Chapter Nine Chemical Reactions. Section One Reactions & Equations

Chapter Nine

Chemical Reactions

Section One

Reactions & Equations

Chemical Reactions

• Chemical Reaction– The process by which the atoms of one or

more substances are rearranged to form different substances

Chemical Reactions

• Some reactions are hard to detect.

• Many provide physical evidence.

• Evidence for Chemical Reactions– Temperature change

• Release energy in the form of heat and light• Absorb heat

– Color change

Chemical Reactions

• Evidence for Chemical Reactions cont’d.– Odor– Gas bubbles– Formation of solid

Representing Chemical Reactions• Equations

– Statements used to represent chemical reactions

– Reactants• The starting substance

– Products• The substances formed during the reaction


– Show direction in which the reaction progresses

– Reactants Products• Read as reactants yield products


– Symbols

+ Separates reactants and products

Separates reactants FROM products

Separates reactants FROM products

Indicates reversible reaction

(s) Solid state

(l) Liquid state

(g) Gaseous state

(aq) Water solution

Representing Chemical Reactions

• Word Equation– Uses elements and compounds full names– Lacks important information – Example

• Reactant 1 + Reactant 2 Product 1• Aluminum(s) + Bromine(l) Aluminum bromide(s)

Representing Chemical Reactions• Skeleton Equation

– Provides important information– Uses chemical formulas– Example

• Al(s) + Br2(l) AlBr3(s)

Representing Chemical Reactions• Practice writing a reaction between carbon

and sulfur (which are solids), to form carbon disulfide (which is a liquid).

Homework

• Practice Problems– #1, 2, 3 on page 284

How to Balance Chemical Equations

Products

Why even bother, aren’t they right once they’re written?

Law of Conservation of Mass– Within a chemical reaction, the mass of the

products is the same as the mass of the reactants.

Reactants

Look at it this way. . .

Just like

ProductsReactants

INGREDIENTS•2 ¾ cups all-purpose flour •1 teaspoon baking soda •½ teaspoon baking powder •1 cup butter, softened •1 ½ cups white sugar •1 egg •1 teaspoon vanilla extract

So how do we balance an equation?

Follow these simple steps. . .

1. Identify a complete chemical equation.

2. Identify and draw boxes around all the chemical formulas.

3. Take an inventory of the elements.

4. Use the inventory in order to determine which numbers need to be written in front of the boxes, so that the inventory is balanced.

1. Have a complete chemical equation.

42SOH NaOH SONa 42

For this example we will write an equation with:

● sodium hydroxide● sulfuric acid● sodium sulfate

● water

We should get something that looks like. . .

OH 2

2. Draw boxes around all the chemical formulas.

OH SONa SOH NaOH 24242

3. Make an inventory of the elements.


Element Before After

Na 1 2

O 5 5

H 3 2

S 1 1

4. Use the inventory in order to determine which numbers need to be written in front of the boxes, so that the

inventory is balanced.


How does writing a number in front of one of these boxes effect the balance of the equation?

•When a number is written in front of a box, anything within that box is multiplied by that number.


element is the same both before and after the reaction.


Na 1 2

O 5 5

H 3 2

S 1 1

So, looking at the inventory, what should we do?




OH SONa SOH NaOH 24242 2


Na 2 2

O 6 5

H 4 2

S 1 1

Now that we put a 2 in front of the NaOH, let’s update our element inventory.

Now what should we do?

OH SONa SOH NaOH 24242 2



2

Once again, we must update our element inventory.


Na 2 2

O 6 6

H 4 4

S 1 1

We have successfully balanced the equation!

Problems You May Encounter

• What happens when you do the inventory, and you find that there are three atoms of element X on the left side of the equation and two on the right.

FeCl3 + Be3(PO4)2 BeCl2 + FePO4

• How can you make those match?


1. Find the least common multiple of those two numbers.

• In the element X example, the least common multiple of two and three is six, so you'd put a “2" in front of the molecule on the left, and a “3" in front of the one on the right.

2FeCl3 + Be3(PO4)2 3BeCl2 + FePO4


2. Put the numbers in front of those two boxes which allow the inventory on both sides to match.

• Element X will then match up, and you can use a new inventory to see what else needs to be done

Inventory Table

Elements Before After

Fe 2 1

Cl 6 6

Be 3 3

PO4 2 1

2FeCl3 + Be3(PO4)2 3BeCl2 + FePO4

2FeCl3 + Be3(PO4)2 3BeCl2 + 2FePO4

Elements Before After

Fe 2 2

Cl 6 6

Be 3 3

PO4 2 2

Inventory Table


• 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?

1. Find the largest molecule in the equation and stick a "2" in front of it.

2. Start the problem over.

Let’s try some problems.

22 BeClNaFBeFNaCl

Is this equation balanced?

Yes No

Let’s look at the equation again.

22 BeClNaFBeFNaCl

Yes No

Is this equation balanced?

Very Good! It’s not balanced!

2BeFNaCl2BeClNaF

Time for Inventory

22 BeClNaFBeFNaCl


Na 1 1

Cl 1 2

Be 1 1

F 2 1

Now what?

NaCl 2BeF NaF 2BeCl2


Na 2 1

Cl 2 2

Be 1 1

F 2 1

It’s Balanced!!

NaCl 2BeF NaF 2BeCl2 2


Na 2 2

Cl 2 2

Be 1 1

F 2 2

Group Work Problem Set 1

1. NaNO3 + PbO Pb(NO3)2 + Na2O

2. AgI + Fe2(CO3)3 FeI3 + Ag2CO3

3. C2H4O2 + O2 CO2 + H2O

4. ZnSO4 + Li2CO3 ZnCO3 + Li2SO4

5. V2O5 + CaS CaO + V2S5

Let’s see how we did!

Group Work Problem Set 21. Mn(NO2)2 + BeCl2 Be(NO2)2 + MnCl2

2. AgBr + GaPO4 Ag3PO4 + GaBr3

3. H2SO4 + B(OH)3 B2(SO4)3 + H2O

4. S8 + O2 SO2

5. Fe + AgNO3 Fe(NO3)2 + Ag

Let’s see how we did!

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Chapter Nine Chemical Reactions. Section One Reactions & Equations