Chemistry – Chapter 2

Chemical Reactions

Section 1: Chemical Formulas and Equations

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

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

Recognizing a Physical Change

Change in size Change in shape Change in state of matter

Solid, liquid, or gas

Practice Identifying Physical Changes

How can you tell a physical change has taken place in each photo?

Recognizing a Chemical Change

Light produced Sound produced Temperature change Gas produced (bubbles) Flavor changes Color changes Smell/odor changes

Practice Identifying Chemical Changes

How can you tell a chemical change has taken place in each photo?

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

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.

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)

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

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

Chemical vs. Word Equations

Chemical Faster Show chemical

formulas

Word Longer Use more familiar

names

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!

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!

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

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???

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

Energy in Chemical Reactions

When a chemical reaction takes place bonds break in the reactants and new bonds form in the products

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

Energy absorbed Products are more stable Bonds have less energy than those of the reactants Endothermic Reactions: absorb heat → temperature

decreases ↓ Ex. Ice pack

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

Section 2: Rates of Chemical Reactions

Vocabulary Words

1. Activation Energy

2. Rate of Reaction

3. Concentration

4. Inhibitor

5. Catalyst

6. Enzyme

7. Proteases

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

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

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

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

Affecting Reaction Rate

6 Factors that affect reaction rate:1. Temperature

2. Concentration

3. Surface Area

4. Inhibitors

5. Catalysts

6. Enzymes

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

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

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

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

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

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

http://www.kentchemistry.com/links/Kinetics/FactorsAffecting.htm

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

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

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

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

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