Define enthalpy. - Chemistry


Define enthalpy.



Enthalpy of a system is sum of internal energy of a system and the energy equivalent to PV work.

H = U + PV

Concept: Enthalpy (H)
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Select the most appropriate option.

The enthalpy of formation for all elements in their standard states is _______.

Select the most appropriate option.

If the standard enthalpy of formation of methanol is –238.9 kJ mol–1 then entropy change of the surroundings will be _______. 

Select the most appropriate option.

Bond enthalpies of H–H, Cl–Cl, and H–Cl bonds are 434 kJ mol–1, 242 kJ mol–1, and 431 kJ mol–1, respectively. Enthalpy of formation of HCl is _______.

Answer in brief.

Obtain the expression for work done in chemical reaction.

Answer the following question.

Calculate the work done in the decomposition of 132 g of NH4NO3 at 100 °C.

NH4NO3(s) → N2O(g) + 2H2O(g)

State whether work is done on the system or by the system.

Answer the following question.

Calculate standard enthalpy of reaction,

Fe2O3(s) + 3CO(g) → 2Fe(s) + 3CO2(g), from the following data.

Δf H°(Fe2O3) = - 824 kJ/mol,

Δf H°(CO) = - 110 kJ/mol,

Δf H°(CO2) = - 393 kJ/mol

Answer the following question.

Calculate ΔU at 298 K for the reaction,

C2H4(g) + HCl(g) → C2H5Cl(g), ΔH = - 72.3 kJ

How much PV work is done?

Calculate the amount of work done in the

1) Oxidation of 1 mole HCl(g) at 200 °C according to reaction.

4HCl(g) + O2(g) → 2Cl2(g) + 2H2O(g)

2) Decomposition of one mole of NO at 300 °C for the reaction

2NO(g) → N2(g) + O2(g)

Answer the following question.

When 6.0 g of O2 reacts with CIF as per 

\[\ce{2ClF_{(g)} + O2_{(g)} -> Cl2O_{(g)} + OF2_{(g)}}\]

The enthalpy change is 38.55 kJ. What is the standard enthalpy of the reaction? (Δr H° = 205.6 kJ)

Write the mathematical relation between ΔH and ΔU during the formation of one mole of CO2 under standard conditions.

An ideal gas expands from the volume of 1 × 10–3 m3 to 1 × 10–2 m3 at 300 K against a constant pressure at 1 × 105 Nm–2. The work done is

The work done by the liberated gas when 55.85 g of iron (molar mass 55.85 g mol–1) reacts with hydrochloric acid in an open beaker at 25°C

Define enthalpy of combustion.

Define enthalpy of neutralization.

Enthalpy of neutralization is always a constant when a strong acid is neutralized by a strong base: account for the statement.

Derive the relation between ∆H and ∆U for an ideal gas. Explain each term involved in the equation.

Calculate the enthalpy change for the reaction \[\ce{Fe2O3 + 3CO -> 2Fe + 3CO2}\] from the following data.

\[\ce{2Fe + 3/2O2 -> Fe2O3}\]; ΔH = −741 kJ

\[\ce{C + 1/2O2 -> CO}\]; ΔH = −137 kJ

\[\ce{C + O2-> CO2}\]; ΔH = −394.5 kJ

The standard enthalpies of formation of SO2 and SO3 are −297 kJ mol−1 and −396 kJ mol−1 respectively. Calculate the standard enthalpy of reaction for the reaction: \[\ce{SO2 + 1/2O2 -> SO3}\]

The difference between heats of reaction at constant pressure and at constanl volume for the reaction

\[\ce{2C6H6_{(l)} + 15O2_{(g)} -> 12CO2_{(g)} + 6H2O_{(l)}}\] at 25°C in kJ

When 4 g of iron is burnt to ferric oxide at constant pressure, 29.28 kJ of heat is evolved. What is the enthalpy of formation of ferric oxide?

(Atomic mass of Fe = 56)

The enthalpy change for two reactions are given by the equations

\[\ce{2Cr_{(s)} + 1.5 O2_{(g)} -> Cr2O3_{(s)}}\];

∆H1 = −1130 kJ ............(i)

\[\ce{C_{(s)} + 0.5 O2_{(g)} -> CO_{(g)}}\];

∆H2 = −110 kJ .........(ii)

What is the enthalpy change, in kJ, for the following reaction?

\[\ce{3C_{(s)} + Cr2O3_{(s)} -> 2Cr_{(s)} + 3CO_{(g)}}\]

Given the bond energies N ≡ N, H – H and N – H bonds are 945, 436 and 391 kJ/mol respectively. The enthalpy of the reaction;

\[\ce{N2_{(g)} + 3H2_{(g)} -> 2NH3_{(g)}}\]

For the reaction, \[\ce{A_{(s)} + 2B_{(g)} -> 5C_{(s)} + D_{(l)}}\], ∆H and ∆U are related as ____________.

If 2 kJ of heat is released from system and 6 kJ of work is done on the system, what is enthalpy change of system?

For the reaction, \[\ce{N_{2(g)} + 3H_{2(g)} -> 2NH_{3(g)}}\], ΔH is equal to ______.

What is the amount of work done when 0.5 mole of methane, CH4 (g), is subjected to combustion at 300 K? (Given, R = 8.314 JK-1mol-1)

The work done during combustion of 9 × 10-2 kg of ethane, C2H6 (g) at 300 K is ______.
(Given R = 8.314 J deg-1, atomic mass C = 12, H = 1)

Calculate the work done during the combustion of 0.138 kg of ethanol, C2H5OH(l) at 300 K.
Given: R = 8.314 Jk−1 mol−1, molar mass of ethanol = 46 g mol−1.

Under what conditions ΔH = ΔU?

Calculate the work done in oxidation of so2(g) at 25°C if, \[\ce{2SO_{2(g)} + O2_{(g)} -> 2SO_{3(g)}}\], R = 8.314 J K−1 mol−1.

Calculate ΔS of the surrounding if the standard enthalpy of formation of methanol is − 238.9 kJ mol−1.

In a particular reaction, 2 kJ of heat is released by the system and 8 kJ of work is done on the system. Determine ΔU.

Calculate work done in oxidation of 4 moles of SO2 at 25°C. (Given: R = 8.314 JK−1 mol−1 ).


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