Chemical Thermodynamics the study of Reaction Feasibility

Reaction Feasibility Thermodynamics is concerned with questions such as: why do some reactions take place while others don’t? can we predict whether or not a reaction will occur? under what conditions will a reaction occur? reversible more likely Increasingly we consider all reactions to be reversible, but, under certain conditions the reaction will be more likely to go in one direction than the other spontaneous non-spontaneous In one direction the reaction will be spontaneous while the other direction will be non-spontaneous

Most spontaneous reactions are exothermic But not ALL ! Spontaneous Exothermic

Spontaneous Endothermic solid ammonium carbonate reacts with conc. ethanoic acid (NH 4 ) 2 CO 3 + 2CH 3 COOH 2NH 4 CH 3 COO + CO 2 + H 2 O

Spontaneous Processes H 2 O (s) ⇋ H 2 O (l) Ice turning to water is spontaneous at T > 0°C, Water turning to ice is spontaneous at T < 0°C. Both Exothermic & Endothermic processes can be spontaneous The direction of a spontaneous process will depend on temperature

Energy Rules! The key to understanding Thermodynamics is the appreciation of the ways in which energy interacts with matter. Temperature Energy doesn’t just determine the speed at which particles move (Temperature) it is part of everything that affects particles.

Energy Rules! no change in temperature Some Processes involve no change in temperature but a major change in the energy of particles is still occurring.

Entropy level of disorder These changes in the energy of particles have an overall affect on the level of disorder shown by a substance. ENTROPY, S The disorder in a substance is known as its ENTROPY, S. The Third Law of Thermodynamics provides a reference against which Entropies can be measured. “the Entropy of a perfect crystal at 0 K is zero”

Entropy - State Molecular Motion TranslationRotationVibration no freedom to move restricted freedom to move total freedom to move no freedom to rotate some freedom to rotate total freedom to rotate free to vibrate

Entropy - Temperature Entropy (S) Temperature Entropy of Fusion Entropy of Vaporisation

Entropy - Dissolving Less RandomnessMore Randomness Less Entropy More Entropy

Entropy - Molecules NO NO 2 N2O4N2O4 Fewer Vibrations Less Entropy More Vibrations More Entropy More Vibrations More Entropy

Entropy - Numbers Less RandomnessMore Randomness Less Entropy More Entropy

Entropy - Mixtures Less RandomnessMore Randomness Less Entropy More Entropy

Entropy Values

Entropy Calculations Similar to a previous formula: ∆S o = ∑ S o products - ∑ S o reactants

Entropy Changes, ∆S spontaneous endothermic reactions tend to have certain characteristics in common the number of moles of product are greater than the number of moles of reactant a large proportion of the products are either liquids or gases reactants are often solids or liquids (NH 4 ) 2 CO 3 + 2CH 3 COOH 2NH 4 CH 3 COO + CO 2 + H 2 O The trend solids liquids gases is associated with an increase in disorder.

Entropy - The Answer? Increase In Entropy Is an Increase In Entropy the driving force behind a spontaneous chemical reaction ? Both ∆S = +ve & ∆S = -ve processes can be spontaneous The direction of a spontaneous process will depend on temperature A spontaneous process will depend on both ∆S and ∆H

Entropy - The Answer? Surroundings The ‘problem’ can be resolved if we take into account changes taking place in the Surroundings. Overall Increase In Entropy The driving force behind a spontaneous process turns out to be an Overall Increase In Entropy

Entropy - The Answer? Overall Increase In Entropy Water freezing is a spontaneous process whenever there is an Overall Increase In Entropy Water freezing leads to a decrease in entropy within the system. Being Exothermic, however, leads to an increase in entropy in the surroundings

Entropy - The Answer? Overall Increase In Entropy Water melting is a spontaneous process whenever there is an Overall Increase In Entropy Being Endothermic leads to a decrease in entropy in the surroundings There will have to be an increase in entropy within the system.

Measuring ∆S o surr Trying to Calculate the effect on the surroundings would appear, at first, an impossible task. Where do the surroundings start & finish? What is the entropy of air? Glass? etc. How many moles of ‘surroundings’ are there? Fortunately it is much, much simpler than that.

Measuring ∆S o surr Firstly the change in Entropy of the Surroundings is caused by the Enthalpy change of the Surroundings, and……. ∆H o surr = -∆H o syst so ∆S o surr ∝ -∆H o syst Temperature has an inverse effect. For example, energy released into the surroundings has less effect on the entropy of the surroundings, the hotter the surroundings are.

Measuring ∆S o surr In fact, it turns out that.. Overall Entropy Change It is the Overall Entropy Change that must be considered. ∆S o surr = - ∆H o syst T ∆S o total = ∆S o syst + ∆S o surr ∆S o total = ∆S o surr -∆H o syst T

Measuring ∆S o tota l Total Entropy ∆S total = 0 We are interested in the point at which the Total Entropy becomes a positive value (ceases being a negative value). We can ‘solve’ for ∆S total = 0 0 = T∆S o syst -∆H o syst 0 = ∆S o surr -∆H o syst T T Multiplying throughout by T gives us

Measuring ∆S o total ∆S total Remember that this is really the formula for ∆S total ∆S o total = T∆S o syst - ∆H o syst ∆S, ∆H T Armed with ∆S, ∆H and values for T we can calculate the overall change in Entropy and a positive value would be necessary for a spontaneous reaction. Gibbs Free Energy, G∆G ∆S However, for reasons that are beyond this Topic, a term called the Gibbs Free Energy, G, is preferred. A negative value for ∆G is equivalent to a positive value for ∆S. This requires a slight adjustment in our final formula.

Gibbs Free Energy ∆G o ∆G o = ∆H o syst - T∆S o syst The convenient thing about this expression is that it allows us to do calculations using only values that can be directly measured or easily calculated. Second Law of Thermodynamics Entropy must increase Strictly speaking, the Second Law of Thermodynamics states that Entropy must increase for a Spontaneous Process. Second Law of ThermodynamicsGibbs Free Energy must decrease In practice, the Second Law of Thermodynamics means that Gibbs Free Energy must decrease for a Spontaneous Process.

Gibbs Free Energy ∆G o

Calculating ∆G o ∆H o = ∑ ∆H f o products - ∑ ∆H f o reactants ∆S o = ∑ S o products - ∑ S o reactants ∆G o = ∆H o - T∆S o Fe 2 O CO ➝ 2 Fe + 3 CO 2 Fe 2 O CO ➝ 2 Fe + 3 CO 2 T in Kelvin

∆G o of Formation ∆G o = ∑ ∆G f o products - ∑ ∆G f o reactants ∆G∆G f The ∆G of a reaction can be calculated from ∆G f values. relative stabilities By themselves, they give useful information about relative stabilities.

Ellingham Diagrams ∆G o = ∆H o - T∆S o y = c + mx

Reversible Reactions ∆G o = ∆H o - T∆S o For a Chemical Reaction ∆G is negativepositive If ∆G is negative for one direction, it must be positive for the reverse. This implies that only one reaction can proceed (spontaneously) under a given set of conditions. ∆S However, ∆S calculations are based on 100% Reactant & 100% Product. ∆S In reality, mixtures exist, so larger ∆S values will be obtained than those calculated.

Reversible Reactions

Equilibrium position ∆G positive negative ∆G for 100% Reac ➝ 100% Prod is positive but 100% Reac ➝ mixture is negative so forward reaction can take place. ∆G and more negative favoured ∆G for 100% Prod ➝ 100% Reac and 100% Prod ➝ mixture is more negative so backward reaction is favoured ∆Gis positive Equilibrium lies over to the left but only slightly since value of ∆G is positive but relatively small

Equilibrium position ∆G very positive slightlynegative ∆G for 100% Reac ➝ 100% Prod is very positive but 100% Reac ➝ mixture is still slightly negative so forward reaction can take place. ∆G and more negative favoured ∆G for 100% Prod ➝ 100% Reac and 100% Prod ➝ mixture is more negative so backward reaction is favoured ∆Gis positive Equilibrium lies well over to the left since value of ∆G is positive and relatively large

Equilibrium position ∆G very negative slightlynegative ∆G for 100% Reac ➝ 100% Prod is very negative but 100% Prod ➝ mixture is still slightly negative so backward reaction can take place. ∆G and more negative favoured ∆G for 100% Reac ➝ 100% Prod and 100% Reac ➝ mixture is more negative so forward reaction is favoured ∆Gis negative Equilibrium lies well over to the right since value of ∆G is negative and relatively large

Equilibrium position

Equilibrium Position ∆G o = - RT ln K There is a mathematical relationship: More simply:

Thermodynamic Limits Thermodynamics can predict whether a reaction is feasible or not. Thermodynamics can predict the conditions necessary for a reaction to be feasible. Thermodynamics can predict the position of equilibrium cannothow fast Thermodynamics cannot predict how fast a reaction might be.

Kinetics Coming Soon to a Board Near YOU!