Chem 1A Fossum
Hints on solving calorimetry problems
The assumption in all calorimetry problems is that the calorimeter is perfectly insulated.
This means that no heat enters or leaves from the outside.
Coffee-cup calorimetry
The reaction is open to the atmosphere. We usually ignore the heat capacity of
the Styrofoam cup itself. The heat produced or absorbed can be used to calculate ∆H of
the reaction (heat produced/absorbed at constant pressure). Typically, the reaction
occurs in solution. To determine ∆H for a reaction given experimental information:
1. Calculate the amount of heat absorbed by or given off by the solution itself
(which is mostly water). Use q=Cm∆T. C is the heat capacity of the entire solution. If C
is not specifically given in the problem and the reaction happens in aqueous solution,
assume C = 4.184 J/g°C or 4.2 J/g°C (water’s heat capacity) – this will be a good
approximation. For ∆T, take Tf – Ti, and be careful with the sign. (If T is increasing, ∆T
will be +. If T is decreasing, ∆T will be -.) For m, use the mass of the entire solution. If
masses are given, add them up. If masses are not given, use the volumes of solutions
mixed – for dilute aqueous solutions, the density will be close to 1 g/mL (again,
because the solutions are mostly water). For example, if you mix 100 mL of 1.0 M HCl
with 100 mL of 1.0 M NaOH, the total volume is 200 mL, and the entire solution will
have a mass of approximately 200 g. Use 200 g as the mass in this case. (Note – the
biggest mistake students make in these types of problems is to calculate the masses of
pure HCl and pure NaOH from volume and molarity, add those, and use that as the
mass. This will not work, because most of the mass of the solution is due to water!
There’s a lot more water than anything else in the solution. Also, if we use the heat
capacity of water in the calculation, it doesn’t make sense to multiply it by the mass of
something that isn’t water and might have a completely different heat capacity.)
2. qrxn = -qsolution. (Opposite signs). Here’s the idea behind this: the calorimeter is
perfectly insulated, so qrxn + qsolution = 0. Therefore, these q’s must have opposite signs.
3. qrxn represents the amount of heat at constant pressure for the amounts that you
used. To find ∆H for a reaction, it has to correspond to the number of moles of
everything in the balanced equation. If the reaction is Mg + 2 HCl → MgCl2 + H2 ,
∆H is implied to be the amount of heat per 1 mole of Mg or per 2 moles of HCl. Here’s
what to do next: divide qrxn by the number of moles of the limiting reactant. The result is
the amount of heat per 1 mole of the limiting reactant. Next, check the coefficients in the
balanced equation. If the limiting reactant has a coefficient of 1, you’re done. If it has a
coefficient other than one, take your result and multiply it by the appropriate coefficient
to get the amount of heat per x moles. For example, if HCl was the LR, you would
divide qrxn by the number of moles of HCl to get heat per mole of HCl. Then, since HCl
has a coefficient of 2 in the balanced equation, multiply your result by 2 to get the
amount of heat per 2 moles of HCl. That will be ∆H for the reaction as written.
Bomb (constant-volume) calorimetry
The reaction is performed in a strong inner chamber surrounded by water and another
container. We cannot ignore the heat capacity of the calorimeter itself, so that heat
capacity will be given in the problem. This setup is often used for combustion reactions.
