Thermodynamics

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Thermodynamics

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Change in Gibbs Free Energy, ΔG

TΔSuniverse is denoted by ΔG

Laws of Thermodynamics

Second Law

the entropy of the universe increases in spontaneous processes and remains unchanged during equilibrium processes. (∆S universe = ∆S system + ∆S surroundings)

spontaneous process, ∆S universe = + ve

ΔG = ∆H system - T∆S system

equilibrium process, ∆S universe = 0

First Law

energy can be converted from one form to another but cannot be created or destroyed. Based on the law of conservation or energy.

Spontaneity

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How to determine spontaneity?

Change in entropy change ΔS

Change in enthalpy ΔH

- both endothermic and exothermic reaction can be spontaneous - sign of ∆H alone is insufficient to predict spontaneity in every process/reaction.

Spontaneous Process

require a 'pus' to get started

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occurs without continuous input of energy

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spontaneous

ball moving down a slope

non-spontaneous

water freezing at 25 °C 1 atm

occurs under a specific set of conditions

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Entropy

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Standard Entropy Change ΔS

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∆rxnS = S (product) - S (reactant)

how to determine ΔS?

by comparing the number of particles of the reactants and products

decrease in the number of particles

increase in the number of particles

by comparing the physical states of the reactants and products

gas to solid

decrease in the number of possible arrangements

solid to gas

increase in the number of possible arrangements

definitions

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possible arrangements or configurations of a system

Examples

least number of possible outcomes

solid state

most number of possible outcomes

gaseous state

"disorder' of a system

Enthalpy

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Standard Enthalpy Change

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standard bond dissociation enthalpy

bond forming

energy released

ΔH bond forming is -ve

bond breaking

energy is absorbed

ΔH bond breaking is +ve

standard enthalpy change of formation, Δf H°

Δf H° of an element & 7 diatomic elements (H2, N2, O2, F2, Cl2, Br2, l2)

Δf H° = 0 kJ/mol

one mole of compound is formed from its element in their reference states

standard enthalpy of reaction, Δrxn H°

∆rxn H° = Sum of Δf H° (product) - Sum of Δf H° (reactant)

unit of Δrxn H° is kJ

only be used with the Δf H° of reactants and products

standard enthalpy change ΔH°

reaction at the same specific temperature

how to determine ΔH?

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products have more energy than reactants

Heat absorbed to surroundings

endothermic reaction

+ve ΔH

products have lesser energy than reactants

Heat released to surroundings

exothermic reaction

-ve ΔH

definition

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Enthalpy change ΔH - amount of heat released or absorbed by a chemical system when a chemical reaction occurs at constant pressure.