1. Chemical Reactions And Equations

2. Chemical Change - objectives
1. List the common indicators of a chemical change and use them to identify when a chemical reaction has probably occurred. They are: color change, change of state (gas bubbles, precipitate, etc), temperature change, pressure change in a closed container, and change in odor. Note the emphasis on change

3. Indications of a Chemical Reaction
There are some easily observed changes that usually indicate a chemical reaction
Color change
Change of state (gas bubbles, precipitate, etc),
Temperature change
Pressure change in a closed container
Change in odor

4. 1. Color change

5. 2. Change of state: gas bubbles
Release of gas bubbles when two substances mixed is often evidence of chemical reaction
Antacid in water

6. 2. Change of state: formation of precipitate
Many reactions happen in solution
If solid appears after two solutions mixed, a reaction has probably happened
Example: Pb(NO3)2 + KI
A bright yellow solid PbI2 appears as the proof that the reaction has happened

7. 2. Change of state: formation of precipitate
The solid that appears is called precipitate
Precipitate  a solid that is made as a result of a chemical reaction in solution and that separates from the solution
PbI2 precipitate

8. 3. Temperature change
Reaction between vinegar and baking soda is accompanied by drop of temperature
Reaction between glycerin C3H5(OH)3 and potassium permanganate KMnO4 produces extensive heat (burning)
Note: Heat by itself NOT necessarily sign of chemical change

9. 4. Pressure change in a closed container
If reaction is going in the gas phase and new gases are formed, the pressure in the closed container will change

10. 5. Odor change
Odor Changes It only takes one experience with a rotten egg to learn that they smell different that fresh eggs.  When eggs and food spoil, they undergo a chemical change.  The change in odor is a clue to the chemical change

11. Understanding Check Is it a chemical or physical change?
“Dry ice”, solid carbon dioxide, is sublimated into gaseous CO2 at room temperature
Salt is dissolved in water
Iron rusts in a damp environment
Gasoline burns in the presence of oxygen
Liquid hydrogen peroxide (H2O2) decomposes to water and oxygen
Rotting of eggs

12. Chemical Equations - objective
2. Given a word equation, write a chemical equation (this requires good formula writing skills)

13. Chemical Reactions and Equations
Chemical reaction is a process by which one or more substances are changed into one or more different substances
Chemical equations represent chemical reaction with symbols and formulas. Chemical equations show the identities, physical phases and relative amounts of the reactants and products in a chemical reaction
Reactants (state) → Products (state)
Reactants are compounds being brought together to react
Products are the new compounds that are made in the reaction

14. Physical Phase Solid (s) Liquid (l) Gas (g)
Aqueous solution (aq) – solution in water
Examples:
Na (s); O2 (g); HCl (l); Ca(NO3)2 (aq)

15. Word Equations
First step in writing chemical equation is to identify facts to be represented
Word equation  “equation” in which reactants and products are represented by words
Does not give quantities of reactants/products
Reactants
==>
Products
chemicals that react
chemicals that are produced
hydrogen + oxygen
water
hydrogen peroxide
water + oxygen
magnesium + oxygen
magnesium oxide
sodium chloride
sodium + chlorine

16. Formula Equations Mg (s) + O2 (g)  MgO (s) Word equation:
hydrogen (gas) + oxygen (gas)  water (liquid)
Formula equation: H2 (g) + O2 (g)  H2O (l)
magnesium (solid) + oxygen (gas)  magnesium oxide (solid)
Mg (s) + O2 (g)  MgO (s)

17. Characteristics of Chemical Equations
The equation must contain correct formulas for the reactants and products.
The Law of Conservation of Mass must be satisfied: mass is neither created nor destroyed in any ordinary chemical reaction.
The quantity of substances produced (products) by a chemical reaction is always equal to the quantity of the reacting substances (reactants).
The “quantity” here means mass or the number of atoms for each element participating in reaction

18. Balancing a Chemical Equation
Write a formula equation
H2O(l)  H2(g) + O2(g)
Create a table showing the number of atoms for each element: This equation is not balanced!
The number of atoms
Reactants
Products H 2 O 1

19. Balancing Equations
Correct the number of O by placing coefficient 2 in front of H2O
2H2O(l)  H2(g) + O2(g)
Correct the table:
Oxygen is now balance with 2 on left and 2 on right. H is not!
Now balance hydrogen – 4 on left, 2 on right. Add coefficient 2 to H2
2H2O(l)  2H2(g) + O2(g) balanced!
The number of atoms
Reactants
Products H 4 2 O
The number of atoms
Reactants
Products H 4 O 2

20. Guidelines for Balancing Equations
Create a table of elements for reactant and product side of the equation
Balance the different types of atoms one at a time.
First balance the atoms of elements that are combined and that appear only once on each side of the equation.
Balance polyatomic ions that appear on both sides as single units.
Balance lone atoms, especially H and O atoms (or any), last.

21. DO NOT!!!.....
DO NOT WRITE INCORRECT FORMULAS, THIS WILL MESS UP YOUR BALANCING
DO NOT CHANGE SUBSCRIPTS IN FORMULAS TO BALANCE THE EQUATION!
H2O(l)  H2(g) + O2(g) H2O(l)  H2(g) + O(g)

22. DO!.....
When you think you have balanced the equation, COUNT THE NUMBERS OF EACH TYPE OF ATOM ON EITHER SIDE OF THE EQUATION
2H2O(l)  2H2(g) + O2(g)
Reactant side
Product side H 4 O 2

23. Sample Problem 1
Write balanced chemical equations for the following reaction: Solid sodium combines with chlorine gas to produce solid sodium chloride.
Word reaction:
Sodium (solid) + chlorine (gas)  sodium chloride (s)
Formula reaction:
Na(s) + Cl2(g) → NaCl(s)
Balanced reaction
2Na(s) + Cl2(g) → 2NaCl(s)

24. Balance Reaction Na(s) + Cl2(g) → NaCl(s) Count atoms:
Na (in compound): 1 on the left, 1 on the right - balanced
Cl (lone element): 2 on the left, 1 on the right – not balanced!
Adjust Cl: Na(s) + Cl2(g) → 2 NaCl(s)
Na: 1 on the left, 2 on the right – not balanced!
Cl: 2 on the left, 2 on the right – balanced
Adjust Na: 2 Na(s) + Cl2(g) → 2 NaCl(s)
Count atoms – all balanced!
Na: 2 on the left, 2 on the right
Cl: 2 on the left, 2 on the right

25. Sample Problem 2
Write word, formula, and balanced chemical equations for magnesium and hydrochloric acid (HCl) react to produce magnesium chloride and hydrogen.
Word equation: Magnesium (solid) + hydrochloric acid (liquid)  magnesium chloride (solid) + hydrogen (gas)
Formula equation: Mg (s) + HCl (l)  MgCl2 (s) + H2 (g)
Count atoms:
Mg (in compound): 1 on the left, 1 on the right
Cl (in compound): 1 on the left, 2 on the right
H (lone element): 1 on the left, 2 on the right
Adjust HCl: Mg + 2HCl  MgCl2 + H2
Recount the atoms – all balanced!

26. Sample Problem 3
Word equation: Methane (gas) + oxygen (gas)  carbon dioxide (gas) + water (gas)
Formula equation:
CH4(g) + O2(g)  CO2(g) + H2O(g)
Reactants
Products
Balanced?
1. C (in compounds in both sides) 1
Yes
2. H (in compound) 4 2 No
3. O (in compound and a lone element) 3

27. Sample Problem 3 (cont.) Adjust H: CH4(g) + O2(g)  CO2(g) + 2H2O(g)
Adjust O: CH4(g) + 2O2(g)  CO2(g) + 2H2O(g)
Reactants
Products
Balanced?
1. C (in compounds in both sides) 1
Yes
2. H (in compound) 4
3. O (in compound and a lone element) 2 No
Reactants
Products
Balanced?
1. C (in compounds in both sides) 1
Yes
2. H (in compound) 4
3. O (in compound and a lone element)

28. Practice Problems
Fe + Cl2 = FeCl3 2Fe + 3Cl2 = 2FeCl3 Zn + HCl  ZnCl2 + H2 Zn + 2HCl  ZnCl2 + H2 Al + O2  Al2O3 4Al + 3O2  2Al2O3