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Chemistry
Chapter 2:Chemical Reactions
Matter can undergo
two kinds of changes:1. Physical
- only affects size, shape, and phase(the solid water is still water when a liquid!)
2. Chemical-produces new substances(the trees and O2 become ash and CO2 )
How to Recognize a Chemical Change
• Light produced• Sound produced• Heat produced• Cold produced• Gas produced• Flavor changes• Color changes• Smell/odor changes
• How can you tell a chemical change has taken place in each photo?
A process thatproduces a chemical change is a Chemical Reaction.
Once a reaction has taken place, you can never go back to the original substances!
Silver nitrate and sodium chloride solutions, 0.01-0.1M, AgNO3 and NaCl
A penny reacts with nitric acid in a test tube.
How do you know a chemical change is taking place?
Can the penny ever change back?
In a chemical reaction, the reactants change into the
products.Reactants ProductsBaking soda + Vinegar → CO2 (gas) + H2O+ white solid
(sodium bicarbonate + acetic acid) (carbon dioxide)+(water)+ (sodium acetate)
Charcoal + Oxygen→ Ash + CO2 (gas) + Heat
Iron + Oxygen + Water → Rust + Heat
Silver + Hydrogen Sulfide (gas) → Black Tarnish + H2
(gas)
Natural gas from stove + Oxygen→ CO2(gas) + Heat
Note: The arrow reads as “produces” or “yields”.
Initially, bare Cu metal atoms react with air to form the pink oxide, cuprite, Cu2O, which has Cu+1
cations. This gradually oxidizes further to the black oxide, tenorite, CuO, with Cu+2 ions. The black sulfide CuS also sometimes forms. In the presence of moisture, the blackish layer slowly reacts with
sulfur dioxide and carbon dioxide from the air to eventually form the patina, which is a mixture of 3
minerals:brochantite, a green, hydrated copper sulfate,
Cu4SO4(OH)6 malachite, the green, hydrated copper carbonate
Cu2CO3(OH)3azurite, the blue, hydrated copper carbonate
Cu3(CO3)2(OH)2In these compounds, copper atoms from the metal surface have been fully oxidized into Cu+2 (cupric)
ions. The rate of patina formation and the proportion of the components depend upon humidity and the amount of air pollution. In
European cities, where sulfur-containing coal has been burned for centuries, brochantite
predominates on older copper roofs and statuary.
Why does copper turn green??
The Law of Conservation of Mass
“The mass of the products in a chemical reaction must be the same as the mass of the reactants.”(discovered by Antoine Lavoisier, French chemist)
*Every atom that appears on the left,must appear on the right!
*If there are 5 atoms of Oxygen at the start, then there must be five at the finish!
* Therefore, atoms are never created nor destroyed!
Oh my, Mrs. Hardy is a fabulous teacher.
CHEMISTRY
The good news and the bad news
Hydrogen
Hydrogen
Carbon-6
Hydrogen
Hydrogen
Compounds are two or more elements that share electrons or have taken or given electrons away.
Atoms combine to form compounds.
Chemical Equations are simple.
2H2 + O2 2H2O
Good news: Your car rolled off the pier but didn’t fall into the water.
Bad news: The damage to the yacht is more than the value of the car.
Imagine counting grains of sand. Not easy but not impossible. Now imagine trying to count grains of sand that are 2,000 miles away. Quite impossible even with the world’s best telescope.
Well this is the challenge in counting atoms. Counting atoms from 1 foot away is the same as counting grains of sand 2,000 miles away.
Chemical Equations are simple.
2H2 + O2 2H2O
Chemical equations are quite easy. Here we count out 4 hydrogens and 2 oxygen atoms and we can make two water molecules. But how many hydrogens and oxygens would we count out to make enough water to fill a one liter bottle of water?
Imagine the number of grains of sand in a handful of sand: thousands or maybe a million? It would be difficult to count that many. Now imagine the number of grains of sand that would be present if we covered all of the Earth (land and sea) 10 miles deep of sand. That number of grains of sands is the same number of water molecules needed to fill a 1 liter bottle.
Balancing Chemical Equations
Ag + H2S → Ag2S + H2
silver + Hydrogen sulfide Silver sulfide + Hydrogen
(gas) (black tarnish) (gas)
• You must count the atoms on each side!How many silver atoms are in the reactants?
How many silver atoms are in the products?
THIS CAN NOT BE!
WHY NOT??
Ag + H2S → Ag2S + H2
silver + Hydrogen sulfide Silver sulfide + Hydrogen
(gas) (black tarnish) (gas)
Place a 2 in front of the Ag in the reactants and see if they balance now.
2Ag + H2S → Ag2S + H2
Ag = 2 Ag = 2
H = 2 H = 2
S = 1 S = 1
It’s Balanced!
NOTE: You will never add
a new subscript!
Only a coefficient!
Try this one!Methane and oxygen react to form carbon dioxide, water, and heat.
CH4 + O2 → CO2 + H2O + Heat
Reactants ProductsC=1 C=1
H=4 H=2 (You need 2 more H)
O=2 O=3-Place a 2 in front on the H2O
CH4 + O2 → CO2 + 2H2O + Heat
Now, multiply 2x2 = 4H and 2x1=2O
Recalculate!
CH4 + O2 → CO2 + 2H2O + Heat
Reactants ProductsC=1 C=1
H=4 H=4O=2 O=4
You need 2 more O in the reactants still!Put a 2 in front of the Oxygen.
Recalculate.
CH4 + 2O2 → CO2 + 2H2O + Heat Reactants Products
C=1 C=1H=4 H=4O=4 O=4
Now it’s balanced!!
Way to go Dudes!
Exothermic & EndothermicReactions
• Exothermic – releases or gives off heat
• Endothermic – absorbs heat (makes cold)
I am Exo-man.I am HOT!
“therm” refers to heat!
Heat can be released rapidly or slowly.
Which is rapid release of heat? Which is slow release of heat? Which is absorption of heat?
Rusting carAlka-seltzer Burning bunk bed
Energy in the Equation
The word “energy” is written on the equation as a necessary ingredient.
Example: CH4 + 2O2 → CO2 + 2H2O + energyMethane Oxygen Carbon Water
dioxide
This shows you that energy is released!
I am releasing my energy! Hi Ya!!
Photosynthesis!!
6CO2 + 6H2O + light →
C6H12O6 + 6O2
Why is light needed as part of this equation???
Section 2Rates of Chemical Reactions
Activation Energy
• Before a reaction can start molecules of the reactants have to collide with each other.
• These collisions must be strong in order for a reaction to occur.
Activation Energy
• To form new bonds in the product, bonds in the reactants must be broken
• Breaking these bonds takes energy
• The amount of energy needed to break these bonds is called activation energy
Ex. Burning gasolineGasoline doesn’t start burning until energy is applied in the form of a spark
Gas Station Activation Energy
• http://www.youtube.com/watch?v=FpdCZirKukg&feature=PlayList&p=45ADAE1FA75E4948&playnext=1&playnext_from=PL&index=32
• YouTube - Man Catches on Fire When Car Explodes
Reaction Rate
• Rate/speed: how much something changes over a period of time
• Rate of Reaction: how fast a chemical reaction occurs– Found by measuring how quickly one of the
reactants disappears or how quickly a product appears
Question: Why would it be beneficial to know the rate of reaction?
Factors that Affect Reaction Rate
• Temperature• Concentration
– The amount of substance present in a certain volume
• Surface Area– The exposed surface area of reactant
particles
Speeding Up Reaction Rate
• Increasing Temperature– Atoms move faster at higher temperatures
• Increasing the concentration– The more atoms present the greater the chance of
collisions
• Increasing the surface area– Only atoms on the surface can react,
this increases the number of surface atoms
Slowing Down Reaction Rate
• Decreasing Temperature– Atoms move more slowly =
less collisions
• Decreasing the concentration– Less atoms = less chances of
collision
• Decreasing the surface area– Decreases number of surface
atoms = less collisionsJust like driving!
Slow down= less collisions,Less cars=less collisions,
Smaller cars= less collisions
Slowing Down Reactions
• Sometimes reactions occur too quickly– Ex. Some medications react too quickly and
lose their effectiveness
• Inhibitor– Substances that slow down a chemical
reaction– Reactions are not stopped but made to take
longer
How are these helpful in slowing down the reaction rate?
Speeding Up Reactions
• Catalyst• Substances that increase reaction rate
– Do not appear in the equation because they are not changed by the reaction or used up
– Reactions using catalysts will not produce more product but will get to the product faster
+ +steel
Road salt
Example: Catalytic Converters– Used in the exhaust system of cars
– Made of platinum or rhodium
– Speeds up the reaction rate turning carbon monoxide (poison) into carbon dioxide (harmless) more quickly and hydrocarbons into carbon dioxide and water = cleaner air!
Enzymes• Enzymes: catalysts in
the body that slow down or speed up reactions– Ex. Helping your body
convert food into fuel
• Proteases (Proh-tee-ays-es)– Special enzymes that
break down proteins• Ex. Meat tenderizer,
contact lens cleaner
