Class Notes (806,850)
BSEN 401 (13)
Lecture

# Chemical Reactions and Chemical Equations.docx

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School
University of Calgary
Department
Course
BSEN 401
Professor
William Huddleston
Semester
Fall

Description
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: Colour change Formation of a solid (precipitate) within a clear solution Evolution of a gas 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 Write the reaction using chemical symbols NO + O  NO 2 2 Balance the chemical equation 2NO + 1O2  2 NO2 *** Balancing equations*** Remember these rules when balancing an equation An equation can be balanced only by adjusting the coefficients of formulas. Never introduce extraneous atoms to balance. NO + O 2NO + O 2 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. - 1 - Problem: Balance C H O6+ 14 4O + H2O 2 2 Solution: Balance C: C H 6 +14 4 6 CO2+ H O 2 2 Balance H: C H 6 +14 46 CO 2 7H O 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 +141542) 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 gaso8in18 component. Solution: 2C H + 25O  16CO + 18H O 8 18 2 2 2 States of Matter The states of matter in a chemical equation are represented by subscript symbols next to the compound. (g) gas (l) liquid (s) solid Example:2C H 8 18(l) 25O 2(g) 16CO 2(g)+ 18H O2 (l) - 2 - Another common symbol which is used for reactants or products disso
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