2Boardworks AS Chemistry Chemical Calculations Teacher notesIn ‘Slide Show’ mode, click the name of a section to jump straight to that slide.
3Boardworks AS Chemistry Chemical Calculations Balancing equationsBoardworks AS ChemistryChemical CalculationsAn 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 ® NaClUnbalanced:1 sodium 2 chlorine1 sodium 1 chlorine2Na Cl ® NaClBalanced:Teacher notesSee the ‘Moles and Formulae’ presentation for more information about moles.2 sodium 2 chlorine2 sodium 2 chlorineThis shows that two moles of sodium react with one mole of chlorine to make two moles of sodium chloride.
4Balancing unfamiliar equations Boardworks AS ChemistryChemical Calculations
5Balancing ionic equations Boardworks AS ChemistryChemical CalculationsEquations 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 Cl → CaCl2Unbalanced:2 calcium 1 chloride2 calcium 2 chloride+1 chargeno chargeCa Cl → CaCl2Balanced:2 calcium 2 chloride2 calcium 2 chlorideno chargeno charge
6Balancing ionic equations problems Boardworks AS ChemistryChemical CalculationsTeacher notesNote that a couple of values have been given in equation 6 to provide students with a starting point.
7Boardworks AS Chemistry Chemical Calculations State symbolsBoardworks AS ChemistryChemical CalculationsState symbols are letters that are added to a formula to indicate what state each reactant and product is in.The four state symbols are:ssolidlliquidggasaqaqueousThese are added after the formula in brackets and subscript. For example:2H2(g) O2(g) ® 2H2O(g)
8Boardworks AS Chemistry Chemical Calculations Adding state symbolsBoardworks AS ChemistryChemical Calculations
9Boardworks AS Chemistry Chemical Calculations Teacher notesIn ‘Slide Show’ mode, click the name of a section to jump straight to that slide.
10Boardworks AS Chemistry Chemical Calculations Reacting massesBoardworks AS ChemistryChemical Calculations
11Calculating reacting masses Boardworks AS ChemistryChemical CalculationsTo 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 / Mr2. Determine mole ratio of reactant to product:ensure the equation is balanced3. Calculate no. moles of product:Teacher notesSee the ‘Moles and Formulae’ presentation for more information about calculating moles from mass and Mr.use the mole ratio4. Calculate mass of product:mass = moles × Mr
12Reacting masses example Boardworks AS ChemistryChemical CalculationsWhat mass of sodium chloride is produced if 2.30 g of sodium is burnt in excess chlorine?1. Calculate no. moles of Na:no. moles = mass / Mr= 2.30 / 23.0= 0.1002. Determine mole ratio of Na to NaCl:2Na + Cl2 ® 2NaClratio = 2:2= 1:13. Calculate no. moles of NaCl:0.100 moles Na = moles NaCl4. Calculate mass of NaCl:mass = moles × Mr= × 58.5= 5.85 g
13Reacting masses calculations Boardworks AS ChemistryChemical Calculations
14More reacting masses calculations Boardworks AS ChemistryChemical Calculations
15Boardworks AS Chemistry Chemical Calculations Teacher notesIn ‘Slide Show’ mode, click the name of a section to jump straight to that slide.
16Boardworks AS Chemistry Chemical Calculations What is concentration?Boardworks AS ChemistryChemical CalculationsThe concentration of a solution is a measure of how much solute is dissolved per unit of solvent.concentration = amount of solute / volume of solventamount of solute is measured in molesvolume of solvent is measured in dm3concentration is measured in mol dm-3.Volumes are often expressed in cm3, so a more useful equation includes a conversion from cm3 to dm3.Teacher notesThe use of the term ‘molarity’ and the symbol ‘M’ to represent mol dm-3 are no longer recommended.It should be clarified to students that if the amount of solute is expressed as a mass, then the equation above calculates the ‘mass concentration’ (measured in g dm-3), which is different from just ‘concentration’.concentration = (no. moles × 1000) / volumemol dm3cm3
17Concentration, moles and volume Boardworks AS ChemistryChemical Calculations
18Concentration calculations Boardworks AS ChemistryChemical Calculations
19Boardworks AS Chemistry Chemical Calculations Standard solutionsBoardworks AS ChemistryChemical CalculationsA 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.Photo credit: Bsip, Alexandre / Science Photo LibraryGloved hand holding a volumetric flask containing a chemical liquid. This flask is used to accurately measure out 100 millilitres of a liquid chemical or solution.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.
20Preparing standard solutions Boardworks AS ChemistryChemical Calculations
21Boardworks AS Chemistry Chemical Calculations What is a titration?Boardworks AS ChemistryChemical CalculationsA 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.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.Photo credit: Andrew Lambert Photography / Science Photo LibraryPotassium permanganate (purple) being added, from a burette, to a known volume of iron(II) solution, in a flask. This is the end point of the reaction. As the potassium permanganate reacts with the iron it becomes colourless. At the end point there are no more iron ions for the potassium permanganate to react with, and so the last drop retains its colour. The burette is graduated to show volumes, allowing the volume of the potassium permanganate added to the iron to be measured. The volume of potassium permanganate is compared to the known volume of iron, giving information on reaction chemistry and chemical concentrations.Teacher notesTitrations can be used to monitor water purity, measure blood composition and for quality control in the food industry.Once the number of moles for the solution is known, the concentration can be easily calculated.
22Carrying out a titration Boardworks AS ChemistryChemical CalculationsTeacher notesThe end point can be based on different observations, depending on the solutions involved in the titrations. These include cessation of precipitation, colour change, and by use of electrical methods.The following precautions/experimental techniques could be discussed with the students:the need to rinse glassware with distilled water followed by the substance it will contain (e.g. acid or alkali)when the pipette is emptied into the conical flask, dipping the end of the pipette helps ensure all the alkali is transferredcheck that the burette does not contain any air bubbles – if it does, running acid through the burette and refilling is necessarythe need to have at least two concordant titres and report the average of these.
23Titration calculations examples Boardworks AS ChemistryChemical CalculationsWhat is the concentration of an NaOH solution if 25.0 cm3 is neutralized by 23.4 cm mol dm-3 HCl solution?1. Calculate no. moles HCl:moles = (conc. × volume) / 1000= (0.998 × 23.4) / 1000=2. Determine ratio of NaOH to HCl:NaOH + HCl ® NaCl + H2Oratio NaOH:NaCl = 1:13. Calculate no. moles of NaOH:moles HCl = moles NaOH4. Calculate conc. of NaOH:conc. = (moles × 1000) / volume= ( × 1000) / 25.0= mol dm-3
24Titration calculations Boardworks AS ChemistryChemical Calculations
25More titration calculations Boardworks AS ChemistryChemical Calculations
26Boardworks AS Chemistry Chemical Calculations Teacher notesIn ‘Slide Show’ mode, click the name of a section to jump straight to that slide.
27What are the different types of yield? Boardworks AS ChemistryChemical CalculationsThe percentage yield of a chemical reaction shows how much product was actually made compared with the amount of product that was expected.To calculate the percentage yield, the theoretical yield and the actual yield must be calculated.The theoretical yield is the maximum mass of product expected from the reaction, calculated using reacting masses.The actual yield is the mass of product that is actually obtained from the real chemical reaction.
28Boardworks AS Chemistry Chemical Calculations Calculating yieldBoardworks AS ChemistryChemical CalculationsThe percentage yield of a reaction can be calculated using the following equation:percentage yield = (actual yield × 100) / theoretical yieldExample: 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= (68 × 100) / 75= 90.7%
29Boardworks AS Chemistry Chemical Calculations What is atom economy?Boardworks AS ChemistryChemical CalculationsAtom 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.Teacher notesSee the ‘Green Chemistry’ presentation for more information about 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.
30Calculating atom economy Boardworks AS ChemistryChemical Calculationsmass of desired products × 100total mass of reactantsatom economy =Example: What is the atom economy of a reaction where the actual yield was tonnes but the mass of the reactants was tonnes?total mass of reactantsmass of desired products × 100atom economy =× 10030 000== 83.3%
31Yield and atom economy calculations Boardworks AS ChemistryChemical Calculations
32Boardworks AS Chemistry Chemical Calculations Teacher notesIn ‘Slide Show’ mode, click the name of a section to jump straight to that slide.
33Boardworks AS Chemistry Chemical Calculations GlossaryBoardworks AS ChemistryChemical CalculationsTeacher notesactual yield – The mass of product obtained in a reaction. This is usually less than the theoretical yield of the reaction.atom economy – A measure of the efficiency of a reaction in terms of the mass of the desired product as a percentage of the total mass of the reactants.concentration – A measure of how much solute is dissolved per unit of solvent. Expressed as moles per decimetre cubed (moldm-3).percentage yield – The amount of product recovered as a percentage of the theoretical yield.standard solution – A solution whose concentration is known accurately, and which is used to determine the concentration of a solution during a titration.state symbols – Letters that are added to the formulae of the reactants and products in an equation to indicate the state they are in during the reaction. They are: s = solid, l = liquid, g = gas, aq = aqueous solution.theoretical yield – The amount of product that can theoretically be made, assuming that all the reactants form the desired product and no product is lost at any stage.titration – A procedure used to determine the concentration of a solution of unknown concentration by reacting it against a standard solution. Also known as volumetric analysis.volumetric flask – A flask with a thin neck and marked with a line so it can be filled accurately to its capacity. Used in the preparation of a standard solution.
34Boardworks AS Chemistry Chemical Calculations What’s the keyword?Boardworks AS ChemistryChemical Calculations
35Boardworks AS Chemistry Chemical Calculations Multiple-choice quizBoardworks AS ChemistryChemical Calculations