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How Can We Describe Chemical Reactions?. Chemistry Unit 9. Main Ideas. Chemical reactions are represented by balanced chemical equations. There are four main types of chemical reactions: synthesis, combustion, decomposition, and replacement reactions.

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How Can We Describe Chemical Reactions?

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How Can We Describe Chemical Reactions?

Chemistry Unit 9


Main Ideas

  • Chemical 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


Objectives

  • Recognize evidence of chemical change.

  • Represent chemical reactions with equations

  • Balance chemical equations.


Chemical Reactions

The 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 Reactions

Chemical 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:

  • Reactantsare the starting substances.

  • Productsare 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


Balancing Chemical Equations

A coefficientin 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.


Balancing Chemical Equations

  • 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


Balancing Chemical Equations

  • 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)


Balancing Chemical Equations

  • Count the atoms of the elements in the products

    • Group intact polyatomic ions as a single substance.

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


Balancing Chemical Equations

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

    • Never change subscripts

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


Balancing Chemical Equations

  • Write the coefficients in their lowest possible ratios

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


Balancing Chemical Equations

  • Go back and check math.

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


Balancing Chemical Equations


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)


Synthesis

A 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.


Synthesis

A synthesis reaction is a reaction in which two or more substances react to produce a single product.

  • When two compounds react:

    • AB + CD  ABCD

    • AB + BC  ABC


Combustion

In 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.


Combustion

In 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


Decomposition

A decomposition reactionis 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.


Decomposition

A decomposition reactionis 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 Series

  • A 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 Series

  • Halogens frequently replace other halogens in replacement reactions.

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


Activity Series

  • Metals/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 1

Which of the following is NOT one of the four types of reactions?

A.deconstructive

B.synthesis

C.single replacement

D.double replacement


Question 1

The 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


Solubility

8.3


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

  • Most nitrate (NO3-) salts are soluble

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

  • Most chloride, bromide, and iodide salts are soluble

    Exceptions: Ag+, Pb2+, Hg22+


Solubility Rules

  • Most sulfate salts are soluble .

    • Exceptions: Bas+, Pb2+, Hg22+, and Ca2+

  • Most hydroxides are only slightly soluble (treat as insoluble).

    • Exceptions: Na+, K+

  • Most sulfide (S2-), carbonate (CO32-), chromate (CrO42-), and phosphate (PO43-) salts are only slightly soluble (treat as insoluble).

    • Exceptions: any containing Alkali metals and ammonium.


Solubility Summary

Soluble

Insoluble

no exceptions

no exceptions

Ag+, Pb2+, Hg22+

Bas+, Pb2+, Hg22+, Ca2+

Hydroxides (OH-)

S2-, CO32-, CrO42- PO43-

  • Nitrates (NO3-)

  • Group 1, NH4+

  • Halogens

  • Sulfates (SO42-)


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 solutioncontains 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)


Types of Aqueous Reactions

Some reactions produce more water molecules.

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

  • Without spectator ions H+(aq) + OH–(aq) → H2O(l)


Types of Aqueous Reactions

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)


Types of Aqueous Reactions

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 Solutions

  • 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:


Types of Reactions in Aqueous Solutions

  • 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:


Types of Reactions in Aqueous Solutions

  • 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


Accumulating Content

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

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


Accumulating Content

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

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


Accumulating Content

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

  • How does lattice energy relate to solubility rules?


Key Concepts

  • Some 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 Concepts

  • Balancing 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 Concepts

  • In 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 Concepts

  • Reactions that occur in aqueous solutions are double-replacement reactions.


Question 1

The 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 1

A 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 1

Ions 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 1

A double replacement reaction produces all of the following except ____.

A.gases

B.solids

C.light

D.water


Question 1

What 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 1

What type of reaction is the following?

2H2(g)+ O2(g) → 2H2O(l)

A.replacementreaction

B.synthesis

C.combustion reaction

D.double replacement reaction


Question 1

A 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 1

A ____ 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 1

Predict the type of reaction.

LiBr2 (aq) + 2NaOH (aq) → ____

A.synthesis reaction

B.combustion reaction

C.single replacement reaction

D.double replacement reaction


Question 1

Which reactions are essentially the opposite of synthesis reactions?

A.single-replacement

B.decomposition

C.combustion

D.double-replacement


The End


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