Chemistry_Grade_10-12 (1).pdf

Write a short test for someone else in the class and

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Write a short test for someone else in the class and then exchange tests with them so that you each have the chance to answer one. 12.2 Writing chemical formulae A chemical formula is a concise way of giving information about the atoms that make up a particular chemical compound. A chemical formula shows each element by its symbol, and also shows how many atoms of each element are found in that compound. The number of atoms (if greater than one) is shown as a subscript. Examples: CH 4 (methane) Number of atoms: (1 x carbon) + (4 x hydrogen) = 5 atoms in one methane molecule H 2 SO 4 (sulfuric acid) Number of atoms: (2 x hydrogen) + (1 x sulfur) + (4 x oxygen) = 7 atoms in one molecule of sulfuric acid A chemical formula may also give information about how the atoms are arranged in a molecule if it is written in a particular way. A molecule of ethane, for example, has the chemical formula C 2 H 6 . This formula tells us how many atoms of each element are in the molecule, but doesn’t tell us anything about how these atoms are arranged. In fact, each carbon atom in the ethane molecule is bonded to three hydrogen atoms. Another way of writing the formula for ethane is CH 3 CH 3 . The number of atoms of each element has not changed, but this formula gives us more information about how the atoms are arranged in relation to each other. The slightly tricky part of writing chemical formulae comes when you have to work out the ratio in which the elements combine. For example, you may know that sodium (Na) and chlorine (Cl) react to form sodium chloride, but how do you know that in each molecule of sodium chloride there is only one atom of sodium for every one atom of chlorine? It all comes down to the valency of an atom or group of atoms. Valency is the number of bonds that an element can form with another element. Working out the chemical formulae of chemical compounds using their valency, will be covered in chapter 4. For now, we will use formulae that you already know. 12.3 Balancing chemical equations 12.3.1 The law of conservation of mass In order to balance a chemical equation, it is important to understand the law of conservation of mass. 224
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CHAPTER 12. REPRESENTING CHEMICAL CHANGE - GRADE 10 12.3 Definition: The law of conservation of mass The mass of a closed system of substances will remain constant, regardless of the processes acting inside the system. Matter can change form, but cannot be created or destroyed. For any chemical process in a closed system, the mass of the reactants must equal the mass of the products. In a chemical equation then, the mass of the reactants must be equal to the mass of the prod- ucts. In order to make sure that this is the case, the number of atoms of each element in the reactants must be equal to the number of atoms of those same elements in the products. Some examples are shown below: Example 1: Fe + S FeS Fe + S Fe S Reactants Atomic mass of reactants = 55.8 u + 32.1 u = 87.9 u Number of atoms of each element in the reactants: (1 × Fe) and (1 × S) Products Atomic mass of product = 55.8 u + 32.1 u = 87.9 u
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