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Course: AP®︎/College Chemistry > Unit 6
Lesson 5: Introduction to enthalpy of reactionWorked example: Measuring enthalpy of reaction using coffee-cup calorimetry
Constant-pressure calorimetry is often used to determine the changes in enthalpy for aqueous reactions. In this video, we'll see how data from a coffee-cup calorimetry experiment can be used to calculate ΔH_rxn for the reaction between AgNO₃(aq) and NaCl(aq). Created by Jay.
Want to join the conversation?
- I don't understand the intuition behind dividing our heat transferred by our mole of AgCl at5:20.(7 votes)
- Just for anyone in the future:
It is because the value for qrxn we get is for this specific reaction we just did, using 0.0025moles of AgCl. But if we want to get the actual qrxn per mole of reaction, we must do the stoichiometry that he did.(2 votes)
- Why are we allowed to assume that 1g = 1mL?(4 votes)
- We’re assuming that the solution has the same density as water does, 1 g/mL. In an aqueous solution the major component is water so it plays a dominant role in determining the density of the solution. Technically since it’s not pure water the density is not exactly 1 g/mL, but the actual difference is too insignificant to affect the calculations.
Hope that helps.(4 votes)
- i cant understand why water is not part of the system since it is part of the solution. Also, why you consider the mass of water be 50g since 50ml is the volume of the solution which is water and the other components? Could you explain it to me?(2 votes)
- We do consider water is part of the solution and therefore the calculation2:03. Jay uses the specific heat capacity of water even for the later calculation2:10. The solution means the solutes, the silver nitrate and the sodium chloride, and the solvent, the water.
Jay is assuming that the density of the solution is that of water, or 1g/mL. Essentially 1 mL of solution has 1 g of mass. So if we have 50 mL of solution, we have 50 g of solution.
Hope that helps.(2 votes)
- I'm a little bit confused about the idea "per mole of reaction".
It feels off to me.
I would have thought that if there was 1 mole of reaction, then there would be half a mole of AgCl, since the ratio of AgCl to the rest of the reaction is 1:2.
I know this doesn't quite make sense, but please can someone give a simple explanation of the idea of moles per reaction?
Thanks(2 votes) - can u stop rambling and get to the point? u explain with filler words that make no sense without elaborating on them.(0 votes)
Video transcript
- [Instructor] A constant
pressure calorimeter can be used to find the change in entropy for a chemical reaction. Let's look at the chemical reaction between an aqueous
solution of silver nitrate and aqueous solution of sodium chloride to form a precipitate of silver chloride and an aqueous solution of sodium nitrate. Let's say we have 25.0 milliliters of a 0.100 molar solution
of silver nitrate and 25.0 milliliters of
a 0.100 molar solution of sodium chloride. Both solutions are
initially at a temperature of 25.000 degrees Celsius. Next, we add our two
solutions to our calorimeter which has made of two coffee cups. And since the top coffee
cup is loose fitting, this reaction is under
the constant pressure of the atmosphere. So this is constant pressure calorimetry. After the two aqueous solutions
mix, the reaction occurs and we watch the thermometer
in the calorimeter. In this case, the temperature
of the solution increases and the final temperature, the highest one reached in our experiment is 25.781 degrees Celsius. So the change in the
temperature of the solution would be the final temperature minus the initial temperature, which is 25.781 minus 25.0 which is equal to positive
0.781 degrees Celsius. The total volume of solution
would be 25 plus 25, which is equal to 50.0
milliliters of solution. If we assume the density of the solution is one gram per milliliter, 50.0 milliliters is equal to 50.0 grams. Next we need to solve for
the heat gained by the water and we can use the Q is
equal to MC delta T equation. So we're solving for the heat, which is symbolized by Q, M is the mass of our solution which we saw was 50.0 grams, So we can plug that in. We can assume that the
specific heat of the solution is the same as the specific heat of water, which is 4.18 joules
per gram degrees Celsius and the change in the
temperature of the solution was 0.781 degrees Celsius. So we can plug that in as well. Grams cancels out, degrees
Celsius cancels out and we find that Q is equal to positive 1.63 times 10
to the second joules. The positive sign means that
energy was gained by the water. But let's think about the distinction between system and surroundings. The system consists of the reactants and products for our particular reaction and the surroundings are everything else which includes the water. So in this case, since the temperature of the
surroundings increased, right? We saw an increase in the temperature, that means that heat flowed from the system to the surroundings and so the surroundings
increased in energy and that's what we see with
this positive sign here. If we assume a perfect transfer of heat from the system to the surroundings, if the surroundings gained energy, that means the system lost energy. So if we're thinking about the heat transferred for the reaction, it's the same in magnitude, 1.63 times 10 to the second joules. However, we need to put
a negative sign in here which indicates that energy
was given off by the reaction. The heat that's transferred
under constant pressure is equal to the change in the entropy of the reaction, delta H. However, let's find the
change in entropy, delta H in terms of kilojoules per mole of silver chloride for our units. Since silver chloride
is one of our products, we first need to find
moles of our reactants and we're gonna do that
using the molarity equation which says that molarity is equal to moles divided by liters. For our silver nitrate solution, the concentration was 0.100 molar and trying to solve for moles so that's X. The volume of our silver nitrate solution was 25.0 milliliters which is 0.0250 liters. So we solve for X and we get 0.00250. So that's how many moles of silver nitrate that we started with and it's the exact same calculation for sodium chloride as well. So this is also how many moles
of sodium chloride we have. Next, we go back to our
balanced chemical equation and we can see we have coefficients of one in front of silver nitrate, in front of sodium chloride and in front of silver chloride. Therefore, we're also going
to produce 0.00250 moles of silver chloride. Next we're gonna calculate the change in the entropy, delta H, for our reaction. The heat that was transferred was negative 1.63 times
10 to the second joules and we're gonna divide that by the moles of silver chloride which was 0.00250 moles
of silver chlorides. This is equal to negative 65,200 joules per mole of silver chloride and we could convert that into kilojoules. And so this is equal to
negative 65.2 kilojoules per mole of silver chloride. We could stop right here and
give this as our final answer, but let's keep going and convert two kilojoules
per mole of reaction. First, let's rewrite this. We have negative 65.2 kilojoules per mole of silver chlorides and what it means by
kilojoules per mole of reaction is how the reaction is written in the balanced equation down here. So if we look at the balanced equation, there's one mole of silver chloride for how the reaction is written. So we can write a conversion factor of one mole of silver chloride per one mole of reaction and writing it this way
for the conversion factor, the moles of silver
chloride would cancel out and give us negative 65.2
kilojoules per mole of reaction. So this could also be our final answer. Finally, the negative sign means that we have an exothermic reaction. The reaction gave off energy and this value when you do a constant pressure calorimetry experiment is often a little bit
lower than the actual value because in reality there's
not always a perfect transfer of heat from the reaction to the water. Often some of the energy
is lost to the environment.