Enthalpy

Q. By what factor does the average velocity of a gaseous molecule increase when the temperature ( in kelvin) is doubled ?? (1) 1.4 (2) 2.0 (3) 2.8 (4) 4.0 \lbrack AIPMT( prelims)-2011\rbrack

Q.

Enthalpy of combustion of $C{H}_4,C_2{H}_6$ and $C_3{H}_8$ are -210.8, -368.4 and -526.2 k cal $mo{l}^{-1}$ respectively. Enthalpy of combustion of hexane can be predicted as ?

1. -840 k cal $mo{l}^{-1}$

2. -684 k cal $mo{l}^{-1}$

3. -1000 k cal $mo{l}^{-1}$

4. None of these

Q.

The enthalpies for the following reactions at 25°C are given.
$\frac{1}{2}{H}_2\left(g\right)+\frac{1}{2}{O}_2\left(g\right)\to OH\left(g\right);△H=10.06Kcal$
$H_2\left(g\right)\to 2H\left(g\right);△H=104.18Kcal$
$O_2\left(g\right)\to 2O\left(g\right);△H=118.32Kcal$

Calculate the OH bond energy in the O - H group

1. -100.19 K cal

2. 100.19 K cal

3. 101.19 K cal

4. -101.19 K cal

Q. Heat of atomisation of $N{H}_3$ and $N_2{H}_4$ are $xkcalmo{l}^{-1}$ and $ykcalmo{l}^{-1}$ respectively. What will be average bond energy of $N-N$?

1. $\frac{-3y-4x}{3}kcalmo{l}^{-1}$
2. $\frac{3y-4x}{3}kcalmo{l}^{-1}$
3. $\frac{3y+4x}{3}kcalmo{l}^{-1}$
4. None of the above

Q.

Consider the reaction:

$4N{O}_2\left(g\right)+O_2\left(g\right)\to 2N_2{O}_5\left(g\right),{\Delta }_{r}H=-111kJ$. If $N_2{O}_5\left(s\right)$ is formed instead of $N_2{O}_5\left(g\right)$ in the above reaction, the ${\Delta }_{r}H$ value will be:

(given, $\Delta H$ of sublimation for $N_2{O}_5$ is $54kJmo{l}^{-1}$)

1. + 219 kJ

2. − 219 J

3. −165 kJ

4. + 54 kJ

Q.

Enthalpy of vaporisation of water is 40, 800 J/Mole at 1 atm. pressure. The mountain, where the pressure is 380 mm of H g, water will boil at

1. 100° C

2. 81.6°C

3. 104° C

4. 50° C

Q. In the process shown in the figure for an ideal diatomic gas, the value of q and $\Delta H$ respectively are:

1. $79.5P_1{V}_1\text{and}94.5P_1{V}_1$
2. $54.5P_1{V}_1\text{and}94.5P_1{V}_1$
3. $9P_1{V}_1\text{and}0$
4. $79.5P_1{V}_1$ and defined $(\because \text{P varies})$

Q. For silver $C_p\left(J{K}^{-1}mo{l}^{-1}\right)=23+0.01T$. If the temperature (T) of 3 moles of silver is raised from 300 K to 1000 K at 1 atm pressure, the value of $\Delta H$ will be close to:

1. 13 kJ
2. 62 kJ
3. 16 kJ
4. 21 kJ

Q.

The heat transferred from a system to its surroundings (or vice versa) when a chemical reaction is run under conditions of constant pressure is equal to:

1. The change in the free energy of the system (∆C)

2. The change in the enthalpy of the system (∆H)

3. The change in the energy of the system (∆E)

4. The change in the entropy of the system (∆S)

Q. For the reaction,
$N_2\left(g\right)+3H_2\left(g\right)\rightleftharpoons 2N{H}_3\left(g\right),△H$ is equal to :
($△U$ = change in internal energy)

1. $△U+2RT$
2. $△U-2RT$
3. $△U+RT$
4. $△U-RT$

Q. White phosphorous is a tetra atomic solid $\left[P_4\right(s\left)\right]$ at room temperature and on strong heating in absence of oxygen, it polymerizes into red phosphorus as:

$\Delta H=-104\text{kJ/mol of}{P}_4$
The enthalpy of sublimation $\left[P_4\right(s)\to P_4(g\left)\right]$ white is $59\text{kJ/mol}$ and enthalpy of atomization is $316.25\text{kJ/mol}$ of $P\left(g\right)$.
Now find the average $P-P$ bond enthalpy in $P_4$ molecule is:

1. $102\text{kJ}$
2. $201\text{kJ}$
3. $104\text{kJ}$
4. $120\text{kJ}$

Q. The standard Gibbs energy change for the reaction H2O<---->H+ + OH–at 25°C is :- Options 1. –100 kJ 2. +100 kJ 3. –90 kJ 4. +90 kJ

Q. 19 The heat of reaction for an endothermic reaction at equilibrium is 1200 calories more than that at constant pressure at 300 K. Calculate the ratio of equilibrium constant Kp and Kc.

Q. Consider an endothermic reaction $X\to Y$ with the activation energies $E_b$ and $E_f$ for the backward and forward reactions respectively. Here

1. $E_b>E_f$
2. $E_b<E_f$
3. $E_b=E_f$
4. Cannot predict

Q. Calculate the lattice formation enthalpy in ${\text{kJ mol}}^{-1}$of $LiF$ given that
Enthalpy of sublimation of lithium = $155.3{\text{kJ mol}}^{-1}$
Dissociation enthalpy of half mole of $F_2=75.3\text{kJ}$
Ionization enthalpy of lithium $=520{\text{kJ mol}}^{-1}$
Electron gain enthalpy of 1 mol of $F\left(g\right)=-333\text{kJ}{\Delta }_f{H}_{\text{overall}}=-594{\text{kJ mol}}^{-1}$

1. $-1011.6{\text{kJ mol}}^{-1}$
2. $-1200.4{\text{kJ mol}}^{-1}$
3. $-984.6{\text{kJ mol}}^{-1}$
4. $-1488.6{\text{kJ mol}}^{-1}$

Q. how is free energy related to entropy and enthalpy of a system?When enthalpy and entropy are favourable will the process be spontaneous or non spontaneous ?explain.

Q. Find out the value of $K_c$ for each of the following equilibria from the value of $K_p$:

Q. What is $△U$ when 2.0 mole of liquid water vaporises at ${100}^{\circ }?$ The heat of vaporisation, $△H$ vap. of water at ${100}^{\circ }C$ is $40.66$ $kJmo{l}^{-1}$

1. 60
2. 75.11
3. 88.2
4. 59

Q. 38.A substance of mass M kg requires input of P watts to remain in the molten state at its molten state at its melting point when the power is turned off, the sample completely solidifies in time t sec . What is the latent heat of fission of the substance?

Q. If $\Delta U$ of combustion of $C{H}_4\left(g\right)$,
$C{H}_4\left(g\right)+2O_2\left(g\right)\to C{O}_2\left(g\right)+2H_{2}O\left(l\right)$
at certain temperature is $-393kJmo{l}^{-1}$. The value of $\Delta H$ is:

1. zero
2. $<\Delta U$
3. $>\Delta U$
4. equal to $\Delta U$

Q.

The diffrence between heat of reaction at constant volume and constant pressure for the reaction

$C_2{H}_2\left(g\right)+\frac{5}{2}{O}_2\left(g\right)\to 2C{O}_2\left(g\right)+H_{2}O\left(l\right).At300KinKcalis:[Given:R=2cal/mol.K]$

1. −0.3 Kcal

2. +0.3 Kcal

3. None of these

4. −0.9 Kcal

Q. The dissociation energy of $C{H}_4$ is 400 Kcal $mo{l}^{-1}$ and that of ethane is 670 Kcal $mo{l}^{-1}$. The C - C bond energy is.

1. 270 kcal

2. 70 kcal

3. 200 kcal

4. 240 kcal

Q.

Generally unit of bond enthalpy is expressed in:

1. cal/mol

2. J/mol

3. kJ/mol

4. MJ

Q. The average energy required to break a P - P bond in $P_4\left(s\right)$ into gaseous atoms is $53.2kcalmo{l}^{-1}.$ The bond dissociation energy of $H_2\left(g\right)$ is $104.2kcalmo{l}^{-1};{△}_f{H}^o$ of $P{H}_3\left(g\right)$ from $P_4\left(s\right)$ is $5.5kcalmo{l}^{-1}.$ The P - H bond energy in $kcalmo{l}^{-1}$ is [Neglect presence of van der Waals forces in $P_4\left(s\right)$]
Given: $P_4\left(s\right)$ contains $6P-P$ single bonds.

1. 85.2 $kcalmo{l}^{-1}$
2. 57.6 $kcalmo{l}^{-1}$
3. 76.9 $kcalmo{l}^{-1}$
4. 63.3 $kcalmo{l}^{-1}$

Q. Which of the following is always negative for exothermic reaction?

1. $\Delta H$
2. $\Delta S$
3. $\Delta G$
4. $Noneofthese$

Q. Energy required to dissociate 4g of gaseous hydrogen into free gaseous atoms is 208 Kcal at ${25}^{o}C$. The bond energy of H - H bond will be:

1. 1.04 Kcal

2. 10.4 Kcal

3. 104 Kcal

4. 1040 Kcal

Q. The dissociation energy of $C{H}_4$ is 400 Kcal $mo{l}^{-1}$ and that of ethane is 670 Kcal $mo{l}^{-1}$. The C - C bond energy is.

1. 270 kcal

2. 70 kcal

3. 200 kcal

4. 240 kcal

Q.

Enthalpy change for the reaction $H_{\left(aq\right)}^{+}+O{H}_{\left(aq\right)}^{-}⟶H_2{O}_{(}l)IS-57.3kJmo{l}^{-1}.$ If 1 L of $0.5MJN{O}_3$ is mixed with 2L of 0.1 MKOH, the enthalpy change would be

1. -28.35 kJ

2. 28.65 kJ

3. -11.46 kJ

4. 11.46 kJ

Q. One mole of a monoatomic ideal gas undergoes the process $A\to B$ in the given p-V diagram. The molar heat capacity for this process is

1. $\frac{13R}{6}$
2. $\frac{3R}{2}$
3. $\frac{5R}{2}$
4. $2R$

Q. A vessel contains $100$ litres of a liquid X. Heat is supplied to the liquid at a constant pressure of $1$ atm in such a fashion that heat given= change in enthalpy. The volume of the liquid increases by $2$ litres. If $202.6$ joules of heat were supplied, then$[\Delta U,\Delta H=\text{change in internal energy and enthalpy}$ is:

1. $△U=0,△H=0$
2. $△U=+202.6\text{J},△H=+202.6\text{J}$
3. $△U=-202.6\text{J},△H=-202.6\text{J}$
4. $△U=0,△H=+202.6\text{J}$