January 17, 2014

Unit 4: Chemical Reactions

What is a chemical reaction?• the process by which the atoms of one or more

substances are rearranged to form different substances

• chemical change

What chemical reactions are important in your life?

January 17, 2014

Evidence of Chemical Reaction (or chemical change)

1. Temperature change

2. Light

3. Color change

4. Production of gasses

5. Production of solids (precipitate)

6. Odor change

What evidence of a chemical reaction do you see in a fire?

What evidence of a chemical reaction do you see when you roast a marshmallow?

January 17, 2014

This arrow separates the reactants from the products (kind of like the equal sign in math)

When you have more than one reactant or product, use a plus sign

Reactants are the starting substances of a reaction.

Products are the substances formed during the reaction.

Representing Chemical Reactions

As chemists, we can describe chemical reactions in different ways.1. In a sentence2. Word equations3. Skeleton equation4. Chemical equations

reactant 1 + reactant 2 product 1 + product 2

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2. Word Equation

iron(s) + chlorine(g) iron (III) chloride(s)

1. In a sentence

"Solid iron and chlorine gas react to produce solid iron (III) chloride."

3. Skeleton Equation: uses chemical formulas

Fe(s) + Cl2(g) FeCl3(s)

The letters in parentheses shows you the state of matter:

(s) = solid state

(l) = liquid state

(g) = gas state

(aq) = water solution

January 17, 2014

Examples

Write skeleton equations for these word equations.

1. hydrogen(g) + bromine(g) hydrogen bromide(g)

2. carbon monoxide(g) + oxygen(g) carbon dioxide(g)

3. potassium chlorate(s) potassium chloride(s) + oxygen(g)

January 17, 2014

Write the word equation and describe in a sentence the following reactions:

1. Mg(s) + FeCl3(aq) Fe(s) + MgCl2(aq)

2. Ba(s) + O2(g) BaO(s)

3. Fe(s) + H2SO4(aq) Fe2(SO4)3(aq) + H2(g)

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skeleton equation: Fe(s) + Cl2(g) FeCl3(s)

chemical equation: 2Fe(s) + 3Cl2(g) 2FeCl3(s)

4. Chemical equation: Balanced equation that shows the reactants and products, and the relative amounts involved in the reaction.• Conservation of mass• Need to have same # of atoms of each element in

the reactants and products.

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How to balance chemical equations

1. Write the skeleton equation.

2. Count the atoms of the elements in the reactants and products.

3. Use coefficients to make the # of atoms of each element equal on both sides of the equation.• coefficients = numbers written in front of a

reactant or product. Tells us the number of particles of the substance involved in the reaction.

skeleton equation: Fe(s) + Cl2(g) FeCl3(s)

chemical equation:

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How to balance chemical equations

1. Write the skeleton equation.

2. Count the atoms of the elements in the reactants and products.

3. Use coefficients to make the # of atoms of each element equal on both sides of the equation.

4. Make sure coefficients are in their lowest possible ratio.

5. Check your work!

January 17, 2014

Practice Problems: Write the balanced chemical equation for each.

1. Hydrogen gas and chlorine gas combine to form hydrochloric acid (HCl)

2. CS2(l) + O2(g) CO2(g) + SO2(g)

January 17, 2014

More practice balancing equations!

1. CaO(s) + H2O(l) Ca(OH)2(s)

2. NaOH(aq) + CaBr2(aq) Ca(OH)2(s) + NaBr(aq)

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Classifying Chemical Reactions

Types of chemical reactions:1. Synthesis 2. Decomposition3. Combustion4. Single-Replacement5. Double-replacement

*Classifying how atoms rearrange during a chemical reaction

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1. Synthesis reaction: 2 or more substances react to produce a single product

A + B AB

Ex:

2Na(s) + Cl2(g) 2NaCl(s)

CaO(s) + H2O(l) Ca(OH)2(s)

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2.Decomposition Reaction: A single compound breaks down into two or more substances. (opposite of synthesis)

AB A + B

Ex:

NH4NO3(s) N2O(g) + 2H2O(g)

Car air bags!: 2NaN3(s) 2Na(s) + 3N2(g)

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3. Combustion reaction: Oxygen combines with a substance and releases energy in the form of heat and light.

Always need O2!

Ex:

2H2(g) + O2(g) 2H2O(g)

C(s) + O2(g) CO2(g)

January 17, 2014

Practice: Classify each of the following reactions

1. CH4(g) + 2O2(g) CO2(g) + 2H2O(g)

2. 2Al(s) + 3S(s) Al2S3 (s)

3. H2O(l) + N2O5(g) 2HNO3(aq)

4. Ni(OH)2(s) NiO(s) + H2O(l)

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4. Single Replacement Reaction: A reaction in which the atoms of one element replaces the atoms of another element in a compound.

A + BX AX + B

Ex:

2Li(s) + 2H2O 2LiOH(aq) + H2(g)

Cu(s) + 2AgNO3

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January 17, 2014

Use activity series to predict whether or not certain reactions will happen.

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Predict whether or not this reaction will occur:

Ag(s) + Cu(NO3)2(aq)

F2(g) + 2NaBr(aq)

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5. Double Replacement Reaction: Reaction in which there is an exchange of ions between compounds.

AX + BY AY + BX

*Cations switch places

*Always produce a precipitate, a gas, or water

Ex: Ca(OH)2(aq) + 2HCl(aq) CaCl2(aq) + 2H2O(aq)

KCN(aq) + HBr(aq)

January 17, 2014

Practice classifying these reactions. There may be more than one classification for each!

1. 3Ni + 2AuBr2 3NiBr2 + 2Au

2. 2HI(g) H2(g) + I2(g)

3. FeO(s) + O2(g) Fe2O3(s)

4. Fe(s) + Sn(NO3)4 (aq) Fe(NO3)3 + Sn(s)

5. LiCrO4(aq) + BaCl2(aq) LiCl(aq) + Ba(CrO4)2(s)

January 17, 2014