C2 Smart Teach 4: Energy Changes and Calculations in Chemistry

Energy Change a) In an endothermic reaction More Heat energy is taken in breaking bonds than given out making bonds The temperature goes down B) In an exothermic reaction More Heat energy is given out making bonds than taken in breaking bonds The temperature goes up

Energy Changes You need to be able to draw these diagrams Heat energy Progress of reaction Reactants Exothermic Products Heat energy Progress of reaction Reactants Endothermic Products

Yield 1) Define theoretical yield The maximum amount of product when all of the reactants react 2)Why is the actual yield obtains often less than the theoretical yield? Reaction is incomplete, Product is lost, Other side reactions take place 3) Write the calculation for % yield Actual yield/theoretical yield * 100

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It may not be copied, sold nor transferred to a third party or used by the school after membership ceases. Until such time it may be freely used within the member school. All opinions and contributions are those of the authors. The contents of this resource are not connected with nor endorsed by any other company, organisation or institution. PiXL Club Ltd, Company number © Copyright The PiXL Club Ltd, 2016 NB: Some of the QWC style questions have more marks than 6 available this is to allow for students to see all of the possible marking points. Commissioned by The PiXL Club Ltd. February 2016 This is a narrated Walking Talking Mark slideshow. It will run automatically when you press start slideshow from the beginning.

Calculate the percentage yield in the following reactions. Question – Percentage yield i.Lucy should have produced a yield of 15g of barium sulfate but when she completed the experiment she only had 6g. ii.Zeenat’s experiment producing calcium chloride should have produced a yield of 156g but only produced a yield of 112g. iii.In Tom’s experiment the yield was 27g of copper sulfate and the theoretical yield was supposed to be 48g. (6 marks)

Calculate the percentage yield in the following reactions. Answers – Percentage yield i. Kirstie should have produced a yield of 15g of barium sulfate but when she completed the experiment she only had 6g. 6/15 x (100) = 40% ii. Zeenat’s experiment producing calcium chloride should have produced a yield of 156g but only produced a yield of 112g. 112/156 x (100) = 72% iii. In Tom’s experiment the yield was 27g of copper sulfate and the theoretical yield was supposed to be 48g. 27/48 (x100) = 56% (6 marks)

Relative formula mass: (Mr) Is the sum of the Relative atomic mass (Ar) for each atom in a molecule E.g: What is the Mr for CO 2 ? 1 x C = 12 2 x 0 = 2 x 16 Therefore the Mr for carbon dioxide is: 44

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It may not be copied, sold nor transferred to a third party or used by the school after membership ceases. Until such time it may be freely used within the member school. All opinions and contributions are those of the authors. The contents of this resource are not connected with nor endorsed by any other company, organisation or institution. PiXL Club Ltd, Company number © Copyright The PiXL Club Ltd, 2016 NB: Some of the QWC style questions have more marks than 6 available this is to allow for students to see all of the possible marking points. Commissioned by The PiXL Club Ltd. February 2016 This is a narrated Walking Talking Mark slideshow. It will run automatically when you press start slideshow from the beginning.

Use a periodic table to calculate the relative formula mass (M r ) of: i.Water H 2 O (2 marks) ii.Potassium hydroxide KOH (2 marks) iii.Nitric acid HNO 3 (2 marks) iv.Sulfuric acid H 2 SO 4 (2 marks) i.Magnesium hydroxide Mg(OH) 2 (3 marks) Question – Relative Formula Mass

Use a periodic table to calculate is the relative formula mass (M r ) of: i. Water H 2 O (2 marks) (1 X 2) + 16 = 18 ii. Potassium hydroxide KOH (2 marks) = 56 iii. Nitric acid HNO 3 (2 marks) (16 x 3) = 63 iv. Sulfuric acid H 2 SO 4 (2 marks) (1x2) (16x4) = 98 v. Magnesium hydroxide Mg(OH) 2 (3 marks) 12 + (16+1) x2 = 46 (5 marks) Answers – Relative Formula Mass H = 1 N = 14 O = 16 Mg = 24 S = 32 K = 39

Calculating Reacting Masses 20g of A typical question

Calculating Reacting Masses 20g of Stage 1: Calculate the relative formula mass for each reactant or product. (for this calculation if there is a big number multiply your answer by the big number ) Cu0 + H 2  Cu + H 2 0 Formula mass

Calculating Reacting Masses 20g of Stage 2 : Calculate how to get from the formula mass to the mass of the substance given in the question Cu0 + H 2  Cu + H 2 0 Formula mass Mass in Q 20 Divide by 80 Times by 20

Calculating Reacting Masses 20g of Stage 3: Now do the same calculation with the mass you are trying to work out Cu0 + H 2  Cu + H 2 0 Formula mass Mass in Q 20 Answer = 16g Divide by 80 Times by 20 Divide by 80 Times by

Empirical formula: all you need to learn is the steps

Percentage composition