A 35.6 g sample of ethanol (C₂H₅OH) is burned in a bomb calorimeter according to the following reaction. If the temperature rose from 35.0 °C to 76.0°C and the heat capacity of the calorimeter is 23.3 kJ , what is the value of ΔU°? The molar mass of ethanol is 46.07 g . C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(g) ΔU°= ?

Question

Quizplus · Accepted Answer

First, we need to calculate the amount of heat released from the combustion of 35.6 g of ethanol. We can do this by using the formula:

q = m*Cp*ΔT

where 
q = heat released
m = mass of ethanol
Cp = heat capacity of the calorimeter
ΔT = change in temperature

Plugging in the values, we get:

q = 35.6 g * (1 mol/46.07 g) * (-136.7 kJ/mol) = -111.1 kJ

Next, we need to calculate the value of ΔU°. We can use the equation:

ΔU° = q + Δngas*R*T

where
Δngas = (moles of gaseous products) - (moles of gaseous reactants)
R = gas constant
T = temperature

From the reaction, we see that Δngas = (2+3) - 3 = 2.
Plugging in the values, we get:

ΔU° = -111.1 kJ + (2 mol)(8.31 J/mol*K)(349 K) = -1237 J/mol = -1.24 x 10^3 J/mol = -1.24 x 10^0 kJ/mol

Finally, converting to the given units, we get the answer as:

Answer: A
 The value of ΔU° is -1.24 × 10^3 J/mol which is equal to -1.24 × 10^0 kJ/mol.

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