copyright © houghton mifflin company. all rights reserved. 6 | 1 chemical reactions reactions...
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Copyright © Houghton Mifflin Company. All rights reserved. 6 | 1
Chemical Reactions
• Reactions involve chemical changes in matter that result in new substances.
• Reactions involve rearrangement and exchange of atoms to produce new molecules.
• Reactants Products
CH4 + 2O2 → CO2 + 2H2O
CH4
O2
CO2
H2O
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Evidence of Chemical Reactions
• A chemical change occurs when new substances are made.
• Visual clues (permanent):– Color change, precipitate formation, gas bubbles,
flames, heat release, cooling, light
• Other clues:– New odor, permanent new state
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Evidence of Chemical Reactions (cont.)
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Chemical Equations
• Shorthand way of describing a reaction
• Provides information about the reaction:– Formulas of reactants and products
– States of reactants and products
– Relative numbers of reactant and product molecules that are required
– CH4(g)+ 2O2(g) → CO2(g) + 2H2O(l)– Can be used to determine weights of reactants used and
of products that can be made
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Symbols Used in Equations
• Symbols used after chemical formula to indicate state:– (g) = gas; (l) = liquid; (s) = solid– (aq) = aqueous, dissolved in water
– e. g. NH3(aq) indicates ammonia dissolved in water
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Conservation of Mass
• Matter cannot be created or destroyed.
• In a chemical reaction, all the atoms present at the beginning are still present at the end.
• Therefore, the total mass cannot change.
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Combustion of Methane
• Methane gas burns to produce carbon dioxide gas and liquid water
CH4(g) + O2(g) CO2(g) + H2O(l)
H
HC
H
HOO+
O
O
C + OH H
1 C + 4 H + 2 O 1 C + 2 O + 2 H + O1 C + 2 H + 3 O
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Combustion of Methane Balanced
• To show a reaction obeys the Law of Conservation of Mass, it must be balanced.
CH4(g) + 2 O2(g) CO2(g) + 2 H2O(l)
H
HC
H
H
OO
+
O
O
C +
OH H
OO
+O
H H
+
1 C + 4 H + 4 O 1 C + 4 H + 4 O
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Writing Equations
• Use proper formulas for each reactant and product.• Proper equation should be balanced.
– Obey Law of Conservation of Mass.
– All elements on reactants side also on product side.
– Equal numbers of atoms of each element on reactant side as on product side.
• Balanced equations show the relationship between the relative numbers of molecules of reactants and products. – Can be used to determine mass relationships
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Balancing Chemical Equations
• If the equation is in words, write the formulas for the reactants and products (include physical states).
• Balance the equation by counting the atoms on both sides of the equation and changing the coefficients as needed. Never change the subscripts!
• This is done by trial and error. Start with the most complicated compound first.
• The best balanced equation is the one with the smallest integer coefficients (not fractions).
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Balance the following equations:
• Solid potassium reacts with gaseous nitrogen to form solid potassium nitride.
• C3H8(g) + O2(g) → CO2(g) + H2O(l)
• CuSO4(aq) + KI(s) → CuI(s) + I2(s) + K2SO4(aq)