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Chemical reactions Wednesday 8th Nov • Chemical reactions happen all the time. • They can be shown by a general equation: Reactants Products • The same amount of chemicals must be present on both sides of the reaction. • The reactants are the chemicals that react together. • The products are the new chemicals that are made. What do the numbers mean? • SO2 – 1 sulphur – 2 oxygen • MgCO3 – 1 magnesium – 1 carbon – 3 oxygen Identifying atoms Compound Salt (NaCl) Element Sodium No of atoms 1 Element Chlorine Carbon dioxide (CO2) Carbon Water (H2O) Hydrogen Oxygen Oxygen Methane (CH4) Hydrogen sulphide (H2S) Calcium carbonate (CaCO3) Carbon Hydrogen Hydrogen Sulphur Calcium Carbon Oxygen No of atoms 1 Identifying atoms Compound Salt (NaCl) Element Sodium Carbon dioxide (CO2) Carbon Water (H2O) Hydrogen Methane (CH4) Hydrogen sulphide (H2S) Calcium carbonate (CaCO3) No of atoms Element 1 Chlorine 1 1 Oxygen Oxygen 2 Carbon Hydrogen Hydrogen Sulphur Calcium Carbon Oxygen No of atoms Identifying atoms Compound Salt (NaCl) Element Sodium Carbon dioxide (CO2) Carbon Water (H2O) Hydrogen Methane (CH4) Hydrogen sulphide (H2S) Calcium carbonate (CaCO3) No of atoms Element 1 Chlorine 1 1 2 Oxygen Oxygen 2 1 Carbon Hydrogen Hydrogen Sulphur Calcium Carbon Oxygen No of atoms Identifying atoms Compound Salt (NaCl) Element Sodium No of atoms Element No of atoms 1 Chlorine 1 Carbon dioxide (CO2) Carbon Water (H2O) Hydrogen 1 2 Oxygen Oxygen 2 1 Methane (CH4) Hydrogen sulphide (H2S) Calcium carbonate (CaCO3) 1 Hydrogen Sulphur 4 Carbon Hydrogen Calcium Oxygen Carbon Identifying atoms Compound Salt (NaCl) Element Sodium No of atoms Element No of atoms 1 Chlorine 1 Carbon dioxide (CO2) Carbon Water (H2O) Hydrogen 1 2 Oxygen Oxygen 2 1 Methane (CH4) Hydrogen sulphide (H2S) Calcium carbonate (CaCO3) 1 2 Hydrogen Sulphur 4 1 Carbon Hydrogen Calcium Oxygen Carbon Identifying atoms Compound Salt (NaCl) Element Sodium No of atoms Element No of atoms 1 Chlorine 1 Carbon dioxide (CO2) Carbon Water (H2O) Hydrogen 1 2 Oxygen Oxygen 2 1 Methane (CH4) Hydrogen sulphide (H2S) Calcium carbonate (CaCO3) Carbon Hydrogen 1 Hydrogen Sulphur 4 Calcium 1 Carbon 1 Oxygen 3 Now try these… • How many atoms of each element are in the following molecules? 1. 2. 3. 4. 5. 6. H2SO4 Ca(OH)2 NaCl (NH3)3P04 3H2O Al2(CO3)3 Representing reactions • When reactions occur the products are turned into reactants. • This is written as a balanced equation. • We also might need to be able to draw this as the molecules, and in more detail as the atoms involved. What is balanced in a chemical equation? Three things are balanced in a chemical equation: 1. Atoms 2. Mass 3. Charge But how do we do it? Na + HCl NaCl + H2 But how do we do it? Na 1 x Na + HCl NaCl 1 x Na + H2 But how do we do it? Na 1 x Na + HCl 1xH NaCl 1 x Na + H2 2xH But how do we do it? Na 1 x Na + HCl 1xH 1 x Cl NaCl 1 x Na + H2 2xH 1 x Cl But how do we do it? Na 1 x Na + HCl 1xH 1 x Cl NaCl 1 x Na + H2 2xH 1 x Cl But how do we do it? Na 1 x Na + HCl NaCl 1 x Na + H2 1xH 2xH 1 x Cl 1 x Cl There are 2 H on the right and only 1 on the left… this isn’t balanced! But how do we do it? Na 1 x Na + HCl NaCl 1 x Na + 1xH 1 x Cl 1 x Cl We need to add more Hydrogen on the left! H2 2xH But how do we do it? Na 1 x Na + HCl 1xH 1 x Cl NaCl 1 x Na + H2 2xH 1 x Cl 2xH 2xH Now we need to add a big number 2 in front of Hydrogen, as we can’t change the compound only how many of the compound there are… But how do we do it? Na 1 x Na + HCl 1xH 1 x Cl NaCl 1 x Na + H2 2xH 1 x Cl 2xH 2xH 2 x Cl 1 x Cl This also means there are now 2 Cl, so we need 2 Cl on the right! Again we have to add a big number 2. But how do we do it? Na 1 x Na + HCl 1xH 1 x Cl NaCl 1 x Na + H2 2xH 1 x Cl 2xH 2xH 2 x Cl 2 x Cl This also means there are now 2 Cl, so we need 2 Cl on the right! Again we have to add a big number 2. But how do we do it? Na 1 x Na + HCl 1xH 1 x Cl 2xH 2 x Cl NaCl 1 x Na + H2 2xH 1 x Cl 2xH 2 x Cl 1 x Na 2 x Na This also means there are now 2 Na, so we need 2 Na on the left! Again we have to add a big number 2. But how do we do it? Na 1 x Na + HCl 1xH 1 x Cl 2xH 2 x Cl NaCl 1 x Na + H2 2xH 1 x Cl 2xH 2 x Cl 2 x Na 2 x Na This also means there are now 2 Na, so we need 2 Na on the left! Again we have to add a big number 2. Na + HCl NaCl + 2Na + 2HCl 2NaCl + H2 H2 Balanced or unbalanced? Remember the rules and it will work like magic! • A small number just talks about the element it is behind. E.g. O2 = 2 x O • Small numbers outside the bracket mean multiply everything inside the bracket by that number E.g. (CO3)3 = 3 x C, 9 x O • Big numbers mean multiply everything behind by it. E.g. 2 H2O = 4 x H, 2 x O Try to balance these equations using the same method: [1] Na + Cl2 NaCl [2] CH4 + O2 CO2 + H2O [3] Li + HNO3 LiNO3 + H2 [4] Al + O2 Al2O3 Here are the answers: [1] 2 Na + Cl2 2 NaCl [2] CH4 + 2 O2 CO2 + 2 H2O [3] 2 Li + 2 HNO3 2 LiNO3 + H2 [4] 4 Al + 3 O2 2 Al2O3