How Can We Describe Chemical Reactions?

How Can We Describe Chemical

Reactions?Chemistry Unit 9

Main IdeasChemical reactions are represented by balanced chemical equations.

There are four main types of chemical reactions: synthesis, combustion, decomposition, and replacement reactions.

Double-replacement reactions occur between substances in aqueous solutions and produce precipitates, water or gases.

Reactions and Equations

8.1

ObjectivesRecognize evidence of chemical change.

Represent chemical reactions with equations

Balance chemical equations.

Chemical ReactionsThe process by which one or more

substances are rearranged to form different substances is called a chemical reaction.

Evidence of a chemical reaction is a chemical change: a process of changing one or more substances into a new substance.

Evidence: temperature changes, color changes, odor, gas bubbles or appearance of a precipitate

Representing Chemical ReactionsChemical equations – statements that

show chemical reactions by the use of chemical formulas and conserved matter with the relative amounts of substances in the reaction.

Parts of an equation reaction:

Reactants are the starting substances.

Products are the substances formed in the reaction.

Common Symbols

Representing Reactions

Word Equations: use of words for reactants and products

aluminum(s) + bromine(l) → aluminum bromide(s)

Skeleton Equations: chemical formulas used for reactants and products but not balanced

Al(s) + Br(l) → AlBr3(s)

Skeleton equations lack information about how many atoms are involved in the reaction.

Representing Reactions

Chemical Equation: Is a statement that uses chemical formulas to show the identities and relative amounts of the substances involved in a chemical reaction.

• 2Al(s) +3Br2 2AlBr3(s)

Balancing Chemical Equations


A coefficient in a chemical equation is the number written in front of a reactant or product, describing the lowest whole-number ratio of the amounts of all the reactants and products.

The most fundamental law is the law of conservation of mass; a balanced equation shows this law.


1.Write the skeleton equation:Make sure chemical formulas are correct.Put in symbols and physical states.

liquid sodium carbonate + aqueous calcium chloride yields solid calcium carbonate + aqueous sodium chloride


2. Count the atoms of the elements in the reactants

Group intact polyatomic ions as a single substance.

Na2CO3(l) + CaCl2(aq) CaCO3(s) + NaCl(aq)


3. Count the atoms of the elements in the products

Group intact polyatomic ions as a single substance.



4. Change the coefficients to make the number of atoms of each element equatl on both sides of the equation

Never change subscripts



5. Write the coefficients in their lowest possible ratios

Na2CO3(l) + CaCl2(aq) CaCO3(s)

+ NaCl(aq)


5. Go back and check math.

Na2CO3(l) + CaCl2(aq) CaCO3(s) + 2 NaCl(aq)


Example

aqueous sodium hydroxide + aqueous calcium bromide yields solid calcium hydroxide and aqueous sodium bromide

2112

Question 1

Which of the following is NOT a chemical reaction?

A. a piece of wood burning

B. a car rusting

C. an ice cube melting into water

D. red litmus paper turning blue

Question 1

What is the coefficient of bromine in the equation 2Al(s) + 3Br2(l) → 2AlBr3(s)?

A. 1

B. 2

C. 3

D. 6

Practice Problems

Page 287 #4-6; page 288 #7-13

Classifying Chemical Reactions

8.2

Objectives

Classify chemical reactions.

Identify the characteristics of different classes of chemical reactions.

Types of Chemical Reactions

Chemists classify reactions in order to organize the many types.

Synthesis

Combustion

Decomposition

Single Replacement

Double Replacement (Metathesis)

SynthesisA synthesis reaction is a reaction in

which two or more substances react to produce a single product.

When two elements react, the reaction is always a synthesis reaction.

SynthesisA synthesis reaction is a reaction in

which two or more substances react to produce a single product.

When two compounds react:AB + CD ABCDAB + BC ABC

CombustionIn a combustion reaction, oxygen

combines with a substance and releases energy in the form of heat and light.

Example: Heated hydrogen reacts with oxygen to produce heat and water in a combustion reaction. This is also a synthesis reaction.

CombustionIn a combustion reaction, oxygen

combines with a substance and releases energy in the form of heat and light.

Element and oxygen react: A + O2 AO

Compound and oxygen react: AB + O2 AO + B

DecompositionA decomposition reaction is one

in which a single compound breaks down into two or more elements or new compounds.

Decomposition reactions often require an energy source, such as heat, light, or electricity, to occur.

DecompositionA decomposition reaction is one

in which a single compound breaks down into two or more elements or new compounds.

Compound breaks down into two elements: AB A + B

Compound breaks down to form new compounds: ABCD AC + BD

Replacement/Displacement

A reaction in which the atoms of one element replace the atoms of another element in a compound is called a single replacement reaction.

A + BX → AX + B

Activity SeriesA metal will not always replace a metal in a compound dissolved in water because of differing reactivities.

An activity series can be used to predict if reactions will occur.

Activity SeriesHalogens frequently replace other halogens in replacement reactions.

Halogens also have different reactivities and do not always replace each other.

Activity SeriesMetals/Halogens are listed in order of reactivity . A less reactive metal/halogen will not replace a more reactive metal/halogen

Practice Problems

Page 291 #14-17; page 292 #18-20

Page 295 #21-24

Double Replacement

Double replacement reactions (also called metathesis) occur when ions exchange between two compounds.

Double Replacement

Metathesis reactions often form one of three products:

The solid product produced during a chemical reaction in a solution is called a precipitate.water – is usually formed with the combination of an acid and a base. A metal salt is also formed.gas – formed when a gas is not a reactant.

Steps to Metathesis

Product Prediction

Question 1Which of the following is NOT

one of the four types of reactions?

A. deconstructive

B. synthesis

C. single replacement

D. double replacement

Question 1The following equation is what

type of reaction?

KCN(aq) + HBr(aq) → KBr(aq) + HCN(g)

A. deconstructive

B. synthesis

C. single replacement

D. double replacement

Practice Problems

Page 297 #25-28; Page 298 #29-34

Solubility8.3

Objectives

Identify new possible ionic compounds in a reaction

Define the terms soluble and insoluble

Predict solids based on solubility rules.

Ionic Compounds in Solutions

Ionic compounds in aqueous solutions mix and exchange partners (double replacement).

example: Na2SO4(aq) + CaCl2(aq)

Some of these products are solids and some of these products remain aqueous.

Solubility

Solubility rules are used to determine the state of matter of products in an aqueous solution.

• Soluble means that the compound dissolves in water.

• Insoluble means that the compound remains intact in the solid state in water.

Solubility Rules

1. Most nitrate (NO3-) salts are soluble

2. Most salts containing the alkali metal ions (Li+, Na+, K+, Cs+, Rb+) and the ammonium ion (NH4

+) are soluble.

3. Most chloride, bromide, and iodide salts are solubleExceptions: Ag+, Pb2+, Hg2

2+

Solubility Rules4. Most sulfate salts are soluble .• Exceptions: Bas+, Pb2+, Hg2

2+, and Ca2+

5. Most hydroxides are only slightly soluble (treat as insoluble). • Exceptions: Na+, K+

6. Most sulfide (S2-), carbonate (CO32-),

chromate (CrO42-), and phosphate (PO4

3-) salts are only slightly soluble (treat as insoluble).• Exceptions: any containing Alkali metals

and ammonium.

Solubility Summary

Soluble

1. Nitrates (NO3-)

2. Group 1, NH4+

3. Halogens

4. Sulfates (SO4

2-)

Insoluble

1. no exceptions

2. no exceptions

3. Ag+, Pb2+, Hg22+

4. Bas+, Pb2+, Hg22+,

Ca2+

5. Hydroxides (OH-)

6. S2-, CO32-, CrO4

2- PO4

3-

Example Problem

Aqueous silver nitrate mixes with aqueous sodium chloride; what solid will be produced from this solution.

Reactions in Aqueous Solutions

9.3

Objectives

Describe aqueous solutions

Write complete ionic and net ionic equations for chemical reactions in aqueous solutions.

Predict whether reactions in aqueous solutions will produce a precipitate, water, or a gas.

Aqueous Solutions

An aqueous solution contains one or more dissolved substances (called solutes) in water.

Aqueous Solutions

Solution – a homogeneous mixture that may contain solids, liquids or gases.

Solutes – the substance that is being dissolved.

Solvent – the substance that is most plentiful and dissolves the other substance.

Aqueous Solutions

There are many possible solutes—sugar and alcohol are molecular compounds that exist as molecules in aqueous solutions.

Molecules that produce hydrogen ions in aqueous solutions are acids.

Aqueous Solutions

Ionic compounds can also be solutes in aqueous solutions.

When ionic compounds dissolve in water, their ions separate in a process called dissociation.

Types of Aqueous Equations

Ionic Equations show ionic detail and dissociation within reactions.

Formula equation:

2NaOH(aq) + CuCl2(aq) → 2NaCl(aq) + Cu(OH)2(s)

Types of Aqueous Equations

2NaOH(aq) + CuCl2(aq) → 2NaCl(aq) + Cu(OH)2(s)

Ionic equations that show all of the particles in a solution as they actually exist are called complete ionic equations.

2Na+(aq) + 2OH–

(aq) + Cu2+ (aq)+ 2Cl–(aq) → 2Na+

(aq) + 2Cl–

(aq) + Cu(OH)2(s)

Ions that do not participate in a reaction are called spectator ions and are not usually written in ionic equations.

Types of Aqueous Reactions

2Na+(aq) + 2OH–

(aq) + Cu2+ (aq)+ 2Cl–(aq) → 2Na+

(aq) + 2Cl–

(aq) + Cu(OH)2(s)

Formulas that include only the particles that participate in reactions are called net ionic equations.

2OH–(aq) + Cu2+

(aq) → Cu(OH)2(s)


Some reactions produce more water molecules.

HBr(aq) + NaOH(aq) → H2O(l) + NaBr(aq)

Without spectator ions H+

(aq) + OH–(aq) → H2O(l)


Gases that are commonly produced are carbon dioxide, hydrogen cyanide (aq-hydrocyanic acid) , and hydrogen sulfide (aq-hydrosulfuric acid).

2HI(aq) + Li2S(aq) → H2S(g) + 2LiI(aq)

HCl(aq) + NaHCO3(aq) → H2CO3(aq) + NaCl(aq)

H2CO3(aq) decomposes immediately.

H2CO3(aq) → H2O(l) + CO2(g)


Two reactions can be combined and represented by a single chemical reaction

Types of Aqueous Reactions Reaction 1

HCl(aq) + NaHCO3(aq) → H2CO3(aq) + NaCl(aq)

Reaction 2

H2CO3(aq) → H2O(l) + CO2(g)

Combined equation

HCl(aq) + NaHCO3(aq) + H2CO3(aq) → H2CO3(aq) + NaCl(aq) + H2O(l) + CO2(g)

Overall equation

HCl(aq) + NaHCO3(aq) → H2O(l) + CO2(g) + NaCl(aq)

Types of Reactions in

Aqueous Solutions1. Precipitate is formed- when a compound

forms from ions, an exothermic reaction takes place. The ions by themselves are less stable and therefore of higher energy than when combined in a compound. This change in energy (increase in stability) ‘drives’ the reaction.

2NaOH(aq) + CuCl2(aq) 2NaCl(aq) + Cu(OH)2(s)

Net:


Aqueous Solutions2. Water is formed- even though a

double reaction takes place, solutions may look the same since water will still be the dominant substance.

HBr(aq) + NaOH(aq) H2O(l) + NaBr(aq)

Net:


Aqueous Solutions3 Gas is formed- for the same reasons

that a precipitate is formed, a gas will also form. Common gases in double replacement reactions are: carbon dioxide, hydrogen cyanide, and hydrogen sulfide.

2HI(aq) + Li2S(aq) H2S(g) + 2LiI(aq)

Net:

Question 1

What is the solvent in an aqueous solution?

A. hydrogen

B. sodium ions

C. water

D. alcohol

Question 2

An equation that includes only the particles that participate in a reaction is called:

A. net ionic equation

B. spectator ions

C. complete ionic equation

D. reduced ionic equation

Practice Problems

Page 302 #35-39; Page 304 #40-44;

Page 306 #45-49; Page #50-56

Accumulating Content

8.5


Objective: Apply knowledge and skills from previous units to content learned in this unit.

• Why do polyatomic ions stay intact in an aqueous solution?



• How do naming rules change when working with gases vs. aqueous acids?



How does lattice energy relate to solubility rules?

Key ConceptsSome physical changes are evidence that indicate a chemical reaction has occurred.

Word equations and skeleton equations provide important information about a chemical reaction.

A chemical equation gives the identities and relative amounts of the reactants and products that are involved in a chemical reaction.

Key ConceptsBalancing an equation involves adjusting the coefficients until the number of atoms of each element is equal on both sides of the equation.

Classifying chemical reactions makes them easier to understand, remember, and recognize.

Activity series of metals and halogens can be used to predict if single-replacement reactions will occur.

Key ConceptsIn aqueous solutions, the solvent is always water. There are many possible solutes.

Many molecular compounds form ions when they dissolve in water. When some ionic compounds dissolve in water, their ions separate.

When two aqueous solutions that contain ions as solutes are combined, the ions might react with one another. The solvent molecules do not usually react.

Key ConceptsReactions that occur in aqueous solutions are double-replacement reactions.

Question 1The law of conservation of mass requires what

in a chemical reaction equation?

A.both sides of the equation to contain the same substances

B. the reactants to have the same amount of molecules as the products

C. both sides to have the same amount of atoms of each element

D.the products to have fewer molecules than the reactant

Question 1A reaction that gives off heat is what

type of reaction?

A. single replacement reaction

B. double replacement reaction

C.synthesis reaction

D. combustion reaction

Question 1Ions that are present in a solution and

do not participate in a chemical reaction when another substance is added are called ____.

A. spectator ions

B. reactants

C.products

D. net ions

Question 1A double replacement reaction produces

all of the following except ____.

A. gases

B. solids

C.light

D. water

Question 1What type of reaction is the following?

2H2O(l) + energy → H2(g) + O2(g)

A. synthesis reaction

B. decomposition reaction

C.combustion reaction

D. replacement reaction

Question 1What type of reaction is the following?

2H2(g) + O2(g) → 2H2O(l)

A. replacement reaction

B. synthesis

C.combustion reaction

D. double replacement reaction

Question 1A precipitate forms in a double

replacement reaction only if:

A. the reactivities of the compounds differ

B. the new compound is denser than water

C.the new compound is soluble in water

D. the new compound is not soluble in water

Question 1A ____ is a statement that uses chemical

formulas to show the identities and relative amounts of the substances involved in a chemical reaction.

A. word equation

B. skeleton equation

C.chemical equation

D. balanced equation

Question 1Predict the type of reaction.

LiBr2 (aq) + 2NaOH (aq) → ____

A. synthesis reaction

B. combustion reaction

C.single replacement reaction

D. double replacement reaction

Question 1Which reactions are essentially the

opposite of synthesis reactions?

A. single-replacement

B. decomposition

C.combustion

D. double-replacement

The End

How Can We Describe Chemical Reactions?

Documents

balanced chemical equations

use of chemical formulas

sure chemical formulas

chemical reactionsthe

equation reaction

reactionsword equations

starting substances

different substances