Introduction to Reactions

Doing reactions—turning one substance into another—is the whole purpose of doing chemistry in the first place.

Under the right conditions, you can turn urine into gunpowder, cotton into explosive films, and household ingredients into a disgusting mess.

(or, you know, do something productive and make a cancer-killing drug or whatnot)

How Do You Know a Reaction Has Happened?

...just something to consider.

(if you see me sprinting out the door, a reaction has happened)

VocabularyA (x) → B (y)

Reactants/Reagents/Starting materials (all the same thing): stuff on left that goes into the reaction (A)

Products: Stuff that gets made (B)

State of matter: s,l,or g for solid, liquid, or gas. Also aq for aqueous—dissolved in water. This is NOT the same as being liquid. (x) and (y)

Catalysts: something that helps the reaction go, but isn't part of the reaction. (Z)

Z

Types of ReactionsMost reactions can be classified as one of the following types:

1. Precipitation

2. Acid-Base

3. Oxidation-Reduction (aka Redox)

4. Organic

PrecipitationAqueous compounds make (at least one) solid. Mostly applicable to ionic compounds:

AB (aq) + CD (aq) → AD (s) + CB (aq)

The only thing you need to see is:

(aq) + (aq) → (s)

The rest is just details.

Sometimes called “double displacement” (because ions sort of do a double swap), however this name is misleading.

Precipitation (Visually)

+-

++

+ + -

-

-

-+

-

+

+

++

--

-

+++

+

--

- -

-+- +-

+

When dissolved (aqueous), ions are separate from each other and move around freely—because there are no bonds in ionic compounds!

Acid-BaseH+ is transferred from one compound (the acid) onto another (the base).

HA + B- → A- + HB

1. There must be an acid (not everything with a hydrogen is one).

2. There must be a base (H+ must get put onto something; can't end up on it's own, or with the same atom it started with)

Recognizing an AcidTake your compound HA (or H

2A, or H

3A) and imagine

breaking it into two parts:

H+ and A- (or A2-, A3-)

Now ask yourself “is A- something I recognize as a normal negative ion?”

If yes—HA is an acid.If no—it's probably not.

Examples: HCl: Cl- is normal, so HCl is an acidH

4C: C4- hasn't been seen before, so H

4C isn't

an acid

Oxidation-Reduction (aka Redox)Charge on at least one element changes.

A + BC → AB + C

Note that A starts out pure (must be neutral) and ends up as part of a compound (has a charge now), while C does the opposite.

If there is a pure element as either reactant or product, it MUST be redox, but not all redox have a pure element.

Likewise, if it's explosive, smoky, or full of flames, it's redox, but not all redox reactions are showy.

Special Types of RedoxNot all redox are one of these, but many are:

1. Synthesis: two (or more) pure elements make one compound

A + B → AB

2. Decomposition: one compound makes two or more products (not necessarily pure elements)

ABC → A + BC

3. Combustion: something reacts with oxygen (O2)

AB + O2 → AO + BO

Products of CombustionCombine each element that gets burned with oxygen:

C3H

4N + O

2 → CO

2 + H

2O + NO

2

In reality, the exact oxide produced can be complicated (you'll often see nitrogen products listed as NO

x because you get a

mix), but for our purposes, the rule of thumb is:

Any element X ends up as XO2

Unless it's...

Hydrogen, which makes H2O

OrganicAnything involving carbon-based covalent compounds.

Some of these fall under other categories, but many do not.

These are a year-long undergraduate course just to get a basic understanding, and people do entire post-graduate degrees in this area. They are very important—biology and many industries are based off organic molecules—but...

...we will not be going into these.

Summary• Four basic types of reaction:

PrecipitationAcid-BaseOxidation-Reduction (Redox)Organic

• Three subtypes of redox:SynthesisDecompositionCombustion