Chemical Equations & Reactions

Ionic compounds can form from transition metals◦ Transition metals do not always follow the patterns that we

discussed Can have more than one oxidation number (charge)

To identify the charges of transition metals, roman numerals are placed in their name, directly after the transition metal◦ The number of the roman numeral is equal to the charge

You will need to be able to write the formulas of these compounds from their name

Roman Numerals in Compound Names

I = +1II = +2III = +3IV = +4

Examples: Iron (II) chloride Iron (III) oxide

Fe+2 Cl-1 FeCl2

Fe+3 O-2 Fe2O3

On page 11, shade in the following metals in ORANGE and update your key

Then write in their possible charges in each box◦ 24 Chromium: +2, +3◦ 25 Manganese: +2, +3, +4◦ 26 Iron: +2, +3◦ 27 Cobalt: +2, +3◦ 28 Nickel: +2, +3 ◦ 29 Copper: +1, +2◦ 30 Zinc: +2◦ 47 Silver: +1 ◦ 48 Cadmium: +2◦ 50 Tin: +2, +4◦ 79 Gold: +1, +3◦ 80 Mercury: +1, +2◦ 82 Lead: +2, +4

Update SNB p. 11

Write the formulas of the following compounds:

1. nickel (II) chloride

2. copper (II) nitrate

3. iron (III) hydroxide

4. Gold (I) oxide

Practice with Roman Numerals

Ni+2 Cl-1 NiCl2

Cu+2 NO3-1 Cu(NO3)2

Fe+3 OH-1 Fe(OH) 3

Au+1 O-2 AuO2

Physical Change A change in the size, shape, state of matter, etc. that

does not change the identity of a substance

◦ A phase change is a physical change even though energy may be removed or added to the substance

In the new state of mater, the substance is still made of the same components, the atoms just have more or less energy (phase changes are the only physical changes that have energy changes)

i.e. if liquid water evaporates, it becomes water vapor if water vapor condenses, it becomes liquid water

Chemical Change

The change of one substance into a new substance (chemical reaction)

◦A chemical change alters the original chemical make-up of the substance

◦Energy changes and physical changes always accompany chemical changes

How can you tell if a chemical change has taken place?

◦ ENERGY Evolution of heat, light, and/or sound (sometimes heat can be absorbed too)

◦ GAS A gas is produced, bubbles (effervescence)

◦ PRECIPITATE When solid particles form from 2 liquids

◦ COLOR – Unexpected Color change (i.e. clear liquid + clear liquid purple liquid)

Indications of a Chemical Reaction

When energy flows out of the system as heat, the process is exothermic.• Usually feels warm Ex: Combustion of a match, hand warmers

Processes that absorb energy from the surroundings are endothermic.◦ Usually feels cool◦ Ex: Melting ice cubes, ice packs

Energy

Reactants – the substances that exist before a chemical change (or reaction) takes place.

Products – the new substance(s) that are formed during the chemical changes.

CHEMICAL EQUATION – a description of a chemical reaction that uses chemical formulas and symbols to indicate the reactants and products of a reaction.

Chemical Equations

REACTANTS PRODUCTS

“Produces,” “Forms,” or “Yields”; indicates result of reaction

“plus”

A reactant or product in the solid state; also used to indicate a precipitate

A reactant or product in the liquid state

A reactant or product in the gaseous state

A reactant or product dissolved in water (in an aqueous solution)

Additional Symbols Used in Chemical Equations

(s)

(l)

(g)

(aq)

+

7 elements◦ Molecules made out of 2 atoms (“di” = 2, “atomic” =

atoms) Some elements are never found alone and will always bond

with another atom of the same type to become stable (O2, N2, H2, etc.)

◦ Hydrogen and the other 6 elements form a “7” shape (nitrogen has an atomic number of 7)

Diatomic Molecules

On page 11,shade these 7 elements in YELLOW and add to your key

Chemical EquationsWrite the equation for the reaction of methane gas (carbon tetrahydride) with oxygen to form carbon dioxide and water.

Reactants Products

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

Notes:• Water will always be a liquid, unless stated otherwise in the problem• Oxygen is a diatomic molecule and will always be a gas• Carbon dioxide will always be a gas, unless stated otherwise in the

problem• If the state of matter is not indicated in the word problem, always

assume the element/compound is in it’s normal state at room temperature

Write an equation for the reaction between chlorineand aqueous sodium bromide to produce bromineand aqueous sodium chloride.

Cl2 (g) + NaBr(aq) Br2(l) + NaCl(aq)

Al2(SO4)3(aq) +

Write the equation for the reaction between solutions of aluminum sulfate and calcium chloride to form a white precipitate of calcium sulfate and aqueous aluminum chloride.

CaCl2(aq) CaSO4(s) + AlCl3(aq)

Notes:• Chlorine and Bromine are a diatomic molecules• Chlorine is normally a gas at room temperature• Bromine is normally a liquid at room

temperature

Solid aluminum oxide is formed when aluminum metal reacts with oxygen. Write the equation for this reaction.

Al (s) + O2 (g) Al2O3 (s)

Counting Atoms Al(s) + O2(g) Al2O3(s)

•The numbers in the front are called coefficients.•Coefficients apply only to the element or compound that comes directly after it.•Just like in math, a coefficient indicates that everything behind it is multiplied by that number

•The numbers behind an atomic symbol are called subscripts•Subscripts apply only to the element that comes directly before it• If parenthesis come directly before the subscript, then the subscript is multiplied by everything within the parenthesis

•Coefficients and subscripts together – multiply by both the coefficient and subscript

aluminum oxide4 3 2

4 Al = 4 aluminum atoms

2(NH4)2 = 4 nitrogen atoms 16 hydrogen atoms

O2 = 2 oxygen atoms(OH)2 = 2 oxygen atoms

2 hydrogen atoms

Meaning of Chemical FormulaChemical Symbol Meaning Composition

One moleculeof water:

Two H atoms and one O atom

Two moleculesof water:

Four H atoms and two O atoms

One moleculeof hydrogenperoxide:

Two H atoms and two O atoms

H2O

2 H2O

H2O2

Chemical Equations

• Matter is not lost in a chemical reaction- Atoms are just rearranged (Remember John Dalton?)

• The same atoms are present in a reaction at the beginning (reactants) and at the end (products)

Law of Conservation of Matter

Kotz web

• This means that when we write our chemical equations, we need to have the same number of atoms on both sides of the reaction arrow! Otherwise matter would not be conserved.

Cl

Cl

ClH

H

H

ClClCl

ClHH

H

H

H2(g) + Cl2(g) HCl(aq) H2(g) + Cl2(g) 2 HCl (aq)

reactants products

H

Cl

reactants products

H

Cl

2

2

2 2

2 2

1

1

(unbalanced) (balanced)

Write the chemical equation for the reaction of hydrogen and chlorine gases to form aqueous hydrogen monochloride.

Balanced Equation – one in which the number of atoms of each element as a reactant is equal to the number of atoms of that element as a product

To balance equations, we add coefficients in front of the element/compound

Do not change the subscripts because then you will change the compound!

Balancing Chemical Equations

1. Identify the most complex substance.2. Beginning with that substance, choose an element

that appears in only one reactant and one product. • Adjust the coefficients to obtain the same number

of atoms of this element on both sides.• Balance polyatomic ions as a unit (if possible).

• Re-write H2O as H-OH if hydroxide is present3. Balance the remaining atoms

• End with the least-complex substance• Leave single elements/diatomic molecules until

last4. Double check - Make sure that the atoms of each

element are the same on both sides of the equation.

Guidelines for Balancing Chemical Equations

Balancing EquationsCH4 (g) O2 (g) CO2 (g) H2O (l)+ +

Cl2 (g) + NaBr(aq) Br2(g) + NaCl(aq)

Al2(SO4)3(aq) + CaCl2(aq) CaSO4(s) + AlCl3(aq)

C = H = O =

C = H = O =

Cl = Na = Br =

Cl = Na = Br =

Al = SO4 = Ca =Cl =

Al = SO4 = Ca =Cl =

22

22

23 3

(NH4)3PO4 + Mg(OH)2 Mg3(PO4)2 NH4OH+ 62 3

Now you try…

AlCl3 + Li2CO3 Al2(CO3)3 + LiCl32 6