chemical reactions chapter 11. describing chemical reactions essential question: how does one write...

Chemical Reactions

Chapter 11

Describing Chemical Reactions

Essential Question:

How does one write a proper word, skeleton or chemical equation?

Writing Chemical Equations

• How do you describe what has happened in a chemical reaction?

• Recall from Chapter 2 the shorthand method of describing a chemical reaction:

Reactants Products

Word Equations

iron + oxygen Iron(III) oxide

hydrogen peroxide water + oxygen

methane + oxygen carbon dioxide + water

What does the arrow in these word equations indicate?

Skeleton Equations

Fe + O2 Fe2O3

H2O2 H2 + O2

CH4 + O2 CO2 + H2O

A skeleton equation is an equation that does not indicate the relative amounts of the substance involved.

Balancing Chemical Equations

• How would you write a word equation for the production of a tricycle?

frame + wheel + handlebar + pedal

tricycle

This doesn’t tell about the relative amounts involved

Skeleton Equations

• How would you write a skeleton equation for the production of a tricycle?

F + W + H + P FW3H2P2

Notice:This still doesn’t tell about the relative

amounts involved

Bicycle Equation

Balanced Equations

• How would you write a balance chemical equation for the production of a tricycle?

F + 3W + 2H + 2P FW3H2P2

Notice:You must use coefficients to indicate the

relative amounts involved.

Balancing Chemical Reactions

Combustion of Methane

Production of Hydrogen Chloride

Rules for Writing Balanced Chemical Equations

See the table at the top of page 327.

Balancing Chemical Equations –

Computer lab activity

Balance These Equations

AgNO3 + H2S Ag2S + HNO3

Zn(OH)2 + H3PO4 Zn3(PO4)2 + H20

FeCl3 + Ca(OH)2 Fe(OH)3 + CaCl2

Ag2S(s) + Al(s) Ag(s) + Al2S3(g)

Types of Chemical Reactions

Essential Question:

What are the five general types of chemical reactions, and how can you predict the products of these

reactions?

Classifying Reactions

Reactions fall into five general categories:

• Combination Reactions

• Decomposition Reactions

• Single Replacement Reactions

• Double Replacement Reactions

• Combustion Reactions

Combination Reactions

• Two or more substances forming a single new substance.

• Our analogy: like a marriage

• What distinguishes combination reactions is the presence of only one product.

Combination Reactions

Combination Reactions• Also known as Synthesis reactions.

Examples:

2Mg(s) + O2(g) 2MgO

2K(s) + Cl2(g) 2KCl(s)

Fe(s) + S(s) FeS(s)

2Fe (s) + 3S(s) Fe2S3(s)

Decomposition Reactions

• A single compound breaks down into two or more simpler products.

• Our analogy: a divorce

• What distinguishes a decomposition reaction is a single reactant.


Examples:

2HgO(s) 2Hg(l) + O2(g)

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

2HI H2(g) + I2(s)

electricity

Single-Replacement Reactions

• One element replaces another element in a compound.

• Our analogy: Date-stealing

• What distinguishes a S-R reaction is an uncombined element in the reactants with a different uncombined element in the products.

Single-Replacement Reactions

Examples:

Fe(s) + Pb(NO3)2(aq)

Fe(NO3)2(aq) + Pb(s)

Cl2(g) + 2NaI(aq)

2NaCl(aq) + I2(aq)

Single-Replacement

Will S-R Reactions Occur?

• Whether one metal will replace another in a compound depends their respective reactivities.

• A reactive metal will replace any metal below it in the reactivity series.

• See Table 11.2 on page 333.

• What about the reactivity of the halogens?

Activity Series of the Metals

The Activity Series of the

Metals

Double-Replacement Reactions

• A chemical change involving an exchange of positive ions between two compounds.

• Our analogy: Date-Swapping

Double-Replacement


Examples:

Na2S(aq) + Cd(NO3)2(aq)

CdS(s) + 2NaNO3(aq)

2NaCN(aq) + H2SO4(aq)

2HCN(g) + Na2SO4(aq)

Ca(OH)2(aq) + 2HCl(aq)

CaCl2(aq) + 2H2O(l)


• For a D-R reaction to occur, one of the following is usually true:

– One of the products is a solid that precipitates out

– One of the products is a gas

– One of the products is a molecular compound, such as water

Combustion Reactions

• An element or a compound reacts with oxygen, often producing light and heat.

• Complete combustion = a hydrocarbon reacting with oxygen to produce ONLY carbon dioxide and water as the products.

Combustion Reactions

CombustionExamples:

2C8H18(l) + 25O2(g)16CO2(g) + 18H2O(l)

2C6H6(l) + 15O2 12CO2(g) + 6H2O(g)

2Mg(s) + O2(g) 2MgO(s)

Which of these are complete combustion?

Reactions in Aqueous Solution

Essential Question:

What does a net ionic equation show, and how can one predict the formation of a precipitate in a double-replacement reaction?

Net Ionic Equations

• A net ionic equation shows only those particles involved in the reaction and is balanced with respect to both mass and charge.

• All ions that are unchanged from the reactants side to the products side of the equation are eliminated.

Net Ionic Equations

• Begin with a double-replacement reaction:

AgNO3(aq) + NaCl(aq)

AgCl(s) + NaNO3(aq)

Write the complete ionic equation:

Ag+(aq) + NO3–(aq) + Na+ (aq) + Cl– (aq)

AgCl(s) + Na+(aq) + NO3– (aq)

Net Ionic Equations, cont.

• Eliminate the ions that are “spectators”





• What remains is the Net Ionic EquationAg+(aq) + Cl– (aq) AgCl(s)

Formation of Precipitates

• One can predict the formation of a precipitate by using the general rules for solubility for ionic compounds.

• See Table 11.3 on page 344, or see the Reference Tables.

Solubility Rules for Ionic Compounds

Predicting the Formation of a Precipitate

Formation of PrecipitatesWill a precipitate form when the following

aqueous solutions are mixed?

AgNO3 and Na2SO4

NH4Cl and Ba(NO3)2

CaCl2 and K2SO4

Hg(NO3)2 and HClYes, No, Yes, No

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