chemistry – chapter 2 chemical reactions section 1: chemical formulas and equations
TRANSCRIPT
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Chemistry – Chapter 2
Chemical Reactions
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Section 1: Chemical Formulas and Equations
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Vocabulary Words
1. Physical Changes
2. Chemical Changes
3. Chemical Reaction
4. Reactants
5. Products
6. Chemical Equation
7. Word Equation
8. Law of Conservation of Mass
9. Endothermic Reactions
10. Exothermic Reactions
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Physical or Chemical Change
Matter can undergo two kinds of changes: Physical Changes
Only affects physical properties Same chemical formula and properties
Chemical Changes Produce new substances with different properties
Different chemical formula and properties
Chemical Reaction: process that produces a chemical change
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Recognizing a Physical Change
Change in size Change in shape Change in state of matter
Solid, liquid, or gas
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Practice Identifying Physical Changes
How can you tell a physical change has taken place in each photo?
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Recognizing a Chemical Change
Light produced Sound produced Temperature change Gas produced (bubbles) Flavor changes Color changes Smell/odor changes
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Practice Identifying Chemical Changes
How can you tell a chemical change has taken place in each photo?
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A penny is added to a test tube of nitric acid
How do you know the penny is undergoing a chemical reaction? Color change Gas Produced - Bubbles
Can the penny ever change back? No
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Chemical Equations
Reactants: Substances that exist before the reaction begins
Products: Substances that form as a result of the reaction
In a chemical reaction, the reactants change into the products.
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Describing What Happens
Chemical Equation: Shorthand form of writing what reactants are used and what products are created Ex. HC2H3O2 + NaHCO3 → NaC2H3O2 + CO2 + H2O
(baking soda + vingear) → (sodium acetate + carbon dioxide + water)
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Reactants ProductsCharcoal + 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”.
Using Words
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Using Chemical Names
Word Equation: Chemical equation that uses chemical names instead of formulas Ex. Sodium Bicarbonate + Acetic Acid →
Sodium Acetate + Carbon Dioxide + Dihydrogen Monoxide
Use chemical names instead of common names Ex. Baking Soda + Vinegar → Sodium Salt + Carbon
Dioxide + Water
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Chemical vs. Word Equations
Chemical Faster Show chemical
formulas
Word Longer Use more familiar
names
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Conservation of Mass
Law of Conservation of Mass: Matter cannot be created or destroyed Created by Antoine Lavoisier Means that you must have the same number of
atoms on both sides of an equation
I created Law of Conservation of Mass, named Oxygen and Hydrogen, wrote the
first extensive list of elements, and introduced the metric system!
Because of all of these accomplishments I am known as the Father of Chemistry!
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Balancing Chemical Equations
When you write a chemical equation, you must observe the Law of Conservation of Mass 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!
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For Example…
2H2 + O2 2H2O
There are 4 Hydrogen and 2 Oxygen in the reactants
There are still 4 Hydrogen and 2 Oxygen in the Products
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Count the Atoms
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 in the reactants? How many Silver atoms in the products?
12
This is IMPOSSIBLE!
WHY???
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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!
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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
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Energy in Chemical Reactions
When a chemical reaction takes place bonds break in the reactants and new bonds form in the products
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Energy released Products are more stable Bonds have less energy than those of the reactants Extra energy is released in the form of light/sound/heat Exothermic Reactions: release heat → temperature
increases ↑ Ex. Burning
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Energy absorbed Products are more stable Bonds have less energy than those of the reactants Endothermic Reactions: absorb heat → temperature
decreases ↓ Ex. Ice pack
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Energy in the Equation
Energy is often written in equations as a reactant or a product Energy as a reactant = necessary for the
reaction to occur Energy as a product = energy released during
the reaction Ex. Cooking food on your gas stove:
CH4 + 2O2 → CO2 + 2H2O + energy
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Section 2: Rates of Chemical Reactions
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Vocabulary Words
1. Activation Energy
2. Rate of Reaction
3. Concentration
4. Inhibitor
5. Catalyst
6. Enzyme
7. Proteases
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How Fast?
Reaction speeds vary greatly Fireworks explode in a few seconds Copper pennies darken slowly as they react with
substances they touch over a number of years
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Activation Energy
Before a reaction can start, molecules must collide with each other
This collision will cause old bonds to break and allow new bonds to form
Molecules must collide with a certain amount of energy or the reaction will not occur Activation Energy: Minimum amount of energy
needed to start a chemical reaction
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Activation Energy Continued….
Reactions that release and absorb energy need activation energy to begin
Ex. Burning gasoline Won’t burn on its own it needs to be ignited
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Reaction Rate
Rate of Reaction: How fast a reaction occurs Measured in 2 ways:
How fast the reactants disappearHow fast the products appear
Helpful inDetermining how fast a product can be madeMeasuring time before food/medicine will spoil
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Affecting Reaction Rate
6 Factors that affect reaction rate:1. Temperature
2. Concentration
3. Surface Area
4. Inhibitors
5. Catalysts
6. Enzymes
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1. Temperature
Decreasing Temperature Decreases rate of reaction (slows down) Food in the fridge lasts longer because
bacteria grows more slowly in colder temperatures
Atoms move slower at lower temperaturesThe more slowly an atom moves the
fewer collisions it will have
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Increasing Temperature Increases rate of reaction (speeds up) Heating foods while cooking them speeds up the
molecules Higher temperatures cook faster than lower ones
Atoms move faster at higher temperatures The faster an atom moves the more collisions it will
have
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2. Concentration
The more molecules present the more likely a collision will be Ex. You’re more likely to bump into someone in a
crowded hallway than an empty one
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Concentration: Amount of substance present in a certain volume Decrease Concentration
Decreases rate of reaction (slows down) Fewer atoms = less collisions
Increase Concentration Increases rate of reaction (speeds up)
More atoms = more collisions
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3. Surface Area
The exposed surface of reactant particles affects how fast a reaction can occur Only molecules in the outer layer can touch
other reactants and react Ex. Small twigs of wood have more surface area for
their size than large logs and would be easier to start a fire with because more wood molecules can react
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Decrease Surface AreaDecreases rate of reaction (slows down)
Fewer surface atoms = fewer reactions Increase Surface Area
Increases rate of reaction (speeds up) More surface atoms = More reactions
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http://www.kentchemistry.com/links/Kinetics/FactorsAffecting.htm
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4. Inhibitors
Inhibitor: Substance that slows down a chemical reaction Adding an Inhibitor
Decreases rate of reaction (slows down) Used to increase the shelf life of foods and
medicines
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5. Catalysts
Catalyst: Substance that speeds up a chemical reaction Reduces the level of activation energy needed
to start a reaction by providing a surface for the reaction to take place
A reaction using a catalyst will not produce more product, it will just produce the same amount of product in shorter time
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Adding a catalyst Increases rate of reaction (speeds up) Ex. Catalytic Converter in a car speeds up the
reactions that change harmful substances like carbon monoxide into harmless substances like water
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6. Enzymes
Enzymes: Protein molecules that speed up or slow down reactions needed for your cells to work properly Adding enzymes make it possible for your body
to function properly or Convert food to fuel Build bone and muscles Convert extra energy into fat Produce other enzymes
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Proteases: Enzymes outside of the body that specialize in protein reactions Ex.
Meat tenderizer: breaks down proteins to make meat more tender
Contact lens cleaner: breaks down proteins from your eyes that could collect on your lens and cloud your vision