Download
1 / 19
230 likes | 667 Vues
Gibbs Free Energy. What’s “free” about Gibbs free energy? The change in free energy for a process equals the maximum work that can be done by the system on the surroundings in a spontaneous process occurring at constant temperature and pressure. D G = w max. Gibbs Free Energy.
E N D
Gibbs Free Energy • What’s “free” about Gibbs free energy? • The change in free energy for a process equals the maximum work that can be done by the system on the surroundings in a spontaneous process occurring at constant temperature and pressure. DG = wmax
Gibbs Free Energy Example:What is the maximum work that can be performed by the combustion of 25.0 g of methanol (CH3OH)? Step 1: Write a balanced equation.
Gibbs Free Energy Step 2: Calculate DGrxn
Gibbs Free Energy • On your exam, you must be able to write a balanced equation for a simple combustion reaction (including predicting the products). • You will then be expected to calculate the maximum work that can be performed using a given number of grams or moles of a reactant.
Gibbs Free Energy • You should be able to write a balanced equation for the combustion of an organic compound or a metal. • Organic compounds: • Metals: Not bal. Not bal.
Gibbs Free Energy • You can use the signs (positive or negative) of DH and DS to predict whether a reaction (or process) will be: • Spontaneous at all temperatures • Spontaneous only at high temperatures • Spontaneous only at low temperatures • Non-spontaneous at all temperatures
Gibbs Free Energy • The sign of DG (and therefore the spontaneity of the reaction) will depend on • the sign of DH and DS • relative magnitude of the enthalpy and the entropy terms. • In some cases, the temperature will impact the spontaneity of a reaction. DG = DH - TDS DG = DH + (- TDS) Enthalpy term Entropy term
Gibbs Free Energy Effect of Temperature of Spontaneity
Gibbs Free Energy Example: Predict whether the following reaction will be spontaneous at low temperature, high temperature, at all temperatures or always non-spontaneous. 2 PbS(s) + 3 O2 (g) 2 PbO (s) + 2 SO2 (g) DH = neg. DS = neg
Gibbs Free Energy Example: Given the standard heats of formation below, predict whether the following reaction will be spontaneous at low temperature, high temperature, at all temperatures or always non-spontaneous. CaO (s) + 3 C (graphite) CaC2 (s) + CO (g) DHfo (CaO) = - 635.1 kJ/mol DHfo (CaC2) = - 59.9 kJ/mol DHfo (CO) = - 110.5 kJ/mol
Gibbs Free Energy • For a system in which the reactants and/or products are not present in their standard states, the values of DG and DGo are related: DG = DGo + RT lnQ where DG = Gibbs free energy change DGo = standard Gibbs free energy change R = 8.314 J/mol.K T = temp. in Kelvin Q = reaction quotient
Free Energy and Equilibrium Constants • For a system at equilibrium, • DG = 0 • Q = K • and the standard free energy change (DGo) for the reaction is directly related to the equilibrium constant for the reaction DGo = -RT ln K
Free Energy and Equilibrium Constants • This equation can be used to calculate DGo for a reaction when the equilibrium constant or the equilibrium concentrations are known. • The equation can also be rearranged and used to find the value of the equilibrium constant if DGo for the reaction is known: K = e- DG /RT o
Free Energy and Equilibrium Constants Example: Find DGo for the following reaction at 25oC if Kp = 7.00 x 105. N2 (g) + 3 H2 (g) 2 NH3 (g)
Free Energy and Equilibrium Constants Example: Calculate the equilibrium constant at 25oCfor the dissolution of barium fluoride if DGo for this process is +32.9 kJ per mole of barium fluoride.
Free Energy and Equilibrium Constants • Once you find the value for the equilibrium constant, you can use the equilibrium constant to : • Calculate the equilibrium concentrations of the products and/or reactants. • How would you calculate the concentrations of barium ions and fluoride ions present in a saturated solution of barium fluoride?
