ANAND CLASSES Study Material and Notes to understand the effect of temperature on normality and molarity in solutions. Learn why normality and molarity vary with temperature, how volume expansion affects concentration, and its impact on chemical reactions.
How Does Temperature Affect Normality?
Normality (N) = Molarity (M) × n-factor
Since normality depends on molarity, any factor that affects molarity will also impact normality. Temperature plays a key role in this because molarity is affected by temperature, but normality remains mostly unchanged in most cases.
Temperature Affects Molarity (M)
- Molarity (M) = Moles of solute / Volume of solution (L)
- Liquids expand with increasing temperature, increasing volume.
- Since the number of moles remains constant but volume increases, Molarity decreases as temperature increases.
Example:
- If 1 M H2SO4 solution is heated, the volume expands, leading to a lower molarity.
- Since N = M × n-factor, a decrease in M leads to a decrease in N.
Why Normality Remains Largely Unaffected?
- For most solutions, Normality changes with temperature only because of volume expansion, just like Molarity i.e. Normality decreases as temperature increases.
- However, in cases where the reaction or n-factor depends on temperature, normality may change significantly.
Cases Where Temperature Affects Normality
| Case | Effect on Normality |
|---|---|
| Dilute Aqueous Solutions | Normality decreases with increasing temperature due to expansion. |
| Concentrated Solutions | Minor changes, as expansion is less significant. |
| Strong Acids/Bases | No change in n-factor, but normality decreases with temperature due to volume expansion. |
| Temperature-Sensitive Reactions (e.g., Weak Acids & Redox Reactions) | If n-factor changes with temperature, normality may increase or decrease. |
Example.1: Effect of Temperature on Sulfuric Acid (H2SO4) Solution
Scenario:
- A 1 N H2SO4 solution at room temperature is heated to 80°C.
- Since H2SO4 completely ionizes in water, its n-factor remains 2.
- However, as temperature increases, the solution expands, increasing volume.
Effect:
- Molarity decreases due to the increased volume.
- Since N = M × n-factor, normality also decreases slightly.
- However, as the n-factor remains unchanged, the relative decrease in normality is the same as in molarity.
Example.2: Weak Acid (Acetic Acid – CH3COOH) in Water
Scenario:
- A 1 M acetic acid solution is prepared at 25°C and heated to 60°C.
- Acetic acid is a weak acid that partially ionizes: $$CH₃COOH ⇌ CH₃COO^- + H^+$$
- The degree of ionization (α) increases with temperature.
Effect:
- Higher temperature leads to more dissociation of CH3COOH.
- The n-factor increases because more H+ ions are released.
- Normality increases due to the increased ionization.
Example.3: Sodium Hydroxide (NaOH) Solution
Scenario:
- A 1 N NaOH solution is stored at different temperatures.
- NaOH undergoes minor self-ionization variations with temperature.
Effect:
- For dilute NaOH solutions, normality slightly decreases due to volume expansion.
- For concentrated NaOH solutions, normality remains nearly constant because expansion is negligible.
Example.4: Redox Reaction of Potassium Permanganate (KMnO4)
Scenario:
- KMnO4 in acidic medium acts as an oxidizing agent: $$MnO_4^- + 8H^+ + 5e^- \rightarrow Mn^{2+} + 4H_2O$$
- At higher temperatures, a different reaction may dominate, changing the number of electrons transferred.
Effect:
- Since the n-factor depends on the oxidation state, the normality changes.
- Higher temperatures may lead to alternate reaction pathways, affecting the redox potential.
Example.5: Precipitation Reaction – Ba(OH)2 in Water
Scenario:
- Ba(OH)2 is sparingly soluble in water.
- Its solubility increases with temperature, leading to more OH– ions in solution.
Effect:
- As more OH– ions dissolve, the n-factor remains constant, but molarity increases.
- Therefore, normality increases as well.
Example.6: Temperature-Dependent Normality in Redox Reactions
- Consider Cr2O72- (Dichromate) in Acidic Medium
- At low temperatures, it follows: $$Cr₂O₇^{2-} + 14H^+ + 6e^- \rightarrow 2Cr^{3+} + 7H₂O$$
- n-factor = 6
- At higher temperatures, it may undergo a different reaction with a different n-factor.
- At low temperatures, it follows: $$Cr₂O₇^{2-} + 14H^+ + 6e^- \rightarrow 2Cr^{3+} + 7H₂O$$
Thus, in redox reactions, temperature may alter the oxidation states involved, changing the n-factor and affecting Normality.
Key Takeaways:
✅ Strong acids and bases: Normality decreases slightly due to volume expansion.
✅ Weak acids and redox reactions: Normality may increase if ionization or reaction pathways change.
✅ Sparingly soluble salts: Normality increases if solubility increases with temperature.
🔷 Best Coaching Center for IIT-JEE, NEET, and Foundations
🔹 ANAND CLASSES – The best coaching institute for JEE, NEET, and Class 11-12 Foundations.
🔹 Buy complete study material at 👉 https://publishers.anandclasses.co.in/
🔹 Proprietor: NIRMAL ANAND Educations
🔹 Written by: Neeraj Anand
🔹 Published by: Anand Technical Publishers under Anand Classes
📞 Contact: +91-9463138669
📧 Email: anandclasses1996@gmail.com
