1. UNIT: Chemical Reactions
TOPIC: Properties of Reactions
Objectives: Lesson 1 of 3
To learn how we recognize a chemical reaction has took place
To learn what a chemical reactions is
To learn how we balance chemical equations
To learn the symbols for each physical state in a chemical reaction

2. Quickwrite In 1-2 sentences answer one of the questions below:
What sort of clues do think indicate that a chemical reaction is happening in your car engine????
What sort of clues indicate a chemical reaction when you drop a piece of alka selzter in water????
What are some by products produced when your body breaks down sugar, C6H12O6????? What are some clues that this reaction is occuring in your body????? Hint: think of temperature!!!!

3. Evidence for a Chemical Reaction
How do you know a chemical reaction has occurred?
Chemists look for visual clues such a color or heat change, a solid forms, bubbles are produced, a flame occurs
However, reactions are not always visible
Sometimes the only signal that a reaction is occurring is a change in temperature as heat is produced

4. What are some clues that a chemical reaction has occurred?
A color change
A solid forms
Bubbles form
A heat change
A flame is produced

5. Chemical Reactions CH4 O2 CO2 H2O C H + O O +
Chemists have learned that a chemical change always involves a rearrangement of the ways in which atoms are grouped and combine with one another
For example, consider the reaction that occurs with your laboratory bunsen burners
In this reaction, methane (CH4) reacts with oxygen (O2) in the air
As you know, a flame is produce and heat is released
As the atoms rearrange themselves, two new chemicals are produced carbon dioxide (CO2), and water (H2O)
CH4 O2
CO2
H2O C H + O O +

6. Chemical Reactions CH4 O2 CO2 H2O + O
Such a change is called a chemical reaction
Notice, the atoms were rearranged, Oxygen combined with Carbon and Hydrogen to produce 2 new chemicals: Water & Carbon Dioxide
Chemical reactions obey the Law of Conservation of Matter which states: Atoms cannot be destroyed, they can only be rearranged
CH4 O2
CO2
H2O + O
Methane (CH4)
Water(H2O)
Carbon Dioxide (CO2)
Oxygen(O2)

7. What is a Chemical Reaction?
A chemical reaction is the change that occurs when atoms rearrange themselves and form new compounds
Chemical reactions obey the first LAW of conservation of MASS or MATTER which states: Atoms and matter cannot be created or destroyed, they can only be rearranged

8. Chemical Equations
We represent chemical reactions by writing a chemical equation in which the chemicals present before the reaction are on the left of the arrow, and the chemicals formed by the reaction are on the right of the arrow
The arrow indicates the direction of the change CH O CO H2O

9. Chemical Equations In the reaction of methane with oxygen, we have:
CH O CO H2O
The chemicals present on the left side of the equation are known as the reactants
The chemicals present on the right side of the equation are known as the products
Reactants
Products

10. What is a Chemical Equation?
An equation that uses the formulas of chemicals compounds and elements to show the reactants and products in a chemical change
Chemicals present before the reaction are on the left of the arrow (reactants), and the chemicals formed by the reaction are on the right of the arrow (products)
Ex: CH O CO H2O

11. Balancing Chemical Reactions
It is important to realize that in a chemical reaction, atoms are neither created nor destroyed
In other words, their must be the same number of each type of atom on the product side as on the reactant side of the arrow

12. Balancing Chemical Reactions
Remember, atoms cannot be created or destroyed in chemical reactions, they can only be rearranged!
The total number of atoms must be the same on both sides of the arrow
Balancing Chemical Reactions
Notice, there are 2 Oxygen atoms on left side and 3 Oxygen atoms on the right side; also there are 4 Hydrogen on the left side and 2 Hydrogen on the right side
We can fix this by putting in 1 more Oxygen molecule on the left (reactant) side and 1 more Water molecule on the right (product) side
The table now has equal number of atoms on both sides of the equation
In other words, the reaction and table below show that 1 Oxygen atom is created and 2 Hydrogen atoms are destroyed
Therefore, the reaction cannot happen this way!
By counting the number of atoms, you can see that the chemical equation is balanced and shows the actual numbers of molecules involved in this reaction
We group molecules together by putting coefficients in front of them:
The reaction below is not balanced
We can see it is not balanced by taking the reactants & products apart:
CH4
CO2
H2O O2 + O2 + + + 2
H2O + + 2 H
H C H
O O
O O
O C O
H O H
H O H
Left
Reactant Side
Right
Product Side
C =
O =
H = 7 6 9 1 1 4 2 3 4 4 4 2
Total Number of Atoms

13. Balancing Chemical Reactions
Notice, there is 1 Oxygen atoms on left side and 2 Oxygen atoms on the right side; therefore the chemical equation is NOT balanced
We can fix this by putting in 1 more Water molecule on the left(reactant) side and 1 more Hydrogen molecule on the right(product) side
The table now has equal number of atoms on both sides of the equation
By counting the number of atoms, you can see that the chemical equation is balanced and shows the actual numbers of atoms and molecules involved in this reaction
In other words, the reaction and table below show that 1 Oxygen atom is created
Therefore, the reaction cannot happen this way!
We group molecules together by putting coefficients in front of them:
Remember, atoms cannot be created or destroyed in chemical reactions, they can only be rearranged!
The total number of atoms must be the same on both sides of the arrow
The reaction below is not balanced
We can see it is NOT balanced by taking the reactants & products apart:
H2O
H2O O2 H2 H2 + 2 2 + +
H O H
H H
H H
O O
H O H 3
Left
Reactant Side
Right
Product Side
O =
H = 6 1 2 4 2 4 2 2 4
Total Number of Atoms

14. Subscripts and Coefficients
When balancing chemical equations, it is important to note that the subscripts can never change
For example, in the equation below, the subscript of 4 in methane cannot be changed, neither can the subscripts 2 in Oxygen, Carbon dioxide and Water
CH O CO H2O
Only coefficients can be added by putting whole numbers in front of the compounds when balancing chemical reactions 2 2

15. How Do we Balance Chemical Equations?
By placing coefficients in front of each molecule or compound
Subscripts cannot be changed!
Ex: CH4 + 2O2  CO2 + 2H2O

16. Balancing Chemical Equations
Remember: When balancing chemical equations, it is important to note that the subscripts can never change!!!!
___O ___O3
___ Mg + ___ O ___ MgO 3 2 2 2

17. Determine how many atoms of each element are in each compound below:
Practice:
Determine how many atoms of each element are in each compound below:
___ Na + ___ Cl ___ NaCl
___ Fe + ___ O ___ Fe2O3
___ Al + ___ O ___ Al2O3
___ Na + ___ F ___ NaF 2 2 4 3 2 4 3 2 2 2

18. Physical States
In a reaction, we often use symbols to represent the physical state of the reactants and products
Look at the reaction below when sodium metal reacts with water: Na(s) + H2O(l) H2(g) NaOH(aq)
Symbol
State
The symbol (s) tells us that Sodium (Na) is in the solid state
The symbol (l) tells us the water (H2O) is in the liquid state
The symbol (g) tells us the hydrogen (H2) is in the gaseous state
Finally, the symbol (aq) tells us that sodium hydroxide (NaOH) is aqueous or dissolved in water
(s)
solid
(l)
liquid
(g)
gas
(aq)
aqueous

19. What do the symbols represent in a chemical equation?
The symbols represent the physical state of the reactants and products
Na(s) + H2O(l) H2(g) NaOH(aq)
Symbol
State
or dissolved in water
(s)
solid
(l)
liquid
(g)
gas
(aq)
aqueous

20. Summary (you can always write your own summary)
Explain the Law of Conservation of Mass in your own words and apply it to chemical reactions:
What is the difference between the reactants and products in a chemical reaction?
Describe the process of balancing chemical reactions, be sure to explain how you count atoms on both sides of the equation
What can you not change when balancing chemical reactions?
Is the reaction below balanced? Explain why or why not:
K + Cl2  KCl