1. Chemical Reactions & Equations+

2. Evidence of Chemical Reactions
Release of a gas
Example: bubbles formed when magnesium and hydrochloric acid were mixed
Color change
Example: color went from clear to yellowish orange when potassium iodide was added to hydrogen peroxide
Formation of a precipitate
Example: cloudiness occurred when CO2 gas passed through limewater
Change in temperature, light, sound, smell
Example: temperature increased when hydrochloric acid and sodium carbonate were mixed

3. Endothermic or Exothermic?
CaO (s) + H2O(l) Ca(OH)2(aq) + 82 kJ
Since energy is a product, this reaction is exothermic.
This reaction will generate enough heat to fry an egg!
NH4NO3(s) + H2O(l) + energy NH4NO3(aq)
Since energy is a reactant, this reaction is endothermic.
This is the cooling reaction of a cold pack.

4. Chemical Equations Reactants Products C3H8 O2 + + CO2 H2O energy
substances before the change
C3H8 O2 + +
Products
substances after the change
CO2
H2O
energy
Propane and oxygen react to produce carbon dioxide, water, and energy. This reaction is not yet balanced.

5. Symbols in Equations + (s) (l) (g) (aq) D N.R.
“plus” or “and” indicates multiple reactants or products
“yields” or “produces” separates reactants & products, arrow points in the direction of the reaction
(s)
“solid”
(l)
(g)
indicate phase of a reactant or product
“liquid”
“gas”
(aq)
“aqueous” dissolved in water (not the same as liquid (l))
indicates reaction is reversible D
over the arrow, indicates heating is necessary
N.R.
“no reaction”

6. Balancing Equations
When balanced, an equation accounts for the conservation of:
mass
charge
energy
Balance the numbers of atoms of each element on both sides of the equation by determining the coefficients of the compounds.
DO NOT CHANGE SUBSCRIPTS!

7. An Example: Forming Water
H2 (g) + O2 (g) H2O (l)
On left: 2 H and 2 O On right: 2 H and 1 O Therefore, not balanced
We could use 1/2 O2 on the left but that is not done, we always want whole number coefficients.
Instead we double the coefficients of H2 and H2O
2H2 (g) + O2 (g) H2O (l)
There are now 4 H’s and 2 O’s on each side.

8. Rules for Balancing Equations
6H2O
coefficient
subscript
Write the formulas for all reactants and products
Don’t change formulas (subscripts), only coefficients.
Balance elements that only occur once on each side of the equation first
To determine the number of atoms of an element, multiply the coefficient by the subscript for the element.
Tips: for reactions involving oxygen, balance it last
polyatomic ions can be balanced as a unit

9. Balancing Example
Write the equation for burning ethylene (C2H4) to produce carbon dioxide and water.
C2H4 + O CO2 + H2O
C and H both appear once on each side of the equation whereas O appears in both compounds on the right hand side.
Multiply CO2 by 2 to balance C
Multiply H2O by 2 to balance H
C2H4 + O CO2 + 2H2O
There are now 6 O’s on the right so multiply O2 by 3 to finish.
C2H4 + 3O CO2 + 2H2O

10. Balancing Practice #1 Unbalanced equation: K + F2 KF Balance F:
Balance K:
2K + F KF

11. Balancing Practice #2 Unbalanced: Notice Ca is fine: Balance O:
Ca + H2O Ca(OH)2 + H2
Notice Ca is fine:
Ca + H2O Ca(OH)2 + H2
Balance O:
Ca + 2H2O Ca(OH)2 + H2
Hydrogen is already taken care of so we’re done.

12. Balancing Practice #3 Cl: N: O: H: 10 H on each side, so we’re done!
NH4Cl + Ca(OH) NH3 + H2O + CaCl2
2NH4Cl + Ca(OH) NH3 + H2O + CaCl2
Cl:
2NH4Cl + Ca(OH) NH3 + H2O + CaCl2 N:
2NH4Cl + Ca(OH) NH3 + 2H2O + CaCl2 O: H:
2NH4Cl + Ca(OH) NH3 + 2H2O + CaCl2
10 H on each side, so we’re done!

13. 5 Types of Chemical Reactions
Combustion
Synthesis
Decomposition
Single Replacement
Double Replacement
Some reactions fall into multiple categories.

14. Combustion Reactions
A substance reacts with oxygen to create products containing oxygen. Heat may be required to initiate. Exothermic.
hydrocarbon + O2 (g) CO2 (g) + H2O(g)
Hydrocarbons (C, H, O) - heat homes, cook food, and power transportation. Always get CO2(g) and H2O(g).
2Mg(s) + O2(g) MgO(s)
Metals produce metal oxides (like rust and tarnishes).

15. Practice Combustion
Write the balanced equation for the combustion of heptane gas (C7H16)
Write out the reactants and products:
C7H16(g) + O2(g)
CO2(g) + H2O(g) + heat
Balance the equation (ignore heat):
C7H16(g) + 11 O2(g)
7 CO2(g) + 8 H2O(g)

16. Synthesis Reactions
The combining of two or more substances to form a more complex substance.
general form
A + B AB
Note: A & B can be elements or compounds
2Na + Cl NaCl
sodium chloride
Examples:
2Mg + O MgO
magnesium oxide

17. Decomposition Reactions
Compound breaks down into two or more simpler substances (elements or compounds).
general form
AB A + B
NH4NO3(s) N2O(g) + 2H2O(g)
Examples:
2H2O(l) H2(g) + O2(g)
electricity

18. Single Replacement Reactions
One element takes the place of another element. Metals replace metals. Non-metals replace non-metals. Not reversible.
A + BC BA + C
general forms
A + BC AC + B
Cl2(aq) + 2KBr(aq) KCl(aq) + Br2(aq)
Example:

19. Double Replacement Reactions
When solutions of two soluble ionic compounds are mixed, an insoluble product (precipitate) forms.
general form
AB + XY AY + XB
Remember “inny outy” rule
Pb(NO3)2(aq) + 2KI(aq) PbI2(s) + 2KNO3(aq)
Example:

20. More on Double Replacement
When 2 ionic solutions are mixed there are several events that could happen:
Nothing, everything is soluble
A precipitate forms (possibly two)
A gas is formed
(like FeS(s) + H2SO4(aq) --> FeSO4(aq) + H2S(g)
A covalent molecule forms (like H+ + OH- --> H2O)
Ions that remain in solution are called spectator ions because they don’t participate in the reaction.

21. Practice - Identifying Reaction Types
Ca(s) + CuSO4(aq) CaSO4(aq) + Cu(s)
2AlCl3(l) Al(l) + 3Cl2(g)
ZnSO4(aq) + SrCl2(aq) ZnCl2(aq) + SrSO4(s)
4Na(s) + O2(g) Na2O (s)
single replacement
decomposition
double replacement
synthesis, combustion