1. Chapter 7 Chemical Reactions

2. Chemical Reactions Chemical reaction (chem. rxn)
Chemical change.
Substances change into other substances.
Key words: Produces, Yields, Forms
Review Question: How is a chemical reaction different from a phase change (melting, freezing, etc)?
Answer: In a phase change, no new substances are formed.

3. Reactants and Products
Enter into rxn (left side of arrow)
“Ingredients”
Products
Produced by rxn (right side of arrow)
Reactants  Products
In a chem rxn:
existing bonds are broken
atoms are rearranged
new bonds are formed

4. A+B→C+D REACTANTS PRODUCTS Chemical Equations Chemical equation
Combination of symbols and numbers
Represents a chem rxn
In a chem equation:
“+” means “and”
“” means “yields” or “produces”
A+B→C+D
REACTANTS
PRODUCTS

5. Examples Br2(aq) + 2NaI(aq) → 2NaBr(aq) + I2(aq)
Bromine and sodium iodide produce sodium bromide and iodine.
2H2(g) + O2(g) → 2H2O(l)
Hydrogen and oxygen yield water.

6. Phase Symbols (s) = solid (l) = liquid (g) = gas
(aq) = aqueous (dissolved in H2O)
Phase symbols are not always necessary, but are sometimes included to provide extra information about the rxn.

7. Balancing Chemical Equations
Law of Conservation of Mass
In a chemical reaction, matter is not created or destroyed.
Atoms can be rearranged.
The amount of each atom that begins a rxn must equal the amount of each atom that ends a rxn 4H 2O 4H 2O

8. Balancing Chemical Equations
If the reaction starts with 2 atoms of H & 2 atoms of O, then it must end with 2 atoms of H & 2 atoms of O
Coefficients
Numbers placed in front of symbols & formulas in chem. equation
Coefficient - # of units of each substance
Pb(NO3)2(aq)+2KI(aq)→PbI2(s)+2KNO3(aq)

9. Structure of an Equation:
yields

10. Describing Coefficients
individual atom = “atom”
2Mg = 2 atoms of magnesium
covalent substance = “molecule”
3CO2 = 3 molecules of carbon dioxide
ionic substance = “unit”
4MgO = 4 units of magnesium oxide

11. Counting Atoms in an Equation
If no subscript present it is assumed to be 1 atom
If elements in brackets or parenthesis, treat same as in math.
Coefficients multiply the entire molecule atoms
You must add all reactant molecules together & compare w/ all molecules in the products
Ca3(PO4)2
Ca=3
P=2
O=8
2Ca3(PO4)2
Ca=6
P=4
O=16
CaCl2
Ca=1
Cl=2
It’s best to list the # of atoms under the molecules as we are doing in these examples

12. Review Counting Atoms What atoms/ions make up 1 unit of Ba3(PO4)2?
3 barium ions
2 phosphorus atoms
8 oxygen atoms
What atoms make up 1 molecule of C6H12O6?
6 carbon atoms
12 hydrogen atoms
6 oxygen atoms

13. Balancing Chemical Equations
Steps:
1) List all elements on the left side.
2) List all elements on the right side.
3) Count the number of atoms of each element on both sides.
4) Compare the numbers.
If the numbers are equal, the equation is already balanced.
If the numbers are not the same, balance using coefficients.
5) A coefficient multiplies an entire formula and all the atoms in it.
Must re-count atoms every time you add or change a coefficient.

14. Balancing Equations, Ex.1
H2(g) + O2(g)  H2O(l)
Hydrogen (gas) + oxygen (gas)  water (liquid)
The phase symbols don’t affect the balancing.
Atoms on left:
H = 2
O = 2
Atoms on right: H O
= 2
= 1
This equation is not balanced!

15. Balancing Equations
2 oxygen atoms on left; 1 on right.
Multiply H2O by 2:
H2(g) + O2(g)  2H2O(l)
Recount:
Left side
H = 2
O = 2
Right side
H = 4
O = 2
Remember: Coefficients multiply every atom in a formula.

16. Balancing Equations The equation is now balanced! Right side H = 4
Now there are 4 hydrogen atoms on the right, but only two on the left.
Multiply H2 by 2.
2H2(g) + O2(g)  2H2O(l)
Recount:
Left side
H = 4
O = 2
Right side
H = 4
O = 2
The equation is now balanced!

17. 2H2(g) + O2(g)  2H2O(l)

18. Check for Understanding
How many oxygen atoms are represented in:
H2O
Answer: 1 oxygen atom
NaNO3
Answer: 3 oxygen atoms
Al2(SO4)3
Answer: 12 oxygen atoms
3KNO3
Answer: 9 oxygen atoms
4CO2 + 5H2O
Answer: 13 oxygen atoms

19. Check for Understanding
Balance the following chemical equations:
Na + Cl2  NaCl
KClO3  KCl + O2
C3H8 + O2  CO2 + H2O
BaCl2 + K3PO4  Ba3(PO4)2 + KCl
Al + CuCl2  AlCl3 + Cu

20. Na + Cl2  NaCl

21. KClO3  KCl + O2

22. C3H8 + O2  CO2 + H2O

23. BaCl2 + K3PO4  Ba3(PO4)2 + KCl

24. Al + CuCl2  AlCl3 + Cu

25. Check for Understanding
Balance the following chemical equations:
Na + Cl2  NaCl
Answer: 2Na + Cl2  2NaCl
KClO3  KCl + O2
Answer: 2KClO3  2KCl + 3O2
C3H8 + O2  CO2 + H2O
Answer: C3H8 + 5O2  3CO2 + 4H2O
BaCl2 + K3PO4  Ba3(PO4)2 + KCl
Answer: 3BaCl2 + 2K3PO4  Ba3(PO4)2 + 6KCl
Al + CuCl2  AlCl3 + Cu
Answer: 2Al + 3CuCl2  2AlCl3 + 3Cu

26. Rates of Change *Massive, bulky molecules react slower
To increase the rate (speed) of a reaction (in most cases):
Increase temperature
Increase surface area
Concentrated solutions
Increase pressure
*Massive, bulky molecules react slower

27. https://www.youtube.com/watch?v=2Juem0lc ifE

28. Balancing equations activity!