OCR unit 5 revision booklet Moles and Molar Mass One mole of any substance contains 6x10 23 particles. For example a mole of carbon atoms would contain 10 x 10 23 carbon atoms. To calculate mole of a substance you add up all the relative atomic masses of the atoms in the formula e.g. H2O Ar values H=1, O=16 The relative formula mass for H2O= (2x1) + (1 x16) = 18 Defining relative atomic mass- 12 C has been chosen as the standard and it has a relative atomic mass of 12. The relative atomic mass of a particular element is the average mass of that element, compared to the mass of 1/12 th of an atom of 12 C. The mass of 1 mole of a substance is its relative formula mass in grams- called the molar mass. The units are g/mol e.g. 1) H2O again Relative formula mass = 18 So 1 mole of water has a mass of 18g 2) What is the molar mass of magnesium hydroxide Mg(OH)2, where Ar of Mg= 24, Ar of O=16, and Ar of H= 1 Mg(OH)2= 1x24 + 2x16 + 2x1= 58 So molar mass = 58 Calculations of moles, mass or molar mass Number of moles = mass / molar mass Example- how many moles of water do you have when you have 36g of water and the molar mass is 18? Number of moles = 38/18= 2
So there are 2 moles of water Reacting masses Mass is conserved during a reaction i.e. nothing is lost. The mass of the reactants is the same as the mass of the products Where could you lose mass? Thermal decomposition- if a substance is heated and breaks down to produce products and one of those products is a gas which can escape, it will appear that mass is lost. You can use the idea of conservation of mass to find out how much of a reactant you would need to thermally break down and produce certain products e.g. Ar of C=12, Ar of O=16, Ar of Ca= 40 CaCO3 100g CaO + CO2 56g + 44g so 100g of calcium carbonate would make 56g of calcium oxide and 44g of carbon dioxide it follows that- 200g of calcium carbonate would make 112g of calcium oxide + 88g of carbon dioxide Exams do not always make it that easy! You could get asked- How much calcium carbonate is needed to make 33g of carbon dioxide 100/44 x 33 =75g Where could reactions gain mass? If a metal was burnt in air it would react with oxygen and gain mass magnesium + oxygen magnesium oxide 2Mg + O2 2MgO So Ar of Mg= 24, Ar of oxygen = 16 2Mg + O2 2MgO 48g + 32g 80g So 48g of magnesium joins with 32g of oxygen
Question- what mass of oxygen is gained by 12g of magnesium when it forms magnesium oxide? Instead of 48g of magnesium you have 12, so 48/12 =4 You then divide the oxygen s 32/4= 8g Question- if you wanted to absorb 4g of oxygen, what mass of magnesium would you need? 48/32 x 4= 6g (of magnesium is needed) Or you could say Instead of 32g of oxygen you have only 4 so 32/4= 8 (you have eight times less), so you need 8 times less of magnesium 48/8= 6g (of magnesium that is needed) Percentage composition Can be found out experimentally e.g. when copper oxide reacts with hydrogen to produce copper metal and water CuO + H2 + Cu + H20 Reduction tube Mass (g) letter A Empty reduction tube 31.9 B Reduction tube + copper oxide 34.9 C Reduction tube + copper 34.3 You can work out how much copper oxide there is by Mass of copper oxide = 34.9-31.9 (B-A)= 3.0g Mass of copper = 34.3-31.9 (C-A) = 2.4g Mass of oxygen = 34.9-34.3 (B-C) = 0.6g (NB mass of copper and oxygen equals the mass of the copper oxide) So you can calculate the percentage mass from experimental data Percentage of copper = mass of copper/mass of copper oxide x 100 = 2.4/3.0 x 100 = 80%
Percentage of oxygen in copper oxide= 0.6/3.0 x 100 = 20% Working out percentage mass when you are given the compound chemical formula and relative atomic masses 1. Calculate formula mass using the relative atomic masses of the atoms in the compound 2. Multiply the relative atomic mass of the element by the number of atoms in the compound 3. Work out the percentage using steps 1 and 2 e.g. calculate the percentage by mass of nitrogen in ammonium nitrate NH4NO3 where Ar of N= 14, Ar of H=1, Ar of O=16 1. Formula mass for NH4NO3= 14 + (4x1) + 14 + (3x16)= 80 2. Mass of nitrogen = 2 x 14= 28 3. So percentage of nitrogen is 28/80 x 100 = 35% Empirical formulae This show the simplest whole number ratio of each atom in a compound e.g. C4H10 can be simplified to C2H5 by dividing both the carbon number and hydrogen number by 2. As there are 5 hydrogens you cannot simplify it further as you would not get a whole number Calculating empirical formulae from experimental data Magnesium + oxygen magnesium oxide Crucible letter Mass (g) A Empty crucible 23.70 B Crucible + magnesium 24.42 C Crucible + magnesium oxide 24.90 Mass of magnesium = 24.42-23.70 (B-A)= 0.72g Mass of oxygen = 24.90-24.42 (C-B)= 0.48g Steps for calculating empirical formulae 1. Write down the symbols for the elements 2. Write down their mass in grams 3. Write down the relative atomic mass (Ar) 4. Finding the number of moles by dividing the mass/ar 5. Divide by the smallest number 6. Check for whole numbers and write formula
So for our example 1. Mg O 2. 0.72 0.48 3. Ar= 24 Ar=16 4. 0.72/24= 0.03 0.48/16 = 0.03 5. 0.03/0.03= 1 0.03/0.03= 1 6. MgO The same can be done if you are given percentages of the elements instead of grams Concentration Working out the concentrations of solutions Solution = Solute (solid) + solvent (liquid solute dissolves in) The more concentrated a solution is the more particles there are in a given volume of solvent The volume of the solvent can be measure in units of cubic decimetre or dm 3 which is the same as 1000cm 3, or cm 3. cm 3 dm 3, divide by a 1000 dm 3 cm 3, multiply by a 1000 Calculating concentrations- Concentration in mol/dm 3 = amount of solute in mol volume of solution in dm 3 so what is the concentration of a solution where 500cm 3 of solution contains 4.5g of sodium chloride? 1. Work out molar mass- 4.5/58.5 (Na=23, Cl= 35.5)= 0.077 mol 2. Work out the volume of the solution- 500/1000= 0.5 dm 3 3. work out the concentration of the solution= 0.077/0.5 = 0.154 mol/dm 3
Dilution and GDA We dilute things all the time by adding water, e.g. diluting concentrated squash to make a drink. We add water depending on whether we like our squash strong or weak but we do not know the actual concentration. Sometimes we do need to work out how much water to add to get a solution to the correct concentration. To do that we use the equation Volume of water to add = x starting volume So to make 20cm 3 of a solution with a concentration of 1.0 mol/dm 3 into a solution of 0.1mol/dm 3 volume of water to add= x 20 = (10-1) x 20 = 9 x 20 =180cm 3 GDA- guideline daily amount- This gives you an amount of a particular food type that we need to eat each day in order to stay healthy and our for us not to become underweight or overweight Energy Sugars (g) Total fat (g) Saturated fat Salt (g) (calories) (g) children 1800 85 70 20 4 women 2000 90 70 20 6 men 2500 120 95 30 6 Food manufacturers include GDA when labelling their food Calculating percentage GDA- if a food contains 5g of fat what is the GDA for a woman Percentage GDA = 5/20 x100= 25%
We need sodium ions (from sodium chloride) in our diet for transmitting nerve signals and muscle contraction. Too much can lead to high blood pressure and a risk of heart disease. GDA of salt (sodium chloride) for adults is 6g (teaspoon)/day. The percentage by mass of sodium in sodium chloride is 39.3% So 1g sodium chloride is the same as 1x 39.3/100= 0.393g However many processed foods contain monosodium glutamate which gives us sodium ions ph curves Neutralisation reaction- acid + alkali salt + water Acid- ph 1-6 (small name small numbers) Neutral- ph 7 Alkali- ph 8-14 You can use a ph meter attached to a data logger to record ph, or use indicators and ph charts Investigating ph- Acid is placed into burette which has a tap at the bottom which can be opened to let acid out and a scale along the side to see how much acid has been released. A known volume of acid is placed in a conical flask and a ph electrode is lowered into the conical flask. Alkali is added from the burette and it takes twice as much acid to alkali to get a neutral solution. ph curve for adding an acid to an alkali
ph curve for adding an alkali to an acid There is a sudden change in ph at the end point when neutralisation occurs. These graphs can be used to see how much acid and alkali need to be added for a neutral solution Titrations Indicators- are chemicals which change colour and indicate the ph of a substance. Single indicators- contain a single substance and give a sudden change in colour e.g. Phenolphthalein- is pink in alkaline solutions and colourless in acidic solutions Litmus- is blue in alkaline solutions and red in acidic solutions Mixed indicators- contain serval different indicators with each one changing colour over a different range of ph values. Universal indicator is a mixed indicator but is not suitable in titration Titrations are used to find out the concentration of acid or alkali using a neutralisation reaction and a single indicator e.g. phenolphthalein. A known volume of acid (25cm 3 ) is placed in a conical flask and a few drops of phenolphthalein which turns pink. Acid is added from a burette slowly until the pink colour disappears. The start volume of acid is subtracted from the end volume to give the titre (volume needed for neutralisation). This is done several times and any anomalous results ignored. Titration calculations- Example- 0.1mol/dm 3 hydrochloric acid and 25cm 3 of unknown concentration of sodium hydroxide. Titration is used to find out the concentration of the sodium hydroxide HCl + NaOH NaCl + H2O If the mean titre of acid required to neutralise is 24cm 3, what is the concentration of sodium hydroxide
Convert all volumes to dm 3 by dividing by 1000 So volume of sodium hydroxide is 25/1000 = 0.025 dm3 The volume of the acid titre is 24/1000 = 0.024 Calculate the amount of hydrochloric acid Amount of HCl= concentration x volume = 0.1 x 0.024= 0.0024mol Looking at the equation you can see that 1mol of HCl reacts with 1mol of NaOH, therefore 0.0024mol of acid will reacts with 0.0024mol of NaOH Calculate the concentration of NaOH= amount /volume= 0.0024/0.025= 0.096mol/dm 3 Measuring gases Use a gas syringe Gases can be collected over water where the gas produced is bubbled into a measuring cylinder which is full of water. The gas produced displaces the water in the measuring cylinder and the volume produced can be read off the scale. This does not work well if the gases is soluble in water e.g. carbon dioxide
Measuring masses Gasses have mass. Using an accurate balance, the mass decrease in a reaction where a gas is produced, is the mass of the gas. (Cotton wool is there to stop any liquid escaping) Hydrogen gas cannot be measured this way as its normal mass is too small for a school balance to be able to measure Gas volumes Mg + 2HCl MgCl2 + H2 The hydrogen gas produced is measured at intervals The rate of reaction is fastest at the start and then decreases as the reaction progresses, and then the line is horizontal and this is where the reaction has stopped. You can work out how much hydrogen is produced in the first 60s by measuring from the graph what volume of gas was produced in 60s, and then divide by 60 to get a rate
Limiting reactants A reaction will stop when one of the reactants is used up but you cannot tell which one. If you add different amounts of magnesium to the hydrochloric acid, and different volumes of gas are produced, it shows that the magnesium must be the limiting factor. If hydrochloric acid was the limiting factor, the same volume of gas would be produced regardless of how much magnesium was added to it. Gas calculations One mole of any gas occupies the same volume at room temperature and pressure (RTP). This molar volume is 24dm 3 and so 1mol of any gas occupies 24dm 3. Calculating the number of moles in a gas can be calculated- Amount of gas = volume of gas at rtp / molar volume at rtp e.g. 1.2dm3 of CO2 at rtp contains 1.2/24 = 0.05mol Equilibria In reversible reaction, there is a forward and a backward reaction. When the forward rate equals the backwards rate, it is called equilibrium. This works in a closed system where substances such as gases can escape. The concentration of reactants and products does not change but the forward and backwards are still occurring. The position of equilibrium depends on the ratio of the concentration of products to the concentration of the reactants. The equilibrium is on the left if the concentration of reactants is greater than the concentration of the products The equilibrium is on the right if the concentration of products is greater than the concentration of the reactants
The position of equilibrium changes if there are changes in temperature, pressure, or concentration of one of the reacting particles If all the substances in a reaction are gases, the equilibrium will move if pressure is increased. It will move in the direction of the side which has the lower moles of gas molecules. 2SO2 + O2 2SO3 In the equation above, there are 3 moles on the left (2 x SO2 + 1 x O2), and only 2 on the right. Therefore if pressure is increased, equilibrium moves to the right. If the concentration of one of the substances increases, equilibrium moves to the opposite side. In the equation above, if oxygen concentration is increased equilibrium moves to the right. If the concentration of one of the reactants decreases, equilibrium moves to its side. That means if oxygen concentration decreases, equilibrium moves to the left. If temperature is increased, the reaction moves in the direction of the endothermic reaction. The production of SO3 is an exothermic reaction so an increase in temperature moves equilibrium to the left. Making sulphuric acid Uses of sulphuric acid
The contact process This is used to make sulphuric acid you it needs the following raw materials Sulphur Air (oxygen) Water The contact process happens in 3 stages As the second stage is reversible, suitable conditions need to be chosen to favour the forward reaction and the production of sulphuric acid. As the production of SO3 is exothermic, more SO3 can be produced if the temperature is reduced. However, the temperature cannot be to low or the rate of reaction would be too low (optimum is around 450 0 C). Most of the SO2 is converted into SO3, and increasing the pressure would increase the amount of SO3 made. However, the cost to produce the pressure would exceed the extra yield. Strong and weak acids Strong acids include- nitric and sulphuric acid Weak acid- ethanoic acid (in vinegar)
Strong and weak acids have different ph values even when they are at the same concentration. Strong acids produce more H + ions. Strong acids completely ionise in water whereas weak ones only partially ionise and so produce less H + ions, with many of their molecules not releasing H + ions. Partial ionisation in weak acids is reversible. Acid strength and concentration Concentration of an acid- is the number of moles of acid in 1dm 3, with more moles in a fixed volume making the acid more concentrated Strength of an acid- is how ionised the acid is in water HCl H + + CL - (hydrochloric acid is completely ionised so stronger acid) CH3COOH CH3COO - + H + (ethanoic acid is weaker as only partially ionised) The ph of a strong acid is lower than a weak one as it has a greater concentration of hydrogen ions Electrolysis of acids Solutions containing ions conduct electricity as the ions are free to move. Acids in solution have ionised and contain hydrogen ions and so conduct electricity. During electrolysis of hydrochloric acid or ethanoic acid, hydrogen gas is made that the cathode (negative electrode that attracts positively charged ions). 2H + + 2e - H2 Ethanoic acid conducts electricity less than hydrochloric acid as it has a lower concentration of hydrogen ions to carry the charge through the ethanoic acid. Reactions of acids
Limescale builds up on the heating elements of appliances such as kettles and washing machines. Descalers contain weak acid e.g. citric acid which reacts with the limescale and so removes it. A stronger acid might work faster but it might damage the metal element. Rate of reaction Ethanoic acid reacts slower with other reactants as it has less hydrogen ions to collide with the reactant particles. Hydrochloric acid has more hydrogen ions and so more collisions are likely. Volume of gas Reactions with ethanoic acid and hydrochloric acid, with say magnesium metal would produce the same volume of hydrogen gas. Being a stronger acid, hydrochloric acid would produce this same volume of gas faster. Ethanoic acid is only partially ionised in water to release hydrogen ions. When these hydrogen ions have been used up in reactions with reactant particles, the position of the equilibrium moves to the right, releasing more hydrogen ions until all the ethanoic acid has reacted. Making insoluble compounds Precipitation reactions- happen when two different solutions react and an insoluble substance is produced
Ionic substances contain ions which are held together by strong ionic bonds and cannot move. However, if the ionic substance is in solution, the ions can move from place to place. In a precipitation reaction, ions from two different solutions react together and form an insoluble precipitate. lead nitrate + potassium iodide potassium nitrate + lead iodide (insoluble) Precipitation reactions happen quickly as the collision frequency between the ions in solution is very large. State symbols in equations Pb(NO3)2 (aq) + 2KI (aq) 2KNO3 (aq) + PbI2 (s) (aq)= aqueous solution or dissolved in water (s)= solid (l)= liquid (g)= gas
Making an insoluble salt Testing ions Medical precipitate If people need an x-ray of their digestive system they are given a barium meal which contains barium sulphate. Barium sulphate is insoluble and is prepared by precipitation reactions. X-rays have difficultly going through barium sulphate so the shape of the digestive system shows up. Making barium sulphate- Barium chloride + sodium sulphate sodium chloride + barium sulphate
BaCl2(aq) + Na2SO4 (aq) NaCl (aq) + BaSO4 (aq) Ionic equation for this is- Ba 2+ (aq) + BaSO4(s) Testing for sulphate ions Add a few drops of hydrochloric acid to the test substance to acidify it. Then add a few drops of barium chloride and if sulphate ions are present you get a white precipitate. Testing for halide ions Halide ions are produced by the halogens (group 7) Add lead nitrate to a test solution If chloride ions present- white precipitate If bromide ions present- cream precipitate If iodide ions are present- yellow precipitate If lead nitrate react together the ions present are Pb 2+, NO 3-, Na +, and Cl - Pb2+(aq) + 2Cl-(aq) PbCl2(s) Na + and NO 3- are called spectator ions as they do not take part