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Maharashtra State BoardHSC Science (Electronics) 12th Standard Board Exam
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टाइम टेबल३३
अभ्यासक्रम

Thermochemistry

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Estimated time: 3 minutes
Maharashtra State Board: Class 12

Key Points: Thermochemistry

  • Thermochemistry deals with heat energy changes during chemical reactions
  • Enthalpy change (ΔH) = difference between products and reactants → ΔH = Hp − Hr
  • Exothermic reactions: ΔH < 0, heat is released to the surroundings
  • Endothermic reactions: ΔH > 0, heat is absorbed by the system
  • Standard enthalpy (ΔH°) measured at 298 K and 1 bar pressure
  • Enthalpy of formation (ΔfH°): formation of 1 mole; for elements = 0
  • Enthalpy of combustion (ΔcH°): always negative (heat released)
  • Bond enthalpy: energy required to break bonds in the gaseous state
  • Reaction enthalpy using bond energy:
    ΔrH° = ΣBE(reactants) − ΣBE(products)
  • Hess’s Law: total ΔH is the same for any path → ΔH = sum of individual steps

Related QuestionsVIEW ALL [41]

Answer the following question.

Calculate ΔrH° for the following reaction at 298 K:

1) 2H3BO3(aq) → B2O3(s) + 3H2O(l), ΔrH° = + 14.4 kJ

2) H3BO3(aq) → HBO2(aq) + H2O(l), ΔrH° = - 0.02 kJ

3) H2B4O7(s) → 2B2O3(s) + H2O(l), ΔrH° = + 17.3 kJ

Standard enthalpy of combustion of a substance is given. Then Write thermochemical equation.

ΔcH0[CH3CHO(l)] = - 1166 kJ mol-1

Which of the following equations has ΔfH° and ΔH° same?

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.

Define standard enthalpy of formation.

Calculate the standard enthalpy of the reaction, \[\ce{SiO2_{(s)} + 3C_{(graphite)} -> SiC_{(s)} + 2CO_{(g)}}\] from the following reactions:

  1. \[\ce{Si_{(s)} + O2_{(g)} -> SiO2_{(s)}}\], ΔrH0 = −911 kJ
  2. \[\ce{2C_{(graphite)} + O2_{(g)} -> 2CO_{(g)}}\], ΔrH0 = −221 kJ
  3. \[\ce{Si_{(s)} + C_{(graphite)} -> SiC_{(s)}}\], ΔrH0 = −65.3 kJ

The enthalpy of combustion of S (rhombic) is − 297 kJ mo1-1. Calculate the amount of sulphur required to produce 29. 74 kJ of heat.

\[\ce{A -> B}\], ∆H = −10 kJ mol−1, Ea(f) = 50 kJ mol−1, then Ea of \[\ce{B -> A}\] will be ______.

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