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| Question | Answer |
| What is theoretical yield? | The mass of product that should be made in a reaction if no chemicals are 'lost' in the process. |
| How do you calculate theoretical yield? | By using the masses of the reactants and a balanced equation. |
| What is actual yield? | The actual mass of the product - it's always less than the theoretical yield, as some chemicals are always lost. |
| Give a couple of ways chemicals can get 'lost'.. | 1) Some could get lost on filter paper 2) Some could get lost during transfers between containers 3) Some could evaporate etc.. |
| What is percentage yield? | The actual amount of product you collect, written as a percentage of the theoretical yield. |
| What is the formula for percentage yield? | Percentage Yield = Actual Yield / Theoretical Yield x 100% |
| Solve: Ethanol can be oxidised to form ethanal: C2H5OH +[O] --> CH3CHO + H2O 9.2g of ethanol was reacted with an oxidising agent in excess and 2.1g of ethanal was produced. Calculate the theoretical yield and the percentage yield? | Theoretical yield: 9.2/46 = 0.2 moles 1 mole:1 mole so, 0.2moles:0.2moles 0.2 x 44 = 8.8g Percentage Yield: 2.1 / 8.8 x 100 = 24% |
| What is atom economy? | A measure of the proportion of reactant atoms that become part of the desired product (rather than by-products) in the balanced chemical equation. |
| What is the formula for atom economy? | % Atom economy = molecular mass of desired product / sum of the masses of all the products x 100% |
| What is the atom economy for addition reactions? | Always 100% as only a single product is formed - no atoms are wasted. |
| What is the atom economy for substitution reactions? | Not 100% as these reactions always result in at least two products, which means you have at least one by-product. |
| Solve: The reaction of bromomethane (CH3Br) with sodium hydroxide (NaOH) to make methanol (CH3OH): CH3Br + NaOH --> CH3OH +NaBr | 32 / 32 +103 x 100 = 23.7% |
| Solve: 0.475g of CH3Br reacts with an excess of NaOH in this reaction: CH3Br + NaOH --> CH3OH + NaBr 0.153g of CH3OH is produced. What is the percentage yield? | 0.475 / 95 = 0.005 moles 1:1 0.005:0.005 0.005 x 32 = 0.160g 0.513/0.160 x 100 = 95.6% |
| Can reactions have high percentage yields and low atom economies? | Yeah! :) |
| Why do companies always choose to use reactions with high atom economies? | Because these reactions have environmental and economic benefits. 1) There are less waste products - so money is saved, as it costs money to separate the desired product from the waste products, and more money to dispose of the waste products safely (so they don't harm the environment). 2) Companies spend a lot of money buying the reactant chemicals, its a waste of money if a high proportion of them end up as useless products. 3) Reactions with high atom economies are more sustainable as the raw materials are being used more efficiently. 4) If all else fails, companies try and find uses for the by-products that get produced, so they aren't just waste products. |
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