Solid urea, (NH2)2CO, burns to give CO2, N2, and liquid H2O. Its heat of combustion is -632.2 kJ/mol.?

Question

Solid urea, (NH2)2CO, burns to give CO2, N2, and liquid H2O. Its heat of combustion is -632.2 kJ/mol.

Balanced equation:
1(NH2)2CO(s) + 3/2O2(g) ---> 1CO2(g) +1 N2 (g) + 2H2O(l)

Calculate the heat generated per mole of H2O formed to 0.1 kJ.

Using this heat of combustion and the appropriate thermodynamic data, determine the heat of formation of urea to 0.1 kJ.

Make sure i have balanced right.. anything you can provide would really help/

Answer 1

Your equation does not need the number "1" anywhere in it, but otherwise it's correct.

Your equation is saying that, when 2 moles of water are formed in this reaction, 632.2 kJ of heat are given out. So merely halve that value for 1 mole.

The enthalpy of formation of urea requires you to work out the heat change during the reaction:

N2 + 2H2 + C + 1/2 O2 ----> (NH2)2CO

Additional information that you need is the enthalpy of combustion of C (= the enthalpy of formation of CO2) and the enthalpy of combustion of H2 (= the enthalpy of formation of H2O). This latter value will need multiplying by 2. Now just apply Hess's Law.

Answer 2

Of course, for something to burn, its atoms or molecules must react with oxygen in the surrounding air, and that is harder for them to do if they are kept in place by the relatively strong cohesive forces that obtain in a solid. If, however, their affinity to oxygen is strong enough, they will. Put a match to the end of a length of very thin steel wool and swing it fast through the air (don't try this indoors!) - it will burn with dazzling sparks, even though the flame of the match is not hot enough to vaporize iron. To get a true flame, on the other hand, as opposed to just sparks and light, you do need a burning gas or vapor.