pH Scale and Acidity

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Acidity Basicity And pH

pH stands for potential of hydrogen. It leads to the hydrogen ion concentration in a solution. It is the measure of the acidity or alkalinity of a solution. Acidic pH ranges from 1 to 14. The scale goes from 0 - 14, with seven being neutral. The acidic pH ranges from 1 to 7, whereas a pH higher than 7 indicates a base. pH is a measure of the corresponding amount of free hydrogen and hydroxyl ions in the water.

The pH of a substance can be represented as the negative logarithm of the hydrogen ion concentration in that matter. Similarly, the pOH of a substance is the negative logarithm of the hydroxide ion concentration in the substance. These quantities can be expressed via the following formulae:

  • pOH = -log[OH]

  • pH = -log[H+]

(Image to be added soon)

The picture denotes the acidity basicity and ph worksheet 


Limitation of pH Scale

  • pH values of the solutions do not provide us with an instant idea of the comparative strengths of the solutions. A solvent of pH = 1 has a hydrogen ion concentration 100 times that of a solution pH = 3. A 4x10⁻¹ N HCl is twice concentrated 2х10⁻¹ N HCl on a solution, but the values of these solutions are (not double) 4.40 and 4.70.

  • pH value 0 is taken in the solution of a strong acid. If the concentration is 2N, 3N, 10N, etc, the corresponding pH values will be negative.

  • A solution of an acid having low concentration, say, 10⁻⁸ N can not have pH 8, as shown by the pH formula but the actual pH value will be less than 7.

pK Value: p is a negative logarithm. Just as Hᐩ and OH⁻ ion concentrations vary over several negative powers of 10, it is accessible to express them as pOH or pH, the dissociation constant value K ranges over multiple negative powers of 10, and it is easy to write them as pK. Therefore, pK is - dissociation K.


Buffers

Most cells in our bodies work inside a small window of the acidic pH range, typically covering only from 7.2 to 7.6. The respiratory system starts malfunctioning, if the pH of the body is outward of this range, also the other organs in the body. The proteins start breaking down, and the cells stop functioning. Deviation outside of the pH range can cause coma or even death. Buffers are the key which quickly absorbs a high amount of Hᐩ or OH⁻, by keeping the pH of the body carefully preserved in the aforementioned close range. Carbon dioxide is part of a central buffer system in the human body; it keeps the pH within the acceptable range. This buffer system includes (carbonic acid) H₂CO₃ and (bicarbonate) HCO₃– anion.


Periodic Variations of Basic and Acidic Properties

  1. Hydracids of Elements of the Same Period 

Analyze the hydracids of the elements of the second period that is CH₄, NH₃, H₂O and HF. These hydrides become extra acidic as we move from CH₄ to HF.

Therefore CH₄ acidic properties are negligible, but NH₃ provides a proton (H+) to strong bases to form, H₂O drops a proton even more quickly, and HF is a relatively strong acid. The increase in acidic properties is because the stability of their conjugate bases develops in the order :

CH⁻₃< NH⁻² < OH⁻ < F⁻

The rise in acidic properties is maintained by the successive increase in the dissociation constant values of these hydrides as shown.

CH₄(=10-58) < NH₃(=10-35) < H₂O(= 10-14) < HF (= 10-4)


  1.  Hydracids of the Elements of a Similar Group.

  • Hydracids of VA group elements such as NH₃, PH₃, AsH₃, SbH₃, BiH₃. All these hydrides show essential character. With the expansion in size and reduction in electronegativity from N to Bi, there is a lessening in electron density in sp3 hybrid orbital and thus electron donor capacity decreases.

  • Hydracids of VI A elements H₂O, H₂S, H₂Se, H₂Te. Aqueous solution of the hydrides behave as weak diprotic acid and ionise as:

          H₂R <–> H+ + HR⁻

          HR⁻ <–> H+ + R₂⁻

strength of the hydrides as acids increase order

H₂O < H₂S < H₂Se < H₂Te.


  1. Oxyacids

The properties of acidic oxyacids of the equal element, which is in other oxidation state gains with an increase in oxidation number.

+ 1            +3          +5            +7

HCIO  <  HC₁O   <   HC₁O₃   <  HCLO₄

+4            +6         +3            +5

H₂SO₃  <   H₂SO₄;    HNO₂    <  HNO₃

The properties of acidic pH range of the oxyacids of separate elements in the same oxidation state decreases as the atomic number increases. This is due to size expansion and decrease in electronegativity.

HC₁O₄ > HBrO₄ > HIO₄

H₂SO₃ > H₂SeO₃

FAQ (Frequently Asked Questions)

1. What is the pH Scale?

pH is the measure of the acidity or alkalinity of a solution. pH stands for the 'potential of Hydrogen'. Values below 7 indicate acidity, which increases as the amount decreases, one being the most acidic. Values above 7 indicate alkalinity, which rises as the number increases, 14 being the most alkaline. The acidic ph range of a substance can be represented as the negative logarithm of the hydrogen ion concentration in that matter. pH values of the solutions do not provide us with an instant idea of the comparative strengths of the solutions. The pH of a substance can be represented as the negative logarithm of the hydrogen ion concentration in that matter. 

2. Define the Measurement of Ion Concentration.

Systems are developing for the measurement of the concentrations of various sorts of ions using the cell potentials of special electrochemical cells. If a voltaic cell is formed with two distinct kinds of metal electrodes, we get a specific cell potential for approved conditions where the electrolytes are at the standard 1 Molar concentration. If ion concentrations are ranging from 1 Molar, then there is a change in the cell potential defined by the Nernst equation. Measurement of that change is a measure of the ion concentration and can be misused for that purpose.