Acid-base dissociation theory and acid-base proton theory

7.
4.
1 Acid-base dissociation theory (Arrhenius theory)

People's understanding of acid and base has gone through a gradual process.
In 1884, Arrhenius defined acid and base on the basis of dissociation theory and discussed the strength of acid and base quantitatively
.

Arrhenius believes that all the positive ions produced by dissociation in aqueous solution are H+ substances, such as HCI, H ₂ SO ₄ , HNO ₃ , and the substances that dissociate and produce all negative ions in aqueous solution are OH ^- are alkalis.
, Such as NaOH, Ba(OH) ₂ , Ca(OH) 2
.

The strength of acid and base can be measured by the size of the dissociation constant.
The larger the value of Ka ^Θ and Kb ^Θ , the stronger the corresponding acid or base
.

The proposal of Arrhenius' dissociation theory has made people's understanding of acid and base have a leapfrog development, and has played a huge role in the development of chemical science
.

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4.
2 Acid-base proton theory (Bronsted theory)

In the 1920s, Bronsted (JNBronsted) and Lowry (TM Lowry) independently proposed the acid-base proton theory
.

Protic acid according to the theory, the reaction is called a proton donating acid substance, such as the HCI, NH2 _.
4 ⁺ , HC03 _.
3 ^- ; proton etc.
can be given, they are protic acid; substance being a proton in the reaction, said an alkali, such as NH2 _.
3 , CI ^- , NO _.
3 ^- , etc.
can be combined with a proton, it proton are alkali; and HC03 _.
3 ^- and H ₂ O and the like in the reaction, and acceptable both proton donating a proton, called Amphiphilic substance
.

The acid-base proton theory believes that acids and bases are not isolated from each other, but have a certain dependence
.

Acid = base + proton

A pair of acid-bases satisfying the above relationship is called a conjugate acid-base pair, such as

HN03 _.
3 = of NO3 ^- + H ⁺

In the reaction formula, NO is the conjugate base of HNO; and HNO; is the conjugate acid of NO
.

Acid-base dissociation reactions and salt hydrolysis reactions are all proton transfer processes between conjugate acid-base pairs
.

The acid-base proton theory is not only applicable to water systems, but also to non-aqueous systems
.

Reactions NH2 _.

In acid-base proton theory, the strength of acid-base can be judged by the ability of a substance to give or accept protons
.
The stronger the ability of the acid to donate protons, the stronger the acidity
.
The stronger the base's ability to accept protons, the stronger the base
.
In the water system, HCI completely dissociates from H ⁺ , HCl is a strong acid; while HAc partially dissociates from H ⁺ , HAc is a weak acid
.
The strength of acid-base can be represented by the equilibrium constants Ka ^Θ and Kb ^Θ of the dissociation reaction
.

In the water system, HCI, HNO ₃ , H ₂ SO ₄ , and HCIO ₄ are all dissociated completely, and they are all strong acids, and the strength of the four acids cannot be distinguished.
This effect of water is called the leveling effect
.
When they are dissolved in HAc, which is more acidic than water, their ability to donate protons is different
.

According to the value of the dissociation equilibrium constant Ka ^Θ in acetic acid, the acidity of these kinds of strong acids can be distinguished.
This effect of acetic acid is called the discrimination effect.
HAc is a discrimination reagent for several kinds of strong acids
.

In the proton acid-base reaction, the direction of the reaction is to generate a weak acid and a weak base from a strong acid and a strong base
.
E.
g

The acid-base proton theory expands the range of acid-base, but it also has limitations and is not applicable to a proton-free system
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7.
4.
3 Acid-base electron theory (Lewis theory) While the acid-base proton theory was proposed, in 1923, GN Lewis proposed the acid-base electron theory, also known as the Lewis theory
.

The substances that can accept electron pairs in the reaction are called acids.
For example, H ⁺ , Na ⁺ , BF ₃ and so on all contain empty orbitals that can accept electron pairs, and they are all Lewis acids; substances that can give electron pairs in the reaction are called alkalis such as OH ^- , the CN ^- , NH2 _.
3 , etc.
can be given pair of electrons, it is a Lewis base
.

Acid is the acceptor of electron pairs.
The stronger the ability to accept electron pairs, the stronger the acidity
.
Alkali is a donor of electron pairs.
The stronger the ability to donate electron pairs, the stronger the alkalinity
.

The acid-base electron theory can divide the reaction into two types, one is the reaction of acid and base forming acid-base complexes
.
E.
g

The other is substitution reaction, including acid substitution reaction, base substitution reaction and double substitution reaction
.
E.
g

The advantage of the acid-base electron theory is that it has a wide range of applications, but the limitation is that the characteristics of the acid-base are unknown, and the relative strength of the acid-base cannot be compared
.