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Acidity of an acid depends upon the nature of the compound, its structure or its position in the periodic table. The acids are normally classified into three categories : (i) Hydro acids: e.g. HCI, H2S, PH3, H2 Te etc. (ii) Oxo acids : e.g. HCIO4, H3PO4, HSO4-etc. (iii) solvated captions: e.g. [AI( H2O)6]3+ NH4+ etc. (a) Strength of Hydro Acids. There is a general increase in hydro acid strength in going form left to right along a period. For example the following trend of acid strength is observed along the second period. HF > H2O > NH3 > CH4 This trend is explained on the basis of increase in electronegative as we more form left to right along a period C < N <O < F With the increase in electronegative of the central atom, the tendency, to pull the electron pair form H increases, and dissociation of H becomes easier and acidic character increases.The above tend based on electro negativity holds true along a period. It does not hold good as we move down a group. In fact, acid in a group increases with decrease in electronegative as we go down the group. For example in the hydro acids of group VII, HI is the strongest acid and HF is the weakest acid which is opposite to the trend along a period. HI > HBr > HCI > HF Similarly n group VI the acid strength increases as we move down the group with H2O as the weakest acid H2O < H2S < H2Se < H2 Te. and H2Te as the strongest acid in spite of H2O being the most polar on the basis of electronegative. For more help in Trends In Acid Strength click the button below to submit your homework assignment You are already aware of the term acids. You have learned about the physical and chemical properties of acids. Some simple ones are – they have a pH below 7, they turn blue litmus paper red, they have a sour taste and they react with alkalis to form salts. One interesting factor about acids is acid strength. In this article, we will talk about acid strength and the factors which affect it. Acid strength is the measure of the ability of the acid to lose its H+ ion. It depends on several factors which we will discuss in the subsequent sections. What are Strong and Weak Acids?
Factors Determining Acid StrengthAs already discussed different acids have different acid strengths. An acid having a greater degree of dissociation behaves as a stronger acid. Now let us understand the factors on which the strength of an acid depends. The degree of dissociation of an acid depends on the following two factors.
In general weaker the strength of H-A bond, stronger is the acid. Similarly, greater the polarity of H-A bond, stronger is the acid. Both these factors make the dissociation of acid molecules into H+ and A- easier thereby increasing the acidity. Acid Strength OrderOrder of acidic strength may be mentioned here that while comparing elements in the same group of the periodic table the strength of H-A bond is a more important factor in deciding the acidity than its polarity. As the size of A increases on descending a group, H-A bond strength decreases and therefore the acid strength increases. For example, the acid strengths of hydrides of group-17 elements increase in the order. HF < HCl < HBr < HI Recommended Videos Factors Affecting Acid Strength
For the most part, strong acids have ions in solution, so bonds holding H and A together must be weak. Strong acids split easily into ions. Weak acids often exist as molecules with just a few ions in solution, hence the bonds that bind H and A together must be solid.
Carborane superacids can be considered the strongest solar acid in the world because fluoroantimonic acid is, in reality, a mixture of hydrofluoric acid and pentafluoride antimony. Carborane comes with a pH value of -18.
The strength of the H-A bond depends on that. The tighter the bond, the more energy it takes to break it down. The acid, then, is solid. The H-A bond polarity determines its acid strength.
Citric acid is a mild acid when compared to sulphuric acid and hydrochloric acid since it has very low concentrations of H+ ions in its solution. Citric acid is an organic acid and weak acids are mostly organic acids.
Seven strong acids are chloric acid, hydrobromic acid, hydrochloric acid, hydroiodic acid, nitric acid, perchloric acid, and sulphuric acid. Getting part of the list of strong acids does not however provide any hint of how dangerous or harmful an acid is. To learn more about acids, their properties, and acidic strength and watch vibrant video lectures on them, download BYJU’S – The Learning App.
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What is the trend across a period left to right for acid strength and what is it based on?
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