Class 10 important chapter 1

1. Chemical Reactions and
Equations
Grade X
CB/X Chemical Reactions and Equations 1

2. CB/X Chemical Reactions and Equations 2
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3. Chemical reaction
How do you know that a chemical
reaction has taken place?
Whenever a chemical change
occurs, we can say that a chemical
reaction has taken place.
A chemical change indicates – the
formation of a new substance
CB/X Chemical Reactions and Equations 3

4. Chemical reaction
During a chemical reaction
atoms of one element do
NOT change into those of
another element
atoms do NOT disappear
from the mixture or
appear from elsewhere
breaking and making of bonds
between atoms to produce
new substances takes place
CB/X Chemical Reactions and Equations 4

5. Chemical reaction
Some
indicators of
a chemical
reaction are:
CB/X Chemical Reactions and Equations 5

6. CB/X Chemical Reactions and Equations 6
What does a generic/basic equation look like?
Chemical reaction

7. CB/X Chemical Reactions and Equations 7
Chemical reactions are written as word equations and
skeleton equations.
Word equations
Chemical reaction
In word equations, the names of reactants and
products are written in words to describe the reaction.
Skeleton equations
In skeleton equations, formulas of reactants and
products are written to describe the reaction.
For example: Magnesium + Oxygen → Magnesium oxide
For example: Mg + O2 → MgO

8. Chemical Equations
Their Job: Depict the kind of reactants
and products and their relative amounts
in a reaction.
Let’s understand more about chemical equations…..
4 Al (s) + 3 O2 (g) 2 Al2O3 (s)
The numbers in the front are called
coefficients
CB/X Chemical Reactions and Equations 8

9. Chemical Equations
Their Job: Depict the kind of reactants
and products and their relative amounts
in a reaction.
Let’s understand more about chemical equations…..
The numbers after the formula are called
subscripts
CB/X Chemical Reactions and Equations 9
4 Al (s) + 3 O2 (g) 2 Al2O3 (s)

10. Chemical Equations
Their Job: Depict the kind of reactants
and products and their relative amounts
in a reaction.
Let’s understand more about chemical equations…..
4 Al (s) + 3 O2 (g) 2 Al2O3 (s)
The letters (s), (g), (l), and aqueous solution (aq)
(dissolved in water) are the physical states of
compounds.
CB/X Chemical Reactions and Equations 10

11. Steps to balancing a chemical equation
 When balancing a chemical reaction you may add coefficients in
front of the compounds to balance the reaction, but you may
not change the subscripts.
 Changing the subscripts changes the compound. Subscripts are
determined by the valence electrons.
 NaCl exists, because Na is +1 and Cl is -1, but NaCl2 does NOT
exist since you would not have a neutral compound! You can’t
just add a number to a formula to balance an equation.
Let’s look at coefficients and subscripts……
CB/X Chemical Reactions and Equations 11

12. Subscripts vs. Coefficients
The subscripts
tell you how
many atoms of a
particular
element are in a
compound.
The coefficients
tell you about,
the number, of
molecules of the
compound.
CB/X Chemical Reactions and Equations 12

13. Why do we need to balance chemical
equations?
Because of the principle of the
conservation of matter (matter
can not be created or destroyed) an equation
must be balanced.
It must have the same number of
atoms of the same kind on both sides.
Law of Conservation of Energy MUST
ALSO BE FOLLOWED!
CB/X Chemical Reactions and Equations 13

14. Steps to Balancing Equations
There are four basic steps to balancing a chemical equation.
1. Write the correct formula for the reactants and the
products.
2. Find the number of atoms for each element on the left side.
Compare those against the number of the atoms of the same
element on the right side.
3. Determine where to place coefficients in front of formulas so
that the left side has the same number of atoms as the right
side for EACH element in order to balance the equation.
4. Check your answer to see if:
 The numbers of atoms on both sides of the equation are
now balanced.
 The coefficients are in the lowest possible whole number
ratios. (reduced)
Let’s see a few
examples…..
CB/X Chemical Reactions and Equations 14

15. Balance this equation!
Na + Cl2
note that the number of sodium atoms balance but
the chlorine does not.
we will have to use coefficients in order to balance
this equation.
Left hand side (LHS) Right hand side (RHS)
Element No. of atoms Element No of atoms
Na 1 Na 1
Cl 2 Cl 1
Let us try to balance
this equation….
First we will write the number of each
element present in the equation.
CB/X Chemical Reactions and Equations 15
NaCl

16. Inserting coefficients
Now the chlorine balances but the sodium does not!
So we go back and balance the sodium.
Left hand side (LHS) Right hand side (RHS)
Element No. of atoms Element No of atoms
Na 1 Na 2
Cl 2 Cl 2
Let us insert
the coefficients
2
CB/X Chemical Reactions and Equations 16
NaCl
Na + Cl2

17. Finally balanced!
2
Left hand side (LHS) Right hand side (RHS)
Element No. of
atoms
Element No of
atoms
Na 2 Na 2
Cl 2 Cl 2
2
CB/X Chemical Reactions and Equations 17
Na(s) + Cl2(g) NaCl(s)
Since the number of each element on the reactant side
and the product side of the equation are equal, the equation
is balanced.
 Now add the state symbols.

18. Balance this equation!
Pb(NO3)2 + NaCl
Left hand side (LHS) Right hand side (RHS)
Element No. of atoms Element No of atoms
Pb 1 Pb 1
Na 1 Na 1
Cl 1 Cl 2
(NO3) 2 (NO3) 1
 Adjust the coefficients not the subscripts. Balance polyatomic ions
as a whole.
 Putting 2 before NaNO3 balances the NO3 but now the Na is not
balanced.
 So we go back and balance it by putting a 2 before NaCl
2 2
CB/X Chemical Reactions and Equations 18
NaNO3 + PbCl2

19. Sodium phosphate + iron (III) oxide  sodium oxide + iron (III) phosphate
Na3PO4 + Fe2O3
Left hand side (LHS) Right hand side (RHS)
Element No. of atoms Element No of atoms
Na 3 Na 2
Fe 2 Fe 1
(PO4) 2 (PO4) 1
O 3 O 1
Balance this equation!
2
2 3
We will have to use coefficients in order to balance this equation.
Remember! Balance the larger molecules first… leave the single atoms until
last!
CB/X Chemical Reactions and Equations 19
Na2O + FePO4

20. Some Suggestions to Help You
Some helpful hints for balancing equations:
Take one element at a time, working left to right except
for H and O. Metals, then nonmetals are a good way,
too. Save H for next to last, and O until last.
IF everything balances except for O, and there is no
way to balance O with a whole number, double all the
coefficients and try again (because O is diatomic as an
element).
Polyatomic ions that appear on both sides of the
equation should be balanced as independent units.
CB/X Chemical Reactions and Equations 20