Chemical Industry Tips – ROH Rules

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This is a simple rule for determining the acidity of different oxyacids. Because each oxyacid can be considered to be formed by the dehydration of its corresponding hydroacid; for example, sulfuric acid (H2SO4) can be regarded as resulting from the dehydration of H6SO6, or S(OH)6, by two molecules of water: S(OH)6 → H2SO4 + 2H2O. Therefore, each oxyacid can be written in the form of R(OH)m, where R is the acid-forming element. The greater the R \"charge\" (which should refer to formal charge, i.e., oxidation state), and the smaller the radius (which should refer to covalent radius), the stronger the attraction between R and O, and thus the stronger the bond formed. Conversely, the O—H bond becomes weaker, making it easier for water to dissociate and release H+, thereby exhibiting acidity. Conversely, the smaller the \"charge\" of R and the larger its radius, the stronger the O—H bond becomes; it is less likely to dissociate to release H+, resulting in weaker acidity or even basicity. These are the ROH rules. G.H. Cartledge combined \"charge\" with radius, expressing it in terms of ion radius measured in units of 10^-10 m, and found that if the value was 3.2, then ROH was an acid; if it was 2.2