# chemistry ?

Urea, NH2CONH2(s), is manufactured on a large scale and used as a fertilizer. To investigate its properties a student places 120.0 mL of water at 25.0°C into a coffee cup calorimeter and immerses a thermometer in the water. After about a minute, the student adds 5.0 g of solid urea to the water and measures the temperature of the solution as the urea dissolves. A temperature drops to 22.9°C.

(a) Calculate the molar enthalpy of solution for urea, NH2CONH2(s).

(b) Write the thermochemical equation for this reaction

Relevance
• To find the heat transferred during the dissolving of the urea, we use the equation

q = mC∆T

q = heat = ?

m = mass of water = 120.0 ml x 1 g/ml = 120.0 g

C = specific heat of water = 4.184 J/gº

∆T = change in temperature = 25.0 - 22.9 = 2.1º

q = (120 g)(4.184 J/gº)(2.1º) = 1054 J and since the temperature dropped, that means the reaction (dissolution of urea) is endothermic so the q has a positive value.

a).  molar enthalpy:

5.0 g urea x 1 mol urea/60.0 g = 0.083 mol

molar enthalpy = 1054 J / 0.083 mol = 12,699 J/mol = 12.7 kJ/mol = 13 kJ/mol

b).  NH2CONH2(s) + H2O(l) ==> NH2CONH2(aq)  ∆H = +13 kJ/mol

or  NH2CONH2(s) + H2O(l) + 13 kJ ==> NH2CONH2(aq)