The mass of a coffee cup calorimeter with its covering is measured to be 5.991 g

Question

The mass of a coffee cup calorimeter with its covering is measured to be 5.991 g. 25.0 mL of 1.00 M aqueous calcium ate is measured into a graduated cylinder and then added to the calorimeter; the mass of the calorimeter with its covering and its contents is 30.834 g. 25.0 mL of 1.00 M sodium phosphate is measured into a graduated cylinder; both solution volumes have a temperature of 22.1 degree C. The sodium phosphate volume is added to the calorimeter causing a precipitation reaction that lowers the temperature of the mixture to 19.6 degree C. The final mass of the calorimeter with its cap and contents is 55.369 g. Calculate the amount of heat generated by the reaction. Calculate the molar enthalpy for the precipitation reaction, based on the limiting reactant and assuming all aqueous solutions are sufficiently dilute that their densities are equivalent to pure water. Combustion reactions are exothermic. Write a balanced reaction for the gas-phase combustion of acetylene. Calculate the AH degree of this reaction based on delta H degree of formation values in Table A4.3.

Explanation / Answer

(1)

Balanced reaction is :

2Na3PO4 + 3Ca(NO3)2--->6NaNO3 + Ca3(PO4)2

Mass of Ca(NO3)2 added = 30.834 - 5.991 = 24.843 g

Mass of Na3PO4 added = 55.369 - 30.834 = 24.535 g

Temperature drop = 22.1 - 19.6 = 2.5 'C

Heat capacity of Na3PO4 = 4.05 J/g.K

Heat capacity of Ca(NO3)2 = 0.91 J/g.K

Heat released = Heat released by Ca(NO3)2 + Heat released by Na3PO4 = 24.843*0.91 + 24.535*4.05 = 121.97 J

Both the compounds have nearly same MW ( = 164 g), and also the amount of each added is also almost same.

Moles of each compound added = 24.6/164 = 0.15 moles

Here, Ca(NO3)2 is the limiting reagent.

So, based on the limiting reagent, molar enthalpy = Heat generated / Moles of limiting reagent = 121.97 / 0.15 = 813.13 J/mol

(2)

The required reaction for complete combustion of acetylene is:

2 C2H2 + 5 O2 = 4 CO2 + 2 H2O

dHorxn = dHof(products) - dHof(reactants)

Switch in the values in the above equation from the table.

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