University:
Carleton University
Course:
CHEM 1001 | General Chemistry I
Academic year:

2024
Views:

484

Pages:

4
Author:

ali

Enthalpy (H) The total heat content of the system at constant pressure is equal to the enthalpy change of the system. [q_p = ΔH] It is a state function and an extensive property. [ΔH = ΣH(products) - ΣH(reactants)] If ΔH = +ve or ΔH > 0 → Enthalpy of system increases. If ΔH = -ve or ΔH < 0 → Enthalpy of system decreases. If ΔH = 0 → Enthalpy of system remains constant. Mathematically, it is a function which includes internal energy and PV. H = E + PV So, ΔH = ΔE + Δ(PV) → General formula At constant pressure, ΔP = 0, so ΔH = ΔE + PΔV At constant volume, ΔV = 0, so ΔH = ΔE + VΔP At constant temperature, enthalpy of the system remains constant. T = constant, so E = constant From Boyle's law, at constant temperature, PV = constant ... H = constant and ΔH = 0 Heat Capacity The quantity or the amount of heat required to increase the temperature of the system by 1 K is called as Heat Capacity. It is denoted by C. It is an extensive property and path function. C = q/ΔT If ΔT = 1K, then C = q Unit of Heat Capacity: J/K or cal/K Molar Heat Capacity (Cn): The amount of heat required to increase the temperature of 1 mole of a substance by 1K is called as molar heat capacity. It is an intensive property. Cn = q/nΔT If n = 1 mole, ΔT = 1K, then Cn = q Specific Heat Capacity (s): The amount of heat required to increase the temperature of 1 gram of a substance by 1K is called as specific heat capacity. It is an intensive property. s = q/wΔT If w = 1g, ΔT = 1K, then s = q Heat Capacity at constant pressure (Cp): It is the amount of heat required to increase the temperature of the system by 1K at constant pressure. Cp = qp/ΔT At Constant pressure, qp = ΔH Cp = ΔH/ΔT ΔH = CpΔT Heat capacity at constant volume (Cv): It is the amount of heat required to increase the temperature of the system by 1K at constant volume. Cv = qv/ΔT At constant volume, qv = ΔE Cv = ΔE/ΔT ΔE = CvΔT During isothermal process, heat capacity is infinity (C = ∞) During adiabatic process, heat capacity is zero (C = 0) Mayer's Relation: We know that at constant pressure, enthalpy change is, ΔH = ΔE + PΔV For 1 mole of gas, ΔH = CpΔT For 1 mole of gas, ΔE = CvΔT For 1 mole of gas at constant pressure, PΔV = RΔT On putting all these values in equation (1), we get CpΔT = CvΔT + RΔT CpΔT - CvΔT = RΔT (Cp - Cv)ΔT = RΔT Cp - Cv = R --- (2) Cp = Cv + R --- (3) Equation (2) and (3) represents Mayer's Relation. Adiabatic coefficient (γ) → It is the ratio of heat capacity at constant pressure to that of heat capacity at constant volume. γ = Cp/Cv Value of γ for monoatomic gas → γ = 5/3 = 1.67 Value of γ for diatomic gas → γ = 7/5 = 1.40 Expansion of gases → It is of two types: 1. Isothermal expansion of gases 2. Adiabatic expansion of gases Isothermal expansion of gases → In this process, the temperature of the system remains constant. Isothermal expansion of gases can occur in two different ways: 1. Isothermal reversible expansion of gases. 2. Isothermal irreversible expansion of gases.