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1 of 35© Boardworks Ltd 2009
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3 of 35© Boardworks Ltd 2009 Balancing equations An important principle in chemical reactions is that matter cannot be created or destroyed. It is important that symbol equations are balanced. A balanced equation has the same number of each type of atom on each side of the equation. Na + Cl 2 NaCl 1 sodium 2 chlorine1 sodium 1 chlorine Balanced: This shows that two moles of sodium react with one mole of chlorine to make two moles of sodium chloride. 2Na + Cl 2 2NaCl 2 sodium 2 chlorine Unbalanced:
4 of 35© Boardworks Ltd 2009 Balancing unfamiliar equations
5 of 35© Boardworks Ltd 2009 Balancing ionic equations Equations containing ions should have the same overall charge on each side in order to be balanced. This can be achieved by balancing the equation in the normal way: Ca 2+ + Cl - → CaCl 2 2 calcium 1 chloride 2 calcium 2 chloride +1 chargeno charge 2 calcium 2 chloride Ca Cl - → CaCl 2 no charge Balanced: Unbalanced:
6 of 35© Boardworks Ltd 2009 Balancing ionic equations problems
7 of 35© Boardworks Ltd 2009 State symbols State symbols are letters that are added to a formula to indicate what state each reactant and product is in. The four state symbols are: These are added after the formula in brackets and subscript. For example: 2H 2(g) + O 2(g) 2H 2 O (g) s l g aq solid liquid gas aqueous
8 of 35© Boardworks Ltd 2009 Adding state symbols
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10 of 35© Boardworks Ltd 2009 Reacting masses
11 of 35© Boardworks Ltd 2009 Calculating reacting masses To calculate the mass of a product given the mass of a reactant, use the following steps: 1. Calculate no. moles of reactant: no. moles = mass / M r 2. Determine mole ratio of reactant to product: ensure the equation is balanced 3. Calculate no. moles of product: use the mole ratio 4. Calculate mass of product: mass = moles × M r
12 of 35© Boardworks Ltd 2009 Reacting masses example What mass of sodium chloride is produced if 2.30 g of sodium is burnt in excess chlorine? 3. Calculate no. moles of NaCl: moles Na = moles NaCl 4. Calculate mass of NaCl: mass = moles × M r = × 58.5 = 5.85 g 1. Calculate no. moles of Na: no. moles = mass / M r = 2.30 / 23.0 = Determine mole ratio of Na to NaCl: 2Na + Cl 2 2NaCl ratio = 2:2 = 1:1
13 of 35© Boardworks Ltd 2009 Reacting masses calculations
14 of 35© Boardworks Ltd 2009 More reacting masses calculations
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16 of 35© Boardworks Ltd 2009 What is concentration? The concentration of a solution is a measure of how much solute is dissolved per unit of solvent. amount of solute is measured in moles volume of solvent is measured in dm 3 concentration is measured in mol dm -3. concentration = amount of solute / volume of solvent Volumes are often expressed in cm 3, so a more useful equation includes a conversion from cm 3 to dm 3. concentration = (no. moles × 1000) / volume mol dm 3 cm 3
17 of 35© Boardworks Ltd 2009 Concentration, moles and volume
18 of 35© Boardworks Ltd 2009 Concentration calculations
19 of 35© Boardworks Ltd 2009 Standard solutions A standard solution is a solution of known concentration. Standard solutions are made by dissolving an accurately weighed mass of solid in a known volume of solvent using a volumetric flask. The volumetric flask has a thin neck, which is marked with a line so it can be filled accurately to the correct capacity. The standard solution can then be used to find the concentration of a second solution with which it reacts. This is known as volumetric analysis or titration.
20 of 35© Boardworks Ltd 2009 Preparing standard solutions
21 of 35© Boardworks Ltd 2009 A titration is a procedure used to identify the concentration of a solution by reacting it with a solution of known concentration and measuring the volume required for a complete reaction. Once the number of moles for the solution is known, the concentration can be easily calculated. The number of moles in the standard solution is calculated. Using a balanced equation for the reaction, the number of moles in the solution of unknown concentration can also be calculated. What is a titration?
22 of 35© Boardworks Ltd 2009 Carrying out a titration
23 of 35© Boardworks Ltd 2009 Titration calculations examples What is the concentration of an NaOH solution if 25.0 cm 3 is neutralized by 23.4 cm mol dm -3 HCl solution? 3. Calculate no. moles of NaOH: moles HCl = moles NaOH 4. Calculate conc. of NaOH: = ( × 1000) / 25.0 conc. = (moles × 1000) / volume = mol dm Calculate no. moles HCl: moles = (conc. × volume) / 1000 = = (0.998 × 23.4) / Determine ratio of NaOH to HCl: NaOH + HCl NaCl + H 2 O ratio NaOH:NaCl = 1:1
24 of 35© Boardworks Ltd 2009 Titration calculations
25 of 35© Boardworks Ltd 2009 More titration calculations
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27 of 35© Boardworks Ltd 2009 What are the different types of yield? The theoretical yield is the maximum mass of product expected from the reaction, calculated using reacting masses. To calculate the percentage yield, the theoretical yield and the actual yield must be calculated. The actual yield is the mass of product that is actually obtained from the real chemical reaction. The percentage yield of a chemical reaction shows how much product was actually made compared with the amount of product that was expected.
28 of 35© Boardworks Ltd 2009 Calculating yield The percentage yield of a reaction can be calculated using the following equation: percentage yield = (actual yield × 100) / theoretical yield Example: What is the percentage yield of a reaction where the theoretical yield was 75 kg but the actual yield was 68 kg? percentage yield = (actual yield × 100) / theoretical yield = 90.7% = (68 × 100) / 75
29 of 35© Boardworks Ltd 2009 What is atom economy? Atom economy is another measure of the efficiency of a chemical reaction. It is the mass of reactants that end up as the desired product – this is calculated as a percentage. This concept is useful to chemical industry, because it takes into account the atoms that end up in unwanted waste products as well as the yield of the reaction. This means a process that produces several worthless by-products could have a high yield but a low atom economy. Reactions with a high atom economy tend to be more environmentally friendly as they tend to produce less waste, use fewer raw materials and use less energy.
30 of 35© Boardworks Ltd 2009 Calculating atom economy Example: What is the atom economy of a reaction where the actual yield was tonnes but the mass of the reactants was tonnes? = 83.3% total mass of reactants mass of desired products × 100 atom economy = mass of desired products × 100 total mass of reactants atom economy = × =
31 of 35© Boardworks Ltd 2009 Yield and atom economy calculations
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33 of 35© Boardworks Ltd 2009 Glossary
34 of 35© Boardworks Ltd 2009 What’s the keyword?
35 of 35© Boardworks Ltd 2009 Multiple-choice quiz
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1)Sodium hydroxide is a strong alkali. In terms of ionisation, what is meant by the word strong? ( 1 mark) 2) You are given solutions of sodium hydroxide.
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Section 1.3. Limiting reactant Excess reactant Yields: Theoretical yield Percentage yield Experimental yield Avogadro’s law Molar volume.
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Copyright©2004 by Houghton Mifflin Company. All rights reserved. 1 Introductory Chemistry: A Foundation FIFTH EDITION by Steven S. Zumdahl University of.
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