Using Hess's Law To Determine K - Video Tutorials & Practice Problems

Using Hess's Law To Determine K

We recall with Psa's law that the entropy of reaction or delta H reaction, it changes proportionally to the coefficients of a reaction. Meaning that if I have a chemical reaction and I multiply it by three, I'd multiply delta H by three. If I divide that reaction by two, I divide delta H by two. If I reverse the direction of the reaction, I reverse the sign of delta H, we're gonna say, however, the relationship between your equilibrium constant K and the coefficients of a reaction is exponential. OK. So when we say exponential, we're gonna say there are three possible rearrangements for changes of a chemical reaction. So here we have our balanced chemical equation which is two moles of sulfur dioxide gas combined with one mole of oxygen gas to produce two moles of sulfur trioxide gas associated with this is our original equilibrium constant value of 71.3. Here, we're gonna take a look at three types of changes that can happen. We can multiply the reaction, we can reverse the reaction or we could divide the reaction by a value. What effect will this have on my equilibrium constant K? Well, for the first one multiplication, we're gonna say if you multiply the reaction, we're gonna raise K to the same factor. So for example, I'm gonna multiply the reaction by three. So I'm multiplying this reaction by three. So all the coefficients get multiplied by three, I'll have six. So two gas plus 302 gas gives me six. So three gas, when I multiply the reaction by a value K is raised by that number. So here our new K would be K cubed. So it'd be 71.3 cubed. So that will come out to be pretty large number. It's gonna be 362467.097. Here we don't care about 66 per se. We're just seeing how the value is being affected. What happens if I reverse the reaction? Well, if I reverse the reaction, then I get the inverse of K. Here, this K value is only have K to the one that's it number. When I reverse, it becomes K to the minus with the inverse. So here our new reaction, I'm gonna reverse it. It becomes two. So three gas gives me two. So two gas +102 gas, I'm just flipping the reaction. My reactants have become products, my product has become a reactant. So now my new KK to the minus one, which is 71.3 to the minus one which is 0.0140 that'd be my new K value. Then finally, we could divide the reaction. When you divide the reaction, you ra you raise K to a reciprocal of that factor. So here I'm dividing by two, dividing by two really means I'm raising it to the half power. If I divide it by three, it'd be K to the one third. If I divide it by four, become K to the 1/4 except right. So here I'm dividing the reaction by two. So this becomes, remember, I'm dividing all the coefficients by two. So this becomes one. So two gas plus half +02 gas give me 103. Yes. So then it's gonna become K to the half power. So that's gonna be 71.3 to the half. Remember half power is the same thing as square root. So I'm taking the square root of 71.3 which gives me an answer of 8.44 as my new equilibrium constant, right? So remember the effects that we, the change that we do to our chemical reaction has an exponential change on my equilibrium constant K. It affects the exponent involved with K. So keep this in mind whether you're multiplying reversing or dividing your chemical reaction, the changes that it does to equilibrium constant K.

Using Hess's Law To Determine K Example

For the phone reaction, we're told that the KP value is 7.5 times 10 to the negative two. Here, we need to calculate KP of the reactions below. So basically what's going on here is we have this original equation with its original KP value and we're changing it either through multiplication uh reversal or division in some way that will produce a whole new KP value. It's up to us to determine what that new KP value is. If we take a look at the first one in option A, we can see that what is the change that occurred? It looks like we divided, this is the original equation by two, right. So we divided the original equation by two. So this becomes one mole, divide this by two, this became one mole, divided this by two, became two and divided one by two. Gives me a half. Remember we say that if you're dividing by a number there that you're doing the reciprocal of that factor, basically meaning that I divided by two. So the new K is gonna be K to the half. So he would say KP to the half. So that's gonna be 7.5 times 10 to negative two to the one half power. Remember one half power is the same thing as square root. So this comes out as an answer of 0.27 for our new KP value for the next one. What's the change that we see? All right. So it looks like our products are now reactants and it looks like our reactants here are not products. So what did we do? We reversed the reaction. So it's a reversal. When we do a reversal, it's gonna be the inverse of our original K value. So it's gonna be KP to the negative one. So that's 7.5 times 10 to the negative two to the negative one, which equals 13. Here, all our answers are 26 fix because 7.5 times 10, negative two has two significant figures. And then finally, let's look at the last one. What's the last one? Well, it looks like in the last one, we still reversed it, our products are now reactants and our reactants are now products. But what's the other change that occurred? Well, here we had a coefficient of four. Now it's 16. This was a one. Now it's a four, this, these two were both twos and now they're eights. So this one will represent a reversal and also we multiplied by four. Well, when we reverse, that's gonna give us the reciprocal. But then when you ra when you multiply you raise that as a power. So it's to the fore, right? So it's gonna be 7.15 times 10, negative two to the negative one which we now gave us 13. And then we raise that 13 to the four. So this comes out to 3.2 times 10 to the four. This would be our final answer for KP. So just remember we affect these chemical reactions either through multiplication, reversal or division. Changing the chemical reaction has an exponential change um on the chemical reaction. OK. So we're gonna change the exponent here in order to get our new K value.

K_c = 6.5 x 10² at a particular temperature for a reaction: 2 NO(g) + 2 H(g) ⇌ N₂(g) + 2 H₂O(g). Calculate K_c at same temperature for the following reaction: 1/3 N₂(g) + 2/3 H₂O(g) ⇌ 2/3 NO(g) + 2/3 H(g).

• A. 8.7
• B. 0.12
• C. 0.54
• D. 3.6×10^–9