10 20 30 40 50 60 70 80 90 100 110 120 37 44 56 63 65

Rate of reaction graph 09/11/2018 Amount of product formed/ amount of reactant used up Slower rate of reaction here due to reactants being used up Fast rate of reaction here Slower reaction Time/s Rate of reaction = amount of product formed/reactant used up (in g/s or cm3/s) time

Using Tangents to calculate rate of reaction (HT ) 09/11/2018 Q. Can we work out the rate of reaction at a specific time using the graph? Amount of product formed Diff. in y For example, what is the rate of reaction here? Diff. in x Step 1 – draw a tangent at that point Step 2 – calculate the gradient of the tangent using: gradient = difference in y / difference in x Time/s

Collision Theory 09/11/2018 Chemical reactions will only happen if particles ________ with each other with enough ________.

Rates of Reaction 09/11/2018 Oh no! Here comes another one and it’s got more energy… Here comes another one. Look at how slow it’s going… No effect! It didn’t have enough energy! 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 and Collision Theory 09/11/2018 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: If a ______ has been used The _______ the reactants are under Their concentration Their surface area The ______ of the reactants – this is the biggest factor as it affects the number of collisions AND the energy in each collision Words – activation, quicker, pressure, temperature, collide, catalyst

Energy level diagrams Energy level 09/11/2018 Energy level Activation energy – the energy needed to start the reaction Energy given out by reaction Reaction progress

New pathway with a catalyst How Catalysts Work 09/11/2018 Catalysts work by providing a different pathway for the reaction – one that needs a lower activation energy: Activation energy – the energy needed to start the reaction Energy level New pathway with a catalyst Energy given out by reaction Reaction progress

Words – different, enzyme, speed up, activation, used up, cheaper Catalyst Summary 09/11/2018 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). Catalysts work by lowering the _______ energy needed for a reaction to happen. An example of a catalyst is a biological catalyst called an _______. These catalyts are used to make alcoholic drinks. Words – different, enzyme, speed up, activation, used up, cheaper

C5.3 Equilibria 09/11/2018

Exothermic and Endothermic Reactions recap 09/11/2018 Photosynthesis Can you put these reactions into groups? Burning Self-heating cans Hand warmer packs Cooling packs

Exothermic and Endothermic Reactions 09/11/2018 Exothermic reactions Ones that GIVE OUT energy to the surroundings Endothermic reactions Ones that TAKE IN energy from the surroundings

Reversible Reactions 09/11/2018 Some chemical reactions are reversible. In other words, they can go in either direction – the reactants will make the products and the products will then make the reactants again: A + B C D heat e.g. Ammonium chloride Ammonia + hydrogen chloride NH4Cl NH3 + HCl cool The direction of the reaction can be affected by changing the conditions

Reversible Reactions and energy changes 09/11/2018 If a reaction is EXOTHERMIC in one direction what must it be in the opposite direction? endothermic Q. Which direction would you push the reaction in if you heated it up? A + B C D exothermic If a reaction is endothermic, it wants to take in energy. Therefore, by heating it, you help it. For example, consider copper sulphate: Hydrated copper sulphate (blue) Anhydrous copper sulphate (white) + Heat + Water endothermic exothermic CuSO4 + H2O CuSO4.5H2O

Reversible Reactions and Equilibrium 09/11/2018 09/11/2018 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”): A + B C D Higher Tier only - What happens if we change the conditions of an equilibrium? What does it do to a reaction? Henry Le Chatelier, 1850-1936 We can use Le Chatelier’s Principle to work out how changes in concentration, temperature and pressure will affect an equilibrium. My principle is: “If the system is at equilibrium and a change is made, the system responds to counteract the change.” Here’s the idea:

Le Chatelier’s Principle 1 – Concentration (HT only) 09/11/2018 09/11/2018 Consider a reaction where reactants A and B make products C and D: A + B C D If the concentration of reactant A was increased, the system is no longer at equilibrium. More products will be formed to restore the equilibrium (and vice versa).

Le Chatelier’s Principle 2 – Temperature (HT only) 09/11/2018 09/11/2018 Increasing the temperature of a system basically means giving the system energy, i.e. you will help an endothermic reaction. Unfortunately, this gets expensive! Endothermic reactions Increased temperature: Decreased temperature: Exothermic reactions Increased temperature: Decreased temperature: A + B C D A + B C D More products Less products A + B C D A + B C D Less products More products

Le Chatelier’s Principle 3 – Pressure (HT only) 09/11/2018 09/11/2018 Consider a reaction where nitrogen and hydrogen are used to make ammonia: Q. How many molecules are on this side of the equation? How many are on this side? Nitrogen + hydrogen Ammonia N2 + 3H2 2NH3 A. 4 A. 2 An increase in pressure will push the reaction towards the side with the lower number of molecules (i.e. you’ll make more ammonia). A decrease in pressure will make more hydrogen and nitrogen.

Summary 09/11/2018 09/11/2018 Consider a reaction where reactants A and B make products C and D: A + B C D So, we want to make lots of product BUT: High temperatures and high pressures are expensive… …and they may cause equipment to break We could also use a catalyst to increase the rate of a reaction (but not the yield) We need to choose the right conditions to get a good yield but not make it too expensive!