Balancing Equations - Kyrene School District...Steps for balancing equations 3. Write numbers in front of each of the boxes until the inventory for each element is the same both before

Balancing Equations

Are you up for the challenge?

Hello

Be Proactive

Take out the supplies you need:

– Science Notebook

– Pencil

– Homework

– Science Folder

Everything else gets put away.

Bell Work

Open your science notebook to the next available page. Title that page “Balancing Equations.”

What is the law of conservation of mass?

Why do you need to balance

equations?

The Law of Conservation of Mass says that the mass of the reactants must equal the mass of the products.

In this example: H2 + O2 ---> H2O

There are 2 hydrogen’s on the left and 2 Oxygen’s, but there is only 1 oxygen on the right.

H2 + O2 ---> H2O

The reactants do not equal the products!

Where did the extra oxygen go?

Atoms can’t be created nor destroyed.

They didn’t go anywhere, you just need to add numbers so the sides become equal

An equation is balanced by changing coefficients in a somewhat trial-and-error fashion

Important Vocabulary- Coefficient

Coefficient- Large number located in front of an element or compound in a chemical formula. 2H= 2 hydrogen's

4H20

Coefficient is 4

H2O H2O H2O H2O

Important Vocabulary- Subscript

Subscript- The small number that is in the lower right corner of an element

H20 Subscript is 2

2 hydrogen’s H H

If there is no coefficient or subscript- assume it is 1

C S2

Coefficient=1

C’s subscript= 1

S’s subscript= 2

1 1

Let’s practice

Na O

H PO

H O

S

K 0

2 2

3 4 4

5

3

6

2 2

8

2

Math Review

Distributive Property- The coefficient times the subscript gives the total number of atoms.

4H20

H= 4 X 2= 8 Hydrogen’s

O= 4 X 1= 4 Oxygen’s

1

Math Review

4Mg(OH)2

Mg= 4 X 1= 4

OH= 4 x 2= 8

1

3KOH

How many K atoms do I have? Coefficient 3

Subscript 1

3 X1= 3

How many O atoms do I have? Coefficient 3

Subscript 1

3 X 1= 3

How many H atoms do I have? Coefficient 3

Subscript 1

3 X 1= 3

3K2O

How many K atoms do I have? Coefficient 3

Subscript 2

3 X 2= 6

How many O atoms do I have? Coefficient 3

Subscript 1

3 X 1= 3

2H2O

How many H atoms do I have? Coefficient= 2

Subscript= 2

2 X 2= 4

How many O atoms do I have? Coefficient= 2

Subscript= 1

2 X 1= 2

K3PO4

How many K atoms do I have? Coefficient= 1

Subscript= 3

1 X 3= 3

How many P atoms do I have? Coefficient= 1

Subscript= 1

1 X 1= 1


Subscript= 4

1 X 4= 4

Mg3(PO4)2

How many Mg atoms do I have? Coefficient= 1

Subscript= 3

1 X 3= 3

How many P atoms do I have? Coefficient= 1

Subscript=1 X 2= 2

1 X 2= 2


Subscript= 4 X 2=8

1 X 8= 8

H

O

H2 + O2 H2O

2

2

2

1

2

2

4

2

4

Step 1: Write formula and draw boxes

Step 2: Make an element inventory

Step 3: Add coefficients to make the inventory balance. Don’t forget to change the inventory as you go!

R P

Steps for balancing equations

1. Draw boxes around all the chemical formulas.

Never, ever, change anything inside the boxes. Ever. Really. If you do, you're guaranteed to get the answer wrong.

H2 + O2 ---> H2O


2. Make an element inventory. How are you going to know if the equation

is balanced if you don't actually make a list of how many of each atom you have? You won't. You have to make an inventory of how many atoms of each element you have, and then you have to keep it current throughout the whole problem.

See example on next slide

H2 + O2 ---> H2O

Reactants Products

Hydrogen (H)

2 2

Oxygen (O) 2 1


3. Write numbers in front of each of the boxes until the inventory for each element is the same both before and after the reaction.

Whenever you change a number, make sure to update the inventory - otherwise, you run the risk of balancing it incorrectly. When all the numbers in the inventory balance, then the equation can balance,

H2 + O2 ---> H2O

H2 + O2 ---> H2O

2

Reactants Products

Hydrogen (H) 2 2

Oxygen (O) 2 1

2 2

4

2

4

Notice- I put the coefficient in front of the box- NOT inside!


4. Find the elements which appear in the fewest numbers of molecules and balance these first.

Continue in sequence until you balance the element which appears in the most molecules last.

Tip: Start by balancing an element that appears in only one reactant and product.

Rules

You cannot change a subscript.

You cannot place a coefficient in the middle of a formula. H2 + O2 ---> H22O

Make sure that your final set of coefficients are all whole numbers with no common factors other than one.

For example, this equation is balanced:

4 H2 + 2 O2 ---> 4 H2O However, all the coefficients have the

common factor of two. Divide through to eliminate common factors like this.

C + S8 CS2

C

S

1

8

1

2 How do I get 8 S’s on this side?

4

8

4

4

4

If you run into problems trying to

figure out the answer…

Find the lowest common denominator of those two numbers, and then put the numbers in front of those two boxes which allow the inventory on both sides to match.

If you run into problems trying to

figure out the answer…

What happens when the only way you can get a problem to work out is to make one of the numbers a decimal or fraction?

When this happens, find the largest molecule in the equation and stick a "2" in front of it.

Then start the problem over.

Will this work all the time? Well, no. But it will work sometimes, and give you a new strategy for hard problems.

H

O

H2 + O2 H2O

2

2

2

1

2

2

4

2

4




R P

C

S

C + S8 CS2

1

8

1

2

4

8

4

4

4




R P

How do I get the S’s in the products to equal 8?

Na

O

Na + O2 Na2O

1

2

2

1

2

2

4

4

4




R P

N

O

N2 + O2 N2O5

2

2

2

5

2

10

4

2

4




R P

5

10

Na

O

Na + O2 Na2O2

1

2

2

2

2

2




R P

How do I get the Na’s in the reactants to equal 2?

K + B2O3 K2O + B 2 6




3

Reactants Products

K

B

O

1

2

3

2

1

1

2

6

3

6

N

H

N2 + H2 NH3

2

2

1

3

2

6

2

3

6




R P

Cs

N

Cs + N2 Cs3N

1

2

3

1

2

2

6

6

6




R P

` Li + AlCl3 LiCl + Al 3 3




R P

Li 1 1

Al 1 1

Cl 3 1

3

3

3

NaOH + H2CO3 Na2CO3 + H2O 2 2




R P

Na 1 2

O 4 4

H 3 2

C 1 1

2

5

4 4

5

C2H6 + O2 CO2 + H2O 6 2




R P

C 2 1

H 6 2

O 2 3

12

8

4

12

4

4

14

7

14

H3PO4 + KOH K3PO4 + H2O 3 3




R P

H 4 2

P 1 1

O 5 5

K 1 3

6

7

3

6

7

Na + NaNO3 Na2O + N2 6 2




10

R P

Na 2 2

N 1 2

O 3 2

12

6

3

2

6

12

H3PO4 + Mg(OH)2 Mg3(PO4)2 + H2O 3 2




6

R P

H 5 2

P 1 2

O 6 9

Mg 1 3 3

9

8 12

14

12

2 14

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Balancing Equations - Kyrene School District...Steps for balancing equations 3. Write numbers in front of each of the boxes until the inventory for each element is the same both before