Definitions of acids and bases
Definitions & Key takeaways
Acids are chemicals that release hydrogen ions (H+) when dissolved in water. Bases are compounds that release hydroxide ions (OH-) when dissolved in water. Acids and bases are classified as strong or weak based on how they dissociate in solutions. Strong acids and bases completely dissociate in water, releasing all their H+ or OH- ions. Weak acids and bases only partially dissociate, releasing some but not all of their H+ or OH- ions.
The pH scale measures how acidic or basic a solution is. The lower the pH, the more acidic the solution; the higher the pH, the more basic the solution. Pure water has a pH of 7, which is considered neutral. Many chemists tried to describe acids and bases and proposed various theories. Some of these chemists are Arrhenius, Bronsted, and Lowry.
Acids and bases are substances that are commonly found in our everyday lives. For example, acids give fruits like oranges and lemons, a sour taste.
There are several definitions of acids and bases that are used in chemistry. Our first definition of acids and bases is from a chemist named Aus an ane acid is a substance that increases the concentration of hydrogen ions in solution when it is dissolved in water.
An example of an rene acid is hydrogen chloride gas which has the chemical formula. HCL.
When gaseous HCL dissolves in water, it forms hydrogen ions. H plus and chloride.
An ions cl minus and aqueous solution. Since dissolving HCL in water increases the concentration of hydrogen ions in solution.
HCL is an arene acid. An aqueous solution of HCL is called hydrochloric acid.
An arene base is a substance that increases the concentration of hydroxide ions when it's dissolved in water. The hydroxide ion has a formula oh minus.
An example of an rene base is sodium hydroxide which is a white solid and has the chemical formula. Naoh when NAOH dissolves in water, it dissociates into na plus and oh minus.
Since dissolving naoh in water. Increases the concentration of hydroxide ions in solution.
Sodium hydroxide is an rene base. Our second definition of acids and bases is from two chemists bronsted and Lowry.
A bronsted Lowry acid is a proton donor and a bronsted Lowry base is a proton acceptor. Before we look at examples of bronsted Lowy acids and bases, we need to understand what is really meant by the word proton.
A neutral hydrogen atom has a proton in the nucleus. And an electron outside of the nucleus, we represent a neutral hydrogen atom with an H and a single dot Next to the H to represent the one valence electron.
If we take this electron away from our neutral hydrogen atom, we are left with a proton. So in acid base chemistry, whenever we are talking about a proton, we are talking about the nucleus of a hydrogen atom.
Since a proton has a positive charge and is the nucleus of a hydrogen atom. We symbolize a proton by writing H plus.
We can also refer to a proton as a hydrogen ion. While it is convenient to write H plus a proton interacts strongly with the source of electron density such as a lone pair of electrons on a water molecule.
If a water molecule picks up a proton, a hydronium ion is formed a hydronium ion has the formula H 30 plus. Chemists use the notation of H plus and H +30 plus interchangeably even if it's more accurate to write H +30 plus.
The browns Lowry definition for acids and bases can be used whenever a reaction involves the transfer of a proton from one substance to another.
Let's look at the reaction between H CL and H2O to form the chloride anion CL minus and the hydronium ion H 30 plus. If we look closely at the reactants and products in the balanced equation, this reaction involves the transfer of a proton from HCL to H2O alone pair of electrons on the water molecule takes a proton from HCL to form the hydronium ion.
H 30 plus the two electrons that used to be in the bond between H and CL end up on the CL to form the chloride anion CL minus.
Remember that a bronsted Lowry acid is a proton donor and a bronsted Lowry base is a proton acceptor. Since H CL donates a proton H cl is a bronsted Lowy acid.
Since water accepts a proton, water is a bronsted Lowry base. When H CL donates a proton, it turns into cl minus.
When water accepts a proton, H2o turns into H 30 plus. Let's look at another example.
In this acid base reaction, ammonia reacts with water to form the ammonium ion. And the hydroxide ion alone pair of electrons on the nitrogen in the ammonia molecule.
Take a proton from a water molecule, adding a proton to NH three forms NH four plus the two electrons that used to be in the bond between oxygen and hydrogen end up on the oxygen which forms the hydroxide ion oh minus.
Since water is donating a proton in this reaction, water is a bronsted Lowy acid. Since ammonia is accepting a proton ammonia is a bronsted Lowy base.
The reverse reaction is also possible. NH four plus can react with oh minus to form NH three and H2O if NH four plus donates a proton, it turns into NH three.
Since NH four plus is donating a proton. Ammonium ion is a bronsted Lowy acid.
If oh minus accepts a proton, it turns into H2O since oh minus accepts the proton, the hydroxide ion is a bronci Lowry base.
Conjugate acid base pairs differ by one proton. So on the left water donates a proton and acts as an acid.
When water donates a proton, it turns into the hydroxide ion. On the right, the hydroxide ion can act as a base and accept a proton.
If the hydroxide ion accepts a proton, it turns back into the water molecule. Since there's only one proton difference between H2O and oh minus water and the hydroxide ion are a conjugate acid base pair.
If we take off an H plus from H2O, we get oh minus. If we add an H plus to oh minus, we get H2O another conjugate acid base pair would be the ammonium ion NH four plus and ammonia.
NH three. If we take off an H plus from NH four plus, we get NH three.
If we add an H plus to NH three, we get NH four plus. So there's only one proton difference between the acid NH four plus and its conjugate base NH three in the reaction between ammonia and water, water donates a proton and therefore acts as a bronsted Lowy acid.
In the previous reaction, water accepted a proton and therefore acted as a bronsted Lowy base. An amphoteric substance can act as an acid or a base.
Since water, as we just showed can act as either an acid or a base. Water is an example of an amphoteric substance.
Sometimes the word amphiprotic is used in place of the word amphoteric. Our last definition of acids and bases comes from the chemist Lewis, a Lewis acid is an electron pair acceptor.
We can remember this definition by realizing, hey, both acid and acceptor start with the letter A since a Lewis acid accepts a pair of electrons, a Lewis acid must be deficient or poor in electrons.
So something with a positive charge like a proton H plus or the iron three plus cat iron could be a Lewis acid boron trifluoride.
BF three can also be a Lewis acid even though it doesn't have a positive charge. If we look at the dot structure for boron trifluoride, the boron in the center of the structure has only six electrons around it instead of a complete octet or eight electrons.
So boron is capable of accepting two more electrons to get a complete octet. Since it readily accepts a pair of electrons, it is able to function as a Lewis acid.
In contrast, a Lewis base is relatively rich in electrons and is an electron paired donor. We can remember this definition by taking the D and donor and flipping it horizontally to form a lower case B like the B and the word base.
An example of a Lewis base is the cyanide anion which has a loan pair of electrons on the carbon. The ammonia molecule has a loan pair of electrons on the nitrogen and can also function as a Lewis base.
Let's look at the reaction between the iron three plus C A ion and six cyanide anions to form the ferricyanide ion. The ferricyanide ion is an example of a coordination complex and has an overall charge of three minus.
Since opposite charges attract the negatively charged electrons on a cyanide and ion are attracted to the positively charged iron cation ion.
Each cyanide ion donates a alone pair of electrons to the iron cation ion. Since cyanide is an electron pair donor cyanide is the Lewis base.
Since the iron C ion is an electron pair acceptor, the iron C A ion is well, you guessed it, it's a Lewis acid. Since there are no protons involved in this reaction, we can't use the bronsted Lowry definition for acids and bases in this example, we can only use the Lewis definition.
So a bond forms between nitrogen and boron. Since ammonia is an electron pair donor, it is the lewis base.
Since boron trifluoride is an electron pair acceptor, it's the lewis acid for the product, the formation of a bond between the nitrogen and the boron creates a formal charge of plus one on the nitrogen and a formal charge of minus one on the boron.
Once again, since there are no protons involved in this reaction, only the lowest definition of acids and bases applies here.
All right, is a quick recap. There are three definitions of acids and bases that are commonly used in chemistry.
An arene acid is a substance that increases the concentration of hydrogen ions when dissolved in water. In comparison, an arene base increases the concentration of hydroxide ions when dissolved in water.
A bronsted Lowy acid is a proton donor and a bronsted lowi base is a proton acceptor. A Lewis acid is an electron pair acceptor while a Lewis base is an electron pair donor.
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