Question

What happens when naoh is added to a buffer composed of ch3cooh and ch3coo−?

Answer 1

Hello!

When NaOH is added to a buffer composed of CH₃COOH and CH₃COO⁻, the following reactions happen:

-First, the NaOH is neutralized by CH₃COOH:

NaOH + CH₃COOH → H₂O + CH₃COONa

-Second, the CH₃COONa dissociates in its ions:

CH₃COONa  → CH₃COO⁻ + Na⁺

-Finally, CH₃COO⁻ (a weak base) reacts with water to form OH⁻ ions and regenerate CH₃COOH

CH₃COO⁻ + H₂O ⇄ CH₃COOH + OH⁻

By this sequence of reactions, the buffer can mitigate the effect of the strong base added.

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Answer 2

pH of buffer solution containing ${\mathbf{C}}{{\mathbf{H}}_{\mathbf{3}}}{\mathbf{COOH}}$ and ${\mathbf{C}}{{\mathbf{H}}_{\mathbf{3}}}{\mathbf{COOH}}$ does not change on addition of NaOH to the buffer.

Further explanation:

Buffer solution:

The aqueous solution that consists of weak acid and its conjugate base is known as buffer solution. Such solutions resist any change in their pH on addition of strong base or acid.

pH is used to determine acidity or basicity of solutions. Solutions with pH less than 7 are acidic in nature, those with pH 7 are neutral and those with pH more than 7 are basic.

Classification of buffers:

Acidic buffer:

Solutions of weak acid and its conjugate base with pH less than 7 are acidic. Mixture of acetic acid and sodium acetate is an example of acidic buffer.

Basic buffer:

Solutions of weak base and its conjugate acid with pH more than 7 are basic in nature. Mixture of ammonium chloride and ammonium hydroxide is an example of basic or alkaline buffer.

${\text{C}}{{\text{H}}_{\text{3}}}{\text{COOH}}$ is a weak acid while NaOH is a strong base so these react with each other to form respective salt and water. Reaction between these two occurs as follows:

 ${\text{C}}{{\text{H}}_{\text{3}}}{\text{COOH}} + {\text{NaOH}} \to {\text{C}}{{\text{H}}_{\text{3}}}{\text{COONa}} + {{\text{H}}_{\text{2}}}{\text{O}}$

The salt formed by reaction of ${\text{C}}{{\text{H}}_{\text{3}}}{\text{COOH}}$ with NaOHis then dissociated to form its ions as follows:

${\text{C}}{{\text{H}}_{\text{3}}}{\text{COONa}} \rightleftharpoons {\text{C}}{{\text{H}}_{\text{3}}}{\text{CO}}{{\text{O}}^ - } + {\text{N}}{{\text{a}}^ + }$  

Ionic identity ${\text{C}}{{\text{H}}_{\text{3}}}{\text{CO}}{{\text{O}}^ - }$ reacts with water to form uncharged ${\text{C}}{{\text{H}}_{\text{3}}}{\text{COOH}}$ again. Reaction for this is as follows:

${\text{CHCO}}{{\text{O}}^ - } + {{\text{H}}_{\text{2}}}{\text{O}} \rightleftharpoons {\text{C}}{{\text{H}}_{\text{3}}}{\text{COOH}} + {\text{O}}{{\text{H}}^ - }$  

By going through above series of reactions, effect of addition of NaOH is neutralized by buffer containing ${\text{C}}{{\text{H}}_{\text{3}}}{\text{COOH}}$ and ${\text{C}}{{\text{H}}_{\text{3}}}{\text{CO}}{{\text{O}}^ - }$. Hence pH of buffer solution does not undergo any change in it.

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Answer details:

Grade: Senior School

Subject: Chemistry

Chapter: Buffer solutions

Keywords: buffer solution, pH, CH3COOH, NaOH, CH3COO-, weak acid, strong base, acidic buffer, basic buffer, less than 7, more than 7, H2O, OH-, CH3COONa, reaction, conjugate base.