Transcript Intro to Chemical Equations note

Chapter 4
Chemical Reactions
1
All chemical reactions
 have
two parts:
–Reactants - the substances
you start with
–Products- the substances you
end up with
 The reactants turn into the
products.
 Reactants  Products
2
In a chemical reaction
The way atoms are joined is changed
 Atoms aren’t created or destroyed.
 Can be described several ways:
1. In a sentence
Copper reacts with chlorine to form
copper (II) chloride.
2. In a word equation
Copper + chlorine  copper (II) chloride

3
Symbols in equations
 the
arrow separates the
reactants from the products
 Read “reacts to form”
 The plus sign means “and”
 (s) after the formula = solid
 (g) after the formula = gas
 (l) after the formula = liquid
4
Symbols used in equations
 (aq)
after the formula - dissolved
in water, an aqueous solution.
5
Symbols used in equations
indicates a reversible
reaction (more later)

heat
 shows that
   ,   
heat is supplied to the reaction
Pt
  
is used to indicate a
catalyst is supplied, in this case,
platinum.

6
What is a catalyst?
A
substance that speeds up a
reaction, without being changed
or used up by the reaction.
7
Skeleton Equation
 Uses
formulas and symbols to
describe a reaction
 doesn’t indicate how many.
 All chemical equations are
sentences that describe
reactions.
8
Convert these to equations

Solid iron (III) sulfide reacts with
gaseous hydrogen chloride to form
iron (III) chloride and hydrogen
sulfide gas.
Fe2S3 (s)
9
+ HCl (g) 
FeCl3 (s)
+ H2S (g)
Nitric acid dissolved in water reacts with
solid sodium carbonate to form liquid
water and carbon dioxide gas and
sodium nitrate dissolved in water.
HNO3 (aq) + Na2CO3 (s)  H2O (l) + CO2 (g) + NaNO3 (aq)
10
Now, read these:

Fe(s) + O2(g)  Fe2O3(s)

Cu(s) + AgNO3(aq) 
Ag(s) + Cu(NO3)2(aq)

11
Pt
NO2 (g)   N2(g) + O2(g)
HALOGEN HON:
When any halogen (Group 17), hydrogen,
oxygen, or nitrogen are by themselves in an
equation, they are shown as DIATOMIC
ELEMENTS.
 H2 O2 N2
 F2
 Cl2
 Br2
 I2
12
Balancing Chemical Equations
13
Balanced Equation
 Atoms
can’t be created or
destroyed
 All the atoms we start with we
must end up with
 A balanced equation has the
same number of each element on
both sides of the equation.
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C
+
O
O

O C
O
+ O2  CO2
 This equation is already balanced
 What if it isn’t?
C
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C
+
O
O

C
O
C + O2  CO
 We need one more oxygen in the
products.
 Can’t change the formula, because it
describes what it is (carbon
monoxide in this example)

16
C
 Must
+
O
O

C
O
C
O
be used to make another
CO
 But where did the other C come
from?
17
C
+
C
 Must
O
O

C
O
C
O
have started with two C
 2 C + O2  2 CO
18
Where the numbers are mean
different things…..
4Pb(NO3)2



19
3- only relates to the oxygen atom (called
a “subscript”)
2-relates to everything inside the
brackets(called a “subscript”)
4-relates to every atom in the compound
(called the “coefficient”)
Rules for balancing:
 Assemble, write the correct formulas
for all the reactants and products
 Count the number of atoms of each
type appearing on both sides
 Balance the elements one at a time
by adding coefficients (the numbers
in front) - save H and O until LAST!
 Check to make sure it is balanced.
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
Never change a subscript to balance an
equation.
– If you change the formula you are
describing a different reaction.
– H2O is a different compound than H2O2

Never put a coefficient in the middle of a
formula
– 2 NaCl is okay, Na2Cl is not.
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Example
H2 + O2  H2O
Make a table to keep track of where you
are at
22
Example
H2 + O2  H2O
R
P
2 H 2
2 O 1
Need twice as much O in the product
23
Example
H2 + O2 
R
P
2 H 2
2 O 1
Changes the O
24
2 H2O
Example
H2 + O2 
2 H2O
R
P
2 H 2
2 O 1 2
Also changes the H
25
Example
H2 + O2 
2 H2O
R
P
2 H 2 4
2 O 1 2
Need twice as much H in the reactant
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Example
2 H2 + O2 
2 H2O
R
P
2 H 2 4
2 O 1 2
Recount
27
Example
2 H2 + O2 
2 H2O
R
P
4 2 H 2 4
2 O 1 2
The equation is balanced, has the same
number of each kind of atom on both sides
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Example
2 H2 + O2 
2 H2O
R
P
4 2 H 2 4
2 O 1 2
This is the answer
Not this
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Balancing Examples

_AgNO3 + _Cu  _Cu(NO3)2 + _Ag
 _Mg
 _P4
30
+ _N2  _Mg3N2
+ _O2  _P4O10
Odd Number Rule

31
If ,when balancing, there happens to
be an odd number (other than 1) of
an element on one side, and an even
number on the other side, DOUBLE
THE COEFFICIENT IN FRONT OF
WHERE IT’S ODD, AND THEN START
OVER (with that element)