Extent and Rate of Reaction 1

Question 1

What does molecular motion give insight into?

Answer 1

• The transfer of energy and variation of pressure
• Collision frequency

Question 2

What do transport properties allow us to understand?

Answer 2

How molecules move through phases:
- Ionic movement
- Effect of viscosity
- Diffusion

Question 3

What is the movement of molecules in solid form treated as?

Answer 3

A vibration about mean lattice position

Question 4

State the ideal gas equation.

Answer 4

PV = nRT

Question 5

What are the four principles of explaining behaviour of an ideal gas?

Answer 5

• A gas is made of a very large number of molecules so small compared to the distances between them
• The molecules are in constant, rapid, straight line motion, colliding frequently
• Collisions are perfectly elastic - no kinetic energy is lost or gained during collision - do not exert any attractive or repulsive forces on each other
• The average kinetic energy is proportional to temperature (K)

Question 6

What is the speed of molecules proportional to?

Answer 6

• The temperature and inversely proportional to the molecular mass

Question 7

What is collision frequency proportional to?

Answer 7

• Temperature at constant volume and to pressure at constant volume

Question 8

What is viscosity?

Answer 8

For a molecule to move it requires energy to escape its neighbour

Question 9

When does viscosity decrease?

Answer 9

With an increase in temp. as molecules have more kinetic energy

Question 10

Describe ionic movement.

Answer 10

• Ions move through a solvent by the application of a potential difference
• Resistance is used to ionic motion and inverse of conductance
• Motion of an ion remains random but the application of an electric field biases its motion and subsequent migration through solution

Question 11

When does conductance decrease and increase?

Answer 11

• Conductance decreases with length and increases with cross sectional area of a medium
• Conductance increases with an increase in the number of ions present, Molar conductivity,
  Lm = k/c

Question 12

What are ion channels?

Answer 12

Highly selective proteins that move ions down a potential gradient driven by the hydrolysis of ATP

Question 13

Give the thermodynamic definition of diffusion.

Answer 13

A force that represents the spontaneous tendency of molecules to disperse to a situation of greater disorder

Question 14

What does Fick’s law state?

Answer 14

• Diffusion is greater when the concentration varies steeply with position

Question 15

Define phenomenological diffusion.

Answer 15

• Diffusion is the movement of a substance from a region of high concentration to one of low concentration without bulk motion

Question 16

Define atomistic diffusion.

Answer 16

• Random walk of molecules, not so random, molecules are propelled by collisions in high
  concentration areas

Question 17

Define kinetic energy.

Answer 17

Energy that an object possesses because of its motion

Question 18

Describe how energy in a molecule is distributed.

Answer 18

• Rotation is the movement of a molecule about an axis
• Translation is the movement from one place to another
• Vibration is the motion of one atom in relation to another within the molecule

Question 19

Which motion is only allowed for molecules in the solid state?

Answer 19

Vibration

Question 20

Why do we need to know about the movement of molecules?

Answer 20

• To assist in understanding reaction mechanisms
  and optimise synthetic reactions
• Improve yield
• Speed up processes
• Minimise by-products
• Predict or extend shelf-life
• e.g. aspirin hydrolysis is pH dependant

Question 21

Describe the mechanism of breathing.

Answer 21

• Lungs expand, increase in volume in alveoli causes a partial vacuum and a pressure gradient
• Get a bulk flow of air down the pressure gradient
• Air arriving in the lungs has a high molecular O2 concentration
  than the blood in the capillaries surrounding the alveoli
• See diffusion of O2 molecules down this concentration
  gradient into the blood

Question 22

Describe rate of reaction.

Answer 22

• the greater the number of collisions, the faster the reaction
• the smaller the activation energy, the greater the number of collisions with sufficient energy to react
• Molecules need to be in the correct orientation

Question 23

How to calculate rate of reaction?

Answer 23

rate of reaction = (number of collisions per unit time) x (fraction with sufficient energy) x (fraction with appropriate orientation)

Question 24

State and describe the factors that affect rate.

Answer 24

• Physical state – reactants have to be in the same phase i.e. dissolved.
• Concentration – increasing concentration increases frequency of collision
• Temperature – increasing the kinetic energy of the atoms and the frequency of collisions provides an increase in the number of collisions with sufficient energy to exceed the activation barrier.
• Catalysts – No effect on overall energies, catalyst effects the activation energy

Question 25

Give the rate equation?

Answer 25

Rate = k[A]^n[B]^m
- n is the order of the reaction with respect to A
- m is the order of reaction with respect to B
- (n+m)

Question 26

State the effect of temp. on reaction rates.

Answer 26

• The rate constants increase with temp.

Question 27

State the Arrhenius equation.

Answer 27

k = Ae^ -Ea/RT
- k is the rate constant (sec-1)
- A the frequency factor, representing the number of collisions occurring with the correct orientation to react
- e^-Ea/RT represents the fraction of collisions with the minimum energy to react
- where Ea is the activation energy (kjmol-1)
- R is the universal gas constant (8.314x10-3 kjmol-1K-1)
- T is temperature (K)

Question 28

Describe the effect of catalysts upon reaction rates.

Answer 28

• A catalyst participates in the reaction but is not consumed by it
• The stability of reactants and products does not change from the un-catalysed reaction
• A catalyst increases the rate of a reaction as it modifies the reaction energy pathway, such
  that ΔG‡ is smaller

Question 29

What are the three ways catalysts increase the rate of reaction?

Answer 29

• Activate the reactant TM chemistry
• Stabilise (and reduce energy of) transition state
• Different reaction mechanism

Question 30

How can we measure the rate of reaction?

Answer 30

• Follow the concentration of a component of the reaction, a reactant, as a function of time
• Decrease in reactant concentration
• Increase in product concentration
  A + B → C
  Rate =−ΔA/Δt =−ΔB/Δt =−ΔC/Δt