Although I" told you that acids and also bases aren" difficult to understand also, I" acquired poor news: There are not one yet three widespread meanings offered to describe acids and also bases: Arrhenius acids and bases, Brnsted-Lowry acids and bases, and Lewis acids and also bases. Though this makes it sound as if you" have to learn around acids and bases three times, the good news is that for many kind of useful functions, these three definitions are around tantamount.

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Arrhenius Acids and also Bases

Way back in the late 1800s, our old frifinish Svante Arrhenius came up through definitions of acids and bases while functioning on kinetics problems.

According to Arrhenius, acids are compounds that break up in water to give off hydronium (H+) ions. A common instance of an Arrhenius acid is hydrochloric acid (HCl):

HCl ? H+ + Cl-

The formulas for acids generally start with hydrogen, though organic acids are a remarkable exemption. The names and also formulas of some widespread acids are given in the table below:

Acid Name Formula
hydrochloric acid HCl
nitric acid HNO3
phosphoric acid H3PO4
sulfuric acid H2SO4
acetic acid C2H4O2

Arrhenius bases are defined as compounds that reason the development of the hydroxide ion when placed in water. One instance of an Arrhenius base is sodium hydroxide (NaOH):

NaOH ? Na+ + OH-

Bases typically have "" in their formulas, though tbelow are exceptions. For example, ammonia (NH3) doesn" contain hydroxide ions but forms them once it reacts with water:

NH3 + H2O ? NH4+ + OH-

The names and also formulas of some prevalent bases are in the following table:

Base Name Formula
ammonia NH3
potassium hydroxide KOH
sodium bicarbonate NaHCO3
sodium carbonate Na2CO3
sodium hydroxide NaOH

Some oxides create acids or bases once water is added. Due to the fact that these compounds don" contain any kind of H+ or OH- ions unmuch less they react through water, they" dubbed "" Usually, oxides of nonmetals are acid anhydrides (they develop acid as soon as put in water), and oxides of metals are base anhydrides (creating a base as soon as put in water).

Brnsted-Lowry Acids and also Bases

In the early 1900s, an alternative meaning for acids and bases was proposed by Johannes Brnsted and Thomas Lowry to account for the truth that ammonia have the right to neutralize the acidity of HCl even if water isn" existing. This phenomenon proved them that ammonia is a base, even as soon as tright here isn" water approximately to create hydroxide ions.

The Mole Says

Tright here are many type of different names and formulas supplied to explain the hydronium ion. Though the formula was shown previously as "", it is periodically composed as "" because this is the ion created once H+ combines with water. Another common means of referring to hydronium ions is simply to call them "" This name comes from the truth that H+ represents a hydrogen atom (one proton and one electron) that has actually shed its electron, leaving only the bare proton behind.

A Brnsted-Lowry acid is characterized as a compound that offers hydronium ions to an additional compound?for example, hydrochloric acid provides H+ ions to compounds it reacts via. Brnsted-Lowry bases are compounds that deserve to accept hydronium ions?as soon as ammonia gets a hydronium ion from HCl, it develops the ammonium ion.

The following equation represents the reaction of a Brnsted-Lowry acid via a Brnsted-Lowry base:

HNO3 + NH3 ? NO3- + NH4+

In this reactivity, nitric acid behaves as an acid bereason it offers a proton to ammonia. Ammonia behaves as a base bereason it accepts the proton from nitric acid.

However, if you take a look at the other side of the equation, we find the nitprice and ammonium ions. Because the nitrate ion have the right to accept proloads from the ammonium ion (to form HNO3), the nitprice ion is a really weak Brnsted-Lowry base. Due to the fact that the ammonium ion has actually an added proton to donate (in this instance to the nitprice ion), it is a Brnsted-Lowry acid.

The nitprice ion is based upon the nitric acid molecule, so we say that it is the conjugate base of nitric acid. Likewise, the ammonium ion is the conjugate acid of ammonia. Together, an acid through its conjugate base (such as HNO3 and also NO3-) or a base via its conjugate acid (such as NH3 and NH4+) is referred to as a conjugate acid-base pair.

Lewis Acids and Bases

In the Brnsted-Lowry meaning of acids and also bases, a base is identified as a compound that can accept a proton. However, exactly how does it accept the proton?

One attribute that Brnsted-Lowry bases have actually in widespread via each various other is that they have actually an uncommon pair of electrons. When a hydronium ion comes wandering by the molecule, occasionally the lone pairs will certainly reach out and grab it. An instance of this is when ammonia accepts a proton in an acidic solution:


Figure 23.1Ammonia deserve to grab a proton from nitric acid with its lone pair of electrons.

One method of looking at this procedure is that the ammonia atom is donating its lone pair to the proton. Since the lone pairs are driving this chemical reactivity, we have actually a brand-new meaning of acidity and also basicity, dubbed " acidity/basicity." A Lewis base is a compound that donates an electron pair to another compound (the ammonia in our example). A Lewis acid is a compound that accepts an electron pair (the H+ ion in our example).

Molecular Meanings

Lewis bases are chemicals that deserve to donate electron pairs. Lewis acids are chemicals that have the right to accept them.

Though we had actually ammonia donating a lone pair to a proton in our example, the lone pair in ammonia have the right to react with most various other compounds as well. For example, ammonia deserve to donate its lone pair of electrons to BH3 by the following process:


Figure 23.2The lone pair on ammonia attaching itself to BH3.

In this process, ammonia is the Lewis base and BH3 is the Lewis acid.

Typically, the Lewis definition of acids and bases is the most helpful because it is the the majority of inclusive of the 3 definitions. For instance, the Brnsted-Lowry meaning of an acid consists of HF but not BH3, which doesn" lose a proton when attached by the lone pairs on a Lewis base.

Excerpted from The Complete Idiot" Guide to Chemistry 2003 by Ian Guch. All rights scheduled consisting of the right of reproduction in whole or in component in any type of form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.

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