HL R1.4 Entropy and Spontaneity PDF
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Xuesi Liu
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These lecture notes cover entropy and spontaneity in chemistry, explaining concepts and calculations. It's appropriate for a university-level course.
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HL R1.4 Entropy and spontaneity Xuesi Liu Learning outcomes Reactivity 1.4.1—Entropy, S, is a measure of the dispersal or distribution of matter and/or energy in a system. The more ways the energy can be distributed, the higher the entropy. Under the same condit...
HL R1.4 Entropy and spontaneity Xuesi Liu Learning outcomes Reactivity 1.4.1—Entropy, S, is a measure of the dispersal or distribution of matter and/or energy in a system. The more ways the energy can be distributed, the higher the entropy. Under the same conditions, the entropy of a gas is greater than that of a liquid, which in turn is greater than that of a solid. Predict whether a physical or chemical change will result in an increase or decrease in entropy of a system. Calculate standard entropy changes, ΔS⦵, from standard entropy values, S⦵ Reactivity 1.4.2—Change in Gibbs energy, ΔG, relates the energy that can be obtained from a chemical reaction to the change in enthalpy, ΔH, change in entropy, ΔS, and absolute temperature, T. Apply the equation ΔG⦵ = ΔH⦵ − TΔS⦵ to calculate unknown values of these terms. Learning outcomes Reactivity 1.4.3—At constant pressure, a change is spontaneous if the change in Gibbs energy, ΔG, is negative. Interpret the sign of ΔG calculated from thermodynamic data. Determine the temperature at which a reaction becomes spontaneous. Reactivity 1.4.4—As a reaction approaches equilibrium, ΔG becomes less negative and finally reaches zero. Perform calculations using the equation ΔG = ΔG⦵ + RT lnQ and its application to a system at equilibrium ΔG⦵ = −RT lnK. Overview Entropy Gibbs energy ΔG and equilibrium Entropy Entropy, S, is a measure of the dispersal or distribution of the total available energy or matter in a system If energy and matter localized in one place within a chemical system, the entropy of the system is low If energy and matter are randomly distributed throughout a system, the entropy of the system is high Entropy is often said to be a measure of the disorder of a system Entropy and physical change Changes in entropy are associated with every physical and chemical process Equilibrium Many physical and chemical changes are reversible, so the interconversion of reactants and products can proceed simultaneously in both directions In equations that represent reversible changes, the arrow is replaced with the equilibrium sign ⇌, which symbolizes the bidirectional nature of the process If a reversible change occurs in a closed system, it eventually reaches the state of dynamic equilibrium, where The change continues at the microscopic level Forward and reverse reaction rates are the same Concentrations of reactants and products remain constant Macroscopic properties of the system, such as color and density, remain unchanged The equilibrium can be achieved form either direction Entropy change The total entropy change that occurs during a reaction is the sum of the entropy changes of the reaction system and the surroundings A reaction is said to be spontaneous when it moves towards either completion or equilibrium under a given set of conditions without external intervention (which may be in the form of a change in temperature, pressure or concentration of a reactant) The second law of thermodynamic s allow us to predict the direction of a spontaneous reaction Predicting entropy change Calculating entropy changes The standard entropy change, ΔS⦵, of a system can be calculated from standard entropy value, ΔS⦵ , of the reactants and products Standard entropy values of some substances are given in section 13 of the data booklet The unit of standard entropy value is J K-1 mol-1 Things to note when performing entropy change calculations Entropy values are state specific: The coefficients used to balance the equation must be considered Examine the chemical reaction and predict whether you expect a positive or negative entropy change. This prediction can be used to check your calculation Enthalpy and entropy An increase in enthalpy within a reaction system will result in increased movement of particles, leading to greater disorder and an increase in the entropy of the system Therefore, we need to consider the effects of both enthalpy and entropy changes Exothermic reactions are more likely to be spontaneous, as this leads to a decrease in enthalpy, and therefore greater stability of the reaction products An increase in entropy makes reactions more likely to be spontaneous, as greater disorder leads to a more random distribution of energy within the system However, reactions that are spontaneous, and therefore thermodynamically favorable, can sometimes be kinetically unfavorable due to their high activation energies Gibbs energy Gibbs energy, G, is a state function which combines enthalpy, entropy and temperature The change in Gibbs energy, ΔG, is calculated as The unit of change in Gibbs energy is kJ mol-1 ΔG takes into account the direct entropy change of the system resulting from the transformation of the chemicals and the direct entropy change of the surroundings resulting from the transfer of heat energy At constant pressure, a reaction is spontaneous if the change in Gibbs energy has a negative value Gibbs energy change & spontaneity ΔG and equilibrium Reaction quotient, Q, is the ratio of the concentration of products to reactants When a chemical system has reached equilibrium, the ratio of concentrations of products to concentrations of reactants is called the equilibrium constant, K Q>K Q=K Q at The system is at [reactants] > at equilibrium, so the equilibrium, so the equilibrium, so the reverse reaction is forward and reverse forward reaction is favored until the reactions occur at equal favored until the equilibrium is reached rates equilibrium is reached ΔG and equilibrium A B Which reaction is forward reverse favored? Relationship between Q & QK K Sign of ΔG - - Relationship between ΔG, Q, K & T At the point of equilibrium, ΔG=0, Q=K, so