Question

In a science fiction, astronaut Diego needs to make a bomb to redirect the Ebes space station towards a new set of coordinates, changing its velocity. Given that he is an on-board chemistry scientist and they do not have any form of explosives or flammables to ignite the engine. Diego has to take thermodynamics into consideration. He decides to burn 12.0 g of pure sucrose inside a very sturdy glass jar that initially contains 10L of O2 gas at 1 atmosphere pressure and 298.15 K. The combustion produces water vapor and carbon dioxide gas. Assumed the combustion is completed and all gases can be considered ideal. If the oxygen is not completely consumed by sucrose, some will be left in the jar after reaction takes place.

The values of Cv are given (J/mol*K): CO2(g)=28.7 , H2O(g)=25.5, O2=21.0 . Sucrose density is 1587 g per liter.

1. What is the change in enthalpy for 12.0 g of sucrose cobustion reaction under standard conditions？

2. Astronaut Diego assumes that all the enthalpy change for the reaction is released as solely heat and used to heat the final gas mixture. What is the problem with this assumption? Please explain in terms of thermodynamics.

Answer 1

Moles of Oxygen present initially = PV/RT = (1x10)/(0.0821x298.15) = 0.4085

Moles of Sucrose added = 12 / (144+22+176) = 0.0351

Moles of Oxygen reacts = 6 x 0.0351 = 0.2105

Moles of Oxygen Left = 0.4085 - 0.2105 = 0.198

First law of thermodynamics says that energy can neither be created nor be destroyed, it can only be converted from one form to another.Mathemetically it can be expressed as -

Now q is the heat released from the combustion which will change the internal energy of the gas mixture which doesn't only include the heat some part will be released in the form of work done on the surrounding, changing the mechanical energy and other forms of internal energy (sound,thermal,phase change).