CAKE! Om nom nom! Ingredients for best ever brownies : 185g unsalted butter 185g best dark chocolate 85g plain flour 40g cocoa powder 50g white chocolate 50g milk chocolate 3 large eggs (each 60g) 275g golden caster sugar Total mass: kg Learning Objectives State that reactions do not give a 100% yield (E/D) | Explain that some reactions are reversible (C) | Calculate the percentage yield for a reaction (C/B) Mass of all baked brownies from this recipe = kg Is there a 100% yield? (i.e. is the weight of the ingredients the same as the final cake?) What is the percentage yield?

Working out percentage yield Cake ingredients = kg (max. theoretical mass of product) Baked cake = kg (total mass of product obtained) Percentage yield: % yield = total mass of product obtained x100 maximum theoretical mass of product % brownie yield = x100 = 0.84 x = 84 % Learning Objectives State that reactions do not give a 100% yield (E/D) | Explain that some reactions are reversible (C) | Calculate the percentage yield for a reaction (C/B)

Learning Objectives State that reactions do not give a 100% yield (E/D) Explain that some reactions are reversible (C) Calculate the percentage yield for a reaction (C/B) Keywords Predicted Yield= Actual Yield= Percentage Yield=

The yield is affected by… List three reasons why we don’t get as much cake as we would ‘expect’… Some of the reasons are similar in chemical reactions. The three reasons are: –Some chemicals get lost along the way – they might get stuck to the test tube (like cake mix gets stuck to the mixing bowl). –Sometimes other reactions take place too, e.g. when methane is burned it doesn’t all form carbon dioxide, some forms carbon monoxide or carbon (soot) –Some reactions are reversible and the products turn back into the reactants Learning Objectives State that reactions do not give a 100% yield (E/D) | Explain that some reactions are reversible (C) | Calculate the percentage yield for a reaction (C/B)

Some reactions are reversible! What does this mean? Is our brownie recipe a reversible reaction? –NO! We can’t turn baked brownies back into their ingredients! What reactions are reversible? –Ice to water? –Ammonia + hydrogen chloride  ammonium chloride NH 3 HCl  NH 4 Cl Learning Objectives State that reactions do not give a 100% yield (E/D) | Explain that some reactions are reversible (C) | Calculate the percentage yield for a reaction (C/B)

The chemical industry is a multi billion pound international industry producing millions of products vital to our civilisation and well being. Chemical Engineers play a crucial role and are much in demand – there are many opportunities and high levels of pay! Chemical Engineers are much concerned with: % YIELD and ATOM ECONOMY..

CaCO 3 = CaO + CO 2 LIMESTONE (calcium carbonate) is used to make QUICKLIME (calcium oxide) for cement making RFM: So, THEORETICALLY, 100 tonnes of limestone should produce 56 tonnes of quicklime. BUT the ACTUAL YIELD is only 48 tonnes So..the PERCENTAGE YIELD is only 48 x 100 = 87.5% 56 Why? – next slide PERCENTAGE YIELD 1 RAM Ca 40 O 16 C 12

Very few chemical reactions have a yield of 100% because: The raw materials (eg limestone) may not be pure Some of the products may be left behind in the apparatus The reaction may not have completely finished Some reactants may give some unexpected products Careful planning and design of the equipment and reaction conditions can help keep % yield high PERCENTAGE YIELD 2

Percentage Yield Complete the questions about percentage yield. Remember: % yield = total mass of product obtained x100 maximum theoretical mass of product Learning Objectives State that reactions do not give a 100% yield (E/D) | Explain that some reactions are reversible (C) | Calculate the percentage yield for a reaction (C/B)