Question

1) Constant-volume calorimeters are sometimes calibrated by running a combustion reaction of known ΔE and measuring the change in temperature. For example, the combustion energy of glucose is 15.57 kJ/g. When a 2.500 g sample of glucose burns in a constant volume calorimeter, the calorimeter temperature increases from 21.45 to 23.34°C. Find the total heat capacity of the calorimeter (in kJ/K).

2)An electrical heater is used to add 18.25 kJ of heat to a constant-volume calorimeter. The temperature of the calorimeter increases by 3.40°C. When 2.25 g of ethanol (C₂H₅OH) is burned in the same calorimeter, the temperature increases by 12.45°C. Calculate the molar heat of combustion for ethanol (enter in kJ).

3)A 0.90 g sample of caffeine, C₈H₁₀N₄O₂, burns in a constant-volume calorimeter that has a heat capacity of 7.85 kJ/K. The temperature increases from 298.25 K to 303.34 K. What is the molar heat of combustion of caffeine (in kJ).

4)Use standard enthalpies of formation to determine ΔH^o_rxnfor:

2Al(s) + 3Cl₂(g) → 2AlCl₃(s)   Find the change in internal energy for this reaction. Enter in kJ.

5)Use standard enthalpies of formation to determine ΔH^o_rxn for:

2NH₃(g) + 3O₂(g) + 2CH₄(g) → 2HCN(g) + 6H₂O(g) Find the change in internal energy for this reaction. Enter in kJ.

Answer 1

1)

Let c be the total heat capacity of calorimeter

enthalpy of combustion of 2.5 g of glucose = heat coonsumed by calorimeter

m x = C x (T₂ - T₁)

enthalpy of reaction = =15.5 kJ/g

2.5 x 15.57 kJ = c x (23.34 - 21.45)

38.925 = c x 1.89

specific heat of calorimeter, C = 20.4 kJ/K

2)

First step: finding the heat capacity of calorimeter

q = c x

q = 18.25 kJ

=3.40

C= q/ = 18.25/3.40 = 5.37 kJ/^oC

second step: Finding enthalpy of reaction

Change in temperature of calorimeter when ethanol burned in calorimeter = = 12.45 ^oC

Enthalpy of reaction = heat capacity of calorimeter x

Therefore, 5.37 kJ/^oC x 12.45 ^oC

66.86 kJ

third step: molar heat of combustion of ethanol

molar heat of combustion of ethanol = 1366.8 kJ/mol

3)

Heat capcity of calorimeter = 7.85 kJ/K

= 303.34 K - 298.25 K = 5.09 K

Enthalpy of reaction = 7.85 x 5.09 = 39.96 kJ

Molar heat of combustion of caffeine (in kJ) =

Molar heat of combustion of caffeine (in kJ) =8621.7 kJ

4)

2Al(s) + 3Cl₂(g) → 2AlCl₃(s)

Enthalpy of reaction =

but enthalpy of formation standard elments is 0

change in internal energy for this reaction =

5)

2NH₃(g) + 3O₂(g) + 2CH₄(g) → 2HCN(g) + 6H₂O(g)

Enthalpy of reaction =

but enthalpy of formation standard elments is 0

change in internal energy for this reaction =

= - 942.3 kJ