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Hess’s Law

Pearson
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So in this video were going to demonstrate some basic principles of calorimetry to measure the enthalpy of reaction and we're going to measure a few enthalpies of reaction to demonstrate Hess's law. So the three reactions we're gonna measure are written here on the board. The first is going to be solid sodium hydroxide that we're going to dissolve in water to form aqueous sodium and aqueous hydroxide. And we're gonna measure delta H1 for that. The second reaction is going to be aqueous sodium hydroxide, it's gonna be a 1 M solution reacting with a 1 M solution of hydrochloric acid to give us water and sodium chloride aqueous and that's going to be delta H2. The third reaction is going to be solid sodium hydroxide reacting with aqueous hydrochloric acid also 1 M to give us water and sodium chloride again. And that's going to be delta H3. So which of the combinations of these delta H1, delta H2, and delta H3 would demonstrate Hess's law? So, to answer that question let's go back to the board and look at our reactions here. If we notice here that the product of dissolving solid sodium hydroxide is going to be sodium ions and hydroxide ions. And we notice here in reaction two that we have the same thing as a reactant. Well notice that we can cancel those right here and if we add these reactions together we'll notice that what's left over here is the solid sodium hydroxide like right here. And then we have the aqueous HCl. Aqueous HCl going to the same products water and sodium chloride. So that means the answer would be delta H1 plus delta H2 must equal delta H3. That's Hess's law. So what we're going to do is we're to actually measure the enthalpies reaction for Reaction 1, Reaction 2 and Reaction 3 using these coffee cup calorimeters. Number 1, Number 2 and number 3. So these are pretty crude calorimeters. They're just coffee cups. One placed inside the other. These are on top of a stir plate. So we have a little stirrer inside. And we have a thermometer. So what we're going to do is we're going to run each reaction inside of these calorimeters and we're going to measure the temperature change. And from the temperature change we can calculate the enthalpy. So right here we're gonna have our reactants. So this is the solid and the water for Reaction 1. Here's the solution for aqueous HCl aqueous sodium hydroxide and here is again some water some aqueous hydrochloric acid and again our solid sodium hydroxide. So we're gonna run these reactions. We're gonna just mix the reactants. Let it go and monitor the temperature and then see what the final temperature is. Take this over here. Put that back in. Take that off. And we'll do this one. The question to ask is "Which one of these three reactions is the most exothermic?" Obviously, reaction number three's temperature went up the most. So there for Reaction 3 is the most exothermic. But how do we actually calculate the enthalpy? To do that we've actually saved this data, this temperature data as a function of time and we're gonna analyze it in a spreadsheet. We have collected time versus temperature data for each of these three reactions. So in other words we've collected the temperature, as a function of the time for each of these three reactions and put them in these columns. And what we've done over here is we've graphed them. So here's Reaction 1. So here's temperature versus time for Reaction 1. Reaction 2 and Reaction 3. So you can notice that we started down here at 25 degrees and then we added the reagents. Reagents go, cause it to be exothermic and the temperature goes up and then we reach this equilibrium. And then what we've done is we've taken a portion of this equilibrium region and fit that to a line and extrapolated that line back in time to time zero when we added the reagents. So this becomes T2 or the temperature the equilibrium temperature after the reagent and this becomes T1. And so the difference becomes delta T. So over here we've calculated delta T. So delta T for Reaction 1 is 5.24˚C. Reaction 2 is 6.08˚C and Reaction 3 is 11.53˚C. So with these delta T's, we can now calculate the enthalpies for each of these three reactions. So here we have the basic equation we used to calculate the enthalpy for a reaction. So this is heat, but if you remember under isobaric conditions or constant pressure conditions enthalpy is equal to heat. So we can calculate the enthalpy using this reaction, where m is the mass of the substance in the calorimeter. C is the heat capacity of the substance in the calorimeter. And delta T is the temperature change. So we've calculated the temperature change already. The mass, we can calculate by assuming that the density of this solution is 1, the density of water. So 1 gram per mL. And the heat capacity we assumed to be the heat capacity of water which is 4.184 joules per gram per Kelvin. So, plugging all these numbers into our equation here we can calculate the enthalpy for Reaction 1, 2 and 3. So we had 200 mL of solution in each case. So that's 200 grams assuming 1 gram per mL. Times the 4.184, times the delta T and we got 4.38 kilojoules for Reaction 1. Now remember, enthalpy determines how much you have. It's an extensive property. We have not corrected per mole here. This is an absolute enthalpy for this measurement. For Reaction 2, same thing, 200 grams, 4.18. But delta T is a little bit different and we get 5.08 kilojoules. And Reaction 3, because the delta T was much bigger, was much larger. But notice here, if we take delta H1 and add it to delta H2. So here's our 4.38 kilojoules, 5.08 kilojoules. Add that together we get 9.46, which is not exactly the same but considering how crude or calorimetry was this agreement is very good and demonstrates Hess's law.
So in this video were going to demonstrate some basic principles of calorimetry to measure the enthalpy of reaction and we're going to measure a few enthalpies of reaction to demonstrate Hess's law. So the three reactions we're gonna measure are written here on the board. The first is going to be solid sodium hydroxide that we're going to dissolve in water to form aqueous sodium and aqueous hydroxide. And we're gonna measure delta H1 for that. The second reaction is going to be aqueous sodium hydroxide, it's gonna be a 1 M solution reacting with a 1 M solution of hydrochloric acid to give us water and sodium chloride aqueous and that's going to be delta H2. The third reaction is going to be solid sodium hydroxide reacting with aqueous hydrochloric acid also 1 M to give us water and sodium chloride again. And that's going to be delta H3. So which of the combinations of these delta H1, delta H2, and delta H3 would demonstrate Hess's law? So, to answer that question let's go back to the board and look at our reactions here. If we notice here that the product of dissolving solid sodium hydroxide is going to be sodium ions and hydroxide ions. And we notice here in reaction two that we have the same thing as a reactant. Well notice that we can cancel those right here and if we add these reactions together we'll notice that what's left over here is the solid sodium hydroxide like right here. And then we have the aqueous HCl. Aqueous HCl going to the same products water and sodium chloride. So that means the answer would be delta H1 plus delta H2 must equal delta H3. That's Hess's law. So what we're going to do is we're to actually measure the enthalpies reaction for Reaction 1, Reaction 2 and Reaction 3 using these coffee cup calorimeters. Number 1, Number 2 and number 3. So these are pretty crude calorimeters. They're just coffee cups. One placed inside the other. These are on top of a stir plate. So we have a little stirrer inside. And we have a thermometer. So what we're going to do is we're going to run each reaction inside of these calorimeters and we're going to measure the temperature change. And from the temperature change we can calculate the enthalpy. So right here we're gonna have our reactants. So this is the solid and the water for Reaction 1. Here's the solution for aqueous HCl aqueous sodium hydroxide and here is again some water some aqueous hydrochloric acid and again our solid sodium hydroxide. So we're gonna run these reactions. We're gonna just mix the reactants. Let it go and monitor the temperature and then see what the final temperature is. Take this over here. Put that back in. Take that off. And we'll do this one. The question to ask is "Which one of these three reactions is the most exothermic?" Obviously, reaction number three's temperature went up the most. So there for Reaction 3 is the most exothermic. But how do we actually calculate the enthalpy? To do that we've actually saved this data, this temperature data as a function of time and we're gonna analyze it in a spreadsheet. We have collected time versus temperature data for each of these three reactions. So in other words we've collected the temperature, as a function of the time for each of these three reactions and put them in these columns. And what we've done over here is we've graphed them. So here's Reaction 1. So here's temperature versus time for Reaction 1. Reaction 2 and Reaction 3. So you can notice that we started down here at 25 degrees and then we added the reagents. Reagents go, cause it to be exothermic and the temperature goes up and then we reach this equilibrium. And then what we've done is we've taken a portion of this equilibrium region and fit that to a line and extrapolated that line back in time to time zero when we added the reagents. So this becomes T2 or the temperature the equilibrium temperature after the reagent and this becomes T1. And so the difference becomes delta T. So over here we've calculated delta T. So delta T for Reaction 1 is 5.24˚C. Reaction 2 is 6.08˚C and Reaction 3 is 11.53˚C. So with these delta T's, we can now calculate the enthalpies for each of these three reactions. So here we have the basic equation we used to calculate the enthalpy for a reaction. So this is heat, but if you remember under isobaric conditions or constant pressure conditions enthalpy is equal to heat. So we can calculate the enthalpy using this reaction, where m is the mass of the substance in the calorimeter. C is the heat capacity of the substance in the calorimeter. And delta T is the temperature change. So we've calculated the temperature change already. The mass, we can calculate by assuming that the density of this solution is 1, the density of water. So 1 gram per mL. And the heat capacity we assumed to be the heat capacity of water which is 4.184 joules per gram per Kelvin. So, plugging all these numbers into our equation here we can calculate the enthalpy for Reaction 1, 2 and 3. So we had 200 mL of solution in each case. So that's 200 grams assuming 1 gram per mL. Times the 4.184, times the delta T and we got 4.38 kilojoules for Reaction 1. Now remember, enthalpy determines how much you have. It's an extensive property. We have not corrected per mole here. This is an absolute enthalpy for this measurement. For Reaction 2, same thing, 200 grams, 4.18. But delta T is a little bit different and we get 5.08 kilojoules. And Reaction 3, because the delta T was much bigger, was much larger. But notice here, if we take delta H1 and add it to delta H2. So here's our 4.38 kilojoules, 5.08 kilojoules. Add that together we get 9.46, which is not exactly the same but considering how crude or calorimetry was this agreement is very good and demonstrates Hess's law.