Calorimetry experiments can also be used to measure the enthalpy changes for var

Question

Calorimetry experiments can also be used to measure the enthalpy changes for various aqueous reactions because the heat given off by the reaction is used to increase the temperature of the water and the calorimeter. Consider the reaction between an acid and a base (say NaOH (aq) + HCl (aq) --> H2O (l) + NaCl (aq)). Usually, it is assumed that the solution has the same specific heat capacity and density of water. With this idea in mind, what is the enthalpy of reaction (in Joules) for NaOH + HCl if 20.0 mL of HCl solution and 20.0 mL of NaOH solution (both starting at 20.0 oC) are mixed in the calorimeter (heat capacity of 10.0 J/ oC) producing a final temperature of 40.0 oC? (Remember that water has a density of 1.00 g/mL. Use this to calculate the mass of water that is absorbing the heat from the reaction.)

Explanation / Answer

Typically, the calculation for heat released or absorbed, q, for the reaction

using the equation

q=s*m*change in temp,

where q is heat or enthalpy at constant pressure (which is generally assumed) s is specific heat and m is mass.

q = mass(water) x specific heat capacity(water) x change in temperature(solution)

The q you get here, though, is for this certain amount of grams.

If you want enthalpy per mole, first calculate how much you have in moles.

20ml x 1moles/L HCl or NaOH will give you moles of either acid or base.

The final step is to divide the q you initially had by the number of moles you just calculated above.

note:

1 Molar solution is by definition 1 mole/liter

density of water is 1.00 g mL-1 and specific heat capacity of water is 4.18 JoC-1g-1

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