As an necessary chemistry college student you will find yourself confronted with acid/based questions again and again.

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From categorizing molecule to ranking their toughness without pKa or pKb values. This isn"t terrible if/when you learn it correctly, however, her professor will certainly assume you already know this.

So s/he won’t teach it.


Because you’re intended to psychic this from earlier in basic chemistry.

Instead your professor will certainly fly through the material and expect you to save up.

But this won"t aid with the acid/base classifications.

Simply memorizing meanings without true understanding won"t do it any type of easier. Specifically when challenged with a molecule that shows up to fit an ext than one acid/base definition.

So let"s take it a look at acid/base definitions in a much more logical way.

Categories that Acids and also Bases

Before us dive in, be certain to download my cost-free acid/base cheat sheet to follow along now and take with you to examine on the walk – necessary Chemistry acid Base Cheat Sheet

Acids and also bases will fall under one or more the the complying with three categories:

Arrhenius acids/basesBronsted-Lowry acids/basesLewis acids/bases

The an essential here is to acknowledge that if each category has a particular definition, any kind of given molecule can fall into much more than one category, some right into all 3. Again, other we"ll watch at later in this article.

Arrhenius Acid

An Arrhenius mountain is a molecule the when liquified in water will certainly donate one H+ in solution. Just put, a proton donor.

The trick come recognizing an Arrhenius mountain is come look for a molecule that starts with an H, and also typically contains an oxygen or halogen.

Common instances of Arrhenius acids include:

Hydrochloric acid – HClNitric acid – HNO3Sulfuric acid – H2SO4Acetic acid – HCH3CO2and so many more…

An mountain dissociating in water go not type a free-floating proton. Instead one that the water molecules in equipment will grab the H+ yielding a hydronium or H3O+ ion. Here"s what happens as soon as nitric acid dissociates in water.


Arrhenius Base

An Arrhenius base is a molecule the when dissolved in water will failure to productivity an OH- or hydroxide in solution. To recognize the Arrhenius base look for a molecule finishing in OH, however not following CHx which describes an alcohol.

Arrhenius base examples include:

Sodium hydroxide – NaOHPotassium hydroxide – KOHMagnesium hydroxide – Mg(OH)2and so numerous more…


But what if the acid/base is not dissolved in water?

The Arrhenius an interpretation for acids and bases only refers to compounds dissolved in water. Go this median that acids and also bases cannot exist the end of water? no quite, that"s wherein the Bronsted-Lowry an interpretation comes in.

Bronsted-Lowry Acid

A Bronsted-Lowry acid, choose an Arrhenius acid, is a compound that breaks down to give an H+ in solution. The only distinction is the the systems does not have to be water. We can still describe the exact same acids as detailed for the Arrhenius acid examples, but this time we"ll change the solvent to ammonia, alcohol, or anything else.

We observed what happens when nitric mountain (HNO3) dissolves in water. Currently let"s see what happens as soon as it disappear in ammonia (NH3) or even methanol (CH3OH)


Nitric acid still liquified to yield an H+ and also NO3-, however this time it to be NH3 and also not water the picked increase the free-floating proton.

Bronsted-Lowry Base

This is where we begin to view the difference between the Bronsted-Lowry and also Arrhenius definitions. When the Arrhenius base referred especially to the hydroxide (OH-) ion, the Bronsted-Lowry basic refers to any atom or ion qualified of agree or bonding to a cost-free proton in solution.

Referring earlier to the HNO3 + NH3 reaction above, once ammonia choose up the totally free H+ the acts as a proton-acceptor. NH3 is the Bronsted-Lowry base in this example.

Additional examples include:

Methanol – CH3OHFormaldehyde – H2COAnd even water – H2O

Lewis Acids and bases

The Lewis meaning for acids and also bases is the many extreme because it"s not dealing with protons specifically. Rather the Lewis definition deals v the motion of electrons.

Lewis Acid/Base Mnemonic

Think the Lewis together ‘lectrons"

Lewis Acid

A Lewis acid refers to an atom or molecule that accepts one electron pair. Think back to your ‘pushing arrows" for orgo mechanisms. Every time you attract an arrowhead representing the motion of electrons, the atom getting assaulted or accepting those electrons is the Lewis acid in the reaction.

Common Lewis Acid examples in organic Chemistry

Borane – BH3 (hydroboration reaction)Aluminum Chloride – AlCl3 (electrophilic aromatic substitution reaction)Iron (III) Bromide – FeBr3 (electrophilic aromatic substitution reaction)and our good friend H+ (keep reading)

The drawing below is component of the EAS aromatic Halogenation reaction i m sorry you"ll view in late Orgo 1 or Orgo 2. Notice how the Fe gets attacked by a lone pair the electrons. By accepting those electrons Fe acts together a Lewis acid


Lewis Base

Since the Lewis an interpretation has to do with the move of electrons, girlfriend can guess by currently that a Lewis base is an electron pair donor. When again think earlier to your reaction mechanisms. The molecule utilizing its electron to attack an additional atom is an electron pair donor and a Lewis Base.

Here is the first step in acid catalyzed hydration. The pi bond attacking H+ renders the alkene a Lewis Base.


What If it Fits an ext Than 1 Category?

After discovering the individual acid/base meanings you"re most likely confused and also overwhelmed.

So lot information and also so very similar!

And what wake up if a molecule shows up to fit much more than one category?

For example:

Is OH- an Arrhenius Base? Or possibly its a Bronsted-Lowry or also Lewis Base?

The prize is possibly all 3!

The definitions above evolved gradually as scientists were beginning to understand much more and an ext details about chemical reactions. The very first is a an easy (no pun intended) definition, but the last 2 simply increase to to the right more complicated solvents and also situations.

Take a look in ~ the reaction listed below where the hydroxide ion assaults a proton on hydronium.


Arrhenius mountain Definition: Hydronium breaks up to yield one H+ in solution.

Arrhenius Base Definition: Hydroxide is one OH- liquified in water.

Bronsted-Lowry acid Definition: Hydronium is one H+ donor regardless of solution

Bronsted-Lowry Base Definition: Hydroxide attacks and accepts the H+ from hydronium.

Lewis mountain Definition: The H+ on Hydronium accepts the attacking electron pair to form a bond.

Lewis base Definition: Hydroxide donates its electron pair to type a bond between itself and also H+

Now the you know the similarities/difference between Arrhenius, Bronsted-Lowry, and Lewis acids and bases, how do you tell i m sorry is the more powerful or weaker acid? which molecule is more reactive? Which side of a reaction will certainly be favored in ~ equilibrium?

Learn this and an ext in my acids and bases tutorial video series:


In conclusion:

As an organic chemistry student you will be forced to recognize and classify 3 different varieties of acids and also bases. Arrhenius, Bronsted-Lowry, and also Lewis. While the technical meanings vary, when you acquire the reasonable behind their meanings you"ll be able to quickly and easily recognize the different varieties of acids and bases.

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Want to test your knowledge of acids and bases? Click to try the free Acid/Base exercise quiz.