Question

A combustion reaction of a hydrocarbon is defined as the reaction of the compound into carbon dioxide and water.

CmHn(g) + (m+(1/4)n)O2(g) ---> mCO2 + (1/2)nH2O(l)

1. Demonstrate that this chemical reaction is balanced.

2a. From this definition, write down the chemical reaction for the combustion of cyclopropane, C3H6(g).

2b. The enthalpy of reaction for the combustion of cyclopropane (i.e., the reaction in 2a) is measured to be ΔH° rxn = −2091 kJ/mol in the standard state at 25° C. Find the standard state enthalpy of formation, ΔH° f , at 25° C for CO2(g) and H2O(l) in a chemistry textbook. Use this information to determine the enthalpy of formation for cyclopropane in the standard state at 25° C.

2c. Propene is an isomer of cyclopropane. At 1bar pressure and a temperature of 25° C, the enthalpy of formation for propene is +20.42 kJ mol-1 . Determine the enthalpy of reaction for the isomerization reaction:

cyclopropane (g) → propene (g) ,

at 1bar pressure and a temperature of 25° C.

Answer 1

CmHn (g) + (m+(1/4)O2 (g)------------> mCO2 +(1/2)n H2O (l),

hydrocarbon hsa the general formuala of alkane=CnH2n+2,

FOR EXAMPLE n=1 then ,formula will be, CH_2*1+2=CH4,

PUTTING THE VALUES IN REACTION, WHERE m=(n)=1, and n=4,(2n+2), 1/4=1

CmHn (g) + (m+(1/4)O2 (g)------------> mCO2 +(1/2)n H2O (l),

CH4 (g) + 1+(1)O2 (g)------------>1 CO2(g) +1/2n=2 H2O (l) i.e,

CH4 (g) +2O2 (g)------------> CO2(g) +2 H2O (l) THIS REACTION IS BALANCED.

2a) C3H6 + O2 -----> 3CO2 + 3H2O from this reaction we can balance the reaction as,

one carbon gives 1 co2, 4 hydrogen gives 2 water molecules ( general reaction),then how many oxygen atoms are required to balance the reaction,

for 1 carbon 2 molecules of oxygens to be require,

but for ,C3H6 + (9/2)O2 ---------> 3CO2 + 3H2O , then for 2 molecules of cyclopropane=

2C3H6 + 9O2 -------> 6CO2 + 6H2O,

2 b) dH= dHf products - dHf of reactants, dH=-2091 KJ /MOL,

-2091= (dHf Co2+dHf H2O) - dHf of reactants,

-2091= (dHf 393.509+dHf 241.818) - dHf of reactants,

-2091=635.323 KJ/MOL -  dHf of reactants, THERE FORE

dHf of reactants=-635.323 KJ/MOL +(-2091)=-1455.67 KJ /MOL,

-2091= (dHf -635.323 +(-1455.67)

c) FOR THE ISOMERISATION OF CYCLOPRORPENE TO PROPENE,

C3H6---------->C2H4+H2,

dH= dHf products - dHf of reactant,

dH= (C2H4+H2)- (C3H6), dH= (20.42 KJ/MOL+0)- (-1455.67 KJ/MOL), dHf of H2 AT 25 ⁰ C=0,

dH= 1476.09 KJ/MOL FOR ISOMERISATION OF CYCLOPROPENE.