Chemical Equilibrium

Chemical Equilibrium Contributions by: John L. Falconer & Will MedlinDepartment of Chemical and Biological EngineeringUniversity of ColoradoBoulder, CO 80309-0424 Supported by the National Science Foundation

The exothermic reaction A  B reaches an equilibrium conversion of 25% in an adiabatic steady-state flow reactor. Which of the following will likely increase conversion? Add a catalyst Raise the inlet temperature Use a bigger reactor Add an inert to the feed All of the above

The endothermic reaction A  B reaches an equilibrium conversion of 25% in an adiabatic steady-state flow reactor. Which of the following will likely increase conversion? Add a catalyst Raise the inlet temperature Use a bigger reactor Remove inert from the feed All of the above

Why are most reactions carried out on a large scale exothermic (not endothermic)? Rate constants higher because temperature increase DS larger Equilibrium constants higher None of the above

Which is the correct energy balance plot (extent of reaction vs. temperature) for an adiabatic, exothermic reaction? A B x C D E Temperature

Consider the two reaction and their equilibrium constants: A + B  C K1 = 1000 A + D  3 E K2 = 78 Which conclusion is correct? Reaction 1 reaches equilibrium first Reaction 2 reaches equilibrium first Reaction 2 has a higher conversion at equilibrium None of these

For a gas phase reaction: A + B  C + D What are the units of the equilibrium constant? bar2 bar bar -1 It is dimensionless

For the gas phase reaction: The equilibrium constant is 10 at 2 atm. What is the equilibrium constant at 10 atm? 250 10 0.40 50 A  3 B

What is the expression for the equilibrium constant for the following reaction? A(l) and C(l) are immiscible, B is an ideal gas. Assume that B does not dissolve in liquids A or C. (xcc)/(xAAPB2) PB-2 xc/(xAPB2) fc/(fAPB2) fc/(fAB2PB2) A(l) + 2 B(g) C(l)

For which reaction would the equilibrium conversion increase if inert gases were added to a flow system at constant total pressure? 2A  B B  2 A A  B None of these

As the total pressure is increased for the following reaction, what will happen to Ka and to the equilibrium extent of reaction? Both with increase Both will decrease Both will stay the same Ka will stay the same, extent of rxn will increase Ka will increase, extent of rxn will stay the same C2H4(g) + ½ O2(g)  C2H3OH (l)

Given the following reaction: If steam is added to the feed and everything else kept the same, the equilibrium conversion ___________. increases decreases remains the same C4H8 C4H6+ H2

This reaction goes to equilibrium at 400°C in a closed container under two starting conditions: • 100 g Pd, 1 mol O2 • 100 g Pd, 1 mol O2, 1 g PdO The initial O2 pressure is high enough to reach equilibrium. The Pd and PdO are in separte phases. The final O2 pressure is _____________. higher for condition A higher for condition B the same for both conditions Pd + 0.5 O2 PdO

N2 CaCO3(s)CaO(s) + CO2(g) A constant volume system contains CaO(s), CaCO3(s), and CO2(g) at chemical equilibrium at 850 K. When 0.1 mol N2 is injected into the system at 850 K and constant pressure, what happens? Assume ideal gases. Some CaCO3 decomposes All the CaCO3 decomposes Some CO2 reacts All the CO2 reacts Nothing changes CO2 CaO(s) CaCO3(s)

N2 CaCO3(s)CaO(s) + CO2(g) A piston-cylinder system contains CaO(s), CaCO3(s), and CO2(g) at chemical equilibrium at 850 K. When 0.1 mol N2 is injected into the system at 850 K and constant pressure, what happens? Some CaCO3 decomposes All the CaCO3 decomposes Some CO2 reacts All the CO2 reacts Nothing changes CO2 CaO(s) CaCO3(s)

CaCO3(s)CaO(s) + CO2(g) A constant volume system contains CaO(s), CaCO3(s), CO2(g), and N2(g) at chemical equilibrium at 850 K. The N2 is removed through a selective membrane at 850K and constant pressure. Assume ideal gas. What happens? Some CaCO3 decomposes All the CaCO3 decomposes Some CO2 reacts All the CO2 reacts Nothing changes CO2 N2 CaO(s) CaCO3(s)

CaCO3(s)CaO(s) + CO2(g) A piston-cylinder system contains CaO(s), CaCO3(s), CO2(g), and N2(g) at chemical equilibrium at 850 K. Some N2 and CO2 are removed at 850K and constant pressure using a syringe. What happens? Some CaCO3 decomposes All the CaCO3 decomposes Some CO2 reacts All the CO2 reacts None of the above CO2 CO2 N2 CaO(s) CaCO3(s)

CaCO3(s)CaO(s) + CO2(g) A piston-cylinder system contains CaO(s), CaCO3(s), CO2(g), and N2(g) at chemical equilibrium at 850 K. All the N2 is removed through a selective membrane at 850K and constant pressure. What happens? Some CaCO3 decomposes All the CaCO3 decomposes Some CO2 reacts All the CO2 reacts Nothing changes CO2 N2 CaO(s) CaCO3(s)

CaCO3(s)CaO(s) + CO2(g) 0.2 mol CaO, 10 mol CaCO3, and 10 mol CO2 are in a piston-cylinder system. Starting at equilibrium, the piston is pushed down at constant temperature until the volume is half its original value. What happens? CO2 pressure almost doubles CaO and CO2 react, so CO2 pressure does not change System is at equilibrium, nothing changes All the CO2 reacts CO2 CaCO3(s) CaO(s)

CaCO3(s)CaO(s) + CO2(g) Staring at equilibrium, the piston is pushed down until the volume is half its original value while temperature is kept constant, what happens? CO2 pressure is higher, amount of CaCO3 is higher CaO and CO2 react, so CO2 pressure does not change System is at equilibrium, nothing changes All the CO2 reacts 0.1 mol CO2 1 mol CaCO3(s) 10 mol CaO(s)

CaCO3(s)CaO(s) + CO2(g) Staring at equilibrium, the piston is pushed down until the volume is half its original value while temperature is kept constant, what happens? CO2 pressure is higher, amount of CaCO3 is higher CaO and CO2 react, so CO2 pressure does not change System is at equilibrium, nothing changes All the CO2 reacts 5 mol CO2 1 mol CaCO3(s) 5 mol CaO(s)

A piston-cylinder system contains C(s), CO2, and CO in equilibrium. 1 kg of carbon is added to the system and it returns to equilibrium at the same temperature and pressure. The solid occupies an insignificant volume. The CO pressure ____________. increases decreases remains the same C(s) + CO2(g) 2 CO(g) CO2 CO C(s)

A mixture of CO, O2, and CO2 gases have been sealed in a container for 3 weeks at 25°C. The composition has not changed measurably. What can you conclude about this system? The gases are at equilibrium The kinetics are very slow There is no leak in the container All of the above

The reaction of CO2(g) with MgO(s) is … Exothermic Endothermic Need more information MgCO3(s) + CO2 log pCO2 (atm) MgO(s) + CO2 + MgCO3(s) MgO(s) + CO2 d 1/T

The exothermic reaction: takes place in a fixed-volume, sealed container. The two solids are in separate phases. At 500 K, equilibrium is obtained when pCO2 = 10.4 atm. As the temperature further increases, pCO2 ___________. increases decreases remains the same MgO(s) + CO2(g) MgCO3(s)

X X X X T T T T For an exothermic reaction, which is the correct plot of equilibrium conversion (fraction of reactant reacted) vs. temperature ? B A C D

What conditions would you carry out this gas-phase reaction: CH4 + H2O  CO + 3 H2 1 bar, 600 K 1 bar, 1200 K 100 bar, 600 K 100 bar, 1200 K kcal/mol

For an exothermic reaction in an adiabatic flow reactor, which statement is true? Decreasing the mass flow rate of an inert componentin the feed increases equilibrium conversion. The temperature will initially go up, but then decrease as equilibrium is approached. Increasing the inlet temperature increases the equilibrium conversion. None of the above

As the temperature increases for an exothermic reaction, which rate of reaction increases faster? Forward reaction Reverse reaction Rates increase at same rate More information is needed to predict kinetics EF reactants ER DH products

The equilibrium extent of reaction for an exothermic reaction will be higher for _______________ an adiabatic reactor an isothermal reactor neither adiabatic nor isothermal

An endothermic reaction takes place in an adiabaticreactor. The reactor is large enough so that conversion is complete. If the inlet flow rate is decreased by a factor of two, the exit temperature ________. increases decreases remains the same

ξ T Consider the endothermic steam reforming of methane: Which trend describes the extent of equilibrium conversion as a function of temperature? CH4 + H2O  CO + 3 H2 C equil. B A

The starting conditions for this reaction are indicated: A(g) + B(g)  C(s) 3 bar 3 bar x mol The reaction proceeds to the right. Which will have the lower pressure of A at equilibrium? x = 0.01 mol x = 2 mol Same pressure of A for both

The exothermic water‐gas shift reaction is often conducted downstream of the reforming reactor to generate excess hydrogen and to remove CO. The reaction is conducted in a sequence of two reactors. What is the appropriate sequence? A high T reactor followed by a low T reactor A low T reactor followed by a high T reactor A high P reactor followed by a low P reactor A low P reactor followed by a high P reactor CO + H2O  CO2 + H2

Which is the correct energy balance for a steady-state, adiabatic, tubular reactor for the reactionA  B ? (conversion is 50%, H is in molar quantities) HAin = 0.5HAout +0.5 HBout + 0.5 DHrxn HAin+ 0.5 DHrxn= 0.5 HAout + 0.5 HBout 0.5 HAin= 0.5 HBout HAin= 0.5 HAout + 0.5 HBout None of the above Adiabatic A A,B A  B

Chemical Equilibrium