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
Representing Chemical Reactions• Equations
– Statements used to represent chemical reactions
– Reactants• The starting substance
– Products• The substances formed during the reaction
Representing Chemical Reactions• Equations
– Show direction in which the reaction progresses
– Reactants Products• Read as reactants yield products
Representing Chemical Reactions• Equations
– 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).
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
3. Make an inventory of the elements.
OH SONa SOH NaOH 24242
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.
OH SONa SOH NaOH 24242
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.
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.
element is the same both before and after the reaction.
Element Before After
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
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.
element is the same both before and after the reaction.
OH SONa SOH NaOH 24242 2
Element Before After
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
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.
element is the same both before and after the reaction.
2
Once again, we must update our element inventory.
Element Before After
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?
Problems You May Encounter
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
Problems You May Encounter
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
Problems You May Encounter
• 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.
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!