This document discusses chemical reactions and how to write and balance chemical equations. It defines chemical reactions as when new substances are formed with new properties, whereas physical changes only alter physical properties. To identify if a reaction occurred, one looks for evidence like new colors, gas formation, or heat/light production. Chemical equations represent reactions, with reactants on the left and products on the right. Formulas replace word names and physical states are indicated. Equations are balanced by adding coefficients to ensure equal numbers of each type of atom/ion on both sides of the equation. Several examples demonstrate how to write word equations, convert to formulas, and balance chemical equations.

1. 1
Chemical Reactions
Chapter 7
Section 7. 1 and 7.4
CHEM 1020 Instructor: Dr. Rekha, MCTC

2. In this chapter you will learn:
• Difference between chemical and physical changes.
(Review from Ch-3)
• How to identify if a chemical reaction has occurred?
• What are some of the reasons for a chemical reaction
to happen (forces that drive a reaction)
• What are the four main types of reactions (specified
for your course)?
• What other ways can we classify these four types of
reactions?
• How to write the chemical equations and balance the
equations

3. Chemical Reactions
Chemical Reactions:
• Elements can combine with each other and form new
compounds, as you learned in chapter-5
• Simple compounds as well as complex compounds can
also react with one another to form other new
substances.
• When new substances are formed with new
properties, and the original substance do not
remain anymore, it is called chemical reactions.

4. Chemical Reactions
In Chemical reactions or chemical changes you cannot
get the original substance back.
• Reactants: Chemicals that come together before the
reaction, are called reactants.
• Products: New chemicals formed after the reaction
are called Products.
• The products will have completely different properties
as compared to the reactants.

5. Chemical Reactions
• But in physical changes, only the physical properties
are changed, and you can get back your original
substance, by reversing the process.
• You learned many examples of physical changes in
previous chapters.
• In your ebook, in chapter-7, several examples are
shown again for physical and chemical changes, in
section 7.2, with practice questions.
• You should review them and work on them, before
moving further in this chapter.

6. Evidence of a Chemical Reaction
What to look for, to determine if a chemical reaction
has occurred = Evidence to check
a) Formation of a solid as a product, when the reactants
before the reaction were not solids!
b) New color: When the reactants before mixing were
not colored, or had completely different color. But
after reactions, new color is seen.
c) Formation of gas (as seen by bubbles or
effervescence) when two solutions or solids react
d) Burning (flame), heat or light production when the
reactants are mixed.

7. Evidence of a Chemical Reaction
• The link to this video is
https://www.youtube.com/watch?v=zKYGGAjh_Ss
• And it is also posted in D2L.

8. How to write chemical equations
• Chemical Equations: Chemical reactions are
represented by chemical equations.
• The first step in writing chemical equations, is to
identify which are the reactants and which are the
products.
• Then write it out in words, the equation with names of
reactants and products. Show the reactants names
first, then an arrow, and then the products.
Reactants Products

9. How to write chemical equations
• The physical states of the reactants and products must
be shown. Solids are shown as (s), liquids (l), gases
(g).
• A precipitate (insoluble substance formed) is also a
solid; If a reaction produces water in the form of ice,
then ice is also a solid (s) is solid;
• Water is liquid (l);
• Vapor, fumes or steam is shown as gas (g)

10. How to write chemical equations
• Example: sodium hydroxide aqueous solution reacted with
hydrochloric acid solution, and formed soluble sodium chloride
and water.
• In the above statement, you recognize that the reactants are
sodium hydroxide and hydrochloric acid are both solutions in
water (acid is aqueous by definition).
• And the products formed are sodium chloride (also now in
aqueous solution) and water (is liquid l).
• Therefore, the word equation along with physical states
would be:
• Sodium hydroxide(aq) + hydrochloric acid (aq)
Sodium chloride (aq) + H2O (l)

11. Next, write the formula equation
• Next convert the word equation into chemical or formula
equation. Continue to note the physical states.
• In our example: The word equation was:
Sodium hydroxide(aq) + hydrochloric acid (aq)
Sodium chloride (aq) + H2O (l)
Formula Equation (or molecular equation) is:
NaOH(aq) + HCl(aq) NaCl (aq) + H2O (l)
• It is Extremely Important that you write the correct
formulas.

12. Balancing the Equation
Next, you must Balance the chemical equation:
• This involves making sure that the number of atoms of EACH
element are SAME on both reactant and product sides.
• If polyatomic ions are involved, then the whole units of such
ions must be same on both sides.
• Balancing is done by adding coefficients in front of the
formulas. Never change the subscripts or never change the
formula itself.
• A coefficient added in front of the formula, applies to all the
atom in that formula.
• Do not add fractions as coefficients. All coefficients must be
whole numbers.

13. Balancing the formula equation
• In the example we are working on, our Formula Equation was
NaOH(aq) + HCl(aq) NaCl (aq) + H2O (l)
(remember! H2O can be considered
as HOH just until balancing, we just don’t show it that way)
• Let us count the atoms or the polyatomic ions on both sides.
• Reactant side: 1 Na, 1 OH, 1 H and 1 Cl.
• Product side: 1 Na, 1 OH and 1 H (from water) and 1 Cl
• All are same number.
• Hence there is no need to add any coefficients! It is already
balanced.
• Next, we will work on an example, where balancing is needed.

14. Another Example
Example-2: Aqueous Calcium nitrate reacts with aqueous sodium
hydroxide, forming soluble sodium nitrate and calcium hydroxide
precipitate (note: Precipitate means solid)
Let us write the Word equation with physical states:
Calcium nitrate(aq) +sodium hydroxide (aq)
Calcium hydroxide(s) + Sodium nitrate (aq)
Formula Equation:
Ca(NO3)2 (aq) + NaOH((aq) NaNO3 (aq) + Ca(OH)2 (s)
(check to make sure formulas are correct as per what you
learned in Chapter-5, before trying to balance)

15. Balancing the Formula Equation
Ca(NO3)2 (aq) + NaOH((aq) NaNO3 (aq) + Ca(OH)2 (s)
Reactant side: 1 Ca, 2 (NO3), 1 Na and 1 (OH)
Product side: 1 Ca, 1 (NO3), 1 Na and 2 (OH)
Therefore, to make the (NO3) as 2 units on product side, I need to
add coefficient 2 in front of NaNO3. This number 2 applies to
both Na and NO3. And to make the (OH) as 2 units on the left side
(reactant side), I need to add 2 in front of the NaOH.
Ca(NO3)2 (aq) + 2NaOH((aq) 2NaNO3 (aq) + Ca(OH)2 (s)
Therefore, now if you recount all the atoms/ion units, they are
the same on both sides of the equation and are balanced.

16. When diatomic molecule involved
Example-3: When diatomic molecule is involved, as stand-alone
free molecular element (Uncombined State), you need to make
sure they are shown as diatomic!! (example: H2, N2, O2, F2, Cl2, Br2,
I2).
Hydrogen and oxygen gases, combine to produce liquid water.
H2(g) + O2(g) H2O(l)
Left side: 2H and 2O; Right side, 2 H and only 1 O. So, you need
to add coefficient of 2 in front of water.
H2(g) + O2(g) 2H2O(l)
Count again! Left side: 2H and 2O; Right side, 4 H and 2 O. So,
you need to add coefficient of 2 in front of hydrogen on reactant side.
2H2(g) + O2(g) 2H2O(l); now balanced

17. Points to Consider while Balancing
• First write correct formulas in the equation before balancing.
• Subscripts in the formulas indicate the number of atoms of
simple elements, or the number of units of the polyatomic ions.
• To balance the equation, use Coefficients numbers in front of
the formula that contains the atom or ion you want to balance.
• Do not use or change the subscripts of the original formulas.
• Identify the most complex substance, and first start balancing
the atoms of that one.
• Tackle the simplest stand alone element last.
• For all polyatomic ions, balance them as one whole unit, not the
individual elements within it.

18. Another Example
Example-4: Gaseous propane (C3H8) burns with oxygen, producing
gaseous carbon dioxide and liquid water.
Word equation:
Propane (g) + Oxygen (g) Carbon dioxide (g) + water (l)
Formula Equation:
C3H8(g) + O2(g) CO2(g) + H2O (l)
Reactant side: 3 C, 8 H, 2 O; Product side: 1 C, 2H, 3 O
To Balance: First consider the most complex one and balance the
atoms in it first, on both sides of the equation. In this case, C3H8
is the complex substance.
So, in the above example, let us first try to balance the C and H on
both sides, by considering the complex molecule C3H8.

19. Another Example
By putting 3 on the product side before CO2 and 4 before H2O
C3H8(g) + O2(g) 3CO2(g) + 4H2O (l)
Count again:
Reactant side: 3C, 8 H, 2 O; Product side, 3C; 10 O , 8 H
So, although the C and the H are balanced, O is still not balanced.
And When diatomic elements are present, and are the simplest stand
alone substance, balance it at the end.
So, put 5 before the oxygen on the reactant side.
C3H8(g) + 5O2(g) 3CO2(g) + 4H2O (l)
Check again:
Reactant side: 3C, 8 H, 10 O; Product side, 3C; 8H, 10 O and
they are all balanced.

20. Work on other examples from book
In your ebook, there are other examples of equations and several
practice exercises shown. Work on them.
In all our examples, we gave the statements, and you determined how
to write the word equation, formula equation and balance it.
You can also try the type of examples where the formula
equation is already given. You balance it, and also write a
statement about which reactants were involved, in what physical
states and what were the products formed, in what physical state.
There are examples shown in the book.