posted by Julie .
Hi Dr. Bob,
I am quite stumped on how to figure out when chemical equations have a REACTION or NO REACTION. I have read the material in my textbook and even took out a chemistry books for the "Utterly Confused" to try to figure this out.
Example: Zn(s) + H3PO4(aq)--> Zn3(PO4)2(s) + H2. This has a reaction, but exactly how do I know that it has a reaction. Are there any tips that you can provide to easily be able to distinguish?
I have just what you need.
There are four rules that will help although it won' cover everything.
There are oxidation-reduction reactions that are not covered here although some are included here. Those are are another issue but will be covered when you cover redox equations.
Reactions will proceed from left to right (another way of saying a reaction will go to completion) if one of the following is true.
a. a gas is formed. Example:
Zn + 2HCl ==> ZnCl2 + H2. H2 is the gas.
b. a ppt is formed. Example:
AgNO3(a) + NaCl(aq) ==> AgCl(s) + NaNO3(aq)
That means you need to know what's soluble in water and what isn't. That's almost impossible except with many years of experience. However, here is a simplified set of rules. Note that AgCl is insoluble according to rule #3.
Note, also, that just below that set of rules is a list of common gases other than H2.
3. Formation of of a slightly ionized substance. These are weak acids or weak bases (weak electrolytes) with the most common one you run into day to day is H2O. Example:
NaOH + HCl ==> NaCl + H2O
(You may hear from time to time that an acid + a base yields a salt + water. This neutralization reaction is such an example).
These three rules will handle probably 99% of what you will run into in the first year of chemistry. Some redox reactions are included in this group but not all of them. For example,
2CuO + C + heat ==> CO2(g) + 2Cu This redox equation still follws the rule of a gas being formed.
and Cl2 + 2KI ==> I2(s) + 2KCl is another one (I2 is a solid) but I don't think that is covered in the simplified rules.