Consider the reaction of Calcium carbide and water as described below. The stand
ID: 487721 • Letter: C
Question
Consider the reaction of Calcium carbide and water as described below. The standard heat of reaction is Hr° = +69.4 kJ.
a. If the reaction ran adiabatically, would the temperature increase or decrease? Are the products or reactants more stable?
CaCl2(s) + 5H2O(l) CaO(s) + 2CO2(g) + 5 H2(g)
b. Suppose you charge 150g of calcium carbide and liquid water into a piston at 25 C, heat thecontainer until the calcium carbide reacts completely, and then cool the products backdown to 25 C, condensing all the unconsumed water, during the process the piston isallowed to expand and compress against a constant atmospheric pressure. Write andsimplify the energy balance for the closed system. Use the energy balance to determine thenet amount of heat that must be transferred to or from the reactor.
c. If you leave 1 mol of solid calcium carbide 5 mol liquid water in a container at 25 C and leftfor several days. You would not find 1 mol of solid calcium oxide, 2 mol of carbon dioxidegas and 5 mol of hydrogen gas. Why not?
Explanation / Answer
a)
If the reaction runs adiabatically, the temperature would decrease as Hr° is positive.
Positive Hr° shows that the heat is absorbed by system to run the reaction. But as the reaction was run adiabatically, no heat was provided from outside. Thereby, decreasing the system temperature.
Reactants will be stable as extra heat needed to run the reaction will not be provided.
b)
The general energy balance for closed system can be written as,
U = Q + W
U= the change in internal energy of this closed system
Q= heat transferred.
Q is positive when system absorbs heat
Q is negative when system rejects heat
W= work done on or by the system
W is positive when the work is done on the system
W is negative when the work is done by the system
Q is positive as heat was provided to the system, as the reaction is not reversible, as the temperature brought back to 25°C no or very little heat was lost by the system. (Q1>Q2)
As the work was done against constant environmental pressure, and cooling down would have brought piston back to position even if gases were formed. (W1=-W2 = 0)
c)
We would not find 1 mole of solid calcium oxide, 2 mole of carbon dioxide gas and 5 mole of hydrogen gas as reaction need to provide heat to run.
Even if the reaction takes place (which is difficult) at 25C, equilibrium will try to arrest reaction in between.