The thermochemical equation which is associated with ∆H°f, the standard enthalpy of formation, for urea, CO(NH2)2 (s), is:
CO (g) + 2NH3 (g) --> CO(NH2)2 (s) + H2 (g)
CO (g) + 2H2 (g) + N2 (g) --> CO(NH2)2 (s)
C (s) + O (g) + N2 (g) + 2H2 (g) --> CO(NH2)2 (s)
C (s) + 1/2O2 (g) + N2 (g) + 2H2 (g) --> CO(NH2)2 (s)
C (s) + 1/2O2 (g) + 2NH2 (g) --> CO(NH2)2 (s)
Please help
To determine the thermochemical equation associated with the standard enthalpy of formation (∆H°f) for urea, CO(NH2)2 (s), we need to identify the equation that represents the formation of urea from its constituent elements in their standard states.
The standard enthalpy of formation is defined as the change in enthalpy when one mole of a compound is formed from its elements in their standard states, with all substances in the gaseous state and at a standard pressure of 1 bar (or 1 atm).
Looking at the given options:
1. CO (g) + 2NH3 (g) --> CO(NH2)2 (s) + H2 (g)
This equation represents the reaction of carbon monoxide (CO) with two moles of ammonia (NH3) to form urea (CO(NH2)2) and hydrogen gas (H2). It seems to be the correct equation.
2. CO (g) + 2H2 (g) + N2 (g) --> CO(NH2)2 (s)
This equation involves the reactants carbon monoxide (CO), hydrogen gas (H2), and nitrogen gas (N2) reacting to form urea (CO(NH2)2). This reaction does not follow the stoichiometry of urea formation and is therefore incorrect.
3. C (s) + O (g) + N2 (g) + 2H2 (g) --> CO(NH2)2 (s)
This equation includes solid carbon (C) reacting with oxygen gas (O2), nitrogen gas (N2), and hydrogen gas (H2) to produce urea (CO(NH2)2). Since carbon is in the solid state instead of gaseous, this equation is not representative of standard conditions and is incorrect.
4. C (s) + 1/2O2 (g) + N2 (g) + 2H2 (g) --> CO(NH2)2 (s)
Similar to option 3, this equation involves solid carbon (C) reacting with oxygen gas (O2), nitrogen gas (N2), and hydrogen gas (H2) to form urea (CO(NH2)2). Again, solid carbon is not in the standard state, and this equation is incorrect.
5. C (s) + 1/2O2 (g) + 2NH2 (g) --> CO(NH2)2 (s)
This equation suggests that solid carbon (C) reacts with nitrogen-hydrogen compound (NH2) and half a mole of oxygen gas (O2) to form urea (CO(NH2)2). The presence of NH2 species does not align with urea's chemical formula, and therefore this equation is incorrect.
In summary, the first option, CO (g) + 2NH3 (g) --> CO(NH2)2 (s) + H2 (g), correctly represents the thermochemical equation and the standard enthalpy of formation (∆H°f) of urea, CO(NH2)2 (s).