Question

Given the following heats of reaction, use Hess’s law to calculate the heat of reaction (ΔH_rxn) for the transformation below:

3 C(s) + 4 H₂(g) à C₃H₈(g)   ΔH_rxn = ?

C₃H₈(g) + 5O₂(g) à 3 CO₂(g) + 4H₂O(g)   ΔH= - 2043 kJ/mol

                        C(s) + O₂(g) à CO₂(g)                        ΔH= - 393.5 kJ/mol

                        2H₂(g) + O₂(g) à 2 H₂O(g)                            ΔH= - 483.6 kJ/mol

	A.	-1165.9 kJ/mol
	B.	- 104.7 kJ/mol
	C.	682.3 kJ/mol
	D.	-2920.1 kJ/mol
	E.	104.7 kJ/mol
	F.	1165.9 kJ/mol
	G.	2920.1 kJ/mol
	H.	-4190.7 kJ/mol

calculate the heat of reaction (ΔH_rxn) for the transformation below:

Ca²⁺(aq) + 2 OH^-(aq) + CO₂(g) à CaCO₃(s)+ H₂O(l)                      ΔH_rxn = ?        

CaCO₃(s) à CaO(s) + CO₂(g)                      ΔH= - 178.1 kJ/mol

CaO(s) + H₂O(l) à Ca(OH)₂ (s)                    ΔH= - 65.3 kJ/mol

Ca(OH)₂ (s) à Ca²⁺(aq) + 2 OH^-(aq) ΔH= - 16.2 kJ/mol

	A.	-519.2 kJ/mol
	B.	-96.6 kJ/mol
	C.	31.3 kJ/mol
	D.	259.6 kJ/mol
	E.	519.2 kJ/mol
	F.	-31.3 kJ/mol
	G.	96.6 kJ/mol
	H.	-259.6 kJ/mol Calculate the value of work w for the internal energy change in the following reactions at 25.0°C. Which direction is the work being applied, by the system or on the system? 2 NH4NO3 (s) à 2 N2 (g) + 4 H2O (g) + O2 (g) A. -1.455 kJ/mol; by system B. 0.000 kJ/mol; neither C. -12.34 kJ/mol; by system D. 171.3 J/mol; on system E. -171.3 J/mol; by system F. 12.34 kJ/mol; on system G. -17.34 kJ/mol; by system H. 1.455 kJ/mol; on system Calculate the value of work w for the internal energy change in the following reaction at 25.0°C. Which direction is the work being applied, by the system or on the system? 2 SiO2 (g) + 3 C (s) à SiC (s) + 2 CO (g) A. -207.9 J/mol; by system B. 415.7 J/mol; on system C. -2478 J/mol; by system D. 0.000 J/mol; neither E. -24.47 J/mol; by system F. -4955 J/mol; by system G. 48.93 J/mol; on system H. 2478 J/mol; on system

Answer 1

3*( C(s) + O2(g) -----> CO2(g)                 ΔH = 3* -393.5 kJ )

2*( 2H2(g) + O2(g) ----> 2 H2O(g)              ΔH = 2*-483.6 kJ )

3 CO2(g) + 4H2O(g) --> C3H8(g) + 5O2(g)      ΔH = + 2043 kJ/mol
------------------------------------------------------------------
3 C(s) + 4 H2(g) -----> C3H8(g)             ΔHrxn = ?
------------------------------------------------------------------

ΔHrxn = 3* -393.5 + 2*-483.6 + 2043

       = -104.7 kj/mol

answer: B. -104.7 kj/mol

Ca2+(aq) + 2 OH-(aq) ----> Ca(OH)2 (s)       ΔH = + 16.2 kJ/mol

Ca(OH)2 (s) ----> CaO(s) + H2O(l)              ΔH = + 65.3 kJ/mol

CaO(s) + CO2(g) ---> CaCO3(s)                 ΔH = + 178.1 kJ/mol
--------------------------------------------------------------------------
Ca2+(aq) + 2 OH-(aq) + CO2(g) ---> CaCO3(s)+ H2O(l)   ΔHrxn = ?
--------------------------------------------------------------------------

ΔHrxn = 16.2+ 65.3+178.1 = 259.6 kj/mol

answer: D

2 NH4NO3 (s) --> 2 N2 (g) + 4 H2O (g) + O2 (g)

as solid reactant converts into gaseous products , results

work is done by the system

w = -P*DV

   = -DnRT

Dn = no of mole changed = 7

R = 8.314 j.k-1.mol-1

T = 25 c = 298 k

w = -7*8.314*298

   = -17.34 kj/mol

answer: G. -17.34 kJ/mol; by system

2 SiO2 (g) + 3 C (s) ---> SiC (s) + 2 CO (g)

Dn = 2-2 = 0

w = 0 , work done = 0

answer: D. 0.000 J/mol; neither