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chem reactions

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University of Windsor
Chemistry and Biochemistry

Chemistry Chapter Four Chemical Reactions and Chemical Equations A chemical reaction is a process in which one set of substances (reactants) is converted to a new set of substances (products). The main evidences that prove that a chemical reaction has occurred are: a) Colour change b) Formation of a solid (precipitate) within a clear solution c) Evolution of a gas d) Evolution or absorption of heat To write the shorthand representation of a chemical equation certain steps must be followed. Consider the chemical reaction: nitrogen monoxide + oxygen  nitrogen dioxide 1. Write the reaction using chemical symbols NO + O 2NO 2 2. Balance the chemical equation 2NO + 1O2  2 NO2 *** Balancing equations*** Remember these rules when balancing an equation 1. An equation can be balanced only by adjusting the coefficients of formulas. 2. Never introduce extraneous atoms to balance. NO + O 2NO + O2 3. Never change a formula for the purpose of balancing an equation. NO + O 2NO 3 When balancing equations, using these strategies will result in efficient balancing of chemical equations.  Balance elements that occur in only one compound of each side first.  Balance free elements last.  Balance unchanged polyatomics (or other groups of atoms) as groups.  Fractional coefficients are acceptable and can be cleared at the end by multiplication. Problem: Balance C H O 6 O14 4O + H2O 2 2 Solution: Balance C: C H 6 +14 4 6 CO2+ H O 2 2 Balance H: C H O + O  6 CO + 7H O 6 14 4 2 2 2 At this point, the right side contains 19 O atoms whereas the left only contains 4. To get 15 more O atoms we need to add 15/2 more O atoms by multiplying it with O . 2 Balance O: C H 6 14(14/2) O  6 CO 2 7H O 2 2 Now remove the fraction by multiplying the whole equation by 2. 2C 6 O14 14 O  12 2O + 14 H O 2 2 Remember to check both sides to see that both sides contain equal amounts of each element present. Problem: Write a balanced equation for the complete combustion of C H , a 8 18 gasoline component. Solution: 2C H8 18+ 25O  26CO + 18H 2 2 States of Matter The states of matter in a chemical equation are represented by subscript
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