Chemistry Equilibrium Partial Pressures

In summary, the problem involves calculating the total pressure in a closed vessel at equilibrium, given the initial amount of PCl5 and the equilibrium constant. The student attempted to solve the problem using the equation Kp=Kc(RT)^change in n, but their calculations did not match the expected answer. They were using the small x approximation, but it is unclear if this is justified based on their calculated value for x. The final answer should be 4.09 atm for Ptotal.
  • #1
Madelin Pierce
24
2

Homework Statement



Kc = 4.15 x 10-2 at 356°C for PCl5(g) ↔ PCl3(g) + Cl2(g). A closed 2.00 L vessel initially contians 0.100 mol PCl5. Calculate the total pressure in the vessel (in atm to 2 decimal places) at 356°C when equilibrium is achieved.

Homework Equations


PV=nRT
Kp= Kc(RT)^change in n
Kp= Pressure products/pressure reactants
Small x approximation

The Attempt at a Solution


The answer key says 4.09 for Ptotal, but I keep getting a wrong answer
 
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  • #2
Show how you arrive at the wrong answer then.
 
  • #3
Borek said:
Show how you arrive at the wrong answer then.
I use the equation Kp=Kc(RT)^change in n. I plug in Kp=(.0415)(.08206)(629.15)^1= 2.1425.
Then, I solve for PV=nRT for PCl5: P(2.00L)=(.100mol)(.08206)(629.15K), equaling 2.5814atm. I use Kp= Products/Reactants which is 2.1425=x^2/(2.5814-x) where the Xs are from an ice table. I was told by my instructor to use small x approximation for this problem, so I eliminated the x on the bottom, so 2.1425*2.5814= x^2. Times them and square root and I got 2.3517 for X. Plugged Ptotal=P(PCl5)+P(PCl3)+P(Cl2) which equaled 4.935 for total pressure. I don’t know how I’m not getting the answer right, I just don’t. The answer’s supposed to be 4.09 Ptotal.
 
  • #4
You got 2.3517 for x - does it justify the small x approximation?
 

FAQ: Chemistry Equilibrium Partial Pressures

1. What is the definition of equilibrium in chemistry?

In chemistry, equilibrium refers to a state in which the forward and reverse reactions occur at equal rates, resulting in no net change in the concentrations of reactants and products.

2. How is equilibrium affected by changes in temperature?

According to Le Chatelier's principle, an increase in temperature will favor the endothermic reaction, while a decrease in temperature will favor the exothermic reaction. This means that equilibrium will shift in the direction that consumes or produces heat in order to maintain a constant temperature.

3. How are partial pressures calculated in equilibrium?

In equilibrium, the partial pressures of each gas involved can be calculated using the ideal gas law, where P is the partial pressure, n is the number of moles, V is the volume, and T is the temperature: P = nRT/V. The total pressure in the system is equal to the sum of all the partial pressures.

4. How do you determine the equilibrium constant for a reaction?

The equilibrium constant, K, is determined by taking the ratio of the concentrations of products to reactants at equilibrium, with each concentration raised to the power of its coefficient in the balanced chemical equation. The value of K indicates the extent to which the reaction favors products or reactants at equilibrium.

5. How does a change in pressure affect equilibrium?

According to Le Chatelier's principle, an increase in pressure will shift equilibrium in the direction that decreases the total number of moles of gas, while a decrease in pressure will shift equilibrium in the direction that increases the total number of moles of gas. This means that equilibrium will shift to the side with fewer moles of gas in order to relieve the pressure change.

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