Thermo.Kinetics Test II

Question 1

Thermodynamics

Answer 1

• Why a chemical reaction or process occurs
• To what extent the reaction or process occurs
• Direction and extent of a reaction or process.

Question 2

Kinetics

Answer 2

• How fast a reaction occurs
• What changes occur at the molecular level to bring about the change from reactants to products
• The rate of reaction.

Question 3

Reversible processes

Answer 3

• occur infinitely slow so takes an infinitely long time to complete
• executed via a large number of very small steps
• completely unrealistic, goal to reach for

Question 4

Irreversible processes

Answer 4

• Happens in one big step
• Happens in finite time, hence are REAL
• Less thermodynamically efficient as less work is done.

Question 5

Spontaneous Processes vs. Non spontaneous processes

Answer 5

Spontaneous

• Occurs without assistance
• Irreversible
• K>1

A process that goes in a direction opposite to that of a spontaneous one in classified as non spontaneous.
Needs energy input to make it go in that direction.

Question 6

1st Law of Thermodynamics

Answer 6

ΔU= q + w
Change in internal energy of a process is equal to the sum of work done and heat transferred.

Energy is never created nor destroyed but simply transferred.

Question 7

2nd Law of Themodynamics

Answer 7

• In any spontaneous process, the total entropy increases.
• Demonstrates the natural order of reactions and processes.

Irreversible
ΔS surr + ΔS sys = ΔS total >O

Reversible
ΔS surr + ΔS sys = ΔS total =O

Question 8

Free energy changes

Answer 8

ΔG sys= ΔH syst - TΔS syst

If ΔG sys is 0 reaction is not spontaneous.

Question 9

3rd Law of Thermodynamics

Answer 9

Entropy of a pure crystalline substance at 0 K is zero.

hence there is a perfect order in a perfect crystal at T=0 K

Question 10

Zeroth Law of Thermodynamics

Answer 10

Two systems which are in equilibrium with a third system must be in equilibrium with each other.

Allows us to effectively measure the temperature of an object with a thermometer.

Question 11

Primary temperature standard

Answer 11

The constant volume gas thermometer (CVGT)

-has a fixed volume and is charged with a fixed number of moles of gas

Question 12

Measuring an unknown “absolute” temperature using the CVGT

Answer 12

1. Prepare CVGT containing given gas at constant V and n
2. Expose gas to unknown T and allow thermal equillibrium to take place and measure its pressure =(Ptp)1
3. Expose gas to a sample of water kept at triple point temp (Ttp) and measure its pressure (Ptp)1
4. Repeat above but decrease moles of gas and hence obtain values P2 and (Ptp)2 etc.
5. Plot (P/Ptp)v vs. Ptp and extrapolate Ptp to zero to give lim Ptp->0 (Ps/Ptp)=(Tunknown/Ttp)=known intercept.

Question 13

James Joule experiment leading to the first law of thermodynamics
ΔU= w + q

Answer 13

Involved a closed calorimeter

(i) An adiabatic experiment where q=O and T was increased by using mechanical action of a paddle inside (W expt)
(ii) A ‘heat’ expt where heat was added by addition of hot objects ∆U=q

Both lead to ∆T, same result but different pathways

∆U= q + w
Heat and work are aspects of the same thing.

Question 14

Reversible isothermal expansion of an ideal gas

Answer 14

Consider an ideal gas which is enclosed in a cylinder with a frictionless, weightless piston.
-Gas is allowed to expand in cylinder by pushing against the frictionless, weightless piston.
-Gas is increasing by infinitesimal amounts
Pext=Pint
-Temp remains the same

Question 15

Irreversible isothermal expansion of an ideal gas

Answer 15

-A situation where the system is not in equilibrium throughout but is at constant temperature.

Question 16

Reversible adiabatic expansion of an ideal gas

Answer 16

An ideal gas undergoing adiabatic expansion is contained in a vessel for which the walls are perfectly insulating so that no heat exchange with the surroundings can occur.

As the internal energy of the gas is known to depend directly on temperature
-Temp of the gas must also decrease

Question 17

Perpetual motion machine

Answer 17

One that supposedly goes on forever without needing any energy input, appears to produce power it needs itself.

Question 18

Explain why perpetual motion machines are impossible to construct in reality

Answer 18

• Violates the 1st and 2nd law of thermodynamics
• In an isolated system, one cannot create new energy. Cannot its own energy, will run down
• Cannot operate with 100% efficiency-friction losses

Question 19

The Carnot Cycle

Answer 19

A thermodynamic cycle which involves taking a system through a series of different states before being returned to its initial state.

Transfers energy from a warn region to a cold region and converts part of that to mechanical energy.

-Regarded as the most efficient cycle possible for converting a given amount of thermal energy into work.

Question 20

4 processes comprising Carnot cycle

Answer 20

1=Reversible isothermal expansion (T1)
2= Reversible adiabatic expansion (T3)
3= Reversible isothermal compression (T3)
4= Reversible adiabatic compression (T1)

Question 21

Explain briefly why the entropy change for the spontaneous formation of water ice from supercooled, liquid water at 265.15 K is greater than merely cooling preformed ice from 273.15 K to 265.15 K`

Answer 21

Its a spontaneous formation of water because it will have a ∆S component due to the phase transition of l->s which is relatively larger than the ‘cooling’ component.

Question 22

Gibbs free energy

G=H-T.S

Answer 22

∆G°system is negative= feasible, equilibrium favours products (K>1)

∆G°system is zero= reaction is balanced (K=1)

∆G°system is positive= reaction not feasible, equilibrium favours the reactants (K

Question 23

Reaction mechanism

Answer 23

1. Determine overall stoichiometric balance of reactants/products
2. Determine the kinetics of each reactant/product
3. Investigate how the reaction varies with temp
4. Propose a mechanism
5. Test and refine mechanism with further experimental investigation

Question 24

Zero order reaction and half life

Answer 24

[A]t= -kt[A]0

t1/2 - 1/2 [A]o/k

Question 25

Explain the practical reason why for most catalyst used on an industrial scale are heterogenous-type catalysts

Answer 25

Ease of separation of solid catalysts from liquid product mix

Question 26

What are some of the strategies that are commonly used to increase the surface area of a solid catalyst?

Answer 26

• Increase the surface area of catalyst by using high internal surface area materials
• Dispersing catalyst on cheap high surface area supports.

Question 27

Briefly describe how competitive inhibition reduces the apparent rate of enzyme catalysed reactions

Answer 27

E + S ES

Competes with E + I EI
This reduces enzyme available. Therefore increases km, relative to km for uninhibited reactions. in Lineweaver burk plots.

Question 28

Homogenous catalysis

Answer 28

The catalysis is in the same phase as the reactants.