Chemistry Essentials - Introductions to chemical reactions

Introduction to Chemical

Reactions

​Directions: Read through the following slides and answer the questions. You have to get each question correct. This is new infomation, I suggest taking notes on this.

What is a Chemical Reaction?

It is a chemical change in which one or
more substances are destroyed and one
or more new substances are created.

BEFORE

H2 gas

and

O2 gas

AFTER

H2O liquid

Parts of a Chemical Reaction

Reactants → Products

Reactants: Substances that are destroyed by the

chemical change (bonds

break).

Products: Substances created by the chemical

change (new bonds

form).

The arrow (→) is read as “yields”.

Other symbols in chemical

reactions

• (s) = solid
• (l) = liquid
• (g) = gas
• (aq) = aqueous solution (the substance is

dissolved in H2O)

• “+” separates two or more reactants or

products

• “→” yield sign separates reactants from

products

Evidence for a Chemical Reaction

1) Evolution of light or heat.

Evidence for a Chemical Reaction

2) Temperature change (increase or

decrease) to the surroundings.

Evidence for a Chemical Reaction

3) Formation of a gas (bubbling or an odor)

other than boiling.

Evidence for a Chemical Reaction

4) Color change (due to the formation of a

new substance).

Evidence for a Chemical Reaction

5) Formation of a precipitate (a new solid

forms) from the reaction of two aqueous
solutions.

Multiple Choice

Which of the following is not a chemical reaction?

• Boiling water on a stove

• Turning on a light

• Burning a candle

• All are examples of a chemical reaction

Word Equations

• Statements that indicate the reactants and

products in a chemical reaction.

• Ex. Iron (s) + chlorine (g) → iron (III) chloride (s)

• This is read as:

“Solid iron and chlorine gas react (combine) to produce

solid iron (III) chloride”

Translating Word Equations to

Skeleton Equations

• A skeleton equation uses chemical formulas

rather than words to identify the reactants and
products of a chemical reaction.

• The word equation

Iron (s) + chlorine (g) → iron (III) chloride (s)

• The skeleton equation

Fe(s) + Cl2(g) → FeCl3 (s)

A skeleton equation is not yet “balanced” by coefficients!

One more example…

•

6 Na (s) + Fe2O3 (s) → 3 Na2O (s) + 2 Fe (s)
–

The numbers preceding the chemical formulae are
coefficients. They are used to balance the reaction.

–

The numbers within the chemical formulae are
subscripts.

–

You can read the above balanced reaction as:

•

“6 atoms of solid sodium plus 1 formula unit of solid
iron (III) oxide yields 3 formula units of solid sodium
oxide and 2 atoms of solid iron” or…

•

“6 moles of solid sodium plus 1 mole of solid iron (III)
oxide yields 3 moles of solid sodium oxide plus 2
moles of solid iron”

•

Chemical reactions can never be read in terms of
grams, only in terms of particles or groups of particles
(moles).

Conservation of Mass

During a chemical reaction, atoms are neither

created nor destroyed (Conservation of
Mass).

Hydrogen and oxygen gas react to form

water:

H2 (g) + O2 (g) → H2O (l)

Conservation of Mass

H2 (g) + O2 (g) → H2O (l)

What is wrong with this equation above? Doesn’t

it appear that one oxygen atom “went missing”?

According to conservation of mass, the proper way

to write this reaction is:

2H2 (g) + 1O2 (g) → 2H2O (l)

The red coefficients represent the # of molecules

(or the # of moles) of each reactant or product.

Not All Properties are Conserved

During Chemical Reactions!

CONSERVED

NOT CONSERVED

Mass

Types of atoms

Number of each atom

Color

Physical state (solid,

liquid, gas)

Volume

Number of moles of
reactants/products

Multiple Choice

The law of conservation of mass states that....

• Matter can be made

• Matter can be destroyed

• Matter can neither be made or destroyed

• Matter can evaporate

Multiple Choice

What is the number before a chemical formula called?

Example: 2H₂ + O₂ ---> 2H₂O

• Coefficient

• Atom

• Subscript

• Equation

Multiple Choice

What is the little number after an element in a chemical equation called.
Example: H₂

• Coefficient 
• Subscript
• Atom
• Equation

Synthesis Reactions (cont’d)

• Metallic and nonmetallic elements react to form ionic

compounds. The resultant compound should be charge
balanced by the criss-cross method.

Ex. 4Li + O2 → 2Li2O

• Nonmetals react with each other to form covalent

(molecular) compounds. You should be able to draw a
valid Lewis Structure for the product.

2H2 + O2 → 2H2O

or

H2 + O2 → H2O2

But NOT

H2 + O2 → 2OH

TRY TO CLASSIFY THESE:

4) 2Ag + S → Ag2S

5) MgCO3(s) → MgO(s) + CO2(g)

6) Cl2 + 2KBr → 2KCl + Br2

Rules for Balancing
Chemical Reactions

__H2 + __ O2 → __H2O

Balancing is about finding the

right coefficients!

Rules for Balancing
Chemical Reactions

1) You can change the coefficients, but

NEVER the subscripts!

__H2 + __ O2 → __H2O

Off Limits!

Rules for Balancing
Chemical Reactions

2) The coefficients must reduced to

represent the lowest possible numbers.

4H2 + 2 O2 → 4H2O

Rules for Balancing
Chemical Reactions

3) It is OK to use fraction coefficients, but

you must get rid of them in the end
(multiply through by denominator).

H2 + ½ O2 → H2O

Rules for Balancing
Chemical Reactions

4) Often, it is helpful to save the following

elements until the end (do other
elements first):

H, C, O

Rules for Balancing
Chemical Reactions

5) Do a final balance check for each

element!

2H2 + O2 → 2H2O

H0

rxn= nH0

f(products) - nH0

f(reactants)

• Not as hard as it looks ☺
• Basically, you just

1) multiply the coefficient of each product times its
standard heat of formation and add together for all
products

2) multiply the coefficient of each reactant times its
standard heat of formation and add together for all
reactants

3) take the difference of 1 and 2

(always products - reactants)

4) If the difference is (-) the reaction is exothermic;

if the difference is (+) the reaction is endothermic.

Multiple Choice

__Na + __Cl₂ --> __NaCl

• 1,1,2

• 2,1,2

• 2,1,1

• already balanced

Multiple Choice

__NaClO₃ --> __NaCl + __O₂

• 2,2,2

• 2,2,3

• 3,2,2

• 1,2,3

Spend the remaining time in class completing the worksheet, "Chemical Reactions: An introduction"

​

​Some text here about the topic of discussion.

​Have a good weekend!

~ Mr. Boyd