Ethylene (CH₂CH₂) is the starting point for a wide array of industrial chemical syntheses. For example, worldwide about 8.0 × 10¹0 kg of polyethylene are made from ethylene each year, for use in everything from household plumbing to artificial joints. Natural sources of ethylene are entirely inadequate to meet world demand, so ethane (CH3 CH3) from natural gas is "cracked" in refineries at high temperature in a kinetically complex reaction that produces ethylene gas and hydrogen gas. Suppose an engineer studying ethane cracking fills a 70.0 L reaction tank with 16.0 atm of ethane gas and raises the temperature to 500. °C. He believes K 10. at this temperature. Calculate the percent by mass of ethylene the engineer expects to find in the equilibrium gas mixture. Round your answer to 2 significant digits. Note for advanced students: the engineer may be mistaken about the correct value of K, and the mass percent of ethylene you calculate may not be what he actually observes.

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Ethylene (CH₂CH₂) is the starting point for a wide array of industrial chemical syntheses. For example, worldwide about 8.0 × 10¹0 kg of polyethylene are made
from ethylene each year, for use in everything from household plumbing to artificial joints. Natural sources of ethylene are entirely inadequate to meet world
demand, so ethane (CH₂CH3) from natural gas is "cracked" in refineries at high temperature in a kinetically complex reaction that produces ethylene gas and
hydrogen gas.
Suppose an engineer studying ethane cracking fills a 70.0 L reaction tank with 16.0 atm of ethane gas and raises the temperature to 500. °C. He believes
= 10. at this temperature.
Calculate the percent by mass of ethylene the engineer expects to find in the equilibrium gas mixture. Round your answer to 2 significant digits.
Note for advanced students: the engineer may be mistaken about the correct value of K, and the mass percent of ethylene you calculate may not be what he
actually observes.
%
01.2
X
Transcribed Image Text:Ethylene (CH₂CH₂) is the starting point for a wide array of industrial chemical syntheses. For example, worldwide about 8.0 × 10¹0 kg of polyethylene are made from ethylene each year, for use in everything from household plumbing to artificial joints. Natural sources of ethylene are entirely inadequate to meet world demand, so ethane (CH₂CH3) from natural gas is "cracked" in refineries at high temperature in a kinetically complex reaction that produces ethylene gas and hydrogen gas. Suppose an engineer studying ethane cracking fills a 70.0 L reaction tank with 16.0 atm of ethane gas and raises the temperature to 500. °C. He believes = 10. at this temperature. Calculate the percent by mass of ethylene the engineer expects to find in the equilibrium gas mixture. Round your answer to 2 significant digits. Note for advanced students: the engineer may be mistaken about the correct value of K, and the mass percent of ethylene you calculate may not be what he actually observes. % 01.2 X
4NO₂ (g) + O₂(g) = 2N₂0, (8)
Using the chemical equation you can write the pressure equilibrium constant expression equation:
Initial
Now comes the challenge. How are we going to find the equilibrium pressures to put into the right-hand side of this equation? One important clue is that you're
told the initial pressures. That suggests solving an equilibrium composition problem might be part of the solution. Let's go down that path a little and see what
happens.
To solve for the equilibrium composition, you'd begin by setting up an ICE table. So let's do that. We'll let stand for the rise in pressure of the NO:
Change
N₂0₂
NO₂
Equilibrium
Po
Jay₂0₂
pV=nRT
n= PV
RT
P
K =
P
4.0-2x
The problem, of course, is that we don't know X, and we can't solve for it without.
it. Are we stuck? Not necessarily. Let's think a moment about why knowing
would let us solve for X: it's because knowing that number gives us a relationship between the expressions in the "equilibrium" row of the ICE table. That is,
we're able to combine these expressions into an equation with just one unknown, the value of X. An equation with just one unknown can always be solved for
the value of the unknown using algebra.
NO₂
4.0
-2x
Is there any other relationship we can imagine between the entries in the ICE table? Some other way to get an equation involving X?
Maybe. The one other fact you're given is the mole fraction of N₂O, at equilibrium. That is, you're told the ratio of the moles of N₂O, to the total moles of
reactants and products. That's almost a relationship between the entries in the ICE table, except that it's in terms of moles. What we need is some way to
transform it into a relationship between pressures.
RT
PN₂0₂
PIOT
When the issue is put like that, hopefully the idea of the ideal gas law comes to mind, because you know the ideal gas law relates pressure and moles for gases.
Let's use the ideal gas law to figure out what relationship there is between the equilibrium pressure of N₂O, and the total pressure, given that we know the
relationship between the moles of N₂O, and total moles:
Px₂0₂
RT
PIOT
PIOT
P
9.1
1
-3x
9.1-x
P
Pos
0
Here's the ideal gas law.
Solving for tells us how moles can be expressed in terms of pressure.
Now we're getting somewhere. You're told ₂0₂² =0.11. That means you now know a relationship between the equilibrium pressure of N₂O, and the total
pressure. And that's all you need to solve for X, using the entries in the "equilibrium" row of the ICE table:
Here's the definition of the mole fraction of N₂O₂.
Here's what happens when we substitute in the ideal gas law expression for moles, top and bottom.
You can solve this equation for X using algebra:
x=0.11 (13.1-x)
1.165x-1.441=0
x= 1.237...
Everything cancels but the pressures!
(4.0-2x)-(9.1-x)+(x)
Clear fractions.
Multiply everything out.
Solve the linear equation.
The hard part is over. Now you just need to substitute this value for X back into the expressions in the "equilibrium" line of the ICE table, to get the equilibrium
pressures. These then go into the pressure equilibrium expression equation to give you
13.1-x
= 0.11
x²
(1.237)
(4.0-23)*(91-3) (4.0-2(1.237))*(9.1-(1.237))
= 0.03327...
21
Ar
H
Transcribed Image Text:4NO₂ (g) + O₂(g) = 2N₂0, (8) Using the chemical equation you can write the pressure equilibrium constant expression equation: Initial Now comes the challenge. How are we going to find the equilibrium pressures to put into the right-hand side of this equation? One important clue is that you're told the initial pressures. That suggests solving an equilibrium composition problem might be part of the solution. Let's go down that path a little and see what happens. To solve for the equilibrium composition, you'd begin by setting up an ICE table. So let's do that. We'll let stand for the rise in pressure of the NO: Change N₂0₂ NO₂ Equilibrium Po Jay₂0₂ pV=nRT n= PV RT P K = P 4.0-2x The problem, of course, is that we don't know X, and we can't solve for it without. it. Are we stuck? Not necessarily. Let's think a moment about why knowing would let us solve for X: it's because knowing that number gives us a relationship between the expressions in the "equilibrium" row of the ICE table. That is, we're able to combine these expressions into an equation with just one unknown, the value of X. An equation with just one unknown can always be solved for the value of the unknown using algebra. NO₂ 4.0 -2x Is there any other relationship we can imagine between the entries in the ICE table? Some other way to get an equation involving X? Maybe. The one other fact you're given is the mole fraction of N₂O, at equilibrium. That is, you're told the ratio of the moles of N₂O, to the total moles of reactants and products. That's almost a relationship between the entries in the ICE table, except that it's in terms of moles. What we need is some way to transform it into a relationship between pressures. RT PN₂0₂ PIOT When the issue is put like that, hopefully the idea of the ideal gas law comes to mind, because you know the ideal gas law relates pressure and moles for gases. Let's use the ideal gas law to figure out what relationship there is between the equilibrium pressure of N₂O, and the total pressure, given that we know the relationship between the moles of N₂O, and total moles: Px₂0₂ RT PIOT PIOT P 9.1 1 -3x 9.1-x P Pos 0 Here's the ideal gas law. Solving for tells us how moles can be expressed in terms of pressure. Now we're getting somewhere. You're told ₂0₂² =0.11. That means you now know a relationship between the equilibrium pressure of N₂O, and the total pressure. And that's all you need to solve for X, using the entries in the "equilibrium" row of the ICE table: Here's the definition of the mole fraction of N₂O₂. Here's what happens when we substitute in the ideal gas law expression for moles, top and bottom. You can solve this equation for X using algebra: x=0.11 (13.1-x) 1.165x-1.441=0 x= 1.237... Everything cancels but the pressures! (4.0-2x)-(9.1-x)+(x) Clear fractions. Multiply everything out. Solve the linear equation. The hard part is over. Now you just need to substitute this value for X back into the expressions in the "equilibrium" line of the ICE table, to get the equilibrium pressures. These then go into the pressure equilibrium expression equation to give you 13.1-x = 0.11 x² (1.237) (4.0-23)*(91-3) (4.0-2(1.237))*(9.1-(1.237)) = 0.03327... 21 Ar H
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