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Moles and reacting masses

Moles and reacting masses. Saying relative atomic mass in grams or relative formula mass in grams is a bit wordy… Shorthand word for it – mole! E.G. the relative atomic mass in grams of carbon (12g) is a mole of carbon atoms. A mole is the A r or M r of any substance expressed in grams.

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Moles and reacting masses

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  1. Moles and reacting masses

  2. Saying relative atomic mass in grams or relative formula mass in grams is a bit wordy… Shorthand word for it – mole! E.G. the relative atomic mass in grams of carbon (12g) is a mole of carbon atoms. A mole is the Ar or Mr of any substance expressed in grams. A mole of any substance always contains the same number of particles (6.02 x 1023)! The mole

  3. Example 1: Mg + F2 MgF2 How many moles of fluorine molecules react with one mole of magnesium atoms? Example 2: Mg + 2 HCl  MgCl2 + H2 How many moles of HCl molecules react with how many moles of magnesium atoms? Example 3: 4 Al + 3 O2 2 Al2O3 How many moles of Al atoms react with how many moles of O2 molecules? The mole

  4. Balancing chemical equations • In a chemical reaction, mass is conserved. • This means that in the symbol equation, we have to have the same number of atoms on the side of the reactants as on the side of the products. • You cannot change theformula of the compound! • You can only change how many “lots” of that substance you have by putting a large number • in front of the formula.

  5. Balancing chemical equations • Example: • Mg + HCl  MgCl2 + H2 Mg Mg Cl Cl H H

  6. Calculations We can use moles and formula mass to work out mass of chemicals needed/used m = n x Mr n = m / Mr Mr = m / n (m) (n) (Ar or Mr)

  7. Example If I have 9g of water, how many moles do I have? n = m / Mr n = 9 g / 18 n = 0.5 moles What is the mass of 0.25 moles N2? (m) m = n x Mr m = 0.25 moles x 28 m = 7g (n) (Ar or Mr)

  8. Task: 1) If I have 9g water how many moles do I have? 0.5 moles 2) If I have 5g NaOH how many moles do I have? 0.125 moles 3) If I have 0.5 moles O2 how many grams do I have? 16g

  9. 1) Calculate the number of moles of 5.6g of CO 0.2 moles 2) Calculate the number of moles of 8.2g calcium nitrate 0.05 moles 3) Calculate the mass of 0.2 moles of magnesium oxide 8g 4) Calculate the mass of 0.25 moles of NaCl 14.6g

  10. 4.0 moles of CO are burned in excess O2. The equation for the reaction which occurs is given below. 2CO (g) + O2(g)  2CO2(g) What is the amount in moles of O2 and the amount in moles of CO2 produced? What are the masses of the reactants and the product?

  11. What mass of hydrogen is produced when 192 g of magnesium is reacted with hydrochloric acid? Mg + HCl  MgCl2 + H2 2 8 moles 8 moles n 24 2 Mr 192g 16g m • Now complete questions 2 – 5 on your worksheet!

  12. Making only as much as we need PERCENTAGE YIELD and ATOM ECONOMY AQA Module C2 (higher)

  13. Most of the substances we use every day are made from RAW MATERIALS, often through complex chemical reactions. Reactants (raw materials) Chemical reactions Products

  14. Braunton Pottington 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.

  15. % YIELD is the amount of product you actually make as a % of the amount you should theoretically make PRODUCT REACTANTS % YIELD ABOUT 75% + ACTUALLYmake this much SHOULD make this much

  16. PERCENTAGE YIELD 2 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

  17. PERCENTAGE YIELD 1 RFM: 10056 44 LIMESTONE (calcium carbonate) is used to make QUICKLIME (calcium oxide) for cement making RAM Ca 40 O 16 C 12 CaCO3 CaO + CO2 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

  18. Atom Economy Compare these two industrial reactions 2Mg + O2→ 2MgO What do you notice about each one? Think raw materials, useful products, waste products USEFUL PRODUCT (antacids, fire resistant coatings, electrical insulators) CaCO3→ CaO + CO2 USEFUL PRODUCT (cement, glass, agriculture etc)

  19. ATOM ECONOMY is the mass of product you want as a % of the mass of all the products you make RAM Mg 24 Fe 56 C 12 O 16 2Mg + O2→2MgO RFM: 48 32 80 Atom Economy = 80 / 80 x 100% = 100 % (obviously) Fe2O3 + 3CO →2Fe + 3CO2 160 84 112 132 Atom Economy = 112 / 244 x 100% = 45.9 %

  20. Real example: Paracetamol The non-prescription analgesic market (paracetamol, aspirin, ibuprofen) is worth about £21 billion annually. Maximising % yield and atom economy in the reactions on the left is vital to save money and conserve energy and resources.

  21. SUMMARY % YIELD is the amount of product you actually make as a % of the amount you should theoretically make ATOM ECONOMY is the mass of the product you want as a % of the mass of all the products you make % YIELD ABOUT 75% Stuff you also get but don’t want Stuff you want ATOM ECONOMY about 50% ACTUALLY make this much SHOULD make this much

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