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Chapters 4 & 5 - Chemical Formulas, Eqtns, Math

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Formulas, Equations, &
Mathematics
CHAPTERS 4 & 5
Overview & Memory Refreshers

Universal language of symbols for identifying all of the elements on earth;
using 1, 2, or 3 letters

1st letter is always capitalized, all others are lower case

Some letters are for the English names, some are for Latin or Greek (K –
kalium, Na – natrium)

Subscripts – written if more than one atom is present

Diatomic molecules – oxygen, hydrogen, nitrogen, Group 17
Chemical Formulas

Compounds are combinations of elements chemically joined/bonded in
definite proportions

Formulas use chemical symbols, subscripts, and numbers to show
qualitative and quantitative information about a substance

Qualitative information relates to things no counted/measured, like what
elements the compound is comprised of

Quantitative information deals with things that can be counted/measured,
like how many atoms of each element are present
Chemical Formulas (cont)

Symbols supply qualitative info – CO: carbon & oxygen (what’s the
difference between CO & Co?)

Subscripts tell you how many of each atom are there – quantitative

Two types of formulas – empirical & molecular

Empirical – all types of compounds

Molecular – compounds formed from sharing atoms (what type of bond?)
Chemical Formulas (cont)

Empirical Formula – represents the simplest integer ratio in which atoms
combine to form the compound (most reduced)

Ionic substances don’t form molecules, but form an array of ions

Ionic formulas indicate the ratio of atoms in a compound (MgCl2)

Ionic formulas are empirical formulas
Chemical Formulas (cont)

Covalently bonded substances form discrete units called molecules

Sometimes the empirical formula represents the simplest & actual ratio
(H2O)

Molecular formula may often be a multiple of the empirical formula

C6H12O6 is the molecular formula of glucose, but the empirical formula is
CH2O
Atoms, Compounds, & Ions

Atoms naturally have a neutral charge

Ions are positively or negatively charged

Positively charged ions attract negatively charged ions in a ratio that will
produce a neutral compound

The charge of an ion is represented by a superscript following a symbol

1+ or 1-, the 1 is omitted (only charge is shown): Na 1+ = Na+, Cl 1- = Cl-

Anything else shows size and sign of charge

Al3+, O2-
Atoms, Compounds, & Ions (cont)

Polyatomic ion – atoms covalently bonded together that has a charge
(Table E in your reference table)

Parentheses are used to enclose polyatomic ions when there is more than
one of the ion in a compound, with a subscript indicating how many

Compounds can form several ways, one such way is by oppositely
charged ions attracting each other in a ratio that produces a neutral
charge

(NH4)3PO4
Atoms, Compounds, & Ions (cont)

Subscripts indicate quantities of atoms present, superscripts indicate
charge, coefficients tell us how many units of the formula are present

2H2O – 2 molecules of water; 4 hydrogen & 2 oxygen atoms

4Mg(NO3)2 – 4 Mg, 8 N, 24 O
Atoms, Compounds, & Ions (cont)

When water from some ionic solutions evaporates, the solute (left over
substance) forms a crystal lattice that binds water within the structure

This is called a hydrate

These crystals have a definite number of water molecules for each unit of
the compound

Barium chloride (BaCl2) traps two water molecules; BaCl2∙2H2O

In a chemical reaction, the water in a hydrate does not react, but it does
add mass to the compound
Writing Formulas & Naming
Compounds

All compounds must be electrically neutral (what does that mean?)

Compounds achieve neutrality by having an equal number of positive
and negative charges; NaCl

When ions have unequal charges, simply write the charge of one ion as
the subscript for the other (but not the sign, just the number)

Ex: MgCl2

polyatomic ions form compounds the same way; NaNO3, Ca(NO3)2
Writing Formulas & Naming
Compounds (cont)

Compounds are named according to the types of elements that form
them

Ionic compounds are named one way, covalent compounds containing
only nonmetals are named another

Binary ionic compounds – containing only 2 elements

Ionic compound – positively charged particle (usually a metal) comes first

Negatively charged ion ends the compound

Name of negative ion changed to end in -ide

NaCl: sodium chloride
Writing Formulas & Naming
Compounds (cont)

Name the following ionic compounds: KCl, MgS, AlN

Naming compounds with polyatomic ions: if positive portion is a metal, use
the unchanged metal name plus the negative polyatomic ion name

KNO3 – potassium nitrate

Most polyatomic ions are negatively charged, but NH4+ is not

If a compound contains the ammonium ion (+), and the negative is a
nonmetal, the ending is –ide, but if it is combined with another polyatomic
ion, they both keep their names

Name these: NH4Cl; NH4NO3
Writing Formulas & Naming
Compounds (cont)

If a binary (2 atom) compound contains 2 nonmetals, it is molecular
(covalent bonds, no ions)

Lower electronegativity value written first

Name of compound will end in –ide

prefixes are used to tell how many atoms of each element are present

CO, CO2

If only one atom of the first atom is present, mono is not used

If element starts with a vowel, final a or o in prefix is not used

NO, N2O4
Writing Formulas & Naming
Compounds (cont)

Prefixes Used
Number of Atoms
Prefix
1
Mono-
2
Di-
3
Tri-
4
Tetra-
5
Penta-
6
Hexa-
7
Hepta-
8
Octa-
Writing Formulas & Naming
Compounds (cont)

Some metals have more than one common oxidation state

Fe: +2, +3

Can be a problem when naming a compound because you don’t know
which iron is being used

Solution is to use roman numerals after the name of the metal for the
oxidation state

Iron(II) chloride; FeCl2

Iron(III) chloride; FeCl3
Chemical Rxns & Equations

A physical change does not form a new substance, but merely the
appearance of the material (melting, boiling, etc)

A chemical change is when the substance produced is different from the
starting material (think of something burning)

A chemical reaction is a well-defined chemical change

A chemical equation shows what takes place during a chemical rxn

A substance that enters into an equation is called a reactant, and is on the
left side of the rxn arrow

A product is a substance that is produced by the rxn, and appears on the
right of the arrow
Chemical Rxns & Equations (cont)

Plus signs separate reactants and products, while the arrow represents a
chemical rxn (read as “produces” or “yields”)
Chemical Rxns & Equations (cont)

Chemical & physical changes involve the loss or gain of energy, usually in
the form of heat

There are two major groups of rxns, depending on whether the rxn gains or
loses energy; what are they?
Chemical Rxns & Equations (cont)
Type of Rxn
Surrounding
Temp
Potential
Energy of
Reactants
Potential
Energy of
Products
Energy
Change
Endothermic
Decrease
Less
More
Positive
Exothermic
Increases
More
Less
negative
Chemical Rxns & Equations (cont)

Endothermic rxns require energy to occur; i.e. energy is entered into the
system

ice melting is endothermic

Energy is absorbed from the surroundings, lowering the surrounding
temperature

Reactants absorb energy as they become products, so the products have
more potential energy than the reactants
Chemical Rxns & Equations (cont)

Exothermic rxns release thermal energy when they occur; i.e. energy is
removed into the system

Water freezing is exothermic

Energy is released and given off to the surroundings, raising the
surrounding temperature

Products have less potential energy than the reactants
Chemical Rxns & Equations (cont) –
Balancing Chemical Equations

A correctly written chemical equation has the same number of each type
of atom on both sides of the equation

The law of conservation of mass states that matter is neither created nor
destroyed in a chemical reaction

An equation that conserves atoms and agrees with the law of
conservation of mass is called a balanced equation

An equation that does not conserve atoms between reactants and
products is called an unbalanced equation, and must be balanced

Can you change the subscripts of an atom/formula in order to balance an
equation?
Chemical Rxns & Equations (cont) –
Balancing Chemical Equations

The only way to balance a chemical equation with the correct formulas is
to change the coefficients in the equation

What is a coefficient?

Think of the equation as a seesaw: both sides must be balanced in order
to have an equilibrium (law of conservation of mass)

How would you balance the following equation?

H2(g) + O2(g) → H2O(g)
Chemical Rxns & Equations (cont) –
Balancing Chemical Equations

H2(g) + O2(g) → H2O(g)

H2(g) + O2(g) → 2H2O(g)

2H2(g) + O2(g) → 2H2O(g)
Chemical Rxns & Equations (cont) –
Balancing Chemical Equations
Symbol
Meaning
+
Separates reactants or products
→
Separates reactants from products;
Reads as yields or produces
(s)
Identifies a substance as a solid
(ℓ)
Identifies a substance as a liquid
(g)
Identifies a substance as a gas
(aq)
Identifies a substance as being dissolved
in aqueous (water) solution
Chemical Rxns & Equations (cont) –
Types of Rxns

It would be difficult to categorize all types of chemical rxns, but there are 4
major types:

Synthesis

Decomposition (analysis)

Single replacement

Double replacement
Chemical Rxns & Equations (cont) –
Types of Rxns: Synthesis

Synthesis: 2 or more reactants combine to form a single product

General equation form is
A + B → AB
Chemical Rxns & Equations (cont) –
Types of Rxns: Decomposition

Decomposition (analysis): is the opposite of a synthesis rxn; a reactant is
broken down (decomposed) into two or more simpler substances
(products)

all decomposition rxns begin with just one reactant
AB → A + B
Chemical Rxns & Equations (cont) –
Types of Rxns: Single Replacement

Single replacement: one element replaces another in a compound

always involves an element and a compound
A + BX → B + AX

How do we know if a reaction will take place?

Activity Series (Table J, pg 5) [RT quiz]

The activity series is arranged so that an element on the table will react
with anything below it

All metals above H2 will react with acids to produce hydrogen gas and
produce a salt
Chemical Rxns & Equations (cont) –
Types of Rxns: Single Replacement

The second column of Table J contains a few nonmetals that will replace a
less active nonmetal:
A + XB → B + XA
Chemical Rxns & Equations (cont) –
Types of Rxns: Double Replacement

Double replacement: usually involves 2 soluble ionic compounds that react
in solution to produce a precipitate, gas, or molecular compound (H2O)
AB + CD → AD + CB

Just like single replacement rxns, not all combinations of reactants will
produce a rxn
Chemical Rxns & Equations (cont) –
Types of Rxns: Double Replacement

3 situations that might cause a double replacement rxn:

1. A rxn will occur if one of the products is a solid (precipitate)

Solubility Guidelines (Table F, pg 2)

Check the solubility of the two ionic products

If one product is insoluble, the rxn will occur (precipitate is formed)

2. a rxn will occur if one of the products is a gas

3. a rxn will occur if a molecular (covalent bond) substance, such as water,
is formed
Chemical Rxns & Equations (cont)

What is the law of conservation of mass?

If you are given an unknown in a balanced equation, you should be able
to use process of elimination and simple math to determine what element
(and how many of it) is missing

The mass of a missing substance can also be found; the total mass of
reactants must be the total mass of the products
The Mathematics of Formulas

Remember that atomic mass is a relative value based on carbon-12 (this is
the standard)

How much larger is Mg than C?

Formula mass is the sum of the atomic masses of all atoms present

Gram formula mass is the formula mass expressed in grams instead of amu

Some substances form molecules, and the gram formula masses are
expressed as gram molecular masses
The Mathematics of Formulas (cont)

Formulas represent the composition of a substance, so using the subscripts
and coefficients of the elements in a substance, you can find the percent
mass of the substance

The percentage composition of a substance is the composition (by %) of
each element compared with the total mass of the compound

What is the % of O in potassium chlorate?

Flash back to hydrates; what are they?
The Mathematics of Formulas (cont)

Ionic substance with trapped water molecules within their crystal structure
are called hydrates, substances without water are called anhydrous

If you need to calculate the percentage of water in a hydrate, count the
water molecule as a single unit (all units of H2O in the hydrate)

What is the % by mass of water in Na2CO3 ∙ 10H2O ?
The Mole

Collective nouns are used to describe quantities that we are familiar with;
a dozen = 12, a gross = 144, and a ream is 500

In chemistry, there is a collective noun used to describe a usable number
of particles

A mole is the number of atoms of carbon present in 12.000 grams of C-12
(everything comes back to carbon-12 being the standard)

The number of particles in a mole of a substance is 6.022 x 1023, which is
referred to as Avogadro’s number
The Mole (cont)

It’s not actually possible to count the number of particles in a mole, the
mass of one mole can be found by determining the gram formula mass

6.02 x 1023 particles of that substance

Gram formula mass of any substance is the mass of one mole of the
substance

The abbreviation for mole is mol
The Mole (cont)

To convert grams to moles and moles to grams:
moles =
# of
grams
grams =
# of
moles
x
X
1 mol
gram formula mass
gram formula mass
1 mol
The Mole (cont)

How many grams are present in 40.5 moles of H2SO4 ?

How many moles are present in 4.75 grams of NaOH ?
The Mole (cont)

When the molecular mass & its empirical formula are known, it’s possible to
determine the correct molecular formula

Propene: molecular mass is 42 amu, empirical formula is is CH2, and the
molecular mass of CH2 is 14 amu

Divide the molecular mass of the compound by the molecular mass of the
empirical formula, and you’ll get an integer (42/14 = 3)

Multiply the empirical formula by the integer found to get the molecular
formula

C3H6
Mole Relations in Balanced Equations

Coefficients in a balanced equation represent the number of mole ratios

The mole ratios from a balanced equation will remain constant no matter
what

How many liters of carbon dioxide gas will be produced from the
complete combustion of 30.0 liters of ethane as shown by the following
equation?

2C2H6(g) + 7O2(g) → 4CO2(g) + 6H2O(g)
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