Chapter 4 Stoichiometry of Chemical Reactions

Chemical Equation

Whenatomsgainorlose electrons to yield ions, or

combine with other atoms to form molecules, their

symbols are modified or combined to generate chemical formulas that appropriately

represent the sespecies. Extending this symbolism to represent both the identities and the relative quantities of substances undergoing a chemical (or physical) change involves writing and balancing a

Fundament aspects of a chemical equation

reactants

The substances undergoing reaction are called reactants, and their formulas are placed on the left side of the equation

products

Thesubstances generated by the reaction are called products, and their formulas

are placed on the right sight of the equation.

plus signs

and long arrow

(represent)

Plus signs (+) separate individual reactant and product formulas, and an arrow (⟶) separates the reactant and product (left and right) sides of the equation

coefficients

The relative numbers of reactant and product species are represented by coefficients (numbers placed immediately to the left of each formula). A coefficient of 1 is typically omitted

common practice to use the smallest possible whole-number coefficients in a chemical equation

that these coefficients represent the relative numbers of reactants and products, and, therefore, they may be correctly interpreted as ratios

Different ways you can interpet this chemical equation: Methane and oxygen react to yield carbon dioxide and water in a 1:2:1:2 ratio

• One methane molecule and two oxygen molecules react to yield one carbon dioxide molecule and two water molecules.

• One dozen methane molecules and two dozen oxygen molecules react to yield one dozen carbon dioxide molecules and two dozen water molecules.

• One mole of methane molecules and 2 moles of oxygen molecules react to yield 1 mole of carbon dioxide molecules and2 moles of water molecules.

balanced

balanced, meaning that equal numbers of atoms for each element involved in the reaction are represented on the reactant and product sides

If an element appears in more than one formula on a

given side of the equation, the number of atoms represented in each must be computed and then added together. For example, both products pecies in the example reaction, CO2 andH2O,contain the element oxygen, and so the number of oxygen atoms on the product side of the equation is

an unbalanced chemical equation:

H2O ⟶ H2+O2

(unbalanced)

The H atom balance was upset by this change, but it is easily reestablished by changing the coefficient for the H2 product to 2. 2H2O ⟶ 2H2+O2 (balanced)

what can you change and not change when balancing an equation

Toachieve balance, thecoefficients oftheequation maybe changed as needed. Keep in mind, of course,

that the formula subscripts define, in part, the identity

of the substance, and so these cannot be changed without altering the qualitative meaning of the equation. For example, changing the reactant formula from H2O to H2O2 would yield balance in the number of atoms, but doing so also

changes there actant's identity(it's now hy drogen peroxide and not water). The O atom balance may be achieved by changing the coefficient for H2O to 2.

Write a balanced equation for the reaction of molecular nitrogen (N2) and oxygen (O2) to form dinitrogen pentoxide.

N2+ O2 ⟶ N2O5 (unbalanced)

2N2+ 5O2 ⟶ 2(N2O5)

=2N2+5O2 ⟶ 2N2O5

Write a balanced equation for the decomposition of ammonium nitrate to form molecular nitrogen, molecular oxygen, and water. (Hint: Balance oxygen last, since it is present in more than one molecule on the right side of the equation.)

NH4NO3 ------- N2 O2 H20

Answer: 2NH4NO3 ⟶ 2N2+O2+4H2O

2(N H4 N O3) 2N2 O2 4H20

C2H6+O2 ⟶ H2O+CO2 (unbalanced)

why problem can be hard: C2H6+ (7/2)O2 ⟶ 3H2O+2CO2

This results in seven O atoms on the product side of the

equation, an odd number—no integer coefficient can be used with the O2 reactant to yield an odd number, so a fractional coefficient, 7 2,

now must get rid of the fraction by multiplying everything by 2

answer: 2C2H6+7O2 ⟶ 6H2O+4CO2

what need to do to balance/finish this equation

3N2+9H2 ⟶ 6NH3

the coefficients are not the smallest possible integers representing the relative numbers of reactant and product molecules. Dividing each coefficient by the greatest common factor, 3, gives the preferred equation:

N2+3H2 ⟶ 2NH

physical states of reactants and products in chemical equations very often are indicated with a parenthetical abbreviation following the formulas

s

l

g

aq

The physical states of reactants and products in chemical equations very often are indicated with a parenthetical abbreviation following the formulas. Common abbreviations include s for solids, l for liquids, g for gases, and aq for substances dissolved in water (aqueous solutions, as introduced in the preceding chapter). These notations are illustrated in the example equation here:

greek letter delta over arrow means

Special conditions necessary for are action are sometimes

designated by writing a word or symbol above or below the equation's arrow. For example, are action carried out by

heating may be indicated by the upper case Greek letter delta (Δ) over the arrow. CaCO3(s) ⟶ Δ CaO(s)+CO2(g)

molar equation

CaCl2(aq)+2AgNO3(aq) ⟶ Ca(NO3)2(aq)+2AgCl(s)

because it doesn't explicitly represent the ionic species that are present in solution.

When ionic compounds dissolve in water, they may dissociate into their constituent ions, which are subsequently dispersed homogenously throughout the resulting solution

complete ionic equation

Explicitly representing all dissolved ions results in a complete ionic equation

Ca2+(aq)+2Cl−(aq)+2Ag+(aq)+2NO3−(aq) ⟶ Ca2+(aq)+2NO3−(aq)+2AgCl(s)

spectator ions

net ionic equation

Examining this equation shows that two chemical

species are present in identical form on both sides

of the arrow, Ca2+(aq)and NO3−(aq). These spectator ions—

ions whose presence is

required to maintain charge neutrality—are neitherchemically nor physically changed by the process, and so they may be eliminated from the equation to yield a more succinct representation called a net ionic equation:

When carbon dioxide is dissolved in an aqueous solution of sodium hydroxide, the mixture reacts to yield aqueous sodium carbonate and liquid water. Write balanced molecular, complete ionic, and net ionic equations for this process.

CO2(aq)+NaOH(aq) ⟶ Na2CO3(aq)+H2O(l)

balance the equation:

CO2(aq)+2NaOH(aq) ⟶ Na2CO3(aq)+H2O(l)

CO2(aq)+2Na+(aq)+2OH−(aq) ⟶ 2Na+(aq)+CO32−(aq)+H2O(l)

get rid spectator ions: CO2(aq)+2OH−(aq) ⟶ CO32−(aq)+H2O(l)

page 190

answer: CO2(aq)+2OH−(aq) ⟶ CO32−(aq)+H2O(l)

NaCl(aq)+H2O(l) ⎯⎯⎯⎯⎯⎯⎯⎯⎯→ electricity NaOH(aq)+H2(g)+Cl2(g)

balance the equation and Write balanced molecular, complete ionic, and net ionic equations for this process.

2NaCl(aq)+2H2O(l) ⟶ 2NaOH(aq)+H2(g)+Cl2(g) (molecular)

2Na+(aq)+2Cl−(aq)+2H2O(l) ⟶ 2Na+(aq)+2OH−(aq)+H2(g)+Cl2(g) (complete ionic

2Cl−(aq)+2H2O(l) ⟶ 2OH−(aq)+H2(g)+Cl2(g) (net ionic)

three of the most prevalent types of chemical reactions:

three of the most prevalent types of chemical reactions: precipitation, acid-base, and oxidation-reduction.

A precipitation reaction

A precipitation reaction is one in which dissolved substances react to form one (or more) solid products.

double displacement

Many reactions of this type involve the exchange of ions between ionic compounds in aqueous solution and are sometimes referred to as double displacement, double replacement, or metathesis reactions. These reactions are common in nature and are responsible for the formation of coral reefs in ocean waters and kidney stones in animals

solubility

The extent to which a substance may be dissolved in water,

or any solvent, is quantitatively expressed as its solubility, defined as the maximum concentration of a substance that can be achieved under specified conditions

soluble

Substances with relatively large solubilities are said to be soluble.

precipitate

A substance will precipitate when solution conditions are such that its concentration exceeds its solubility.

insoluble

Substances with relatively low solubilities are said to be insoluble, and these are the substances that readily precipitate from solution

The solubility guidelines in Table 4.2 may be used to predict whether a precipitation reaction will occur when solutions of soluble ionic compounds are mixed together. One merely needs to identify all the ions present in the solution and then consider if possible cation/anion pairing could result in an insoluble compound

Predict the result of mixing reasonably concentrated solutions of the following ionic compounds. If precipitation is expected, write a balanced net ionic equation for the reaction.

(a) potassium sulfate and barium nitrate

(b) lithium chloride and silver acetate

(c) lead nitrate and ammonium carbonate

(a)ThetwopossibleproductsforthiscombinationareKNO3 andBaSO4,bothofwhicharesolubleperthe tabulated guidelines. No precipitation is expected.

(b) The two possible products for this combination are LiC2H3O2 and AgCl. The solubility guidelines indicate AgCl is insoluble, and so a precipitation reaction is expected. The net ionic equation for this reaction, derived in the manner detailed in the previous module, is Ag+(aq)+Cl−(aq) ⟶ AgCl(s)

(c) The two possible products for this combination are PbCO3 and NH4NO3, both of which are soluble per the tabulated guidelines. No precipitation is expected

Which solution could be used to precipitate the barium ion, Ba2+, in a water sample: sodium chloride, sodium hydroxide, or sodium sulfate? What is the formula for the expected precipitate?

sodium sulfate, BaSO4

acid base reactions

an acid-base reaction is one in which a hydrogen ion, H+,

is transferred from one chemical species to another.

an acid

an acid is a substance that will dissolve in water to yield hydronium ions, H3O+.

As an example, consider the equation shown here: HCl(aq)+H2O(aq) ⟶ Cl−(aq)+H3O+(aq)

When dissolved in water, H3O+ ions are produced by a chemical reaction in which H+ ions are transferred from HCl molecules to H2O molecules

strong acid

Virtually every HCl molecule that dissolves in water will undergo this reaction. Acids that completely react in this fashion are called strong acids, and HCl is one among just a handful of common acid compounds that are classified as strong

weak acid

A far greater number of compounds behave as

weak acids and only partially react with water,

leaving a large majority of dissolved molecules in

their original form and generating a relatively small

amount of hydronium ions. Weak acids are commonly

encountered in nature, being the substances partly

responsible for the tangy taste of citrus fruits, the stinging sensation of insect bites, and the un

pleasant smells associated with body odor

base

A base is a substance that will dissolve in water to yield hydroxide ions, OH−. The most common bases are ionic compounds composed of alkali or alkaline earth metal cations(groups1and2)combined with the hydroxide ion—for example, NaOH and Ca(OH)2. When these compounds dissolve in water, hydroxide ions are released directly into the solution

strong bases

These bases, along with other hydroxides that completely dissociate in water, are considered strong bases.

weak bases

. In all cases, these compounds react only partially

and so are classified as weak bases. These types of

compounds are also abundant in nature and important

commodities in various technologies. For example, global

production of the weak base ammonia is typically well over

100 metric tons annually, being widely used as an agricultural fertilizer, a raw material for chemical synthesis of

other compounds, and an active ingredient in household cleaners

water can serve as both a solvent and a reactant

The chemical reactions described in which acids and

bases dissolved in water produce hydronium and hydroxide ions, respectively, are, by definition, acid-base reactions. In these reactions, water serves as both a solvent and a reactant.

neutralization reaction

A neutralization reaction is a specific type of acid-base reaction in which the reactants are an acid and a base, the products are often a salt and water, and neither reactant is the water itself: acid+base ⟶ salt+water

Writing Equations for Acid-Base Reactions

Write balanced chemical equations for the acid-base reactions described here:

(a) the weak acid hydrogen hypochlorite reacts with water

(b) a solution of barium hydroxide is neutralized with a solution of nitric acid

chemical formula HOCl.

HOCl +H20 ----

(a)The two reactants are provided,HOClandH2O.

Since the substance is reported to be an acid, its reaction with water will involve the transfer of H+ from HOCl to H2O to generate hydronium ions, H3O+ and hypochlorite ions,

OCl−. HOCl(aq)+H2O(l)⇌OCl−(aq)+H3O+(aq) A double-arrow is appropriate in this equation because it indicates the HOCl is a weak acid that has not reacted completely.

(b) The two reactants are provided, Ba(OH)2 and HNO3. Since this is a neutralization reaction, the two products will be water and a salt composed of the cation of the ionic hydroxide (Ba2+) and the anion generated when the acid transfers its hydrogen ion (NO3−). Ba(OH)2(aq)+2HNO3(aq) ⟶ Ba(NO3)2(aq)+2H2O(l)

Check Your Learning

Write the net ionic equation representing the neutralization of any strong acid with an ionic hydroxide. (Hint: Consider the ions produced when a strong acid is dissolved in water.)

H3O+(aq)+OH−(aq) ⟶ 2H2O(l)

oxidation

oxidation was originally used to describe chemical reactions involving O2, but its meaning has evolved to refer to a broad and important reaction class known as oxidation-reduction (redox) reactions.

Starting over with Classification of chemical reactions