What is chemical equilibrium ?

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Q. What is chemical equilibrium ?

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Step 1. A reversible reaction can proceed in both directions at once -- reactants form products (forward reaction) while products simultaneously re-form reactants (reverse reaction).

Step 2. In a closed system, the forward rate starts high (only reactants present) and falls as reactants are used up; the reverse rate starts near zero and rises as products build up.

Step 3. Eventually the two rates become exactly equal. From that point on, reactants are converted to products at exactly the rate products are converted back to reactants, so the measurable concentrations of everything present stop changing.

Step 4. This state -- both reactions still occurring, but at matched rates so nothing measurable changes -- is chemical equilibrium; it is dynamic, not static.

✓Final answer

Chemical equilibrium is the dynamic state, reached in a closed system, at which the forward and reverse reaction rates are equal, so reactant and product concentrations remain constant even though both reactions continue.

Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CBSE 2025Set ANNUAL3 marks
$^{-4}$
›Reveal solutionSolution

$pOH=-\log(2\times10^{-4})=3.7$ ; $pH=14-3.7=10.3$ .

Conjugate acid-base pair: by the Bronsted–Lowry theory, when an acid donates a proton it becomes its conjugate base, and this acid–base pair (differing only by $H^+$ ) is called a conjugate acid-base pair, e.g. $CH_3COOH$ (acid) / $CH_3COO^-$ (its conjugate base).

pH calculation: given $[OH^-] = 2\times10^{-4}\ mol\,dm^{-3}$

$pOH = -\log(2\times10^{-4}) = 4 - \log 2 = 4 - 0.301 = 3.7$

Using $pH + pOH = 14$ :

$pH = 14 - 3.7 = 10.3$

✓Final answer
$CH_3COOH/CH_3COO^-$
CBSE 2024Set ANNUAL3 marks
$Q. Define acids and bases according to Bronsted-Lowry theory. Derive relationship between pH and pOH.$
›Reveal solutionSolution

Bronsted acid donates H $^+$ , base accepts it. From $K_w=[H^+][OH^-]$ : $pH+pOH=pK_w=14$ at 25°C.

Bronsted–Lowry theory: an acid is a species that donates a proton ( $H^+$ ) to another species; a base is a species that accepts a proton. Every acid, on losing a proton, forms its conjugate base, and every base, on gaining a proton, forms its conjugate acid.

Relationship between pH and pOH: water undergoes self-ionisation:

$H_2O \rightleftharpoons H^+ + OH^-$ , with $K_w = [H^+][OH^-] = 10^{-14}$ at 25°C.

Taking $-\log$ of both sides:

$-\log K_w = -\log[H^+] - \log[OH^-]$

$pK_w = pH + pOH$

Since $K_w = 10^{-14}$ at 25°C, $pK_w = 14$ , so:

$pH + pOH = 14$

✓Final answer
$H^+$

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Q.What is chemical equilibrium ?

Concept understanding — Ionization of Acids and Bases

Q.What is chemical equilibrium ?

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