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Names and formulae of compounds Naming compounds Most of the substances around you are compounds which have been made by combining some of the 100 or so elements mentioned earlier. Compounds are two or more different elements chemically bonded together. The name of a chemical compound comes from simply combining the names of the elements you reacted together to make your compound. E.g. sodium + chlorine sodium chloride iron + oxygen iron oxide The -ide is added to the end to tell us that 2 elements have joined together, forming a compound. The metal always appears first in the name (if there is one). If the compound in made of two non-metals joined together, the element with the lower group number comes first. For naming compounds, hydrogen is considered to be part of group 1. E.g. hydrogen + chlorine hydrogen chloride carbon + oxygen carbon dioxide the prefixes mon- and di- are used to identify different oxides where more than 1 exists. When a compound contains oxygen and 2 other elements, the compound was not made by reacting all three elements together directly but by another route. The name of compounds of this type (i.e with 2 elements and also oxygen) contains the name of the first two elements (metal first) and end in -ate to indicate the presence of oxygen. E.g. A compound containing copper, sulphur and oxygen is called copper sulphate. iGCSE TOPIC 10.5: Reacting masses and chemical equations 1 Formulae Writing chemical formulae correctly is not as difficult as people think. The chemical formula is made up using the symbol on the periodic table. Make sure you get the correct symbol. For simple molecules the formula shows the actual number of each type of atom present. However for ionic substances and macromolecules, which have giant structures, the formula shows the ratio of ions or atoms of each type. We can use valencies to help us work out the formula of a compound. The valency of an element is the number of electrons an atom has to lose, gain or share to form a compound and is therefore depends on the group it is in on the periodic table. Some valencies are shown in the table below. Element Group 1 Group 2 Group 3 Group 4 Group 5 Group 6 Group 7 To form a compound Lose 1 eLose 2 eLose 3 eShare 4 eShare or gain 3 eShare or gain 2 eShare or gain 1 e- Valency 1 2 3 4 3 2 1 To work out the formula using valencies e.g. sodium oxide 1 Write down the valencies of the two elements. Sodium = 1 2 Write down the symbols of the elements in the order they appear in the name. Na 3 oxygen = 2 O Add subscript numbers after each element so that the valencies balance. We need two Na’s since O is already 2 iGCSE TOPIC 10.5: Reacting masses and chemical equations 2 Answer Na2O Another way to work out formulae is by balancing charges on ions since the final compound is electrically neutral. E.g Aluminium oxide 1 Write down the charges on the two ions. Aluminium = 3+ 2 oxygen = 2- Imagine them on a ‘charge balancing see-saw’ and add ions to both sides until the total on the positive side is equal to the total on the negative side. Al3+ Al3+ O2- O2- O2- Hence the formula contains 3 x Al and 2 x O Answer Al2O3 Some compounds have common names which are not helpful in understanding which elements they are made from and you simply have to learn these. E.g. Water H2O, methane CH4 and ammonia NH3. Once you have worked out chemical formulae you will most likely want to use them to write balanced equations. Balancing is done by placing numbers called coefficients in front of the formulas for the compounds/elements. For example, ‘O2‘ means there is one oxygen molecule involved in a reaction but ‘2O2’ would mean there are two. Example: CH4(g) + O2(g) CO2(g) + H2O(g) Starting with the first element (C) we can see that there is 1C on the left and also 1 on the right so they are balanced. The equation is still unbalanced as there are 4 ‘H’ on the left but only 2 ‘H’ on the right. This must be corrected by placing a ‘2’ in front of the ‘H2O’ so there are now 2 waters: CH4(g) + O2(g) CO2(g) + 2H2O(g) Note: You can’t change the little numbers (ie the 2 in H2O ) as this would no longer be water. iGCSE TOPIC 10.5: Reacting masses and chemical equations 3 Now the ‘H’ balances but there 4 ‘O’ on the right and only 2 on the left. This must be balanced by placing a ‘2’ in front of the ‘O2’ so that there are 2 oxygen molecules: CH4(g) + 2O2(g) CO2(g) + 2H2O(g) Now there is 1 ‘C’, 4 ‘H’ and 4 ‘O’ on each side so it balances. In ionic equations, we tend to look only at the ions that actually change. For example, when iron reacts with copper sulphate to form iron sulphate and copper the equation is: Fe(s) + Cu2+(aq) + SO42-(aq) Fe2+(aq) + SO42-(aq) + Cu(s) Remember that when a salt is in solution the lattice breaks up and its ions separate. In this case, the sulphate ion (SO42-) remains unchanged (we call it a spectator ion) so it can be left out of the equation to give: Fe(s) + Cu2+(aq) Fe2+(aq) + Cu(s) THE MASSES OF ATOMS AND MOLECULES MASS NUMBER AND RELATIVE ATOMIC MASS The mass number of an isotope is equal to the number of protons plus neutrons in one atom and is, therefore, a whole number. In round numbers, mass number and relative atomic mass are equal, but the relative atomic mass is not an exact whole number, partly because protons and neutrons do not have exactly the same mass and partly because of the existence of isotopes. iGCSE TOPIC 10.5: Reacting masses and chemical equations 4 RELATIVE ATOMIC MASS Because of the incredibly small size of atoms, it is not possible to weigh them directly. Instead the masses of atoms are related to an arbitrary standard; for this purpose the mass of one atom of 12C is taken to be 12.0000. The mass of any other atom compared to that of the carbon atom is called it’s RELATIVE ATOMIC MASS and is given the symbol Ar. The relative atomic mass of an element is defined as the average mass of one atom of the element compared with one twelfth the mass of an atom of 12C. N.B. Relative atomic mass is a ratio, so it is simply a number and has no units. The 12C atom contains 6 protons, 6 neutrons and 6 electrons. The hydrogen atom contains 1 proton and 1 electron. Since the mass of an electron is negligible compared to that of a proton or a neutron, the hydrogen atom has only 1/12 the mass of a carbon atom; therefore the relative atomic mass of hydrogen is 1. Similarly, the relative mass of an oxygen atom, which contains 8 protons, 8 neutrons and 8 electrons, is 16. RELATIVE FORMULA MASS We can use the Ar to calculate the relative formula mass (given the symbol Mr) of compounds regardless of whether they exist as individual molecules or as a giant ionic lattice. For CO2 the molecule contains a carbon atom and two oxygen atoms and so 1 molecule of CO2 has a relative formula mass of 12+16+16=44. Similarly the formula of NaCl shows 1 sodium atom for every chlorine atom and so the Mr of NaCl is 23+35.5=58.5. The method for calculating Mr is straightforward even if the formula of a substance in not. Ammonium sulphate has the formula (NH4)2SO4 Mr = (2x14)+(8x1)+32+(4x16) = 132 iGCSE TOPIC 10.5: Reacting masses and chemical equations 5 Calculating quantities During chemical reactions elements always react in the same ratio to form compounds. Mass is also conserved so the total mass of reactants = total mass of products. Consider copper + oxygen copper oxide If 64g of copper react with 16g of oxygen to form copper oxide What mass of oxygen will 32g of copper react with? Ans ………………… What mass of copper will 64g of oxygen react with? Ans ………………… What mass of copper oxide forms when 64g of copper react with 16g of oxygen? Ans ……………… What mass of copper and what mass of oxygen must react to produce 8.0g of copper oxide? Ans ……………… Consider calcium carbonate calcium oxide + carbon dioxide If 100g of calcium carbonate react to produce 56g of calcium oxide and 44g of carbon dioxide What mass of calcium oxide will 60g of calcium carbonate produce? Ans ………………… What mass of carbon dioxide will 20g of calcium carbonate produce? Ans ………………… What mass of calcium carbonate will be needed to form 5.6g of calcium oxide and 4.4g of carbon dioxide Ans ……………… iGCSE TOPIC 10.5: Reacting masses and chemical equations 6 THE PERCENTAGE COMPOSITION OF COMPOUNDS A pure chemical compound has a fixed chemical composition and is represented by a fixed formula. This formula conveys information about: a) the elements present in the substance b) the mass of 1 mole of the substance (Mr expressed in g) c) the number of moles of each element present in 1 mole of the substance The percentage composition of a compound is the % contribution by mass which each component element makes to the substance. CALCULATION OF % COMPOSITION Example: Calculate the % composition by mass of ammonium nitrate. 1. Work out the formula of the compound. The formula of ammonium nitrate is NH4NO3. 2. Calculate the relative molecular/formula mass of the compound. Mr (NH4NO3) = 14 + (4x1) + 14 + (3x16) = 80 80g NH4NO3 contains 28g nitrogen 4g hydrogen 48g oxygen 3. Express the mass of each element as a percentage of the molar mass. %N = 28 x 100 = 35.0% 80 %H = 4 x 100 = 5.0% 80 %O = 48 x 100 = 60.0% 80 Answers are usually given to 1 d.p. 4. Check that the percentages add up to 100%. N.B. Errors from rounding up or down may give answers between 99.8 and 100.2%, How pure are substances? which is satisfactory. iGCSE TOPIC 10.5: Reacting masses and chemical equations 7 PURITY We make use of raw materials such as limestone, crude oil and metal ores which are obtained from the earth’s crust. These materials contain other substances and are therefore not pure. Purity is normally stated as a percentage. Percentage purity = mass of pure substance present total mass (including impurities) x 100 Example 20 tonnes of impure copper is purified by electrolysis. 18 tonnes of pure copper is obtained. What is the % purity of the copper before it was refined. Percentage purity = 18 20 x 100 = 90% An example for you to do Limestone is an impure form of calcium carbonate. If 315g of limestone contain 300g of calcium carbonate, calculate the % purity of the limestone. iGCSE TOPIC 10.5: Reacting masses and chemical equations 8 Topic 5 Reacting masses and equation Summary questions 1 2 3 Write the correct chemical formulae for the following compounds. Sodium chloride ………………………………………………… Sodium oxide ………………………………………………… Magnesium fluoride ………………………………………………… Potassium sulphate ………………………………………………… iGCSE TOPIC 10.5: Reacting masses and chemical equations 9 4 5 Write down the ratio of atoms present in each of the following compounds. Copper oxide CuO ………………………………………………………….. Potassium oxide K2O ………………………………………………………… Calcium hydroxide Ca(OH)2 …………………………………………………. Ammonium sulfate (NH4)2SO4 ……………………………………………….. 6 Balance the following equations 1 Li + S Li2S 2 K + P K3P 3 H2 + Br2 HBr 4 Cu + O2 CuO C2H6 + O2 CO2 5 iGCSE TOPIC 10.5: Reacting masses and chemical equations 10 + H2O 7 Consider the following reaction C + O2 CO2 If 12g of carbon burns completely to produce 44g of carbon dioxide, how much carbon dioxide can be made by burning 1.2g of carbon? ……………………………………………………………………………………… ……………………………………………………………………………………… ……………………………………………………………………………………… 8 Consider the following reaction S + O2 SO2 How much sulfur dioxide can be made when 8g of sulfur burns completely in oxygen? Show your working. ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………………………… 9 Consider the following reaction 4K + O2 2K2O How much potassium oxide can be made when 39g of potassium burns completely in oxygen? Show your working. ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………………………… iGCSE TOPIC 10.5: Reacting masses and chemical equations 11 10 Calculate the % by mass of hydrogen, sulphur and oxygen in sulphuric acid (H2SO4). Show your working ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………………………… iGCSE TOPIC 10.5: Reacting masses and chemical equations 12