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114. Representing chemical change Chemistry: Chemical change Grade 10 Everything Science www.everythingscience.co.za
2Chemical equationsA chemical equation uses symbols to describe a chemical reaction.In a chemical equation, reactants are written on the left hand side of the equation and theproducts on the right. The arrow is used to show the direction of the reaction.When representing chemical change, it is important to be able to write the chemical formulaof a compound.Chemical equations allow us to concisely show what happens in a reaction:Sentence: Iron reacts with sulphur to form iron sulphideWord equation: Iron + sulphur → iron sulphideChemical equation: FeS FeS Everything Science www.everythingscience.co.za
3Conservation of massThe law of conservation of mass states that the mass of a closed system of substances willremain constant, regardless of the processes acting inside the system. Matter can changeform, but cannot be created or destroyed.In any chemical reaction, the law of conservation of mass applies. This means that the totalatomic mass of the reactants must be the same as the total atomic mass of the products.This also means that the total number of atoms of the reactants must be the same as thetotal number of atoms of the product. Everything Science www.everythingscience.co.za
4Balancing chemical equationsIf the number of atoms of each element in the reactants is the same as the number of atoms ofeach element in the product, then the equation is balanced.The equation below is not balanced. To balance it the co-efficients of the molecules needto be changed. Everything Science www.everythingscience.co.za
5Balancing chemical equations continuedOnce a chemical equation is balanced we can add state symbols to the equation. We use g forgas, l for liquid, s for solid and aq for aqueous.Using the example on the previous slide we get: CH (g)2 O (g) CO 2 (g)2 H 2 O (l) 4 2We can use this equation in chemical calculations. Everything Science www.everythingscience.co.za
6For more practice or to ask an expert for help on thissection see:www.everythingscience.co.zaESADY Everything Science www.everythingscience.co.za