chemical reactions bellwork briefly write about a scientific observation you made recently
TRANSCRIPT
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