Worksheet: Enthalpy and Internal Energy

In this worksheet, we will practice calculating enthalpy changes from changes in internal energy and changes in volume, at constant pressure.

Q1:

A 1.50-mole sample of an ideal gas at 245 K and a pressure of 0.75 bar is altered by some process that takes it to a state in which its temperature is increased to 295 K and its pressure is increased to 0.90 bar. What will be the enthalpy change ( Δ 𝐻 ) of the gas in this process?

Q2:

Consider the reaction 2 C ( ) + C l ( ) + 2 F ( ) C F C l C F C l ( ) s g g g 2 2 2 2 , for which the standard reaction enthalpy has a value of Δ 𝐻 = 8 9 0 . 4 / r k J m o l at a temperature of 298 K. What is the value of Δ 𝑈 r for this reaction at 𝑇 = 2 9 8 K ?

Q3:

Consider the reaction C H C O O H ( ) + ( 1 5 / 2 ) O ( ) 7 C O ( ) + 3 H O ( ) 6 5 2 2 2 s g g l , for which the standard reaction enthalpy is Δ 𝐻 = 3 2 2 8 . 2 / r k J m o l . Calculate the value of Δ 𝑈 r for this reaction and determine whether any thermodynamic work is done on ( 𝑤 > 0 ) or by ( 𝑤 < 0 ) the reaction system if the reaction is carried out at a constant pressure. Assume that the gases in this reaction exhibit ideal-gas behavior at 𝑇 = 2 9 8 . 1 5 K and 𝑃 = 1 b a r .

  • A Δ 𝑈 = 3 2 3 0 . 7 / r k J m o l and 𝑤 = 0
  • B Δ 𝑈 = 3 2 3 0 . 7 / r k J m o l and 𝑤 < 0
  • C Δ 𝑈 = 3 2 2 8 . 2 / r k J m o l and 𝑤 = 0
  • D Δ 𝑈 = 3 2 2 7 . 0 / r k J m o l and 𝑤 < 0
  • E Δ 𝑈 = 3 2 2 7 . 0 / r k J m o l and 𝑤 > 0

Q4:

The standard heat of combustion of solid glucose ( C 6 H 1 2 O 6 ) at 298 K is Δ 𝐻 c ( 2 9 8 K ) = 2 8 0 1 kJ/mol. What is the value of Δ 𝑈 c , the change in internal energy accompanying the combustion at 298 K and a fixed pressure of 1 bar?

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