Thermal Properties of Matter Flashcards

Internal Energy

• Internal energy (U) is the energy store comprising the total kinetic energy (KE) and potential energy (PE) of all particles in a substance.

• U = KE{total} + PE{total}

• Kinetic energy is due to the motion (including vibration) of particles.

• Potential energy is due to the distances and forces between particles.

• Temperature change affects KE; a rise increases KE, while a fall decreases KE. Energy is transferred to the surrounding medium through heating or electromagnetic waves (infrared radiation).

• Change of state affects PE; as particles move further apart (solid to liquid/gas), PE increases, thus increasing internal energy, and vice versa.

• Internal energy is measured in joules (J).

• Conservation of energy dictates that heating a substance increases the KE of particles, raising its temperature. Further heating can cause a change in state, increasing the PE of particles.

Heat Capacity and Specific Heat Capacity

Heat Capacity

• Heat capacity (C) is the measure of a body's ability to store internal energy.

• The capacity depends on:

  • Number of particles: More particles allow more energy storage; larger mass implies larger heat capacity.

  • Nature and strength of forces between particles: Stronger forces require more energy transfer for particles to gain KE.

• Heat capacity (C) is defined as the change in internal energy per unit change in temperature.

• C = \frac{\Delta U}{\Delta \theta}

• Q = C\Delta\theta

• SI unit: J K⁻¹ (or J °C⁻¹)

Specific Heat Capacity

• Specific heat capacity (c) is the change in internal energy per unit mass for each unit change in temperature.

• c = \frac{C}{m} = \frac{\Delta U}{m\Delta \theta}

• Q = mc\Delta\theta

• SI unit: J kg⁻¹ K⁻¹ (or J kg⁻¹ °C⁻¹)
  *Solids typically have lower specific heat capacities than liquids, and gases have higher specific heat capacities.
  *Water has a very high specific heat capacity, which leads to several applications:
  *Oceans act as heat sinks influencing global climate.
  *It moderates sea and land breezes due to slow temperature changes.
  *Water is used in central heating systems, car engines, and hot water bottles.
  *Sweat cools the skin during hot weather because its major component is water.
  *Cookware is made of different materials depending on use; metal pots have low heat capacities and clay pots maintain heat for longer periods.

Determination of Specific Heat Capacity of a Solid

1. Insulate a metal block of known mass m using lagging.

2. Insert a heater into a hole drilled in the metal block.

3. Insert a thermometer into a second hole to measure temperature rise \Delta \theta .

4. Record time t using a stopwatch.

   • Energy transferred electrically: Q = mc\Delta\theta

   • If P is power and IV is electrical power, then Pt = mc\Delta\theta or IVt = mc\Delta\theta

   • c = \frac{Pt}{m\Delta\theta} = \frac{IVt}{m\Delta\theta}

Determination of Specific Heat Capacity of a Liquid

1. Insulate a calorimeter of known mass mc and specific heat capacity cc.

2. Measure mass of liquid m_l using an electronic balance.

3. Insert heater into the liquid.

4. Insert thermometer to measure the temperature rise \Delta \theta .

5. Record time t using a stopwatch.

   • Reduce energy loss via evaporation using an insulating lid.

   • A polished copper calorimeter minimizes energy transfer due to radiation.

   • Use a copper stirrer to ensure uniform temperature.

   • Energy transferred electrically = Internal energy gained by liquid and calorimeter: Q = (mc\Delta\theta)l + (mc\Delta\theta)c

   • Pt = (mc\Delta\theta)l + (mc\Delta\theta)c

   • IVt = (mc\Delta\theta)l + (mc\Delta\theta)c

   • cl = \frac{IVt - (mccc\Delta\theta)}{(ml\Delta\theta)}

Latent Heat and Specific Latent Heat

Melting and Solidification

• Melting is the change from solid to liquid state with energy transfer without a temperature change. The temperature is called the melting point.

• Solidification is the reverse process.

• Melting occurs when intermolecular forces weaken, allowing molecules to move. During melting, energy transferred is the latent heat of fusion.

Latent Heat of Fusion

• Latent heat of fusion (L_f) is the energy transferred during a change between solid and liquid states at constant temperature.

• SI unit: J

Specific Latent Heat of Fusion

• Specific latent heat of fusion (l_f) is the energy transferred per unit mass during a change between solid and liquid states at constant temperature.

• lf = \frac{Lf}{m}

• SI unit: J kg⁻¹

Determination of Melting Point/Solidification Point (of Naphthalene)

1. Place naphthalene in a boiling tube.

2. Lower the tube into boiling water until naphthalene melts.

3. Insert a thermometer.

4. When the temperature reaches 90°C, remove the tube from the beaker.

5. Record the temperature every minute until it falls to about 65°C.

6. Plot a temperature-time graph.

7. Deduce the solidification point from the graph.

   • The melting/solidification point of naphthalene is identified from the graph where temperature remains constant during the change of state.

Boiling and Condensation

• Boiling is the change from liquid to gaseous state with energy transfer, without a change in temperature.

• The temperature is called the boiling point.

• Condensation is the reverse of boiling.

• During boiling, energy is used to weaken intermolecular forces, allowing molecules to move far apart. Energy is also used to push back against atmospheric pressure.

• The energy absorbed is called the latent heat of vaporization.

Latent Heat of Vaporization

• Latent heat of vaporization (L_v) is the energy transferred during a change between liquid and gaseous states at constant temperature.

• SI unit: J

Specific Latent Heat of Vaporization

• Specific latent heat of vaporization (l_v) is the energy transferred per unit mass during a change between liquid and gaseous states at constant temperature.

• lv = \frac{Lv}{m}

• SI unit: J kg⁻¹

Molecular Explanation of Latent Heat

• Latent heat refers to the change in internal energy hidden from a thermometer.

• At melting point, energy weakens intermolecular forces, allowing molecules to slide past each other, thus melting occurs. The temperature remains constant as molecules gain potential energy instead of kinetic energy.

• Latent heat of fusion weakens intermolecular forces in a solid.

• On cooling a liquid, intermolecular forces strengthen, binding molecules into a solid. Potential energy decreases, and the latent heat of fusion (solidification) is transferred to the surrounding medium. Temperature remains constant.

• During boiling, energy increases molecular kinetic energy, overcomes intermolecular forces, and does work against atmospheric pressure.

Applications of Latent Heat

Refrigerator

1. Compressor compresses warm coolant vapor.

2. Vapor flows through the external coil at high pressure.

3. Coolant transfers internal energy to the surrounding air.

4. Coolant vapor condenses into liquid.

5. Coolant enters the refrigerator.

6. The interior transfers energy to the coolant, cooling down the contents.

7. Warmer coolant flows back to the compressor, repeating the cycle.

Boiling and Evaporation

• Boiling occurs at a specific temperature throughout the liquid, forming bubbles, and requires an external energy source.

• Evaporation occurs at any temperature only at the surface of the liquid, without bubbles, and energy is supplied by the surrounding medium.

Explanation of How Evaporation Occurs

1. Molecules near the surface with higher KE escape into the air.

2. The average KE of liquid molecules decreases, lowering temperature.

3. Energy from the surrounding air is transferred to the liquid.

4. The KE of the molecules increases, repeating the process.

Applications of Evaporation

• Evaporation cools the skin.

• Evaporation dries wet laundry.

• Evaporation is used in the perfume industry.

Factors Affecting Evaporation

1. Temperature: Higher temperature increases evaporation rate.

2. Area of exposed surface: Greater area increases evaporation rate.

3. Humidity of surrounding air: Less humidity increases evaporation rate.

4. Motion of air: Greater air speed increases evaporation rate.

5. Atmospheric Pressure: Lower pressure increases evaporation rate.

6. Boiling Point of Liquid: Lower boiling point increases evaporation rate.