Advertisements
Advertisements
प्रश्न
State and explain Hess’s law of constant heat summation.
उत्तर
Hess’s law of constant heat summation states that, “Overall the enthalpy change for a reaction is equal to sum of enthalpy changes of individual steps in the reaction”.
Explanation:
- The enthalpy change for a chemical reaction is the same regardless of the path by which the reaction occurs. Hess’s law is a direct consequence of the fact that enthalpy is a state function. The enthalpy change of a reaction depends only on the initial and final states and not on the path by which the reaction occurs.
- To determine the overall equation of reaction, reactants and products in the individual steps are added or subtracted like algebraic entities.
- Consider the synthesis of NH3,
| 1. | \[\ce{2H_{2(g)} + N_{2(g)} -> N2H_{4(g)}}\], | `Delta_"r""H"_1^0` = +95.4 kJ |
| 2. | \[\ce{N2H_{4(g)} + H_{2(g)} -> 2NH_{3(g)}}\], | `Delta_"r""H"_2^0` = −187.6 kJ |
| \[\ce{3H_{2(g)} + N_{2(g)} -> 2NH_{3(g)}}\], | ∆rH° = – 92.2 kJ |
The sum of the enthalpy changes for steps (1) and (2) is equal to enthalpy change for the overall reaction.
APPEARS IN
संबंधित प्रश्न
Select the most appropriate option.
Which of the following reactions is exothermic?
Answer in brief.
How will you calculate reaction enthalpy from data on bond enthalpies?
Answer in brief.
How much heat is evolved when 12 g of CO reacts with NO2? The reaction is:
4CO(g) 2NO2(g) → 4CO2(g) + N2(g), ΔrH° = - 1200 kJ
The standard enthalpy of formation of water is - 286 kJ mol-1. Calculate the enthalpy change for the formation of 0.018 kg of water.
Calculate the standard enthalpy of formation of liquid methanol from the following data:
- \[\ce{CH3OH_{(l)} + \frac{3}{2} O_{2(g)} -> CO_{2(g)} + 2H2O_{(l)}}\] ∆H° = – 726 kJ mol–1
- \[\ce{C_{(Graphite)} + O_{2(g)} -> CO_{2(g)}}\] ∆cH° = – 393 kJ mol–1
- \[\ce{H_{2(g)} + \frac{1}{2} O_{2(g)} -> H2O_{(l)}}\] ∆fH° = – 286 kJ mol–1
Calculate the standard enthalpy of the reaction.
\[\ce{2Fe_{(s)} + \frac{3}{2} O_{2(g)} -> Fe2O_{3(s)}}\]
Given:
| 1. | \[\ce{2Al_{(s)} + Fe2O_{3(s)} -> 2Fe_{(s)} + Al_2O_{3(s)}}\], | ∆rH° = –847.6 kJ |
| 2. | \[\ce{2Al_{(s)} + \frac{3}{2} O_{2(g)} -> Al2O_{3(s)}}\], | ∆rH° = –1670 kJ |
Write an application of Hess’s law.
Classify the following into intensive and extensive properties.
Pressure, volume, mass, temperature.
Daily requirement of energy of a person is 'x' kJ. If heat of combustion of food material (Molecular mass = 100 g) is 'y' kJ, his daily consumption of the food in gram would be ____________.
The standard heats of formation in kcal mol−1 of NO2(g) and N2O4(g) are 8.0 and 2.0 respectively. The heat of dimerization of NO2 in kcal is ____________.
\[\ce{2NO2_{(g)} ⇌ N2O4_{(g)}}\]
The heat evolved in the combustion of benzene is given by
\[\ce{C6H6 + 7 1/2O2 -> 6CO2_{(g)} + 3H2O_{(l)}}\]; ΔH = −3264.6 kJ
Which of the following quantities of heat energy will be evolved when 39 g C6H6 are burnt?
Enthalpy of formation of two compounds x and y are −84 kJ and −156 kJ respectively. Which of the following statements is CORRECT?
Given the reaction,
\[\ce{CH2O_{(g)} + O2_{(g)} -> CO2_{(g)} + H2O_{(g)}}\] ΔH = −527 kJ
How much heat will be evolved in the formation of 60 g of CO2?
Combustion of glucose takes place as
\[\ce{C6H12O6_{(s)} + 6O2_{(g)} -> 6CO2_{(g)} + 6H2O_{(g)}}\]; ΔH = −72 kcal mol−1
The energy needed for the production of 1.8 g of glucose by photosynthesis will be ___________.
An ideal gas expands isothermally and reversibly from 10 m3 to 20 m3 at 300 K performing 5 .187 kJ of work on surrounding. Calculate number of moles of gas undergoing expansion. (R = 8.314 J K-1 mol-1)
Calculate the standard enthalpy of:
\[\ce{N2H_{4(g)} + H_{2(g)} -> 2NH_{3(g)}}\]
If ΔH0(N – H) = 389 kJ mol–1, ΔH0(H – H) = 435 kJ mol–1, ΔH0(N – N) = 159 kJ mol–1.
When the enthalpy of combustion of carbon to carbon dioxide is - 360 kJ mol-1, then the enthalpy change for the formation of 18 g of CO2 from carbon and dioxygen at the same temperature in kJ will be ______.
What is enthalpy of formation of NH3 if bond enthalpies as (N ≡ N) = - 941 kJ/mol.
\[\ce{(H - H)}\] = 436 kJ/mol and \[\ce{(N - H)}\] = 389 kJ/mol?
The enthalpy change that accompanies a reaction in which 1 mole of its standard state is formed from its elements in their standard states
When 0.5 gram of sulphur is burnt to form SO2, 4.6 kJ of heat liberated. Calculate enthalpy of formation of SO2(g). (Atomic mass : S = 32, O = 16)
Define and explain the term, enthalpy of reaction.
Calculate the standard enthalpy of the reaction:
SiO2(s) + 3C(graphite) → SiC(s) + 2CO(g) from the following reactions:
- Si(s) + O2(g) → SiO2(s), ΔrH° = −911kJ
- 2C(graphite) + O2(g) → 2CO(g), ΔrH° = −221kJ
- Si(s) + C(graphite) → SiC(s), ΔrH° = −65.3kJ
Standard enthalpy of combustion of a substance is given. Then Write thermochemical equation.
ΔcH0[C2H5OH(1)] = - 1409 kJ mol-1
Heat of combustion of methane is - 890 kJ/mol. On combustion of 12 gm of methane in excess of oxygen, ______ heat is evolved.
Draw energy profile diagram and show:
- activated complex
- energy of activation for forward and backward reactions
- enthalpy of reaction
The heat evolved in the combustion of 6.022 x 1021 carbon particles is 3.94 kJ. The heat of combustion of carbon is ______.
Calculate heat evolved for combustion of 13 gm of acetylene (C2H2).
Given: \[\ce{C2H2_{(g)} + 5/2O_{2(g)}-> 2CO_{2(g)} + H2O_{(l)} \Delta_{(c)}H^{0} = - 1300 kJ}\]
