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Boardworks AS Chemistry Chemical Calculations

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Presentation on theme: "Boardworks AS Chemistry Chemical Calculations"— Presentation transcript:

1 Boardworks AS Chemistry Chemical Calculations

2 Boardworks AS Chemistry Chemical Calculations
Teacher notes In ‘Slide Show’ mode, click the name of a section to jump straight to that slide.

3 Boardworks AS Chemistry Chemical Calculations
Balancing equations Boardworks AS Chemistry Chemical Calculations 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 ® NaCl Unbalanced: 1 sodium 2 chlorine 1 sodium 1 chlorine 2Na Cl ® NaCl Balanced: Teacher notes See the ‘Moles and Formulae’ presentation for more information about moles. 2 sodium 2 chlorine 2 sodium 2 chlorine This shows that two moles of sodium react with one mole of chlorine to make two moles of sodium chloride.

4 Balancing unfamiliar equations
Boardworks AS Chemistry Chemical Calculations

5 Balancing ionic equations
Boardworks AS Chemistry Chemical Calculations 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 Cl → CaCl2 Unbalanced: 2 calcium 1 chloride 2 calcium 2 chloride +1 charge no charge Ca Cl → CaCl2 Balanced: 2 calcium 2 chloride 2 calcium 2 chloride no charge no charge

6 Balancing ionic equations problems
Boardworks AS Chemistry Chemical Calculations Teacher notes Note that a couple of values have been given in equation 6 to provide students with a starting point.

7 Boardworks AS Chemistry Chemical Calculations
State symbols Boardworks AS Chemistry Chemical Calculations 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: s solid l liquid g gas aq aqueous These are added after the formula in brackets and subscript. For example: 2H2(g) O2(g) ® 2H2O(g)

8 Boardworks AS Chemistry Chemical Calculations
Adding state symbols Boardworks AS Chemistry Chemical Calculations

9 Boardworks AS Chemistry Chemical Calculations
Teacher notes In ‘Slide Show’ mode, click the name of a section to jump straight to that slide.

10 Boardworks AS Chemistry Chemical Calculations
Reacting masses Boardworks AS Chemistry Chemical Calculations

11 Calculating reacting masses
Boardworks AS Chemistry Chemical Calculations 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 / Mr 2. Determine mole ratio of reactant to product: ensure the equation is balanced 3. Calculate no. moles of product: Teacher notes See the ‘Moles and Formulae’ presentation for more information about calculating moles from mass and Mr. use the mole ratio 4. Calculate mass of product: mass = moles × Mr

12 Reacting masses example
Boardworks AS Chemistry Chemical Calculations What 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 = 0.100 2. Determine mole ratio of Na to NaCl: 2Na + Cl2 ® 2NaCl ratio = 2:2 = 1:1 3. Calculate no. moles of NaCl: 0.100 moles Na = moles NaCl 4. Calculate mass of NaCl: mass = moles × Mr = × 58.5 = 5.85 g

13 Reacting masses calculations
Boardworks AS Chemistry Chemical Calculations

14 More reacting masses calculations
Boardworks AS Chemistry Chemical Calculations

15 Boardworks AS Chemistry Chemical Calculations
Teacher notes In ‘Slide Show’ mode, click the name of a section to jump straight to that slide.

16 Boardworks AS Chemistry Chemical Calculations
What is concentration? Boardworks AS Chemistry Chemical Calculations The concentration of a solution is a measure of how much solute is dissolved per unit of solvent. concentration = amount of solute / volume of solvent amount of solute is measured in moles volume of solvent is measured in dm3 concentration 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 notes The 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) / volume mol dm3 cm3

17 Concentration, moles and volume
Boardworks AS Chemistry Chemical Calculations

18 Concentration calculations
Boardworks AS Chemistry Chemical Calculations

19 Boardworks AS Chemistry Chemical Calculations
Standard solutions Boardworks AS Chemistry Chemical Calculations 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. Photo credit: Bsip, Alexandre / Science Photo Library Gloved 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.

20 Preparing standard solutions
Boardworks AS Chemistry Chemical Calculations

21 Boardworks AS Chemistry Chemical Calculations
What is a titration? Boardworks AS Chemistry Chemical Calculations 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. 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 Library Potassium 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 notes Titrations 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.

22 Carrying out a titration
Boardworks AS Chemistry Chemical Calculations Teacher notes The 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 transferred check that the burette does not contain any air bubbles – if it does, running acid through the burette and refilling is necessary the need to have at least two concordant titres and report the average of these.

23 Titration calculations examples
Boardworks AS Chemistry Chemical Calculations What 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 + H2O ratio NaOH:NaCl = 1:1 3. Calculate no. moles of NaOH: moles HCl = moles NaOH 4. Calculate conc. of NaOH: conc. = (moles × 1000) / volume = ( × 1000) / 25.0 = mol dm-3

24 Titration calculations
Boardworks AS Chemistry Chemical Calculations

25 More titration calculations
Boardworks AS Chemistry Chemical Calculations

26 Boardworks AS Chemistry Chemical Calculations
Teacher notes In ‘Slide Show’ mode, click the name of a section to jump straight to that slide.

27 What are the different types of yield?
Boardworks AS Chemistry Chemical Calculations The 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.

28 Boardworks AS Chemistry Chemical Calculations
Calculating yield Boardworks AS Chemistry Chemical Calculations 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 = (68 × 100) / 75 = 90.7%

29 Boardworks AS Chemistry Chemical Calculations
What is atom economy? Boardworks AS Chemistry Chemical Calculations 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. Teacher notes See 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.

30 Calculating atom economy
Boardworks AS Chemistry Chemical Calculations mass of desired products × 100 total mass of reactants 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? total mass of reactants mass of desired products × 100 atom economy = × 100 30 000 = = 83.3%

31 Yield and atom economy calculations
Boardworks AS Chemistry Chemical Calculations

32 Boardworks AS Chemistry Chemical Calculations
Teacher notes In ‘Slide Show’ mode, click the name of a section to jump straight to that slide.

33 Boardworks AS Chemistry Chemical Calculations
Glossary Boardworks AS Chemistry Chemical Calculations Teacher notes actual 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.

34 Boardworks AS Chemistry Chemical Calculations
What’s the keyword? Boardworks AS Chemistry Chemical Calculations

35 Boardworks AS Chemistry Chemical Calculations
Multiple-choice quiz Boardworks AS Chemistry Chemical Calculations


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