Definitions [4]
The specific values of macroscopic variables that completely describe every equilibrium state of a thermodynamic system are called thermodynamic state variables.
The thermodynamic state variables that depend on the size of the system (e.g., internal energy, volume) are called extensive variables.
The thermodynamic state variables that do not depend on the size of the system (e.g., pressure, temperature) are called intensive variables.
Define the Enthalpy of atomization.
The enthalpy change accompanying the dissociation of all the molecules in one mole of gaseous substance into atoms is called enthalpy of atomization.
Formulae [1]
Write the mathematical equation of the first law of thermodynamics for an isochoric process.
By substituting equation W = −pex . ΔV in the equation ΔU = q + W, we get
ΔU = q − pex . ΔV ...(1)
If the reaction is carried out in a closed container so that the volume of the system is constant, then Δ = 0. In such a case, no work is involved.
The equation (1) becomes ΔU = qv
Equation (1) suggests that the change in internal energy of the system is due to heat transfer. The subscript v indicates a constant volume process. As U is a state function, qv is also a state function. We see that an increase in the internal energy of a system is numerically equal to the heat absorbed by the system in a constant volume (isochoric) process.
Theorems and Laws [3]
Write the mathematical equation of the first law of thermodynamics for an isochoric process.
By substituting equation W = −pex . ΔV in the equation ΔU = q + W, we get
ΔU = q − pex . ΔV ...(1)
If the reaction is carried out in a closed container so that the volume of the system is constant, then Δ = 0. In such a case, no work is involved.
The equation (1) becomes ΔU = qv
Equation (1) suggests that the change in internal energy of the system is due to heat transfer. The subscript v indicates a constant volume process. As U is a state function, qv is also a state function. We see that an increase in the internal energy of a system is numerically equal to the heat absorbed by the system in a constant volume (isochoric) process.
Statement:
The net heat energy supplied to a system is equal to the sum of the change in internal energy of the system and the work done by the system. It is based on the law of conservation of energy.
Formula:
where Q = heat added, ΔU = change in internal energy, W = work done by the system.
State and explain Hess’s law of constant heat summation.
The law states that, “Overall, the enthalpy change for a reaction is equal to the sum of enthalpy changes of individual steps in the reaction”.
The enthalpy change for a chemical reaction is the same regardless of the pathway taken during the reaction. Hess’s law is a direct result of the principle that enthalpy is a state function. The enthalpy change of a reaction depends only upon the initial and final states, independent of the reaction path.
To determine the overall reaction equation, the reactants and products from the different steps are combined or subtracted as algebraic entities.
Consider the synthesis of NH3:
i. \[\ce{\phantom{...}2H2_{(g)} + N2_{(g)} -> N2H4_{(g)}, \Delta_rH^0_1 = +95.4 kJ}\]
ii. \[\ce{N2H4_{(g)} + H2_{(g)} -> 2NH3_{(g)}, \Delta_rH^0_2 = -187.6 kJ}\]
_______________________________________________
\[\ce{\phantom{.....}3H2_{(g)} + N2_{(g)} -> 2NH3_{(g)}}\], ΔrH0 = −92.2 kJ
The sum of the enthalpy changes for steps (i) and (ii) is equal to the enthalpy change for the overall reaction.
Key Points
First Law: Energy of system + surroundings remains constant → ΔU = q + W
ΔU: change in internal energy, q: heat, W: work done on system
Sign convention:
- Work by system (−)
- on system (+)
- Heat absorbed (+)
- released (−)
ΔU > 0: energy enters system; ΔU < 0: energy leaves system
- Isothermal: ΔU = 0 → q = −W
- Adiabatic: q = 0 → ΔU = W
- Isochoric: W = 0 → ΔU = q
- Isobaric: ΔU = q + W
Important Questions [53]
- Write Features of Reversible Process.
- Write the Conditions for Maximum Work Done by the System.
- Distinguish Between Isothermal Process and Adiabatic Process.
- The Process in Which the Value of δU = 0 is __
- For a Certain Reaction, ∆H = − 50 Kj and ∆S = − 80 J K-1, at What Temperature Does the Reaction Turn from Spontaneous to Non-spontaneous?
- Which of the Following Pairs is an Intensive Property?
- Determine whether the reactions with the following ΔH and ΔS values are spontaneous or non-spontaneous. State whether the reactions are exothermic or endothermic.
- Derive the equation : W = - PextAV
- 5 Moles of Helium Expand Isothermally and Reversibly from a Pressure 40 × 10-5 N M-2 to 4 × 10-5 N M-2 at 300 K. Calculate the Work Done, Change in Internal Energy and Heat Absorbed During
- What is the action of heat on potassium permanganate?
- Absolute Entropies of Solids, Liquids and Gases Can Be Determined by
- 300 M mol of perfect gas occupies 13 L at 320 K. Calculate the work done in joules when the gas expands
- One Mole of a Gas Expands by 3l Against a Constant Pressure of 3 Atmosphere. Calculate the Work Done
- 55 L Atm of Work is Obtained When 1.0 Mole of an Ideal Gas is Compressed Isothermally from a Volume of 28.5 L to 18.5 L, the Constant External Pressure is
- For a Chemical Reaction, A→ Products, the Rate of Reaction Doubles When the Concentration of ‘A’ is Increased by a Factor of 4, the Order of Reaction is
- Derive an expression for maximum work in isothermal reversible expansion of two moles of an ideal gas.
- Write the mathematical equation of the first law of thermodynamics for an isochoric process.
- Write mathematical equation of first law of thermodynamics for Adiabatic process
- Write one statement of first law of thermodyamics and its mathematical expression
- Calculate the internal energy at 298K for the formation of one mole of ammonia, if the enthalpy change at constant pressure is – 42.0 kJ mol-1. (Given: R = 8.314 J K-1 mol-1)
- Prove that ΔH=ΔU+ΔnRT. what is the condition under which ΔU=ΔH?
- What will happen to the internal energy if work is done by the system?
- How is Enthalpy of Sublimation Related to Enthalpy of Fusion and Enthalpy of Vaporization
- Define the Enthalpy of Fusion
- Define the Enthalpy of atomization.
- If the Enthalpy of Vaporisation of Water at 100oc is 186.5 J.Mol-1, the Entropy of Vaporization Will Be
- For a chemical reaction ΔS=0.035 kJ/k and ΔH=20kJ.
- A system absorbs 640 1 heat and does work of 260 J, the change in internal energy of the system will be
- Define the Term ‘Enthalpy’.
- Define Enthalpy of Vaporization
- A System Absorbs 6 Kj of Heat and Does 1.5 Kj of Work on Its Surroundings. the Change in Internal Energy is
- Hess’s law of constant heat summation
- State and explain Hess’s law of constant heat summation.
- Calculate ΔH° for the reaction between ethene and water to form ethyl alcohol from the following data
- Calculate the Standard Enthalpy of the Reaction,From the Following δH° Values
- Calculate ∆H° for the Following Reaction: 2h3bo3(Aq) → B2o3(S) + 3h2o(L) Given That,
- Calculate C-Cl bond enthalpy from following reaction: CH3Cl(g) + Cl2(g) → Ch2Cl2(g) + HCl(g) ΔH° = -104KJ
- Calculate the standard enthalpy of combustion of CH3COOH(l) from the following data:
- For the Reaction: Cl2(G) → 2cl(G)
- Define Enthalpy of Sublimation.
- Calculate the Work Done in the Following Reaction at 50o C. State Whether Work is Done on the System Or by the System.
- The Standard Enthalpy of Combustion of Formaldehyde δ0h0 = -571 Kj. How Much Heat Will Be Evolved in the Formation of 22 G of Co2?
- Write Applications of Standard Molar Entropy.
- A Substance Which Shows the Highest Entropy is ___________.
- What Are the Conditions for Spontaneous and Non-spontaneous Reactions in Terms of Free Energy Change?
- Write Chemical Reactions Involved in Van Arkel Method for Refining Titanium
- The equilibrium constant Kp for the reaction, H2(g) + I2(g) → 2HI(g) is 130 at 510 K.
- Explain the Relationship Between Gibb'S Standard Energy Change of the Reaction and Equilibrium Constant.
- Amongst the Following Identify the Criterion for a Process to Be at Equilibrium
- Derive the relation between ΔG° and equilibrium constant (K) for the reaction - aA_bB ⇌ cC+dD.
- Give Two Uses Of Third Law of Thermodynamics
- State Third Law of Thermodynamics
- No Machine Has an Efficiency Unity', is Stated In-
Concepts [17]
- Chemical Thermodynamics and Energetic
- Concepts of System
- Types of Systems
- Surroundings
- Work, Heat, Energy, Extensive and Intensive Properties
- State Functions
- First Law of Thermodynamics
- Internal Energy and Enthalpy
- Hess’ Law of Constant Heat Summation
- Enthalpy of Bond Dissociation
- Combustion, Formation, Atomization, Sublimation
- Phase Transition
- Ionization and Solution
- Dilution Introduction of Entropy as a State Function
- Free Energy Change for Spontaneous and Non Spontaneous Processes
- Equilibrium Constant
- Second and Third Law of Thermodynamics
