Question

9.) As mentioned below, you are required to design your own procedure for determining the heat of dissolution of your chosen salt with a coffee cup calorimeter. Answer the following questions and consider them in designing this procedure:

     a.) When you find your initial temperature T_i, should you take one measurement from the thermometer as soon as you place it in the water in the calorimeter? Why or why not? If not, what should you do instead?

     b.) Do you expect the temperature of the water in the calorimeter to rise, fall, or stay the same after you add your salt? If the temperature will change, will it continue to change indefinitely or not?

     c.) How will you know your dissolved salt solution has reached it's maximum or minimum temperature, i.e. what will you observe in your data to indicate this has happened?

     d.) Should you repeat your experiment? Why or why not? If so, what could you change from trial to trial? Give two possible suggestions, and determine which one you should change (as well as which one you should not!) in any subsequent trials you decide to perform.

*Information previously calculated is below*

1. Calculate the amount of heat (q) produced by the combustion of 4.05 g CH₄ (ΔH_comb = -890.4 kJ).

225.38 kj

2. Consider that the 4.05 g methane is burned and all of the heat from this combustion is absorbed by 1.0 kg of 20 °C water (use 4.18 g^-1 ºC^-1 for the specific heat of the water). What would be the final temperature of the water?

73.8°C

Answer 1

a)Yes, initial temperature must be noted , as soon as the salt is added into water because we need accurate change in temperature (∆T) to measure the heat of dissolution by calorimetry.

∆T=T(final)-T(initial)

Else the cup may be sealed ,so that heat is not lost into the atmosphere using its styroform lid

     b)The temperature may rise or fall depending on the nature of salt added. Most salts have endothermic dissolving reactions, so temperature decreases initially and begin to rise later.

But some salts may even have exothermic dissolving reaction for which temperature increases.

     c) The temperature becomes stable after complete dissolution of salt.When the temperature doesn’t rise in a fairly linear manner, its an indication of completion of dissolution.

     d) Yes , the experiment must be repeated until a consistent value of initial and final temperature is obtained.

The calorimeter should not be changed as the value of specific heat may change.Also the mass of water must be kept constant.

-QRxn =Qcalorimeter+ Qwater (where Q=heat )

The mass of salt can be changed if we intend to calculate heat of dissolution per unit mass as the value would not change if the trials are conducted under similar conditions.

Answer 1) moles of methane=mass/molar mass=4.05g/16g/mol=0.2531 mol    [molar mass CH4=12+4*1=16 g/mol]

CH4+2O2→CO2+2H2O (combustion of methane)   , ΔHcomb = -890.4 kJ

So 1 mole methane burns to give out heat =-890.4 kJ

0.25 mol methane burns to give heat=q=-890.4*0.2531=-225.36 kj

2) moles of methane=(4.05g)= 0.2531 mol (see part 1)

Heat produced=-225.36 kj

Qwater=heat absorbed by water=Qrxn (heat released by reaction)

Qwater=mass (water)*Cwater*∆T=(1.0 kg*1000g/kg) *(4.18 J g-1C-1)*(Tf-20C)

                                                                   =1000g *(4.18 J g-1C-1)*(Tf-20C)

                                                                 =4180 JC-1 (Tf-20C)

                                                                  =4.18 kj C-1 (Tf-20C)

Also Qwater=Qrxn

Or, 4.18k J (Tf-20C)= 225.36

4.18 Tf-83.6 =225.36

4.18 Tf=225.36 +83.6 =308.96

Tf=308.96 kj/4.18=73.91C

Temperature final of water=73.91C