WINDSOR UNIVERSITY SCHOOL OF MEDICINE

CHEMICAL REACTIONS AND EQUATIONS“THE EASY PATH LEADS TO THE HARD LIFE. THE HARD PATH LEADS TO THE EASY LIFE. ”RAINER MARIA RILKE

Ch7. J.C. Rowe

LEARNING OBJECTIVES

Chemical Reactions Recognizing reactions Writing reactions and chemical equations Balancing chemical equations

Types of Chemical Reactions Decomposition Single-Replacement Double-Replacement

Precipitation Gas Formation Neutralization

Combustion Reaction

The Meaning of a Chemical Equation

Chemical equations give information in two major areas.

First, they tell us what substances are reactants (those being used up) and what substances are products (those being made).

Second, the coefficients of a balanced equation tell us in what ratio the substances react or are produced.

CHEMICAL REACTIONS

A chemical reaction occurs when a substance is changed (chemically) into another substance.

In other words, substances change their chemical composition and energy changes occur at the same time.

We typically look for some physical signs that a reaction has occurred, such as:

the release or absorption of heat emission of light formation of a solid (observation of a "precipitate") formation of a gas (observation of bubbles) change of color

However, some reactions are subtle enough that it is difficult to determine whether a reaction has occurred.

CHEMICAL REACTIONS Cont’d. Chemical reactions consist of reactants and products.

Reactants undergo chemical change to form products

CH4(g) + 2O2(g)   CO2(g) + 2H2O(l) methane gas and oxygen gas react to form carbon dioxide gas and water liquid .

 C4H8(g) + 6O2(g)  4CO2(g) + 4H2O(g) 

 butane gas and oxygen gas react to form carbon dioxide gas and water gas.

Ca(H2PO4)2 + CaSO4 + HF ---> Ca10F2(PO4)6 + H2SO4

are chemical substances written on the left of the reaction arrow.

are chemical substances written on the right of the reaction arrow.

reactants products

small, subscript numbers are part of the chemical formula, they indicate the relative ratio of atoms within a chemical formula

large coefficients to the left of the formula indicate the number of molecules ( or "moles") of the substance that undergo reaction or are formed relative to all other substances

small subscript letter in parentheses indicates the state of the substance reacting or formed

2KClO3(s)  2KCl(s) + 3O2(g)

•potassium chlorate solid reacts with heat (Delta) and a catalyst (MnO2) to form potassium chloride  solid and oxygen gas

•symbols indicating conditionsreaction arrow indicates that a chemical reaction has occurred to the reactants which has resulted in the products and is read "reacts to form"symbols over or under the arrow indicate other conditions of the reaction (often whether the reaction is heated and/or if a catalyst is used)

Heat &catalyst (MnO2)

Law of Conservation of Matter  According to the Law of Conservation

of Matter – atoms are neither created nor destroyed by a chemical reaction but only rearranged to form new substances

All chemical equations which represent a chemical reaction must be balanced so that the reactions, and the equations which represent them, obey the Law of Conservation of Matter.

The Balanced Equation

A balanced chemical equation represents the conversion of the reactants to products such that the number of atoms of each element is conserved.

Does this remind you of any Thermodynamic Law ??

Rules for Balancing Equations

1. Determine the correct formulas for all the reactants and products in the reaction.

2. Write the formulas for the reactants on the left & the formulas for the products on the right with an arrow in between. If two or more reactants or products are involved, separate their formulas with plus signs.

3. Count the number of atoms of each element in the reactants a products. A polyatomic ion appearing unchanged on both sides of the equation is counted as a single unit.

4. Balance the elements one at a time by using coefficients. A coefficient is a small whole number that appears in front of a formula a an equation. When no coefficient is written, it is assumed to be 1. It is best to begin with an element other than hydrogen or oxygen. These two elements often occur more than twice in an equation. You must not attempt to balance an equation by changing the subscripts in the chemical formula of a substance.

5. Check each atom or polyatomic ion to be sure that the equation is balanced.

6. Finally, make sure that all the coefficients are in the lowest possible ratio.

Balancing Chemical Equations you cannot change the subscripts once the

formula is correctly written remember, the numerical subscripts indicate the

relative ratio of atoms within a molecule or formula--if you change that ratio, the molecule is changed and is no longer what was originally intended

you can change the coefficient to the left of the chemical formula (in other words, you can only change the relative number of molecules or formula units that participate in the reaction)

Remember: the total number of each type of atom must be the same on both sides of the equation but you cannot change the formula to make the equation balance

C(s)        +     2H2(g)      ®              CH4(g) 

1 atom of carbon solid and 2 molecules of hydrogen gas react to form 1 molecule of methane gas            reactants                                products           1 C atom                                1 C atom           4H atoms                               4H atoms                            balanced!

PCl3(l)     +    3H2O(l)         ®           H3PO3(aq)  +   3HCl(aq) 

1 molecule of phosphorus trichloride and 3 molecules of water liquid react to form 1 molecule of phosphorus acid and 3 molecules of hydrochloric acid             reactants                                     products           1 P atom                                     1 P atom           6 H atoms                                    6 H atoms           3 O atoms                                   3 O atoms                                  balanced!

Hints for Balancing Chemical Equations

begin by balancing the element which has the fewest atoms present in only one of the chemical formulas For example:      C2H6   +   7/2 O2      ®        2 CO2   +   3 H2O

balance the C atoms first (there are only 2 C atoms in C2H6),

then the H atoms, then, finally, the O atoms (since O atoms

occur in more than one chemical formula on the product side, we leave them to last)

C2H6  +  7/2 O2      ®        2 CO2  + 3 H2O

the above reaction is balanced, although it is, strictly speaking, incorrectly balanced

    2 [C2H6   +   7/2 O2    ®        2 CO2    +    3 H2O]o

not only should the coefficients be in the smallest possible ratio, they should also be whole numbers--to get the whole number, simplest ratio of coefficients,

we multiply all coefficients by two (the common denominator of any fractions), which, in effect, removes the 7/2 fraction.

    2 C2H6   +   7 O2      ®       4 CO2    +     6 H2O

the ratio 2 : 7/2 : 2 : 3 is the same ratio as 2 : 7 : 4 : 6 , but the latter is the correct value of the simplest ratio of whole number coefficients

TYPES OF CHEMICAL REACTIONSSummary of Types: decomposition

one chemical compound changes to two or more different chemical compounds

combination two or more chemical compounds change to one

chemical compound of a different type single replacement

an element or an elemental molecule changes to a compound in which it is chemically combined with another element

Summary of Types Cont’d. double replacement

the ion partners of two different ionic compounds exchange, there are several physical occurrences which indicate a double replacement reaction: formation of a solid (precipitation) formation of a gas (bubbling occurs) formation of a molecular (non-ionic) compound

(neutralization)

combustion reaction with oxygen (O2)

Decomposition

A compound decomposes into two or more different substances 

General Form:     A            B    +    C

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

           CaCO3(s)       CaO(s)  +  CO2(g)

Combination

Two or more substances combine to form a third, different, substance.

General Form:   A +  B               C

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

4Fe(s)   +    3O2(g)       2Fe2O3(s)

Single Replacement:

 occurs between an element and a compound such that the element replaces another element in the compound.

General Form:   A +  BX                AX   +   B 

Examples:    C(s)  +   PbO(s)          CO(g)   +    Pb(s) 

 element    compound          compound     element

                                                                 2Al(s)  +   Cr2O3(s)        Al2O3(s)   +    2Cr(s)

 element    compound            compound         element

Double Replacement

these reactions occur between two ionic compounds in which the compounds exchange ion partners.

General Form:  AX  +  BY            AY   +   BX  where A & B are cations and X & Y are anions

precipitation – (formation of a solid) AgNO3(aq) + NaCl(aq)          AgCl(s) + NaNO3(aq) 

Double Replacement Cont’d. gas formation:

CaCO3(s)  + 2HCl(aq)   ®     CaCl2(aq) +  H2O(l)  +  CO2(g) 

                                                       K2SO3(aq) + 2HCl(aq)         2KCl(aq)  + H2O(l)  +  SO2(g) 

                                                       Na2S(aq)  +  2HCl(aq)            2NaCl(aq)   +   H2S(g) 

please note: anytime a carbonate (any ionic compound with CO3

2- anion), or a sulfite (any ionic compound with SO32- anion),

or a sufide (any ionic compound with S2- anion) react with any acid, CO2(g), SO2(g) and H2S(g) will be formed, respectively

Double Replacement Cont’d. molecular compound formation

(neutralization):

HCl(aq)+ NaOH(aq)           H2O(l) +  NaCl(aq)

  note that H2O is a molecular compound--

not an ionic compound--while NaOH is an ionic compound, NaCl is an ionic compound and HCl(aq) is an acid which when dissolved produces H+ and Cl- ions

Combustion

The reaction of anything (element or compound) with oxygen (O2)

 General Form:  A   +   O2               AO2   

or AX4   +  2O2              AO2   +   2X2O

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

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

 2H2(g) +  O2(g)         2H2O(g)

Solubility vs. Insolubility

In order to be able to predict double replacement precipitation reactions, you must know which substances are soluble and which substances are insoluble.

The term "soluble" means that the substance dissolves in water. In other words, if the substance is ionic, such as salt (NaCl), then the ions (Na+ and Cl-) are separated by the water molecules and the white solid dissolves (disappears) and a solution is formed, as when you put a teaspoon of salt in a glass of water.

The term "insoluble" means that the substance does not dissolve in water, but remains in solid form--much like coffee grounds in water, they remain solid.

Few general rules for predicting whether a substance is soluble or insoluble -- know these rules!

soluble ionic compounds

insoluble ionic compounds

1. All common compounds of Grp. IA ions and NH4

+ ions are soluble1. All common metal hydroxides are inso-luble, except Grp. IA and the larger Grp. IIA starting with Ca+2 ion

2. All common nitrates (NO3-),

acetates (CH3COO-), and most perchlorates (ClO4

-) are soluble

2. All common carbonates (CO3-2)

and phosphates (PO4-3) are

insoluble, except those of Grp. IA and NH4

+

3. All common chlorides (Cl-), bromides (Br-), and iodides (I-) are soluble, except those of Ag+, Pb+2, Cu+ and Hg2

+2 

3. All common sulfides are insoluble, except those of Grp. IA, NH4

+, and Ca2+ and Ba2+

4. All common sulfates (SO4-2) are

soluble, except those of Ca+2, Sr+2, Ba+2 and Pb+2

Quick Review

There are three important things to remember about reading an equation.

1. Reactants are on the left and products are on the right of the arrow.

2. Coefficients are the numbers in front of each formula. If no number is shown, a one is understood.

3. The coefficients tell us how many molecules (moles) of each reactant used and how many molecules (moles) of each product made.

Anyone who has been seriously engaged in scientific work of any kind realizes that over the entrance to the gates of the temple of science are written the words: we must have faith. It is a quality which the scientist cannot dispense with.

Max Planck