Chemical Equations

Chemical Reactions

Signs of a Chemical Reaction/Change Precipitation (a solid forms from two liquids or a liquid

and a gas)

Energy Change

Releases heat - exothermic

Absorbs heat - endothermic

Odor Changes

Gas release

Sounds

(but not the most reliable)

Chemical Equations

Forms of Equations:

Word equations – show the names of each

component in a reaction

Chemical equation – show the chemical

formulas and amounts of each component in a

chemical reaction

2H2(g) + O2(g) 2H2O(l)

Hydrogen and oxygen reacted to form water.

Writing Chemical Equations Reactant = substances that undergo chemical

change

Product = substances that are produced by a chemical change

Arrow defines chemical change – “yields”, “produces”, “makes”

Chemical equation:

Think of reactant as “before”, product as “after” change

Can write both word equations and symbol equations

Reactant Product

Types of Reactions

1. Synthesis Reactions

A + B AB

2. Decomposition Reactions

AB A + B

3. Combustion Reactions

CXHY + O2 CO2 + H2O

4. Single Replacement Reactions

A + BC AC + B

5. Double Replacement Reactions

AB + CD AD + CB

Synthesis Reactions

(Also called “combination”)

A compound is formed by the combining

two or more simpler substances

A + B AB

Example---

Magnesium + oxygen → magnesium oxide

2Mg(s) + O2(g) → 2MgO(s)

Synthesis Reactions

Practice

Write the chemical equations for the following:

1. carbon + oxygen → carbon dioxide

C(s) + O2(g) → CO2(g)

2. sodium oxide + water → sodium hydroxide

Na2O(s) + H2O(l) → 2NaOH(aq)

Decomposition Reactions

A compound breaks down to form 2 or

more simpler substances

AB A + B

Example--

Calcium carbonate, when heated, forms calcium

oxide and CO2(g).

CaCO3(s) → CaO(s) + CO2(g)

Mercury (II) oxide breaks down upon heating

to form elemental mercury and oxygen.

HgO (s) Hg (l) + O2(g)

Decomposition Reactions

Decomposition Reactions

Practice

Write the chemical equations for the following:

1. Potassium chlorate, when heated, decomposes into

potassium chloride and oxygen.

2KClO3(s) → 2KCl(s) + 3O2(g)

2. Sulfuric acid, when heated, decompose into sulfur

trioxide and water.

H2SO4 → H2O(l) + SO3(g)

D

D

Combustion Reactions

A hydrocarbon is burned with sufficient

oxygen supply. The products are always

carbon dioxide and water vapor.

Hydrocarbon

(CxHy) + O2(g) → CO2(g) + H2O(g)

Combustion Reactions

Practice

Write the chemical equations for the

following:

1. Methane (CH4) is burned in air.

CH4(g) + O2(g) → CO2(g) + H2O(g)

2. Butane (C4H10) is burned in air.

C4H10(g) + O2(g) → CO2(g) + H2O(g)

2 2

10 8 13 2

Single Replacement

Reactions

A lone element replaces another atom in an

ionic compound.

Cation is replaced

A + XC → AC + X

Or

Anion is replaced

A + BY → BA + Y

A Single Replacement Reaction

Single Replacement

Examples

Replacement of a copper in copper sulfate

by iron (II).

Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)

Replacement of bromine in sodium bromide

by chlorine.

Cl2(g) + 2NaBr(aq) → 2NaCl(aq) + Br2(l)

Single Replacement

Practice

Replacement of hydrogen in water by

sodium.

2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)

Reminder---

Don’t forget the diatomic molecules. “HBrONClIF” Instead of lone elements,

these like to be in pairs!!!

The Activity Series

Not ALL Single Replacement Reactions will

occur.

We can predict if a reaction will occur, however,

by using the activity series

Activity Series of Metals

Metal higher in

activity series will

displace lower

metal

Examples:

Mg + HgCl2 MgCl2 + Hg

Cu + HgCl2 CuCl2 + Hg

Ag + HgCl2 No Reaction!

The Activity Series

Single

replacement

reactions will

occur ONLY if

the lone

element is

above the

element it

replaces on

this list.

Example

Iron is

above

hydrogen

on the

activity

series, so

it WILL

replace

hydrogen

in this

reaction.

Double Replacement

Reactions

A pair of ions come together. Both cations switch

places or both anions.

AX + BY → AY + BX

Example---

Sodium chloride reacts with silver nitrate to form

sodium nitrate and silver chloride.

NaCl (aq) + AgNO3(aq) → NaNO3(aq) + AgCl(s)

A Double Replacement Reaction

Double Replacement

Practice

Write the chemical equations for the following:

1. Hydrochloric acid reacts with iron(II) sulfide to form iron(II) chloride and dihydrogen sulfide.

HCl(aq) + FeS(s) → FeCl2(aq) + H2S(g)

2. Sodium hydroxide reacts with hydrochloric acid to form sodium chloride and water.

Na(OH)(aq) + HCl(aq) → NaCl(aq) + H2O (l)

Practice Identifying the Type

of Reaction

2NH3(g) N2(g) + 3H2(g) Type: __________

S8(s) + 12O2(g) 8SO3(g) Type: __________

2C5H10+ 15O2(g)10CO2(g) +10H2O Type: __________

2HgO(s) 2Hg(l) + O2(g) Type: __________

2H2O2(l) 2H2O(l) + O2(g) Type: __________

3Zn(s) + 2AlCl3(aq)3ZnCl2(aq)+2Al Type: __________

K2O(s) + H2O(l) 2KOH(aq) Type: __________

2Na(s) + CaO(s) Na2O(s) + Ca(s) Type: __________

Single Replacement

Single Replacement

Synthesis

Synthesis

Decomposition

Decomposition

Decomposition

Combustion

Coefficients

The numbers to the left of the chemical formulas

are called coefficients.

Show how many moles or molecules are used or made

in the reaction.

Always whole numbers

2 H2 + O2 → 2 H2O

Coefficients and

Compounds

More about Coefficients….

Law of conservation of mass

(Lavoisier): matter cannot be gained or lost in

any chemical reactions.

Chapter 3

O

Atoms in a Reaction

Reaction: Methane gas burns in oxygen to form

carbon dioxide and water

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

O=

H=

C=

1 atom of carbon

4 atoms of hydrogen

4 atoms of oxygen

1 atom of carbon

4 atoms of hydrogen

4 atoms of oxygen

Reactants: Products:

Symbols Used in Equations

+ “plus” or “combined”.

Used between each molecule to indicate

they are combined.

“yields” or “produces”. Used to separate the

reactants from the products.

Pt. Indicates a catalyst is used in a reaction.

Indicates heat is used in the reaction.

⇄ Indicates that the reaction is reversible.

N.R. “No reaction” – The given reactants do not

react with each other.

∆

Symbols Used in Equations

(s) “solid”. Indicates the chemical is a solid. “precipitate”

(l) “liquid”. Indicates the chemical is a liquid.

(aq) “aqueous”. Indicates the chemical is

dissolved in water.

Other symbols used…

Indicates a precipitate falls out of solution.

Indicates a gas is formed.

↓

↓

Symbols used to show the state of a component…

Writing Word and Formula

Equations

Write a word equation by :

Write the names of the reactants

Draw an arrow to separate the reactants from the products

Write the names of the product

If there’s more than one product or reactant, plus signs should be

between them

Remember, reactants are on the left, and products are on the right!

Write a formula equation by substituting the chemical

formula for the names

Show any energy changes

If requested, show states

Writing & Balancing

Chemical Equations

Step #1: Write each chemical in an equation form. (Include the state symbols!)

Step #2: Write the chemical formulas for each reactant and each product.

Step #3: Balance using coefficients in front of the reactants and products until each side of the equation has the same number of each atom.

Step #4: Recheck your result to make sure atoms on both sides are equal

Tidbits: 1. Begin by balancing the atom that is used the least in

the reaction.

2. Balance H and O last.

Balancing Chemical

Equations

Law of conservation of matter applies for reactions

- End up with the same number of atomic elements

that you start with = balanced equation

Coefficient = whole numbers that are placed in

front of formulas to balance an equation

NEVER change the subscript

in a formula when balancing

equations!

Method for Balancing

Equations

• Count the number of atoms of each

element on each side of the equation.

• Change the coefficients to make the

number of atoms of each type match on

both sides

• Multiply the subscript by the coefficient

to calculate new numbers

• All elements in a compound must be

multiplied by the new coefficient!

Method for Balancing

Equations

Number of H:

Number of O:

Hydrogen + Oxygen → Water

H2 + O2 → H2O 2

2 = 4

1

2 2

2

2 = 4

2 = 2

2

H H

O O

H

O

H

H

O

H

H H

+

Balanced!!!

An Example from

A Word Equation

Hydrogen gas reacts with oxygen gas to

produce water.

Step 1.

hydrogen(g) + oxygen(g) water(l)

Step 2.

H2(g) + O2 (g) -----> H2O(l)

Step 3.

2 H2(g) + O2(g) -----> 2 H2O(l)

Another Example

Iron(III) oxide reacts with carbon monoxide to produce the iron (II) oxide (FeO) and carbon

dioxide.

iron(III) + carbon iron (II) + carbon

Oxide monoxide(g) oxide dioxide (g)

Fe2O3 + CO (g) FeO + CO2 (g)

Fe2O3 + CO (g) 2 FeO + CO2 (g)

Tricks For Balancing

Combustion Reactions---

In balancing combustion reactions, balance

them in the following order:

If you get stuck, 1. Balance Carbon, then Hydrogen,

2. Multiply all coefficients through by 2.

3. Then balance the oxygen.

Give it a try:

C3H6 + O2(g) → CO2(g) + H2O(g) 3 3 6 6

2 9

CHO

Practice Balancing Reactions

__NH3(g) __N2(g) + __H2(g)

__S8(s) + __O2(g) __SO3(g)

__C5H10+ __O2(g)__CO2(g) +__H2O

__HgO(s) __Hg(l) + __O2(g)

__H2O2(l) __H2O(l) + __O2(g)

__Zn(s) + __AlCl3(aq) __ZnCl2(aq)+ __Al

__K2O(s) + __H2O(l) __KOH(aq)

__Na(s) + __CaO(s) __Na2O(s) + __Ca(s)

2 3

12 8

2

2

15

2

10

3

10

2

2 2

3 2

2

2