LECTURE                             CHEMICAL REACTIONS

In this lecture you will learn:

MOST ELEMENTS ARE NOT FOUND PURE IN NATURE
So when two elements react to form a molecule or compound, they do it by certain "rules".  The nucleus is not involved, the inner electrons are not involved, and they do not merge into one larger atom.  Each element involved in the reaction maintain their distinct atomic structure. 
 
Only the outermost electrons are involved in chemical reactions.   Chemistry is all about the movement of electrons.  And how the electrons will movement is determined by the element and the number of electrons in the outer shell. 
 

The following examples are from:
http://www.chemtutor.com/react.htm#examp
How to remove tarnish from silver - the easy and "green" method.  Click here

"Using a heat safe plate or dish, place a piece of aluminum foil on the bottom. For larger pieces, use a Pyrex bowl.

Place the piece to be cleaned on top of the foil. Cover the piece liberally with baking soda. 

Then pour very hot water (near boiling is okay) to cover the piece. It will bubble a bit and smell not so pleasant. It is important that the silver piece is completely covered with water and that it comes in contact with the aluminum foil." 

A silver spoon tarnishes. The silver reacts with sulfur in the air to make silver sulfide, the black material we call tarnish. 

     2 Ag + S =  Ag2S (AKA tarnish)

3 Ag2S           +   6 Al    -->   6 Ag +   3  Al2S
silver sulfide   aluminum =    silver   aluminum sulfide

The baking soda is the "solution" that provides the path for the electrons that the aluminum gives up to combine with the silver.  The silver and aluminum must be in contact also for the reaction to go and heating the water speeds the reaction. 

An iron bar rusts. The iron reacts with oxygen in the air to make rust. 

4 Fe + 3 O2 --->  2 Fe2O3


 
CHEMICAL REACTIONS ARE GROUPED INTO "TYPES" 

 
 1. OXIDATION - REDUCTION REACTIONS aka REDOX
In Redox reactions:
1. one element GRABS electron(s)
2. the other GIVES UP electron(s)

-- The element that grabs electrons (often oxygen) is said to be a good OXIDIZER. 

OVERALL Reaction: 2 Ag + S =  Ag2S
Silver gives up electrons to Sulfur
Ag (elemental silver or (s))  -> Ag+ (ionic silver (aq)) - an electron(e-)
S (elemental sulfur(s))  -> S2-  (ionic sulfur (aq)) + 2 e- 

s= solid   aq = aqueous

RULES: 
1. There must always be a pair of reacting compounds. 
2. One gives, one gets. 

The electrons will flow from those with weakest hold on the electrons to those with the strongest hold.  There is a list compiled by experimentation to determine which elements gives to which and which takes from which. 

At least for metals when a metal has all its electrons it is a solid.  When it loses its electrons it goes into water solution as an ION.

There must be some kind of "salt bridge" which is nothing more than water with some "salts"  connecting the two chambers containing the elements.  Everything starts with a 0 zero charge, but ions wont move freely in distilled water, they need an established ionic or salty water. 

Redox reactions will not occur just by putting two pieces of metal on top of each other. There must be moisture and salts of some kind. This is the reason that salting streets in winter results in cars rusting so badly.  In southern states that dont need to salt cars do not readily rust. 

Classic example of a redox reaction =  a battery

When a zinc (Zn) bar is inserted in a salt solution,  Zn tends to loose electrons according to the reaction, 
Zn = Zn2+ + 2 e-.

Copper (Cu) is inserted in a copper salt solution,  copper tends to loose electrons according to the reaction, 
Cu = Cu2+ + 2 e-.

However,  Zn wants to loose electrons MORE than copper, so when the two cells are connected by a salt bridge and a wire, an electrical current occurs  as electrons flow from the zinc chamber to where the copper ions (Cu2+) are and copper by gaining electrons become copper metal. This is a galvanic cell or battery.
 http://www.science.uwaterloo.ca/~cchieh/cact/c123/battery.html

  Electrolysis is a redox reaction  driven in reverse by a source of electrons and input of energy.

for example:  it can be used to separate compounds like oxygen and hydrogen from water.  The best electrodes for this are platinum!
 

 


http://library.thinkquest.org/3659/electrochem/electrolysis.html
Another use for electrolysis is to gold plate something.

What is vermeil?? 


 
2. PRECIPITATION REACTIONS
In precipitation reactions:
1. two soluble compounds are mixed together
2. the compounds in solution SWITCH partners to form new more energetically stable arrangements in which on of the products is NOT soluble.

The insoluble compound precipitates out of solution. The most usual reason is the compound is too large to remain in solution. 

Classic example: 

- Sodium carbonate (Na2CO3) when titrated into calcium chloride (CaCl2) forms calcium carbonate which precipitates out of solution... this is the "chalk" as in the White Cliffs of Dover.

 Na2CO3 (aq) + CaCl2 (aq) --> CaCO3 (solid)  + NaCl (aq)

aq = aqueous

the precipitation of silver nitrate click here

http://www.iun.edu/~cpanhd/C101webnotes/chemical%20reactions/precipitation.html


 
3. ACID-BASE REACTIONS
In acid-base reactions: 
1. An acid gives up H+ ions 
2. A base gives up OH- ions
3. The two ions neutralize each other to make water

     H+ (aqueous) + OH- (aqueous) <=> H2O

hydrochloric acid + sodium hydroxide = water and sodium chloride
HCl (aq) + NaOH (aq) <=>  H2O (l) + NaCl (aq)
 

... baking soda, NaHCO3 (weak base) and vinegar (organic weak acid with H+ ions to contribute)  react and release CO2 (gas) and H2O, which is the leavening agent for biscuits.. 
 

baking POWDER, however is soda + tartaric acid which donates the H+ ions so no vinegar is needed to make the biscuits rise!

Classic example is baking soda and vinegar

"Baking soda, a pure chemical called sodium bicarbonate, has the chemical formula:  NaHCO3

When dissolved in water baking soda separates into sodium (Na+) and bicarbonate ions (HCO3- ): NaHCO3 ---> Na+(aq) + HCO3-(aq)

Vinegar, a weak (5%) solution of acetic acid in water, partially dissociates into hydrogen ( H+) and acetate ions (CH3COO-): 

CH3COOH <--> H+(aq) + CH3COO-(aq)

When the two ingredients are mixed, hydrogen ions ( H+) from the vinegar react with the bicarbonate ions (HCO3- ) from the baking soda to form a new chemical called carbonic acid (H2CO3). 

H+ + HCO3- ---> H2CO3

The carbonic acid thus formed then immediately decomposes into carbon dioxide gas (CO2) and water (H2O).

H2CO3 ---> H2O + CO2

It's this carbon dioxide gas that you see bubbling and foaming as soon as you mix baking soda and vinegar together.

The overall reaction is  written as follows:

NaHCO3 (aq) + CH3COOH (aq) ---> CO2 (g) + H2O (l) + CH3COONa (aq)"
http://www.apple-cider-vinegar-benefits.com/baking-soda-and-vinegar.html
 

acid base youtube
http://www.iun.edu/~cpanhd/C101webnotes/chemical%20reactions/acidbase.html

 
4. POLYMERIZATION REACTIONS
In polymerization reactions:
1. Two organic molecules are joined by a covalent bond
2. A water is removed

-- purely organic - formation of loooooonnng chains of carbon based molecules

- in biological systems, the addition of carbons is by "dehydration synthesis" or the removal of water.

- in forming plastics, there can be other compounds given off, like HCl (hydrochloric acid), etc.

This is very close to an acid base type reaction, but it is ORGANIC. 



REVIEW QUESTIONS:

Fill in the table

  
main type of reaction  EXAMPLE 
REDOX    
PRECIPITATION    
ACID-BASE    
POLYMERIZATION    
Na --> Na+ and e- ... Na is _______________
Cl + e-  --> Cl- ..... Cl is _______________
What does oxygen want?
What does sodium want?
Why is it called a REDOX reaction, why must there be a reduction when there is an oxidation?
What is a precipitation reaction, give an example
What is an acid-base reaction, give an example
What is a polymerization reaction