Thermal Energy vs Intermolecular Forces - UNSOLVED PRACTICE SET
Chapter: States of Matter | Topic: Thermal Energy vs Intermolecular Forces
THERMAL ENERGY VS INTERMOLECULAR FORCES - UNSOLVED PRACTICE SET
Topic: Thermal Energy vs Intermolecular Forces
Multiple Choice Questions
Q1. The three states of matter are determined by the balance between:
- Mass and volume
- Thermal energy and intermolecular forces
- Pressure and temperature
- Kinetic energy and potential energy
Q2. In solids, the intermolecular forces are:
- Much weaker than thermal energy
- Much stronger than thermal energy
- Equal to thermal energy
- Zero
Q3. When thermal energy exceeds intermolecular forces, the substance exists as:
- Solid
- Liquid
- Gas
- Plasma
Q4. The temperature at which the thermal energy of particles is sufficient to overcome the intermolecular forces holding them in fixed positions is called:
- Boiling point
- Melting point
- Critical temperature
- Triple point
Q5. In gases at high temperatures:
- Intermolecular forces dominate
- Thermal energy dominates
- Both are equal
- Neither has any effect
Q6. Which state of matter has particles with thermal energy approximately equal to intermolecular forces?
- Solid
- Liquid
- Gas
- All states
Short Answer Questions
Q7. Explain the balance between thermal energy and intermolecular forces in solids, liquids, and gases.
Q8. Why do solids have definite shape and volume, while gases have neither? Explain in terms of thermal energy and intermolecular forces.
Q9. Explain why increasing temperature can cause a solid to melt and then vaporise, using the concept of thermal energy overcoming intermolecular forces.
Q10. Why do substances with stronger intermolecular forces generally have higher melting and boiling points?
Q11. At a given temperature, neon exists as a gas while water exists as a liquid. Explain this difference in terms of the relative strengths of thermal energy and intermolecular forces.
Q12. What happens to the motion of particles when a substance is heated? How does this affect the intermolecular forces holding the particles together?
Long Answer Questions
Q13. (a) Explain how the balance between thermal energy and intermolecular forces determines the physical state of a substance.
(b) Draw a simple diagram or describe the arrangement of particles in solids, liquids, and gases, highlighting the relative strengths of thermal energy and intermolecular forces in each state.
(c) Explain why liquids have definite volume but no definite shape, while gases have neither definite volume nor definite shape.
Q14. (a) Define thermal energy and intermolecular forces. How do they compete with each other to determine the state of matter?
(b) Explain the process of melting and boiling in terms of thermal energy overcoming intermolecular forces.
(c) Why does increasing pressure favour the conversion of gas to liquid, even at constant temperature? Explain using the concept of intermolecular forces.
Q15. (a) Compare the relative magnitudes of thermal energy and intermolecular forces in:
(i) Ice at β10Β°C
(ii) Water at 25Β°C
(iii) Steam at 150Β°C
(b) Explain why substances with strong hydrogen bonding (like water) require more thermal energy to change state compared to substances with only London dispersion forces (like methane).
(c) In the Indian context, explain why LPG (liquefied petroleum gas) is stored as a liquid under pressure in cylinders, even though it is a gas at room temperature. What does this tell you about the relationship between pressure, thermal energy, and intermolecular forces?
Numerical / Application-Based Problems
Q16. The following table gives data for three substances:
| Substance | Melting Point (Β°C) | Boiling Point (Β°C) | ΞHfus (kJ/mol) | ΞHvap (kJ/mol) |
|---|---|---|---|---|
| A | -114 | 78 | 5.0 | 38.6 |
| B | 0 | 100 | 6.0 | 40.7 |
| C | β183 | β89 | 0.9 | 6.8 |
(a) Identify which substance has the strongest intermolecular forces and which has the weakest. Give reasons.
(b) Calculate the ratio ΞHvap/ΞHfus for each substance. What does this ratio tell you about the energy needed to overcome intermolecular forces in different phase changes?
(c) Substance B requires 40.7 kJ/mol to vaporise but only 6.0 kJ/mol to melt. Explain why vaporisation requires so much more energy than fusion.
(d) If 100 g of substance B is heated from β10Β°C to 110Β°C, sketch a rough heating curve and label the regions where thermal energy is overcoming different levels of intermolecular forces.
Q17. Consider the following substances and their intermolecular forces:
| Substance | Type of Intermolecular Forces | Melting Point (K) |
|---|---|---|
| NaCl | Ionic bonds | Data |
| HβO | Hydrogen bonding | Data |
| CHβ | London dispersion | Data |
| HCl | Data | Data |
(a) Explain why NaCl has the highest melting point despite not having intermolecular forces in the traditional sense.
(b) Calculate the ratio of melting points of HβO to CHβ. How does this ratio reflect the relative strength of their intermolecular forces?
(c) HCl has a higher molecular mass than HβO but a lower melting point. Explain this observation.
(d) A student argues that since ionic bonds are much stronger than hydrogen bonds, NaCl should exist as a solid even at 2000 K. Is this reasoning correct? What other factor must be considered?
Q18. In India, understanding the balance between thermal energy and intermolecular forces has practical applications:
(a) During summer, road surfaces sometimes develop cracks. Explain this in terms of thermal energy causing expansion and overcoming intermolecular forces in asphalt.
(b) Why does a pressure cooker cook food faster? Explain using the concept that increased pressure raises the boiling point by making it harder for thermal energy to overcome intermolecular forces.
(c) In cold hilly regions of India, water pipes sometimes burst in winter. Explain this using the concept of hydrogen bonding and the unusual behaviour of water when it freezes.
(d) A student observes that coconut oil is liquid in summer but solid in winter. Explain this phase change in terms of the competition between thermal energy and intermolecular forces (mainly London dispersion forces in triglycerides).