Review from yesterday’s lesson We are learning how to balance equations because the conservation of mass says that the mass before a reaction must be.

Review from yesterday’s lessonReview from yesterday’s lesson

We are learning how to balance equations We are learning how to balance equations because the conservation of mass says because the conservation of mass says that the mass before a reaction must be that the mass before a reaction must be the same as the mass after a reaction. the same as the mass after a reaction. Atoms cannot be created nor destroyedAtoms cannot be created nor destroyed

HH22 + O + O22 ---> H ---> H22OO

Review from yesterday’s lessonReview from yesterday’s lesson We must know how to count the number of We must know how to count the number of

atoms in a substance before we can balanceatoms in a substance before we can balance

6Na6Na22OO33 Coefficient = 6Coefficient = 6 Subscript Na= 2Subscript Na= 2 Subscript O= 3Subscript O= 3

Total number of Na= 6 X 2= 12Total number of Na= 6 X 2= 12

Total number of O= 6 X 3= 18Total number of O= 6 X 3= 18

Is the blue number a coefficient or Is the blue number a coefficient or a subscript?a subscript?

NaNa22COCO3344KK22POPO44 6H6H220033LiLi

2H2H33POPO44

2Mg(OH)2Mg(OH)22

66LiClLiCl44

Count the number of atomsCount the number of atoms

5H5H22COCO33

How many H atoms?How many H atoms?

5 X 2= 10 5 X 2= 10

How many C atoms?How many C atoms?

5 X 1= 55 X 1= 5

How many O atomsHow many O atoms

5 x 3= 155 x 3= 15

Click here to go to balancing equations steps

Balancing EquationsBalancing Equations

Are you up for the Are you up for the challenge?challenge?

Why do you need to balance Why do you need to balance equations?equations?

Let’s read page 80-81Let’s read page 80-81 The Law of Conservation of Mass The Law of Conservation of Mass

says that the mass of the reactants says that the mass of the reactants must equal the mass of the products.must equal the mass of the products.

In this example: In this example: HH22 + O + O22 ---> H ---> H22OO

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

HH22 + O + O22 ---> H ---> H22OO

The reactants do not equal the products!The reactants do not equal the products! Where did the extra oxygen go?Where did the extra oxygen go? Atoms can’t be created nor destroyed. Atoms can’t be created nor destroyed. They didn’t go anywhere, you just need to They didn’t go anywhere, you just need to

add numbers so the sides become equaladd numbers so the sides become equal An equation is balanced by changing An equation is balanced by changing

coefficients in a somewhat trial-and-error coefficients in a somewhat trial-and-error fashionfashion

Important Vocabulary- CoefficientImportant Vocabulary- Coefficient

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

4H4H2200Coefficient is 4Coefficient is 4HH22O O HH22O O HH22O O HH22OO

Important Vocabulary- SubscriptImportant Vocabulary- Subscript

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

HH2200 Subscript is 2Subscript is 2 2 hydroden’s 2 hydroden’s HH H H

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

C SC S22

Coefficient=1Coefficient=1C’s subscript= 1C’s subscript= 1S’s subscript= 2S’s subscript= 2

11

Let’s practiceLet’s practice

NaNa OO H POH PO HH OO

SS KK 00

2 2

3 44

5

3

6

22

8

2

Math ReviewMath Review

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

4H4H2200

H= 4 X 2= 8 Hydrogen’sH= 4 X 2= 8 Hydrogen’s

O= 4 X 1= 4 Oxygen’sO= 4 X 1= 4 Oxygen’s

1

Math ReviewMath Review

4Mg(OH)4Mg(OH)22

Mg= 4 X 1= 4Mg= 4 X 1= 4

OH= 4 x 2= 8OH= 4 x 2= 8

1

3KOH3KOH

How many K atoms do I have?How many K atoms do I have?Coefficient 3Coefficient 3Subscript 1Subscript 13 X1= 33 X1= 3

How many O atoms do I have?How many O atoms do I have?Coefficient 3Coefficient 3Subscript 1Subscript 13 X 1= 33 X 1= 3

How many H atoms do I have?How many H atoms do I have?Coefficient 3Coefficient 3Subscript 1Subscript 13 X 1= 33 X 1= 3

3K3K22OO

How many K atoms do I have?How many K atoms do I have?Coefficient 3Coefficient 3Subscript 2Subscript 23 X 2= 63 X 2= 6

How many O atoms do I have?How many O atoms do I have?Coefficient 3Coefficient 3Subscript 1Subscript 13 X 1= 33 X 1= 3

2H2H22OO

How many H atoms do I have?How many H atoms do I have?Coefficient= 2Coefficient= 2Subscript= 2Subscript= 22 X 2= 42 X 2= 4

How many O atoms do I have?How many O atoms do I have?Coefficient= 2Coefficient= 2Subscript= 1Subscript= 12 X 1= 22 X 1= 2

KK33POPO44

How many K atoms do I have?How many K atoms do I have? Coefficient= 1Coefficient= 1 Subscript= 3Subscript= 3 1 X 3= 31 X 3= 3

How many P atoms do I have?How many P atoms do I have? Coefficient= 1Coefficient= 1 Subscript= 1Subscript= 1 1 X 1= 11 X 1= 1

How many O atoms do I have?How many O atoms do I have? Coefficient= 1Coefficient= 1 Subscript= 4Subscript= 4 1 X 4= 41 X 4= 4

MgMg33(PO(PO44))22

How many Mg atoms do I have?How many Mg atoms do I have? Coefficient= 1Coefficient= 1 Subscript= 3Subscript= 3 1 X 3= 31 X 3= 3

How many P atoms do I have?How many P atoms do I have? Coefficient= 1Coefficient= 1 Subscript=1 X 2= 2Subscript=1 X 2= 2 1 X 2= 21 X 2= 2

How many O atoms do I have?How many O atoms do I have? Coefficient= 1Coefficient= 1 Subscript= 4 X 2=8Subscript= 4 X 2=8 1 X 8= 81 X 8= 8

Steps for balancing equationsSteps for balancing equations

1. Draw boxes around all the 1. Draw boxes around all the chemical formulas.chemical formulas.

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

H2 + O2 ---> H2O

Steps for balancing equationsSteps for balancing equations

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

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

See example on next slideSee example on next slide

HH22 + O + O22 ---> H ---> H22OO

ReactantsReactants ProductsProducts

Hydrogen Hydrogen (H)(H)

22 22

Oxygen (O)Oxygen (O) 22 11

But what if you have more than one formula But what if you have more than one formula on each side of the equation? Let’s practice on each side of the equation? Let’s practice

with the REACTANTS!with the REACTANTS! NaOH + HNaOH + H22COCO33 NaNa22COCO33 + H + H22OO

How many Na atoms do I have?

1 X 1 =1

How many O atoms do I have?

1 X 1= 1 1 X 3=3 1 + 3 = 4

How many H atoms do I have?

1 X 1= 1 1 X 2= 2 1 + 2= 3

How many C atoms do I have?

1 X 1= 1

Steps for balancing equationsSteps for balancing equations

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

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

HH22 + O + O22 ---> H---> H22OO

ReactantsReactants ProductsProducts

Hydrogen Hydrogen (H)(H)

22 22

Oxygen Oxygen (O)(O)

22 1 1

2

4

2

4

2

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

Steps for balancing equationsSteps for balancing equations

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

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

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

HH

OO

HH22 + O + O22 H H22OO

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

RulesRules You cannot change a subscript.You cannot change a subscript. You cannot place a coefficient in the middle of a You cannot place a coefficient in the middle of a

formula. Hformula. H22 + O + O22 ---> H ---> H222O2O Make sure that your final set of coefficients are Make sure that your final set of coefficients are

all whole numbers with no common factors other all whole numbers with no common factors other than one. than one.

For example, this equation is balanced:For example, this equation is balanced:

4 H4 H22 + 2 O + 2 O22 ---> 4 H ---> 4 H22OO However, all the coefficients have the common However, all the coefficients have the common

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

If you run into problems trying to If you run into problems trying to figure out the answer…figure out the answer…

Sometimes, you will need to find the lowest Sometimes, you will need to find the lowest common multiple in order to make an common multiple in order to make an inventory balance!inventory balance!

HH 22 22

OO 22 77

R P The lowest common multiple between 2 and 7 is 14

14 14

If you run into problems trying to If you run into problems trying to figure out the answer…figure out the answer…

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

When this happens, find the largest When this happens, find the largest molecule in the equation and stick a "molecule in the equation and stick a "22" in " in front of it. front of it.

Then start the problem over. Then start the problem over. Will this work all the time? Well, no. But it Will this work all the time? Well, no. But it

will work sometimes, and give you a new will work sometimes, and give you a new strategy for hard problems.strategy for hard problems.

CC

SS

C + SC + S88 CS CS22

1

8

1

2

4

8

4

4

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

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

NaNa

OO

Na + ONa + O22 Na Na22OO

1

2

2

1

2

2

4

4

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

NN

OO

NN22 + O + O22 N N22OO55

2

2

2

5

2

10

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

5

10

HINT: Find the lowest common multiple of the Oxygen molecules

NaNa

OO

Na + ONa + O22 Na Na22OO22

1

2

2

2

2

2

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

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

K + BK + B22OO33 K K22OO + B+ B26

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!

3

ReactantsReactants ProductsProducts

KK

BB

OO

1

2

3

2

1

1

26

3

6

HINT: Start on the products side- and start with the element with the lowest number of molecules

NN

HH

NN22 + H + H22 NH NH33

2

2

1

3

2

6

2

3

6

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

CsCs

NN

Cs + NCs + N22 Cs Cs33NN

1

2

3

1

2

2

6

6

6

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

`Li + AlClLi + AlCl33 LiCl + Al LiCl + Al33

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!

3

3

3RR PP

LiLiAlAl

ClCl

1

1

3

1

1

1

NaOH + HNaOH + H22COCO33 Na Na22COCO33 + H + H22OO22

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!

2

54 4

5

Hint: We are going to start with the Na on the reactant side because it is not balanced and has the lowest number of molecules

RR PP

NaNa

OO

HH

CC

1

4

3

1

2

4

2

1

CC22HH6 6 + O + O22 CO CO22 + H + H22OO62

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!

12

8

412

4

4

14

7

14

RR PP

CCHH

OO

2

6

2

1

2

3

Hint: We are going to look for the common factor of H’s

HH33POPO4 4 + KOH K + KOH K33POPO44 + H + H22OO33

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!

6

7

3

6

7

RR PP

HH

PP

OO

KK

4

1

5

1

2

1

5

3

Hint: We are going to look for the lowest number of molecules that are not balanced.

Na + NaNONa + NaNO3 3 Na Na22O + NO + N2262

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!

10

12

6

3

2

6

12

RR PP

NaNaNN

OO

2

1

3

2

2

1

Hint: We are going to look for the next common factor of 1 and 3

HH33POPO4 4 + Mg(OH) + Mg(OH)22 Mg Mg33(PO(PO44))22 + H + H22OO32

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!

6

310

9 12

14

12

214

RR PP

HH

PP

OO

MgMg

5

1

6

1

2

2

9

3