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Chapter 11

On May 6, 1937, the huge airship Hindenburg erupted into a fireball. Within a short time, 210,000 cubic meters of hydrogen had burned and the airship was destroyed. The chemical reaction that occurred is “hydrogen combines with oxygen to produce water.” You will learn to represent this chemical reaction by a chemical equation.

Hindenberg video

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Word Equations To write a word equation, write the names of the

reactants to the left of the arrow separated by plus signs; write the names of the products to the right of the arrow, also separated by plus signs.

Reactant + Reactant Product + Product

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Methane + Oxygen Carbon dioxide + Water

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iron + oxygen iron(III) oxide

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Hydrogen Peroxide Water and Oxygen

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Chemical Equations A chemical equation is a representation of a chemical

reaction; the formulas of the reactants (on the left) are connected by an arrow with the formulas of the products (on the right).

Various symbols are used when writing equations.

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A catalyst is a substance that speeds up the reaction but is not used up in the reaction.

Without Catalyst With Catalyst

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Skeleton Equations A skeleton equation is a chemical equation that does not

indicate the relative amounts of the reactants and products.

Here is the skeleton equation for rusting: Fe + O2 Fe2O3

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1. Write a sentence that describes this chemical reaction.Na (s) + H2O (l) NaOH (aq) + H2 (g)

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Balanced Chemical Equations A chemical reaction is also described by a balanced

equation in which each side of the equation has the same number of atoms of each element and mass is conserved.

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◦ To write a balanced chemical equation:

1. Write the skeleton equation with correct formulas.2. Use coefficients to balance the equation so that it

obeys the law of conservation of mass. NEVER CHANGE SUBSCRIPTS OF CORRECT FORMULAS

3. If necessary, adjust coefficient ratios to be whole numbers in lowest terms.

* If it won’t seem to balance then one or more formulas are probably wrong.

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3. Balance each equation.a. AgNO3 + H2S Ag2S + HNO3

b. Zn(OH)2 + H3PO4 Zn3(PO4)2 + H2O

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5. Balance each equation.a. FeCl3 + NaOH Fe(OH)3 + NaClb. CS2 + Cl2 CCl4 + S2Cl2

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The heat and smoke of burning charcoal are the products of a combustion reaction. Combustion is one of the five general types of chemical reactions. If you can recognize a reaction as being a particular type, you may be able to predict the products of the reaction.

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Classifying Reactions◦ What are the five general types of reactions?

1. combination (synthesis), 2. decomposition (analysis), 3. single-replacement, 4. double-replacement, 5. combustion.

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◦ Combination Reactions (synthesis) A combination reaction is a chemical change in which

two or more reactants combine to form a single new product.

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13. Complete and balance this equation for a combination reaction.

Be + O2

◦ Decomposition Reactions (analysis) A decomposition reaction is a chemical change in

which a single reactant breaks down into two or more simpler products.

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16. Write the formula for the binary compound that decomposes into the products H2 and Br2.

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◦ Single-Replacement Reactions A single-replacement reaction is a chemical change in

which one element replaces a second element in a compound.

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The activity series of metals lists metals in order of decreasing reactivity.

Reference table J

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◦ Double-Replacement Reactions A double-replacement reaction is a chemical change

involving an exchange of positive ions between two compounds.

To happen, one of the products must be a gas, a precipitate or water.

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19. Write a balanced equation for each reaction.a. KOH (aq) + H3PO4 (aq) b. H2SO4 (aq) + Al(OH)3 (aq)

◦ Combustion Reactions (oxidation) A combustion reaction is a chemical change in which

an element or a compound reacts with oxygen, often producing energy in the form of heat and light.

combustion video

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20. Write a balanced equation for the complete combustion of each compound.

a. Formic acid (HCOOH)b. Heptane (C7H16)

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How can you predict the products of the five general types of reactions?

The number of elements and/or compounds reacting is a good indicator of possible reaction type and thus possible products.

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Structures in limestone caverns are formed when carbon dioxide converts calcium hydrogen carbonate into calcium carbonate. The calcium carbonate precipitates and forms dramatic stalactites and stalagmites. You will learn to predict the formation of precipitates and write equations to describe the reactions that produce them.

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Ionic compounds dissociate in water.◦ NaCl (s) Na+

(aq) + Cl-(aq)◦ AgNO3 (s) Ag+

(aq) + NO3-(aq)

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A complete ionic equation shows dissolved ionic compounds as dissociated free ions.

The reaction between silver nitrate and sodium chloride:

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A net ionic equation shows only those particles that are directly involved in the chemical change. Spectator ions are left out. An ion that appears on both sides of an equation and is

not directly involved in the reaction is called a spectator ion.

A net ionic equation is also balanced with respect to both mass and charge.

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Sodium ions and nitrate ions are not changed during the chemical reaction of silver nitrate and sodium chloride so they’re left out.

The net ionic equation is

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CONCEPTUAL PROBLEM 11.9

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29. Write the complete ionic equation and net ionic equation for the reaction of aqueous calcium hydroxide a with phosphoric acid. The products are calcium phosphate and water.

How can you predict the formation of a precipitate in a double-replacement reaction? You can predict the formation of a precipitate by using

the general rules for solubility of ionic compounds. See reference table F

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Will a precipitate form when a sodium carbonate solution is mixed with a barium nitrate solution?

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Sodium nitrate is soluble but barium carbonate is insoluble. The net ionic equation is

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