A student runs two experiments with a constant-volume "bomb" calorimeter containing 1100.g of water (see sketch at right).First, a 8.000g tablet of benzoic acid C6H5CO2H is put into the "bomb" and burned completely in an excess of oxygen. (Benzoic acid is known to have a heat of combustion of /26.454kJg.) The temperature of the water is observed to rise from 11.00°C to 56.81°C over a time of 14.2 minutes.Next, 5.690g of acetylene C2H2 are put into the "bomb" and similarly completely burned in an excess of oxygen. This time the temperature of the water rises from 11.00°C to 67.44°C.Use this information, and any other information you need from the ALEKS Data resource, to answer the questions below about this reaction:A "bomb" calorimeter.→+2C2H2g 5O2g + 4CO2g 2H2OgBe sure any of your answers that are calculated from measured data are rounded to the correct number of significant digits.Note for advanced students: it's possible the student did not do these experiments sufficiently carefully, and the values you calculate may not exactly match published values for this reaction.Is this reaction exothermic, endothermic, or neither? exothermicendothermicneitherIf you said the reaction was exothermic or endothermic, calculate the amount of heat that was released or absorbed by the reaction in the second experiment. kJCalculate the reaction enthalpy ΔHrxn per mole of CO2. kJmol
Question
A student runs two experiments with a constant-volume "bomb" calorimeter containing 1100.g of water (see sketch at right).First, a 8.000g tablet of benzoic acid C6H5CO2H is put into the "bomb" and burned completely in an excess of oxygen. (Benzoic acid is known to have a heat of combustion of /26.454kJg.) The temperature of the water is observed to rise from 11.00°C to 56.81°C over a time of 14.2 minutes.Next, 5.690g of acetylene C2H2 are put into the "bomb" and similarly completely burned in an excess of oxygen. This time the temperature of the water rises from 11.00°C to 67.44°C.Use this information, and any other information you need from the ALEKS Data resource, to answer the questions below about this reaction:A "bomb" calorimeter.→+2C2H2g 5O2g + 4CO2g 2H2OgBe sure any of your answers that are calculated from measured data are rounded to the correct number of significant digits.Note for advanced students: it's possible the student did not do these experiments sufficiently carefully, and the values you calculate may not exactly match published values for this reaction.Is this reaction exothermic, endothermic, or neither? exothermicendothermicneitherIf you said the reaction was exothermic or endothermic, calculate the amount of heat that was released or absorbed by the reaction in the second experiment. kJCalculate the reaction enthalpy ΔHrxn per mole of CO2. kJmol
Solution
The reaction is exothermic. This is because the temperature of the water in the calorimeter increases when the reaction occurs, indicating that heat is being released.
To calculate the amount of heat released in the second experiment, we first need to calculate the heat absorbed by the water using the formula q = mcΔT, where m is the mass of the water, c is the specific heat capacity of water (4.18 J/g°C), and ΔT is the change in temperature.
m = 1100 g c = 4.18 J/g°C ΔT = 67.44°C - 11.00°C = 56.44°C
So, q = (1100 g)(4.18 J/g°C)(56.44°C) = 259,000 J = 259 kJ
This is the heat absorbed by the water, but since the reaction is exothermic, this is also the heat released by the reaction.
To calculate the reaction enthalpy ΔHrxn per mole of CO2, we first need to calculate the number of moles of CO2 produced in the reaction. The reaction equation tells us that 4 moles of CO2 are produced for every 2 moles of C2H2 reacted. The molar mass of C2H2 is approximately 26 g/mol, so the number of moles of C2H2 reacted is (5.690 g)/(26 g/mol) = 0.219 mol. Therefore, the number of moles of CO2 produced is (4/2)(0.219 mol) = 0.438 mol.
So, ΔHrxn = q/n = (259 kJ)/(0.438 mol) = -592 kJ/mol
The negative sign indicates that the reaction is exothermic.
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