Equilibrium Constant K - Video Tutorials & Practice Problems
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1
concept
Equilibrium Constant Expressions
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Hey everyone. The equilibrium constant which is depicted as capital K specifies the favored direction of a reaction. Remember with chemical equilibrium we're talking about reversible reactions. So we're talking about is the forward direction favored or the reverse direction favored. While the equilibrium constant talks about direction, our rate constant, which is depicted by lowercase k, deals with the speed of our chemical reaction. Here when we say rate constant we're talking about, chemical kinetics, so we're looking at how fast a chemical reaction proceeds, How quickly can my reactants break down to provide me products? Here when we say our equilibrium constant K it typically is connected to our concentration units. But when we talk about our rate constant, it's not only concentration, but also time as a units. Here time could be in seconds inverse, days inverse, years inverse, any unit of time. So just remember, looking at a chemical reaction to get the complete picture, we look at the equilibrium constant capital K to determine its direction, and we look at the rate constant lowercase k to determine the speed in which it proceeds.
2
concept
Equilibrium Constant Expressions
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Accompanying the equilibrium constant is the idea of equilibrium expressions. Now, first we're going to say that our equilibrium constant K can be represented by K or Keq and we're going to say that it represents a ratio of product to reacting concentrations at equilibrium. We're going to say that the equilibrium constant is temperature dependent. So if we change the temperature that's going to change the value of our equilibrium constant k. Now here we've talked about expressions, but what exactly does that mean? If we take a look here, we're going to say we have our chemical reaction represented by our reactants A and B and our newly formed products C and D. With them we have different coefficients, so the numbers that go in front. We're going to say that our equilibrium constant again is a ratio of products to reactants, so it's products over reactants and it equals the concentrations of our products. And notice here that our coefficients are the exponents involved. So here are x coefficients a and b are the exponents involved here. We're going to say that our equilibrium constant is known as an umbrella term, so it's just k, but we can break it down into further types of k's. So throughout this course, we'll be looking at different types of k's. For now we're going to be concerned with kp and kc. Kp is used when our concentration units are in atmospheres, and kc will be used when our concentration units are in molarity. Now, talking about our chemical reaction, we need to remember that solids and pure liquids are not or they are excluded, so they're excluded from our k expressions. So when we do our equilibrium expression, which is represented by this portion, we would not include solids and liquids. We're going to say here that adding a solid or a pure liquid does not change their concentrations. They're unaffected by external factors such as pressure, so that's typically why we don't include them within our equilibrium expressions. Alright, so let's try to set up an equilibrium expression for the equation given below. In this equation given below, we say that magnesium hydroxide solid reacts with 2 moles of hydrochloric acid to produce 2 moles of water as a liquid and magnesium chloride aqueous. Remember, we don't include solids and liquids, so these are going to be excluded. We're going to have here K equals products over reactants, so here we have the concentration of magnesium chloride, and then here we have the concentration of HCl, hydrochloric acid because it's a reactant. Remember the coefficient forms the exponent. The coefficient here is just 1, so we don't need to include it it's understood as being there, But the coefficient here for hydrochloric acid is 2, so we put a 2 here. This here would represent the equilibrium expression for the chemical balance chemical reaction given above.
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example
Equilibrium Constant K Example
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What is the equilibrium constant expression for the following reaction? Now remember, with equilibrium expressions we exclude solids and liquids. If we take a look here, here's a solid so we're going to exclude this, exclude, exclude, and exclude. We're going to say our equilibrium expression K, or equilibrium constant K equals products over reactants. So the products over reactants represents our equilibrium expression. We're going to include here we have the concentration of hypochlorous acid HOCl, and then here we have Cl2, fluorine gas. Remember their coefficients will be our exponents, and since both of them have a coefficient of 2 this will be hypochlorous acid squared over Cl2 gas also squared. This here represents the equilibrium expression for the chemical reaction given within this example question.
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Problem
Problem
Provide K expression for the reverse of the following reaction:
Fe2O3(s) + 3 H2(g) ⇌ 2 Fe(s) + 3 H2O(g)
A
B
C
D
5
concept
Magnitude of Equilibrium Constant
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Here we say that the magnitude of our equilibrium constant k indicates how far to the left or to the right of reaction lines at equilibrium. Remember that overall value of k though is also based at a given temperature, because remember equilibrium constant k is temperature dependent. When we're talking about the magnitude of our equilibrium constant, we're referring to k being either greater than 1, less than 1 or equal to 1. When K is greater than 1 we're going to say here that our products and the forward reaction are favored. Remember k is products over reactants. Here we could say, for example, the amount of product is equal to 10, our amount of reactant is equal to 2, so overall k equals 5. We can see that there is more product than reactant. Why would there be more product? Because we're moving in the forward direction to create it. Conversely, if we're going the opposite way, if k is less than 1, this would mean the opposite where reactants now are more favored And which direction would we have to, lead or move into so that more reactants are created? The reverse direction, so the reverse reaction would be favored. Here we'd have 10 reactants to just 2 products giving us a k less than 1. But let's say k is equal to 1. Well, if k is equal to 1, then neither direction is favored. Okay, so you're making the same amount of reactants and products, so here k would be equal to 1. Now, in addition to this we can say that the magnitude of k can also be determined from your rate constants. Remember, you have your rate constant which is lowercase k and it could be for the forward or the reverse of reactions. For now, just realize that the connection between your equilibrium constant and your rate constants is given by this equation, where your equilibrium constant k equals the rate constant of the forward direction divided by the rate constant of the reverse direction or reaction. So again, capital k is your equilibrium constant, lowercase k is your rate constant. So when we're talking about magnitude it just means what side is more favored, Forward side to make more product or reverse side to make more reactants? Or does k equal 1? Where neither side is really favored over the other, they're both equal.
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example
Equilibrium Constant K Example
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In the following example question it says, When this reaction comes to an equilibrium, which will be higher in pressure, reactants or products? So, here we have 2 moles of CH4 reacting with 2 moles of H2S. To produce, we have here CS2 and H2. We're told that our equilibrium constant in the form of kp is 1.3 times 10 to the 3rd. Here we can see that k is greater than 1. Remember when k is greater than 1 we say that the product side is favored and the forward direction is favored. So the side that's favored are our products, which means we're making more of them. And when it comes to the pressure of gases, the more particles or molecules of gases present the higher the pressure. Since we have more products being favored they would have an overall greater pressure. This would mean that out of all of our options, option B is the correct choice. Product side is favored, there's more products, therefore they will result in higher pressure.
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Problem
Problem
Which of the reactions is likely to produce more O2 at equilibrium at 25°C?
A
2 NO(g) ⇌ N2(g) + O2(g) Keq = 2.4 x 1030
B
2 NO2(g) ⇌ 2 NO + O2(g) Keq = 5.8 x 10–5
C
2 CO2(g) ⇌ O2(g) + 2 CO(g) Keq = 3.1 x 10–5
D
2 H2O(g) ⇌ 2 H2(g) + O2(g) Keq = 5.1 x 10–82
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Problem
Problem
Consider the reaction A(g) ⇌ B(g) + C(g), with kforward of 5.7 x 10–2 and kreverse of 3.8 x 10–4. Which would be greater at equilibrium, partial pressure of A or partial pressures of B and C?
A
partial pressure of A
B
partial pressure of B & C
C
partial pressures will be equal
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