2 Heat, Work, Internal Energy, Enthalpy, First Law of Thermodynamics

Question 2

What is internal Energy?

Answer 2

Energy relative to the system rather than to a paticular frame of reference. e.g. A still glass of water has no energy from the point of view of an observer but at the microscopic level, it is a mass of high energy molecules. Symbol for internal energy is U.

Question 3

State the forms that constitutes internal energy.

Answer 3

1. Kinetic energy of molecules. 2. Potential energy of constituents of system. 3. energy stored in the form of molecular vibrations and rotations. 4. Energy stored in the form of molecular bonds.

Question 4

State the first law of thermodynamics.

Answer 4

Internal energy of an isolated system is constant. ΔU(total) = ΔU(system) + ΔU(surroundings) = 0. Hence First law of thermodynamics: ΔU(system) = -ΔU(surroundings). For any change in ΔU(system), ΔU(surroundings) must change oppositely in the same amount.

Question 5

How does energy change in a closed system with no chemical reactions and phase changes?

Answer 5

Energy evolved from the system can be classified as work, heat, or a combination of both. ΔU = q + w.

Question 6

Define ‘isothermal’.

Answer 6

Constant temperature.

Question 7

Define ‘isobaric’

Answer 7

Constant pressure.

Question 8

Define ‘isochoric’

Answer 8

Constant volume.

Question 9

Define ‘Work’.

Answer 9

In thermodynamics, work is any quantity of energy that flows across the boundary between system and surroundings that can be used to change the height of a mass in the surroundings.

Question 10

What are the characteristics of work?

Answer 10

1. Work is transitory. It only appears during a change in the state of the system and surroundings, unlike energy which is associated with initial and final states of system. 2. Changes U of system according to the first law of thermodynamics. 3. Quantitative value of work can be calculated from the change in potential energy of mass. E(potential) = mgΔh. 4. Sign convention: if height of mass in surroundings is lowered, w is positive, if mass is raised, w is negative. In short w>0 when ΔU>0. w

Question 11

State the different types of work.

Answer 11

Volume expansion/compression, Stretching/compressing, Surface expansion, Electrical.

Question 12

State the equation for work of volume expansion.

Answer 12

w = - ∫P(external) dV, wher P is pressure, V is volume.

Question 13

State the equation for work of stretching a mass.

Answer 13

w = - ∫F dl, where F is force acting perpendicular to system, l is length of extension/compression.

Question 14

State the equation for work for surface expansion.

Answer 14

w = - ∫γ dσ, where γ is surface tension, σ is area.

Question 15

State the equation for Electrical work.

Answer 15

w = - ∫Φ dQ, where Φ is electrical potential, Q is electrical charge.

Question 16

Define ‘Heat’.

Answer 16

Quantity of energy that flows across boundary between system and surroundings because of a temperature difference across the boundary. Heat always flow spontaneously from regions of high temperature to low temperature.

Question 17

State the characteristics of heat.

Answer 17

1. Heat is transitory, it only appears during a change in state of system and surroundings. Unlike energy which which is associated with initial and final states of system and surroundings. 2. Net effect of heat is to change the internal energy of system and surroundings in accordance with first law. Characterized by a change in temperature. 3. Sign convention: If temperature of surroundings is lowered, q is positive. If raised, q is negative. In other words, when q0, heat is withdrawn form the surroundings and deposited in the system.