Conceptual knowledge - active recall

Question 1

Why is the continual motion of particles vital?

Answer 1

Their resultant kinetic energy can be used as chemical energy to drive reactions. i.e. for many biomolecules, their motions defines their activity.

Question 2

What is the difference between weight and mass?

Answer 2

Weight is a vector (W=g*m -> with direction), while mass is a scalar quantity.

Question 3

What are Newton’s laws?

Answer 3

1. What is at rest will remain at rest (consider net force)
2. F=m*a
3. every action has an equal and opposite reaction

Question 4

In a two-dimensional system, what mathematical concept is similar to the magnitude of a vector and thus how can it be calculated?

Answer 4

A two-dimensional vector is similar to a hypotenuse and can thus be calculated via Pythagoras’ theorem from its coordinates (same applies for any dimensions).

Question 5

What is required for a successful molecular collision?

Answer 5

A molecular collision is successful, if the particles have kinetic energies higher than the activation energy of the reaction, as well as the correct orientation for the reaction to proceed.

Question 6

Which quantities are considered to always be conserved?

Answer 6

Energy and momentum.

Question 7

What is the definition of an elastic collision?

Answer 7

Both the total kinetic energy and the total momentum are maintained and none are lost as other forms of energy. (“The total energy and momentum are the same before and after the collision.”)

Question 8

What is the definition of an inelastic collision?

Answer 8

Total kinetic energy is not maintained. Some energy is lost as other forms of energy (heat, friction, etc.).

Question 9

What makes an inelastic collision ‘perfect’?

Answer 9

If the two colliding objects have equal final velocities.

Question 10

In the case of chemical reactions, can successful collisions be expected to elastic, perfect inelastic or imperfect inelastic?

Answer 10

They would be expected to be imperfectly inelastic. If the collision is successful, kinetic energy will be lost converted to chemical to drive the reaction, making it inelastic. Having them have equal final velocities is very unlikely, thus it cannot be perfect.

Question 11

In what relative spacial arrangement does the cross-product of two vectors lie?

Answer 11

The cross product of two vectors produces a third vector orthogonal (at a 90 degree angle) to their plane.

Question 12

When looking at motion in a 3D system, how is the kinetic energy of a particle affected?

Answer 12

Ekin = 1/2 * m * |v|^2
|v|= magnitude (calculated with Pythagoras from vx, vy, and vz)

Question 13

Define potential energy:

Answer 13

The energy a body stores as a result of position or configuration.

Question 14

What is the difference between chemical potential and chemical potential energy?

Answer 14

Chemical potential is the energy stored as a result of a chemical system being out of equilibrium. Chemical potential energy is the energy stored in chemical bonds.

Question 15

What are two types of potential energy?

Answer 15

Coulomb potential (between two charges) and gravitational potential (between two masses).

Question 16

Define librations vs vibrations:

Answer 16

While vibrations are internal molecular motions of atoms relative to one another, librations are motions of an entire molecule, meaning that librations are translation motions such as rotations or moving a distance through space.

Question 17

What are a particle’s purely translational degrees of freedom?

Answer 17

A particle has 3 dfs when looking at its movement through space: in the x, y, and z direction.

Question 18

How can degrees of freedom for a single molecule in motion be calculated?

Answer 18

df = 3 * n (n = number of atoms in the molecule)

Question 19

Put the following intermolecular forces in order from strongest to weakest: hydrogen bonding, pi-pi, cation-pi, induced dipole - induced dipole, ion - dipole, dipole - induced dipole, ion - induced dipole, dipole - dipole

Answer 19

ion - dipole
cation - pi
hydrogen bonding
dipole - dipole
ion - induced dipole
pi - pi
dipole - induced dipole
induced dipole - induced dipole

Question 20

Explain the hydrophobic effect:

Answer 20

Non-polar and polar molecules will exclude one another, since the polar molecules have a lower entropy when in contact with non-polar molecules, which is energetically unfavourable.

Question 21

Define an open, closed, and isolated system in thermodynamics.

Answer 21

In an open system, energy and matter are transferred while in a closed system, only energy is transferred. An isolated system contains no transfer between system and surroundings.

Question 22

What is an adiabatic system?

Answer 22

A system in which no heat is transferred between system and surroundings. Adiabatic systems essentially do not exist in reality.

Question 23

What is meant when referring to a system as isothermal, isobaric, or isochoric?

Answer 23

Constant temperature, constant pressure, or constant volume

Question 24

How can the most probable speed (vp) and the mean speed (vm) be found from a Maxwell-Boltzmann curve?

Answer 24

vp can be found by setting dfv/dv = 0
vm can be found by utilising the integral

Question 25

How do we define the internal energy of a system?

Answer 25

The internal energy of a system is the sum of all the kinetic and potential energies within the system.

Question 26

What are the assumptions made by the ideal gas law?

Answer 26

1. There are no non-bonding interactions between the particles
2. The particles are sizeless/massless
3. all collisions are elastic

Question 27

Why is the ideal gas constant not always the same for different molecules and at different pressures?

Answer 27

As the pressure is increased, the van-der-Waals equation becomes less accurate and R begins to deviate from 8.324 J K-1 mol-1.

Question 28

What are the main two path functions and what do their signs constitute?

Answer 28

The two main path functions, heat and work, describe how a system can change its internal energy. If they are negative, this means the system is losing heat or doing work, while if they are positive, the system is being heated or work is being done on the system.

Question 29

If the external pressure is constant but a volume changes, how is the work done on/by the system defined?

Answer 29

w.d. = - pext * (Vf - Vi)

Question 30

If the expansion is reversible, the change in volume is usually quite small. How is the work done described in this case?

Answer 30

w.d. = -nRT * ln(Vf/Vi)

Question 31

What is the equipartition theorem?

Answer 31

For a system of n particles at a temperature of T, the energy stored in each degree of freedom is 1/2 * kB * T

Question 32

How many degrees of freedom in translational motion does molecule X have?

Answer 32

Three translational degrees of freedom corresponding to the x, y, and z direction.

Question 33

How many df does molecule X with N atoms have in total?

Answer 33

df = 3 * N

Question 34

How many vibrational df does molecule X with N atoms have?

Answer 34

df = 3 * N - 5

Question 35

Is it possible for a change to result in a decrease in entropy?

Answer 35

Yes, local increases in entropy may result in an overall decrease in entropy.

Question 36

How can the specific heat capacity of a gas be calculated at a constant pressure / volume?

Answer 36

constant pressure: cp = 5/2 * n * R
constant volume: cv = 3/2 * n * R

Question 37

What three conditions exist for equilibria of any kind?

Answer 37

No net change of the physical concept being considered.
This concept is evenly distributed.
It does not change over time.

Question 38

What are the two equations that contribute to the Eyring-Evans-Polanyi equation?

Answer 38

k = kB * T * K# / h
K# = exp(-G# / RT)

Question 39

Why do we sometimes use the van-der-Waals equation and what do a and b denote?

Answer 39

The van der Waals equation is used to adjust for more real circumstances given that the ideal gas equation is a rough approximation. With a it takes into consideration the intermolecular forces ( Pa m6 = J m3), while b accounts for concentration of the gas (m3 mol-1).

Question 40

What does Brownian motion refer to?

Answer 40

Brownian motion is the constant random motion of particles.

Question 41

What is Boyle’s law?

Answer 41

If temperature remains constant, p1V1=p2V2

Question 42

What is Gay-Lussac’s law?

Answer 42

If volume remains constant, p1/T1 = p2/T2

Question 43

What is Charles’ law?

Answer 43

If pressure remains constant, V1/T1 = V2/T2

Question 44

What is Avogadro’s law?

Answer 44

If pressure and temperature are constant, V1/n1 = V2/n2

Question 45

What is the combined gas law?

Answer 45

p1V1/T1 = p2V2/T2

Question 46

What is the general basis of the kinetic model?

Answer 46

If a particle is not hitting the wall orthogonally, only the x-component of its velocity is relevant.

Question 47

what is the internal energy a function of?

Answer 47

The number of moles n and the temperature T.