Theories+of+acids+and+bases

You must have heard the term acids and bases in your daily life, but what actually is an acid or a base?? There are different ways to define whether a substance is an acid or a base. These two definitions are the **//Bronsted-Lowry//** definition and the **//Lewis//** definition.

According to this definition: So an acid will give out hydrogen ions in solution.
 * The Bronsted-Lowry definition **
 * //An acid is a donor of hydrogen ions // (they are proton donors—since a hydrogen ion is just like a proton).
 * //A base is an acceptor of hydrogen ions// (they are proton acceptors).

Eg: the reaction NH 4 +  + NH 2 - ---> 2NH 3  is an acid-base reaction since **NH** ** 4 + ** **acts as an acid** donating a proton to the NH 2 -  to from NH 3  (which has one less hydrogen than NH 4 + ). The **NH** ** 2 - ** <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">**is a base** since it accepts a proton from NH 4 + <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> to from NH 3 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">.

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Eg. H 2 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">SO 4 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> ---> H + <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> + HSO 4 - <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Here, H 2 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">SO 4 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> is an acid since it has lost one hydrogen ion to form HSO 4 - <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">. <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">HSO 4 - <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">which has been formed as a result of the acid losing a hydrogen ion is known as //conjugate base//. (and this makes sense since HSO 4 - <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> can gain one H + <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> ion, like a base, to form H 2 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">SO 4 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> again) <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">(H 2 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">SO 4 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> and HSO 4 - <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> are known as an acid-base pair).
 * When an acid loses a hydrogen ion the species which it forms is known as a **// its conjugate base // . **

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Eg. NH 3 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> + H + <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> ---> NH 4 + <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Here, NH 3 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> is the base since it gains a hydrogen ion to form NH 4 + <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">. NH 4 + <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> which has been formed as a result of the base gaining a hydrogen ions is known as conjugate acid. (and this makes sense since NH 4 + <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> can lose one H + <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> ion, like an acid, to form NH 3 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">again) <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">(NH 3 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> and NH 4 + <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> are known as an acid-base pair)
 * When a base gains a hydrogen ion the species which it forms is known as **//its conjugate acid//** //<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">. //

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Therefore, any acid-base reaction will involve two acid-base pairs. For example: <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> CH 3 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">COOH (aq) + H 2 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">O (l) ---> H 3 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">O + <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> (aq) + CH 3 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">COO - <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> (aq) <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">CH 3 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">COOH is an acid since it loses an H + <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> ion to form CH 3 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">COO - <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">. CH 3 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">COO - <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> is the conjugate base of CH 3 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">COOH since it can gain an H + <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> ion to form CH 3 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">COOH. (Therefore, CH 3 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">COOH and CH 3 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">COO - <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> are known as an acid-base pair)

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Similarly, H 2 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">O acts as a base, since it gains an H + <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> ion to form H 3 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">O + <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">. H 3 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">O + <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> is the conjugate acid of H 2 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">O since it can lose one H + <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> ion to form H 2 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">O again. (Therefore, H 2 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">O and H 3 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">O + <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> are known as an acid-base pair)

// Thus we can conclude that an acid/base pair means are two species that are related by the loss/gain of a single H+ ion. // //<span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> If you are given a chemical species, it is very easy to find its conjugate acid or conjugate base. //

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Eg: The conjugate acid of HSO 4 - <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> is H 2 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">SO 4 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">. HSO 4 - <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">+ H + <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> ---> H 2 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">SO 4
 * <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">To find the ** conjugate acid **<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">, you need to //add an H// // + //<span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> //ion// to the species.

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Eg: The conjugate base of HSO 4 - <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">is SO 4 2- <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">. HSO 4 - <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">---> SO 4 2- <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> + H +
 * <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">To find the ** conjugate base **<span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> you need to //remove an H// // + //<span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> //ion// from the species.

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">When a base gains an H + <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> ion from an acid, it forms a dative covalent bond with the H + <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> ion. //<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Therefore an acid/base can be defined in terms of the formation of dative covalent bonds: //
 * The Lewis definition **

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">To understand this, look at the following example: <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">
 * <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">An acid is a species that //accepts a pair of electrons// to form a dative covalent bond.
 * <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">A base is a species that //donates a pair of electrons// to the H + <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">ion to form a dative covalent bond.

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">As can be seen, H 2 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">O is a base here since it donates its lone pair of electrons to a hydrogen ion to from H 3 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">O + <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">. Therefore, H + <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> is the acid here, since it accepts the lone pair of electrons to form the dative bond with H 2 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">O.

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">__//Remember that all Bronsted-Lowry acids and base are also Lewis acids and bases.//__ <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">If you look in the above example, H 2 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">O has accepted an H + <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> ion so it is a base according to the Bronsted-Lowry definition. According to the Lewis definition it is a still a base since it donates a lone pair to form a dative covalent bond.

However the reverse is not true; All Lewis acids are not Bronsted-Lowry acids. <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Basically the two definitions are different ways of looking at acids and bases (good point for TOK discussion!!). But the Lewis definition is a much broader one, since if we go according to the Lewis definition, any reaction that involves the formation of a dative bond can be considered to be an acid-base reaction (although it may not involve any transfer of hydrogen ions).

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Look at the following reaction: <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">In the above reaction, BF 3 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> is the acid since it accepts the lone electron pair and NH 3 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> is the base since it donates the lone electron pair. In accordance with the Lewis definition the above reaction is an acid-base reaction, even though there is no transfer of H + <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> ions.

By the end of this lesson you should be able to:
 * Define acids and bases according to the Bronsted-Lowry and Lewis definitions
 * Know whether a species can act as a Bronsted-Lowry and/or Lewis acid or base
 * Deduce the formula of the conjugate acid/base of a species which is a Bronsted-Lowry base/acid itself


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