Chemical Reactions

BELLWORK

BRIEFLY WRITE ABOUT A SCIENTIFIC OBSERVATION YOU MADE RECENTLY

Chemical equations

WE USE CHEMICAL EQUATIONS TO DESCRIBE CHEMICAL REACTIONS

Burning Coal

This is a chemical reaction

The purpose of the equation is to show what was present before the reaction (carbon and oxygen) and afterwards (carbon dioxide)

The substances that undergo change are reactants (carbon and oxygen)

New substances formed as a result of that change are called products

REACTANTS PRODUCTS

CARBON + OXYGEN CARBON DIOXIDE

SIMPLIFY:C+O2 CO2

CHEMICAL EQUATION

A CHEMICAL EQUATION IS A REPRESENTATION OF A CHEMICAL REACTION IN WHICH THE REACTANTS AND PRODUCTS ARE EXPRESSED AS FORMULAS.

CONSERVATION OF MASS

THE LAW OF THE CONSERVATION OF MASS: MASS IS NEITHER CREATED NOR DESTROYED IN A CHEMICAL REACTION

ALTHOUGH THE MASS OF THE COAL WAS REDUCED DURING THE REACTION, IT WAS NOT LOST

THE MASS OF THE CARBON DIOXIDE CREATED WAS EQUAL TO THE MASS OF THE COAL THAT WAS LOST

BALANCING EQUATIONS

ACCORDING TO THE LAW OF CONSERVATION OF MASS, EACH SIDE OF AN EQUATION MUST BE EQUAL

H2 + O2 H2O

WHAT IS WRONG WITH THIS EQUATION?

TO BALANCE WE SIMPLY ADD COEFFICIENTS:

THE NUMBERS THAT APPEAR BEFORE THE EQUATION

H2 + O2 H2O =

H2 + O2 2H2O

WHAT IS WRONG WITH THIS EQUATION?

HOW CAN WE FIX IT?

ANSWER

2H2 + O2 2H2O

WHICH MEANS:TWO MOLECULES OF HYDROGEN REACT WITH ONE MOLECULE OF OXYGEN TO YIELD TWO MOLECULES OF WATER

http://PHSchool.com/webcodes10/index.cfm?wcprefix=ccp&wcsuffix=1071&area=view&x=13&y=7

MATH SKILLS

READ OVER THE MATH SKILLS EXAMPLES AND STEPS FOR BALANCING EQUATIONS ON PAGE 195

THEN, TRY THE MATH PRACTICE 1-3

Balancing equations tutorials

http://www.youtube.com/watch?v=RnGu3xO2h74

http://www.youtube.com/watch?v=gskm-dfKv5g

BELLWORK

BALANCE THE FOLLOWING CHEMICAL EQUATION:

H2+O2 H2O

WHAT ARE SOME UNITS OF MEASURE THAT YOU KNOW OF?

A. What is the Mole?A counting number (like a dozen)

Avogadro’s number (NA)

1 mol = 6.02 1023 items

A large amount!!!!

Counting with Moles

MOLE: (MOL) AN AMOUNT OF A SUBSTANCE THAT CONTAINS APPROXIMATELY 6.02 X 1023

PARTICLES OF THAT SUBSTANCE.

KNOWN AS AVOGADRO’S NUMBER

A MOLE OF A SUBSTANCE GENERALLY CONTAINS 6.02 X 1023 ATOMS, MOLECULES, OR IONS OF THAT SUBSTANCE

1 mole of hockey pucks would equal the mass of the moon!

A. What is the Mole?

1 mole of pennies would cover the Earth 1/4 mile deep!

1 mole of basketballs would fill a bag the size of the earth!

Molar Mass

THE MASS OF DIFFERENT ELEMENTS ARE DIFFERENT, AS IS THEIR MOLAR MASS

THE MOLAR MASS OF AN ELEMENT IN THE SAME AS ITS ATOMIC MASS EXPRESSED IN GRAMS

MASS OF COMPOUNDSYOU CAN FIND THE MASS OF COMPOUNDS BY ADDING THE MASS OF THEIR COMPONENTS

A CARBON DIOXIDE MOLECULE:

CARBON= 12 GRAMS

OXYGEN = 16 GRAMS X 2 = 32 GRAMS

12 + 32 = 44 GRAMS

CARBON DIOXIDE HAS A MOLAR MASS OF 44 GRAMS

LETS TRY

MOLAR MASS OF C

MOLAR MASS OF Fe

MOLAR MASS OF H2O

MOLE-MASS CONVERSIONS

TO FIND HOW MANY MOLES ARE IN A CERTAIN # OF GRAMS OF AN ELEMENT OR MOLECULE:

Y GRAMS x 1/z grams= x moles

Example

WE HAVE 55 GRAMS OF CO2

HOW DO WE FIND HOW MANY MOLES WE HAVE?

55 X 1 mol/44 g

1 mol/44 g=.02272727 mol

.02272727mol X 55 g = 1.25 mol CO2

BELLWORK

WHAT IS A MOLE?WHY DO WE USE MOLES?HOW MUCH IS ONE MOLE OF BARIUM IN GRAMS?CARBON DIOXIDE?WATER?

REVIEW:

A mole is just a number, nothing else

That number is 6.02 x 1023

We use this number to make otherwise incredibly small numbers (such as the mass of an atom) easy to use in chemical formulas

Review

There are two conversions we will do with moles:

Find how many moles there are in a certain number of grams (mass) an element or moleculeFind the the number of grams (mass) there are in a certain number of moles of an element or molecule

Two conversion factors

a) Grams b)1 Mol

1 Mol or Grams

How many grams, use conversion factor a

How many moles, use conversion factor b

Practice

Suppose we have 55 grams of CO2

How do we find how many moles of CO2 there are in 55 grams of CO2?

Step 1: find the molar mass of CO2

44 grams/mole

Step 2: set up the equation55 grams CO2 x 1 mol CO2 = ? mol CO2

44 grams CO2

Step 3: Solve the equation55 grams CO2 x 1 mol CO2 = ? mol CO2 44 grams CO2

1/44= .0227

55 x .0227 = 1.25

There are 1.25 moles of CO2 in 55 grams of CO2

Flip it!

Now, suppose we have 2.4 moles of sulfur.

How do we find how many grams there are in 2.4 moles of sulfur?

Step 1: Find the molar mass of sulfur.32.07 grams/mole

Step 2: Set up the equation:

2.4 moles x 32.07 grams S = ? Grams S

1 mol S

Step 3: solve the equation:2.4 x 32.07 = 77 grams sulfer

There are 77 grams of sulfur in 2.4 moles of sulfur

Get into the groups I assigned the other day.

Work on the worksheet together.

Do as many problems as you can.

Types of Chemical Reactions

http://www.youtube.com/watch?v=i-HHvx1VC_8

http://www.youtube.com/watch?v=nsEkKIiOz7Q

7.2 Types of Chemical Reactions

Just like states of matter, you can classify chemical reactions based on reactants and products.

Types:SynthesisDecompositionSingle-replacementDouble replacementCombustion

Flip Book Time!

Make a flip book for the different types of chemical reactions.

You should have 6 flaps, 1 title and 5 different reactions

Each flap should have:The type of reaction (Synthesis)

Definition/explanation

Real example (Sodium reacts with chlorine to produce sodium chloride)

Chemical example (A + B -> AB)

Finished?

Complete your workbook up to section 7.2

We will have an open note quiz over balancing chemical equations and mole conversions tomorrow.

Synthesis

Reaction where two or more substances react to form a single substance.

A + B -> AB

Reactants may be elements or compounds

Examples: sodium reacts with chlorine to make sodium chloride

Chemical Example: 2NA + Cl2 -> 2NaCl

Synthesis

Synthesis

DECOMPOSITIONA COMPOUND BREAKS DOWN INTO TWO OR MORE SIMPLER SUBSTANCES

AB -> A + B

REACTANTS MUST BE COMPOUNDS

PRODUCTS MAY BE COMPOUNDS OR ELEMENTS

EXAMPLE: WATER DECOMPOSES INTO HYDROGEN AND OXYGEN GAS WHEN ELECTRICITY IS PASSED THROUGH IT

2H2O -> 2H2 + O2

Decomposition

Decomposition

SINGLE REPLACEMENTREACTION WHERE ONE ELEMENT TAKES THE PLACE OF ANOTHER IN A COMPOUND

A + BC -> B + AC

EXAMPLE: COPPER WIRE IS DIPPED INTO SILVER NITRATE AND WATER. COPPER REPLACES THE SILVER IN SILVER NITRATE TO FORM COPPER (II) NITRATE.

Cu + 2 AgNO3 -> 2Ag + Cu(NO3)2

Double ReplacementTwo different compounds exchange positive ions and form two new compounds

AB + CD -> AD +CB

Example: Lead (II) nitrate and potassium iodide (colorless). Yellow precipitate forms

Pb(NO3)2 + 2KI -> PbI2 + 2KNO3

Combustion

A substance reacts rapidly with oxygen, often producing heat and light

Example: burning of natural gas

CH4 + 2O2 -> CO2 + 2H2O

Oxidation - Reduction

A reaction in which electrons are transferred from one reactant to another is a oxidation-reduction reaction

Oxidation and reduction always occur together

It can involve full transfers, or partially charged elements.

Oxidation

Any process in which an element loses electrons during a chemical reaction is oxidation

Reduction

The process in which an element gains electrons during a chemical reaction is called reduction.

Reactants are said to be reduced if they gain electrons

Bellwork

What is all involved in a chemical equation?

Reactants

Products

Energy

7.3 Energy Changes in Reactions

Key concepts:What happens to chemical bonds during a chemical reaction?What happens to energy during a chemical reaction?

Example

Lighting a gas grill, this is a chemical reaction

What are the reactants?

Propane and oxygen

What are the products?

Carbon dioxide and water

What else is there?

Heat!

Where does the heat come from?

Chemical energy is stored in the chemical bonds of a substance.

Chemical reactions involve the breaking of chemical bonds in the reactants and the formation of chemical bonds in the products

Combustion of Propane

*During the combustion of propane the bonds in propane and oxygen molecules are broken, while the bonds in carbon dioxide and water molecules are formed.

Breaking bonds

8 C-H, 5 O=O 6 C=O 8 O-H

2 C-C

Breaking bonds

Breaking bonds requires energy

This is why a propane grill must have an igniter, to provide energy to get the reaction started

Forming Bonds

Once the bonds of the reactants are ignited, the bonds are broken, a chemical reaction occurs, and products come together.

The products created from a chemical reaction are bonded together

Energy is released when these bonds are created.

This energy is heat and light.

Question

Does breaking bonds require energy or release energy?

It requires energy.

ReviewIn an Endothermic reaction (such as freezing) energy is released into the surroundings

In an Exothermic reaction (such as melting) energy is absorbed from the surroundings.

Exothermic/Endothermic ReactionsDuring a chemical reaction, energy is either released or absorbed

A chemical reaction that releases energy into its surroundings is an exothermic reaction

Exothermic

Exothermic reactions release more energy than is required to break the bonds of the reactants

Combustion is an example of an exothermic reaction

Exothermic

In any reaction, the chemical energy reaches a peak before the reactants change into products.

For example, at room temperature the propane molecules and oxygen molecules will not collide with enough energy to cause a reaction

They need the help/heat/energy of a spark to begin the reaction

Exothermic

In your book on pg 208, figure A shows a graph that describes the reaction process.

The equation is written

A+B-> AB J

Endothermic ReactionsA chemical reaction that absorbs energy from its surroundings is an endothermic reaction

In an endothermic reaction, more energy is required to break the bonds in the reactants than is released by the formation of the products.

EndothermicLook at 208, figure B

You can see the difference between the reactants and products

Decomposition is a type of endothermic reaction

The equation is written

A + B J -> AB

Conservation of Energy

The total amount of energy before and after the reaction is the same.

Complete 7.3 in your workbook for homework or in class if we have time