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At 300K , the standard enthalpies of for...

At `300K` , the standard enthalpies of formation of `C_(6)H_(5)COOH_((s),CO_(2(g))` and `H_(2)O_((l))` are `-408,-393` and `-286kJ mol^(-1)` respectively. Calculate the heat of combustion of benzoic acid at
`(i)` constant pressure,
`(ii)` constant volume. `(R=8.31J mol^(-1)K^(-1))`

Text Solution

Verified by Experts

Given,
`7C_((s))+3H_(2(g))+O_(2(g))rarr C_(6)H_(5)COOH_((s)), DeltaH^(@)=-408kJ ...(1)`
`C_((s))+O_(2(g))rarr CO_(2(g)), DeltaH^(@)=-393kJ ....(2)`
`H_(2(g))+(1)/(2)O_(2(g)) rarr H_(2)O_((l)), DeltaH^(@)=-286kJ …(3) `
`C_(6)H_(5)COOH_((s))+(15)/(2)O_(2(s))rarr 7CO_(2(g))+3H_(2)O_((l)), DeltaH=? ...(4)`
By `eq. (2)xx7+eq. (3)xx3-(1)`
`C_(6)H_(5)COOH_((s))+(15)/(2)O_(2(g))rarr 7CO_(2(g))+3H_(2)O_((l)), DeltaH^(@)=[-393xx7-286xx3+408]`
`implies DeltaH^(@)=-3201kJ`
Also `DeltaH=DeltaU+DeltanRT`
`:. -3201=DeltaU+(-0.5)xx8.314xx10^(-3)xx300`
`( :' Deltan=-0.5 and R=8.314xx10^(-3)kJ)`
`:. DeltaU=-3201+1.2471=-3199.7529`
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At 300 K, the standard enthalpies of formation of C_(6)H_(5)COOH(s),CO_(2)(g) and H_(2)O(l) are -408,-393 and -286kJ mol^(-1) respectively. Calculate the heat of combustion of benzoic acid at (i) constant pressure and (ii) constant volume. (R=8.31J" "mol^(-1)K^(-1))

At 300 K, the standard enthalpies of formation of C_(6)H_(5)COOH (s), CO_(2)(g) & H_(2)O (l) are , -408, -393 & -286" kJ mol"^(-1) respectively. Calculate the heat of combustion of benzoic acid at : (i) constant pressure (ii) constant volume.

Knowledge Check

  • The standard enthalpy of formation of C_(2)H_(4)(l) , CO_(2)(g) and H_(2)O(l) are 52 , 394 and -286 kJ mol^(-1) respectively. Then the amount of heat evolved by burning 7g of C_(2)H_(4)(g) is

    A
    `1412 kJ`
    B
    `9884 kJ`
    C
    `353 kJ`
    D
    `706 kJ`

  • if enthalpies of formatuon of C_(2)H_(4)(g),CO_(2)(g) and H_(2)O(l)at 250^(@)and 1 atm pressure be 52 ,-394 and -286 kJ mol ^(-1) respectively,the enthalpy of combustion of C_(2) H_(4)(g) will be

    A
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    C
    `+ 141 .2 kJ mol ^(-1)`
    D
    ` -141 .2 kJ mol ^(-1)`

  • If enthalpies of formation of C_(2)H_(4)(g), CO_(2)(g) and H_(2)O(l) at 25^(@)C and 1 atm pressure be 52, - 394 and - 286 kJ mol^(-1) respectively, the enthalpy of combustion of C_(2)H_(4)(g) will be

    A
    `+1412 kJ mol^(-1)`
    B
    `-1412 kJ mol^(-1)`
    C
    `+141.2 kJ mol^(-1)`
    D
    `141.2 kJ mol^(-1)`

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