Energy Changes

Question 1

When is the forward and reverse reaction favoured?

Answer 1

G positive = reverse

G negative = forward

Question 2

What is fugacity?

Answer 2

The effective partial pressure used to account for non-ideal behaviour in real gases.

Question 3

What is activity?

Answer 3

The effective mole fraction, used to account for non-ideal behaviour in real solutions.

Question 4

What is chemical potential?

Answer 4

The energy change when 1 mole of a substance is added to a system at constant conditions.

Question 5

At equilibrium, are the stoichiometric coefficients of the reactants positive or negative?

Answer 5

Negative

Question 6

Derive the equation for free energy at STP.

Answer 6

1. G = sum (vu) = 0
2. G (STP) = sum (vu)
3. GAS: u - u(STP) = RT ln(f/f(STP))
   LIQUID: u - u(STP) = RT ln(a)
   2 - 1
   G - G(STP) = sum[v(u - u(STP))]
   SUB IN 3
   G - G(STP) = v(RT ln(a))
   G - G(STP) = RT lnK

Question 7

Assuming H and D are independent of temperature, write an equation for H.

Answer 7

d(ln K)/d T = H(STP)/RT^2

Question 8

Assuming H and D are independent of temperature, write an expression for H(T2).

Answer 8

H(T2) = H(T1) + Cp

Question 9

For an exothermic and endothermic reaction is H positive or negative?

Answer 9

H positive - endothermic

H negative - exothermic

Question 10

Describe what happens to conversion in an exothermic reaction at equilibrium when T increases.

Answer 10

Conversion decreases with increasing T (LHS) - system acts to reduce stress on system by favouring reverse.

Question 11

Describe what happens to conversion in an endothermic reaction at equilibrium when T increases.

Answer 11

Conversion increases with increasing T (RHS) - system acts to reduce stress on system by favouring forward.

Question 12

State the Arrhenius equation

Answer 12

kC = A exp(-E/RT)

Question 13

Derive the equation for the conversion rate in a CSTR accounting for kinetics and equilibrium.

Answer 13

In = out + reacted
QCA0 = QCA + V(kfCA - kr(CA0-CA))
xA = 1/[(1/tkf)+1+(1/K)]

Question 14

What happens in a CSTR for a reversible exothermic reaction?

Answer 14

High T = kinetics fast, conversion/rate no effect. Conversion always achieved.
Low T = kinetics slow, K is high but conversion is not achieved.

Question 15

What happens in a CSTR for a reversible endothermic reaction?

Answer 15

Lower T required to achieve a given conversion. High T preferred as kinetics are favourable but conversion is always achieved.

Question 16

What happens in a CSTR for a irreversible reaction?

Answer 16

100% conversion always reached over infinite time as reaction is irreversible. Lower T for higher residence time and higher T for lower residence time but same conversion achieved so high preferred because of kinetics.

Question 17

How can you determine H from temperature and K data?

Answer 17

plot 1/T (x) vs lnK (y)

gradient = -H/R

Question 18

Does changing reaction temperature effect product distribution?

Answer 18

Yes