1

Chemical Equations and Reactions

What are they?

2

The Two Sides of a Chemical Reaction

• Definition: In a chemical reaction one or more substances are changed into new substances.

Na + Cl NaCl

Reactant – About to be changed

Product – The new substance

3

Symbol for Chemical Equations

“Pb(NO3)4” is a formula.

Means produces (or yields)

+ Means added together

Coefficients – Number of molecules Subscripts - Number of a particular atom in a

molecule

Pb(NO3)4 + 2KI PbI2 + 2KNO3

4

First Described the “Law of Conservation of Mass”

Father of Modern Chemistry

1743 - 1794

5 Conservation of MassAntoine Lavoisier found that the mass of the

reactants and the products are equal, even when the states of matter change.

This is the Law of the Conservation of Mass –Matter is neither created nor destroyed.

HgHg + + OO22HgHgOO

He started with: 10g of Mercury Oxide (HgO)

He ended up with:and 9.3g Mercury…

0.7

g O

xygen b

eca

me a

gas

10 g.10 g. = = 0.7 + 9.3 0.7 + 9.3 g.g.

...But what happened to the

O2?

6

Lavoisier’s Chemical Equation:

Hg (mercury) can exist by itself... but, oxygen will need to bond with another oxygen to make O2

HgO Hg + O222

To balance the atoms we need to:

This balances the equation!

Put the coefficient of 2 in front the product Hg.

Put the coefficient of 2 in front of reactant HgO.

7

Not that easy?

Let’s try...

Easy, Right?

8

VISUALIZING to Balance VISUALIZING to Balance an Equation…an Equation…

9 Equations Must Be Balanced

HH22 + + OO22

HH22 OO

Law of Conservation of Mass

10 Equations Must Be Balanced

2 2 HH22 + + OO222 2 HH22

OO

Matter is neither created nor destroyed.

Is this reaction possible?

NN NN HH HHNN

HH

HHHH

N2 NH3Why or Why Not?

Let’s Count the Atoms:

There are 2 nitrogen atoms

There are 2 hydrogen

atoms

1 nitrogen and 3 hydrogen…

H2

11

12

Before we begin balancing, we will look at a couple of rules.

13 Atoms can only bond in certain ways..

That’s why we can’t change the subscripts.subscripts.

HHHH

NNNN

NN

NN

NNNN

NNNN

HH

HHHH

HH

HHHH

HH

HH

NN

NN

NN NN HH HH

H2

N2

N2

N2

N2

N2

H2

H2

H2

H2

14

Here is what it means...

Subscripts - Small #’s below an element.

Coefficients - Large #’s in front of the formulas.

2H2H2When balancing equations,

we can only change the coefficients!

HHHH

HHHH

HH

HH

I can’t live without

you!

Now, back to the balancing...

NN NN HH HHNN

HH

HHHH

N2 NH3We can only change coefficients

before the symbols.

1 molecule of nitrogen

1 molecule ofhydrogen

1 molecule of ammonia

H21 1 1

15

16

NN NN HH HHNN

HH

HHHH

After counting the atoms we know...

There are too many nitrogen atoms

and not enough hydrogen atoms.

17

NN NN

HH HH NN

HH

HHHH

A little common sense math goes a long way in making this equation

balance…

Count the atoms. Is it balanced?

There are 2 nitrogen atoms. We will get 2 more

molecules for a total of 6 hydrogen

atoms. That will make 2 NH3.

HH HH

HH HHNN

HH

HHHH

18

N2 + 3 H2 2NH3

NN NN

HH HH NN

HH

HHHH

HH HH

HH HHNN

HH

HHHH

Then it is a balanced equation.

Do both sides have the same amount of atoms?

19

Four Steps to Balance Equations:

1. Set up your equation.Write the metals, nonmetals, oxygen, and

hydrogen.

2. Count and then write the number of atoms you have on both sides.

3. Balance by changing the coefficients and recounting.

4. Start the process again if it still does not balance.

20

1. Set up your equation.

HH22 + O + O2 2 H H22OOHH22 + O + O2 2 H H22OOWrite the elements in this order:

Metals, Nonmetals, Oxygen, and Hydrogen

OOHH

21

2. Count and write the number of atoms you have of each on both sides.HH22 + O + O2 2 H H22OOHH22 + O + O2 2 H H22OO

OOHH

22 112222

22

HH22 + O + O2 2 H H22OOHH22 + O + O2 2 H H22OO

OOHH

22 112222

(2)(2)22

(4)(4)

22

(4)(4)

3. Balance by changing the coefficients and recounting.

Need to have at least 2 “O”

But it changes the number of

“H”

How are you going to make “H” add up

to 4?

Is this balanced?Yes!

23

Mg + HCl Mg + HCl H H2 2 + MgCl + MgCl22Mg + HCl Mg + HCl H H2 2 + MgCl + MgCl22

MgMgClCl

11 112211

(2)(2)

22

(2)(2)

Let’s try another:

Need to have at least 2 “Cl”

Changing the Cl changes the “H”?

Is this balanced?Yes!

HH 2211

24

Na + HCl Na + HCl NaCl + H NaCl + H22Na + HCl Na + HCl NaCl + H NaCl + H22

NaNa

ClClHH

11 11

112211

11(2)(2)

22

In this case, we will start with

hydrogen since it is the only one

unbalanced.

Let’s try another:

25

Na + Na + HCl HCl NaCl + H NaCl + H22Na + Na + HCl HCl NaCl + H NaCl + H22

NaNa

ClClHH

11 11

112211

11(2)(2)

22

(2)(2) (2)(2)

22

But, changing the hydrogen in HCl affects the number of chlorine atoms.

26Changing the chlorine on the product side affects the sodium (Na) on the reactants side. So we must now change sodium as well.

Na + Na + 22HCl HCl NaCl + HNaCl + H22Na + Na + 22HCl HCl NaCl + HNaCl + H22

NaNa

ClClHH

11 11

112211

11(2)(2)(2)(2) (2)(2)

2222(2)(2) (2)(2)

27

http://www.chem.vt.edu/RVGS/ACT/notes/scripts/bal_eq1.html

http://www.wfu.edu/~ylwong/balanceeq/balanceq.html

Click on the web sites below and balance the equations.Click on the web sites below and balance the equations.

28

4 Types of Chemical Reactions

SynthesisDecompositionSingle DisplacementDouble Displacement

29

Synthesis

AA + + BB AABB

22HH22 + + OO22 2 2HH22OO

Two or more substances combine to form a new one.

30

Decomposition

AAB B A A + + BB

22MgMgOO MgMg ++ OO22

Decomposition the opposite of synthesis; a substance breaks down into 2 or more simpler

substances.

31

Single Displacement

AA + + BBC C A ACC + + BB

ZnZn + + CuCu(NO(NO33))22 ZnZn(NO(NO33))22 ++

CuCuThis occurs when one element replaces another in a

compound; a zinc strip is put into a copper nitrate solution and the

copper precipitates out (it turns solid).

32

Double Displacement

AAB B + + CCDD A ADD + + CCBB

HHClCl + + NaNaOHOH HHOHOH ++ NaNaClCl(H2O)

This occurs when two elements “switch” with one another to form two new compounds.