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Chemical Equations & Reactions
Chapter 8
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Chemical Reactions A chemical reaction is a process by which one or more substances is changed into one more different substances Reactants: original substances Products: resulting substances
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Indications of a chemical reaction
Evolution of heat & light Production of a gas Formation of a precipitate Precipitate: a solid that is produced as a result of a chemical reaction
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Chemical equation Describes a chemical reaction
Represents with symbols & formulas the identities & relative amounts of the reactants & products
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Characteristics of a chemical equation
Must represent known facts Must contain the correct formulas Law of conservation of mass must be satisfied The same number of atoms of each element must appear on each side To equalize number of atoms, coefficients are added Placing a coefficient in front of a formula specifies the number of moles of a substance
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Equations Word equation: an equation in which the reactants & products in a chemical reaction are represented by words Only qualitative meaning Formula equation: represents the reactants & products by their symbols or formulas Must be balanced!
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Problem Write word & formula equations for the chemical reaction that occurs when solid sodium oxide is added to water at room temperature and forms sodium hydroxide.
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Significance of a chemical equation
Coefficients indicate relative, not absolute, amounts of reactants & products as moles The relative masses of the reactants & products of a chemical reaction can be determined from the reactions coefficients. The reverse reaction for a chemical equation has the same relative amounts of substances as the forward reaction.
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Cont’d… An equation gives no indication of whether a reaction will actually occur. An equation can be written for a reaction that may not even occur. An equation gives no info about the speed of the reaction.
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Balancing equations Write a word equation. Write a formula equation.
Balance the formula equation Balance atoms that appear only once on each side. Balance polyatomic ions that appear on both sides as single units. Balance the different types of atoms one at a time. Balance H & O atoms after all other atoms are balanced. Count atoms to make sure the equation is balanced.
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Common mistakes Writing incorrect formulas
Trying to balance an equation by changing the subscripts
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Problem The reaction of zinc with aqueous hydrochloric acid produces a solution of zinc chloride & hydrogen gas. Write a balanced chemical equation for the reaction.
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Balance the equations:
_Al + _Fe2O3 _Al2O3 + _Fe _Zn(NO3)2 _ZnO + _NO2 + _O2 _KClO3 _KCl + _O2 _H3PO4 _H2O + _P4O10 _C4H10 + _O2 _CO2 + _H2O _Cu + _AgNO3 _Cu(NO3)2 + _Ag _N2 + _H2 _NH3
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Types of chemical reactions
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(1) Synthesis Two or more substances combine to form a new compound
A + X AX Na + Cl NaCl
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(2) Decomposition A single compound breaks down into two or more simpler substances AX A + X NaCl Na + Cl
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(3) Single-replacement
One element replaces a similar charged element in a compound A + BX AX + B Na + KCl K + NaCl
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(4) Double-replacement
The ions of two compounds exchange places in an aqueous solution to form two new compounds AX + BY AY + BX NaCl + KBr NaBr + KCl
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(5) Combustion A substance combines with oxygen, releasing a large amount of energy in the form of light & heat. A compound reacts with oxygen to form carbon dioxide & water C2H8 + O2 CO2 + H2O
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Identify the type of reaction
C3H6O + 4O2 3CO2 + 3H2O 2KClO3 2KCl + 3O2 2CaO + MnI4 MnO2 + 2CaI2 O2 + 2H2 2H2O 5. LiNO3 + Ag AgNO3 + Li
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