AQA AS Physical Chemistry - Empirical Form, AE & Yield PDF

AS CHEMISTRY 3.1.2 AMOUNT OF SUBSTANCE EMPIRICAL FORMULA The empirical formula is the simplest ratio of atoms in the compound. This can be different to the molecular formula. e.g. Molec...

AS CHEMISTRY 3.1.2 AMOUNT OF SUBSTANCE EMPIRICAL FORMULA The empirical formula is the simplest ratio of atoms in the compound. This can be different to the molecular formula. e.g. Molecular Empirical Compound Formula Formula Water H2O H2O Methane CH4 CH4 Ethane C2H6 *CH3 Ethanol C2H5OH C2H5OH Hydrogen H2O2 *HO Peroxide *Notice how the empirical formula is differ when we can simplify the molecular formula. Other times, the molecular formula cannot be simplified, so they are the same. You need to be able to calculate the empirical formula of a compound when given the % by mass of each element present in a compound. How To Calculate & Use Empirical Formula 1. Divide the % for each element by its Ar value 2. Divide each answer by the smallest value obtained in step 1. 3. This gives you the simplest ratio of the elements (the empirical formula). 4. If you know the Mr of the compound, you can use this and the empirical formula to then find the molecular formula of the compound. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.2 AMOUNT OF SUBSTANCE STOICHIOMETRY (MOLAR RATIOS) Stoichiometry is the process of using the relative ratios of reactants and products in an equation to find the number moles of a substance. Take the following reaction: Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g) Having a balanced equation means that, if you know the number of moles of one substance in the equation, you can deduce the number of moles of all other substances in the equation. e.g. the equation tells us that, in theory, one mole of Mg reacts with 2 moles of HCl. Therefore, in real terms, if we know that 0.006 moles of Mg entered the reaction… It reacted with 0.012 moles of HCl (0.006 x 2 = 0.012) Since it is a 1 : 2 ratio, the number of moles of HCl that reacts is always double that of Mg. It produces 0.006 moles of MgCl2 and 0.006 moles of H2. The ratio of Mg to both products is 1 : 1. So the same number of moles is produced. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.2 AMOUNT OF SUBSTANCE STOICHIOMETRY IN CONTEXT Of course, in an exam question they won’t just simply give you the number of moles of a substance present in a reaction! You need to work it out! What they will give you, however, is a mass, a concentration, a volume etc. You will then have to use the mole calculation equations to find the number of moles of a substance present so you can use stoichiometry to find the moles of another substance! THE 3 STEP METHOD! 1. Calculate the number of moles of one substance in the equation using one of the equations below (depending on what information you have). 2. Use the equation to deduce the number of moles of another substance (whatever the question is about) 3. Use that number of moles to calculate what the question demands using one of the equations below (a mass, Mr, concentration, volume etc) How To Use Stoichiometry & Mole Calculations AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.2 AMOUNT OF SUBSTANCE % ATOM ECONOMY % Atom economy is a measure of how efficient a reaction is. In other words, what percentage of the total mass of the atoms in the reactants go to producing the desired / useful product. Mr of desired / useful product % Atom Economy = X 100 Sum of Mr values of all reactants HINTS | TIPS | HACKS Chemists use % atom economy as a way of measuring how efficient a reaction / process is. The higher the % atom economy, the fewer waste products that are produced. A reaction / process that does not produce any waste products will have a 100% atom economy. Be prepared to rearrange this equation! How To Calculate % Atom Economy AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.2 AMOUNT OF SUBSTANCE % YIELD % Yield is a measure of how much of your expected product did you actually produce. In other words, what percentage of the theoretical yield (found by calculation) is actually produced? Actual Yield % Yield = X 100 Theoretical Yield We can work with theoretical and actual mass OR theoretical and actual number of moles here. HINTS | TIPS | HACKS If you calculate an amount of product made using an equation, you are calculating the “theoretical yield” as this assumes that the reaction ice 100% efficient. More often than not, reactions are not 100% efficient. Some of the reactants do not react to form the products or some of the products are lost / further reacted. Be prepared to rearrange this equation! How To Calculate % Yield AQA www.chemistrycoach.co.uk © scidekick ltd 2024