Question
Fri March 04, 2011 By: Garima Srivastava

(i)What are the factors affecting acid strength. (ii)define thermochemical equation.

Expert Reply
Fri March 04, 2011
Dear Student
 
(i) The acidity of any molecule depends upon the electronegativity of the atom holding hydrogen, the polarity of the bond between the atom holding hydrogen and hydrogen, the solvent.
 
A. If all other factors are constant, the polarity of the X-H bond is directly proportional to the strength of the acid.  Polarity is measured by the difference in electronegativities between the atoms in a bond.
When an acid dissociates in water, the X-H bond is broken to form the ions H+ and X- ions.  The greater the polarity of the bond, the easier it is to form these ions, and thus the stronger the acid.
 
B. The strength of acids increases is directly proportional to the size of the X atom increases because the strength of the X-H bond diminishes as the size of the X atom increases.  The relative strengths of the X-H bond in various compounds can be compared by measuring the bond-dissociation enthalpy.
 
C. Compounds becomes less acidic as the negative charge increases on the molecule. For example:
 
D. As the oxidation number of the central atom in an oxyacid increases, the strength of the acid increases significantly.  As the oxidation state of an atom increases, its tendency to draw electrons in a bond toward itself increases.  In an oxyacid, the central atom pulls electrons away from the oxygen, consequently making the oxygen more electronegative.  The O-H bond, therefore becomes more polar, making it easier to form ions and thus increasing the strength of the acid.
 
 
(ii) Thermochemical equation

Thermochemical Equation is a balanced stoichiometric chemical equation that includes the enthalpy change, ΔH. In variable form, a thermochemical equation would look like this:

A + B → C
ΔH = (±) X, where X is the value of enthalpy.
 
 
We hope that clarifies your query.
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