Tag: energy cycles

Questions Related to energy cycles

The standard formation reaction for aluminium oxide is $4Al(s) + 6O(g)\rightarrow 2Al _2O _3(s)$. This statement is false because:

  1. aluminium is not in its standard state

  2. the reaction should be written for one mole of Al

  3. The reaction is prevented by a thin film of $Al _2O _3$ formed on the surface of Al

  4. none of these


Correct Option: B
Explanation:
$2Al+3O _2\rightarrow Al _2O _3$

The enthalpy of the above reaction is enthalpy of formation as one mole of $Al _2O _3$ is forming from elemental aluminum and oxygen gas. 

In the balanced equation for combustion of 1 mole of butane, $C _4H _{10}(g)$, the coefficient of oxygen is:

  1. 5/2

  2. 9/2

  3. 5

  4. 13/2


Correct Option: D
Explanation:
Balanced chemical reaction of butane:
$C _4H _{10}+\frac{13}{2}O _2\rightarrow4CO _2+5H _2O$

Standard enthalpy of formation $(\Delta H _f)$ of which of the following is zero at $25^0C$ ?

  1. White phosphorous

  2. Red phosphorous

  3. Red lead $(Pb _3O _4)$

  4. $H^+(g)$


Correct Option: A
Explanation:

White phosphors is the elemental form of phosphorus for which enthalpy of formation is zero, while, $H^+$ and $Pb _3O _4$ are not elemental forms.

The enthalpy of hydrogenation for $1-pentene$ is $+126\ kJ/mol$. The enthalpy of hydrogenation for $1, 3-pentadiene$ is $+230\ kJ/mol$. Hence estimate the resonance magnitude of (delocalization) energy of $1, 3-pentadiene$.

  1. $22\, kJ/mol$

  2. $104\, kJ/mol$

  3. $252\, kJ/mol$

  4. cannot be calculated from this information


Correct Option: A
Explanation:

Enthalpy of hydrogenation per bond$=+126\ kJ/mol$


Enthalpy of hydrogenation for 2 bonds$=+252\ kJ/mol$


Resonance energy$=252-230=22\ kJ/mol$

The least stable in amongst the following is:

  1. $Li^-$

  2. $Be$

  3. $B$

  4. $C$


Correct Option: A
Explanation:

$Li$ is alkali metal hence, the negative charge on $Li$ is least stable as it will have the lowest electronegativity.

The heats of neutralization of $CH _3COOH.HCOOH, HCN$ and $HClO$ are 13.2, 13.4,2.9 and 3.6 kcal/eq respectively. Then, the degree of hydrolysis for the respective ions will be in the order :

  1. $CH _3COO^- < HCOO^- < CN^- < ClO^-$

  2. $HCOO^- < ClO^- < CN^- < CH _3COO^-$

  3. $CH _3COO^- < CN^- < ClO^- < HCOO^-$

  4. $HCOO^- < CH _3COO^- < ClO^- < CN^-$


Correct Option: D
Explanation:

Strong acid have high heat of neutralization and are very much stable so required high energy for dissociation. Hence, weak acids are hydrolysed at faster rate than strong acids.

The heat change for the reaction: $C(s) + S(s)\rightarrow CS _2(l)$, known as:

  1. heat of transition

  2. heat of fusion

  3. heat of vapourisation

  4. heat of formation


Correct Option: D
Explanation:
$C(s)+2S(s)\rightarrow CS _2(l)$
The heat change of the reaction is enthalpy of formation as 1 mole of $CS _2$ is being formed from elemental state of carbon as well as Sulphur.

Enthalpy of a solution of $ CsBr(s)$ is $10\ kJ/mol$.If the enthalpies of hydration of $Cs^+(g)$ and $Br(g)$ are 475 and 655 kJ.mol, what should be the lattice energy of $CsBr(s)$ in $kJ/mol$:

  1. $1120$

  2. $1130$

  3. $1140$

  4. $1150$


Correct Option: C

The stability of an ionic compound is mostly due to:

  1. ionization energy

  2. electron affinity

  3. lattice energy

  4. electronegativity


Correct Option: C
Explanation:

Lattice energy is a measure of energy released in ionic crystals when the ions are brought together from infinity. It measures the attraction between ions so that they are held in a position. Ionic compound stability is mostly due to lattice energy.

Decreasing order of lattice energy of $FeO, Fe _2O _3, NaCl$ is:

  1. $NaCl>FeO>Fe _2O _3$

  2. $Fe _2O _3>FeO>NaCl$

  3. $NaCl>FeO=Fe _2O _3$

  4. $Fe _2O _3>NaCl>FeO$


Correct Option: B