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Standard XII

Chemistry

Heat of Formation

On burning ...

Question

On burning $4.0 g$ of iron to ferric oxide at constant pressure, the heat evolved is $29.28 k J$ . The enthalpy of formation of ferric oxide is: (At. wt. of $F e = 5$ )

+ 81.98 K J {m o l}^{- 1}

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- 81.98 K J {m o l}^{- 1}

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+ 819.8 K J {m o l}^{- 1}

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- 819.8 K J {m o l}^{- 1}

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Solution

The correct option is B $+ 81.98 K J {m o l}^{- 1}$
Ferric oxide $({F e}_{2} O_{3})$ contain $2$ moles of $F e$ & $3$ moles of $O$
$2$ moles of $F e = 2 \times 56 = 112 g$
When $4 g$ of $F e$ is burnt, heat liberated is $29.28 K J$
$∴$ When $112 g$ of $F e$ is burnt, heat liberated is $= \frac{29.28 \times 112}{4} = - 819.8 K J / m o l e$ ( $- v e$ sign indicates the liberation of heat)

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Similar questions

Q. When

4 g

of iron is burnt to ferric oxide at constant pressure,

29.28 k J

of heat is evolved. What is the enthalpy of formation of the formation of ferric oxide? (Atomic Mass of

F e = 56 g / m o l e

)

Q. When

4.0

g of iron is burnt at high temperature, to ferric oxide, at constant pressure,

29.28

kJ of heat is evolved. The amount of heat evolved for producing one mole of ferric oxide is:

1 m o l

of iron

(F e)

reacts competely with

0.65 m o l O_{2}

to give a mixture of only

F e O

and

F e_{2} O_{3}

. Mole ratio ferrous oxide to ferric oxide to ferric oxide is:

Q. Becker and Ruth measured the heat evolved in the following processes at

20^{o} C

: (1)

1

mole of solid

{({C O O N H}_{4})}_{2} H_{2} O

is burned in oxygen, (2)

1

mole of solid

{(C O O H)}_{2} {(H_{2} O)}_{2}

is burned in oxygen, (3)

1

mole of solid

{({C O O N H}_{4})}_{2} H_{2} O

is dissolved in a large excess of water, (4)

1

mole of solid

{(C O O H)}_{2} {(H_{2} O)}_{2}

is dissolved in a large excess of water, (5)

1

mole of oxalic acid in dilute solution is neutralized with gaseous ammonia. They found (1)

189.86 k c a l

, (2)

53.10 k c a l

, (3)

- 11.47 k c a l

, (4)

- 8.62 k c a l

, (5)

43.13 k c a l

; (1) and (2) were measured at constant volume, the others at constant pressure. The end products of (1) and (2) were nitrogen, carbon-di-oxide and water. The heat of formation for

1

mole of water from the elements had previously been determined as

68.35 k c a l

at constant pressure and

20^{o} C

. Find the change in enthalpy when

1

mole of

{N H}_{3}

is formed from the elements at

20^{o} C

(only magnitude in nearest integer in kcal)

1 mol of iron( Fe) reacts completely with 0.65 mol of O₂ to give a mixture of only FeO and Fe₂O₃ . mole ratio of ferrous oxide to ferric oxide is

Give the solution using some short cut method

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On burning 4.0g of iron to ferric oxide at constant pressure, the heat evolved is 29.28kJ. The enthalpy of formation of ferric oxide is: (At. wt. of Fe=5)

The correct option is B +81.98KJmol−1Ferric oxide (Fe2O3) contain 2 moles of Fe & 3 moles of O2 moles of Fe=2×56=112gWhen 4g of Fe is burnt, heat liberated is 29.28KJ∴ When 112g of Fe is burnt, heat liberated is =29.28×1124=−819.8KJ/mole (−ve sign indicates the liberation of heat)

On burning $4.0 g$ of iron to ferric oxide at constant pressure, the heat evolved is $29.28 k J$ . The enthalpy of formation of ferric oxide is: (At. wt. of $F e = 5$ )