REACTION RATES AND EQUILIBRIUM Spontaneous process is the time- evolution of a system in which it releases free energy (most often as heat) and moves to a lower, more thermodynamically stable, energy state. A spontaneous process is capable of proceeding in a given direction, as written or described, without needing to be driven by an outside source of energy. Nonspontaneous process takes place only as the result of some cause or stimulus. Sisyphus (SIS-i-fus) Sinner condemned in Tartarus to an eternity of rolling a boulder uphill then watching it roll back down again. Chemistry 21A Dr. Dragan Marinkovic

REACTION RATES AND EQUILIBRIUM Endergonic means absorbing energy in the form of work. Endergonic process is one wherein the system absorbs energy from the surroundings. Exergonic means to release energy in the form of work. Exergonic process is one that releases energy from the system, of which it is a part, to the surroundings. Entropy is a measure of the disorder or mixed-up character (randomnes) of a system. Entropy, symbolized by S, is a measure of the unavailability of a system’s energy to do work. It is a measure of the disorder of molecules in a system Ice melting is a classic example of entropy increasing Melting of ice absorbs heat. Chemistry 21A Dr. Dragan Marinkovic

REACTION RATES AND EQUILIBRIUM If the energy decreases and entropy increases Always spontaneous process If spontaneous process is accompanied By an entropy decreases Then energy also must decrease. By the energy increase then there is also large entropy increase If spontaneous process accompanied (example – water freezing) (ice melting) (wood burning) STABLE SUBSTANCE does not undergo spontaneous changes under surrounding conditions. Chemistry 21A Dr. Dragan Marinkovic 1 2 3

REACTION RATES AND EQUILIBRIUM REACTION RATE is the speed of a reaction. ∆C C t - C 0 REACTION RATE = ∆ t ∆ t ∆C change in concentration C t concentration at the end of reaction C 0 concentration at the beginning of reaction ∆ t time change A + B → C Chemistry 21A Dr. Dragan Marinkovic Reaction rate tends to increase with concentration - a phenomenon explained by collision theory

REACTION RATES AND EQUILIBRIUM Chemistry 21A Dr. Dragan Marinkovic REACTION MECHANISM is a detailed explanation of how a reaction actually takes place. 1.Reaction particles must collide with one another in order for reaction to occur. (exception: e.g. decomposition) 2.Particles must collide with at least a certain amount of energy if the collision is to result in a reaction. 3.In some cases, colliding reactants must be oriented in a specific way if a reaction is to occur. Reactions involving solids usually take place only on the solid surface. Kinetic Energy – The energy of motion. E k = kinetic energy m = mass v = velocity (of the particle)

REACTION RATES AND EQUILIBRIUM Chemistry 21A Dr. Dragan Marinkovic 2O → O 2 Particles with unpaired electrons react spontaneously and rapidly with each other. The energy level of products is much lower than the energy of reactants. (FREE RADICALS)

REACTION RATES AND EQUILIBRIUM Energy = Stretching Energy + Bending Energy + Torsion Energy + Non-Bonded Interaction Energy Chemistry 21A Dr. Dragan Marinkovic INTERNAL ENERGY The energy associated with vibrations within molecule.

Chemistry 21A Dr. Dragan Marinkovic REACTION RATES AND EQUILIBRIUM A chemical reaction is the breaking of bonds and/or the formation of new bonds between atoms. INTERNAL ENERGY The energy associated with vibrations within molecule. When INTERNAL ENERGY Is high enough the bond between atoms breaks.

REACTION RATES AND EQUILIBRIUM Chemistry 21A Dr. Dragan Marinkovic ACTIVATION ENERGY Energy needed to start some spontaneous process. Once started, the process continues without further stimulus or energy from the outside source.

REACTION RATES AND EQUILIBRIUM Chemistry 21A Dr. Dragan Marinkovic H 2 + Cl 2 -> 2 HCl When two molecules colide (like hydrogen and chlorine molecules) they can either bounce back unchanged, or, if there is enough energy, the “original” bonds will break and new bonds (new molecules) will be formed.

REACTION RATES AND EQUILIBRIUM NO + NO 3 → 2NO 2 importance of molecular orientations during collisions Chemistry 21A Dr. Dragan Marinkovic If one or both reacting molecules are unsymmetrical, orientation effects play extremely important role. Molecules have to be properly oriented in order for reaction to take place.

REACTION RATES AND EQUILIBRIUM Chemistry 21A Dr. Dragan Marinkovic Low activation energy. High activation energy. 4P (s) + 5O 2(g) → P 4 O 10(s) S (s) + O 2(g) → SO 2(g) EXOTHERMIC REACTIONS

REACTION RATES AND EQUILIBRIUM Chemistry 21A Dr. Dragan Marinkovic ENDOTHERMIC REACTION

REACTION RATES AND EQUILIBRIUM Chemistry 21A Dr. Dragan Marinkovic REACTION RATE depends on: 1.The nature of reactants 2.The concentration of reactants 3.The temperature of reactants 4.The presence of catalyst While ionic reactions in solutions are instantaneous, such as e.g.: Ba 2+ (aq) + SO 4 2- (aq) → BaSO 4(s) Reactions involving covalent bonds take time – for bonds of reactants to be broken and for the new bonds in products to be formed.

REACTION RATES AND EQUILIBRIUM Chemistry 21A Dr. Dragan Marinkovic REACTION RATE depends on: 1.The nature of reactants 2.The concentration of reactants 3.The temperature of reactants 4.The presence of catalyst While ionic reactions in solutions are instantaneous, such as e.g.: Ba 2+ (aq) + SO 4 2- (aq) → BaSO 4(s) Reactions involving covalent bonds take time – for bonds of reactants to be broken and for the new bonds in products to be formed. Effective collision A collision that causes a reaction to occur between the colliding molecules. NO + NO 3 → 2NO 2 importance of molecular orientations during collisions

REACTION RATES AND EQUILIBRIUM Chemistry 21A Dr. Dragan Marinkovic REACTION RATE depends on: 1.The nature of reactants 2.The concentration of reactants 3.The temperature of reactants 4.The presence of catalyst Effective collision A collision that causes a reaction to occur between the colliding molecules. NO + NO 3 → 2NO 2 importance of molecular orientations during collisions The reaction rate generally doubles for every 10 o C increase in temperature.

REACTION RATES AND EQUILIBRIUM Chemistry 21A Dr. Dragan Marinkovic Generic potential energy diagram showing the effect of a catalyst in a hypothetical exothermic chemical reaction X + Y to give Z. The presence of the catalyst opens a different reaction pathway (shown in red) with a lower activation energy. The final result and the overall thermodynamics are the same. Solid heterogeneous catalysts such as in automobile catalytic converters are plated on structures designed to maximize their surface area REACTION RATE depends on: The presence of catalyst

REACTION RATES AND EQUILIBRIUM Chemistry 21A Dr. Dragan Marinkovic CATALYST is a substance that changes (usually increases) reaction rates without being used up in the reaction. INHIBITOR is a substance that decreases reaction rates. HOMOGENEOUS CATALYST a catalytic substance that is distributed uniformly throughout the reaction mixture. HETEROGENEOUS CATALYST a.k.a. SURFACE CATALYST A catalytic substance normally used in a form of solid with a large surface area on which reaction takes place. A catalyst works by providing an alternative reaction pathway to the reaction product. The rate of the reaction is increased as this alternative route has a lower activation energy than the reaction route not mediated by the catalyst. Catalysts generally react with one or more reactants to form intermediates that subsequently give the final reaction product, in the process regenerating the catalyst. The following is a typical reaction scheme, where C represents the catalyst, X and Y are reactants, and Z is the product of the reaction of X and Y: X + C → X C (1) Y + X C → XY C (2) XY C → C Z (3) C Z → C + Z (4) Although the catalyst is consumed by reaction 1, it is subsequently produced by reaction 4, so for the overall reaction: X + Y → Z

REACTION RATES AND EQUILIBRIUM Chemistry 21A Dr. Dragan Marinkovic CHEMICAL EQUILIBRIUM the creation of products is called the forward reaction the creation of reactants is called the reverse reaction H 2 + I 2 → 2 HI"synthesis of hydrogen iodide" 2 HI → H 2 + I 2 "dissociation of hydrogen iodide" The equilibrium state is independent of the direction from which it is approached. Whether we start with an equimolar mixture of H 2 and I 2 (left) or a pure sample of hydrogen iodide (shown on the right, using twice the initial concentration of HI to keep the number of atoms the same), the composition after equilibrium is attained (shaded regions on the right) will be the same.

REACTION RATES AND EQUILIBRIUM Chemistry 21A Dr. Dragan Marinkovic CHEMICAL EQUILIBRIUM 2 NO 2 D N 2 O 4 + ↓Pressure + ↑ temperature (nitrogen dioxide becomes dinitrogen tetroxide) When the reaction is in equilibrium, a ratio is established between the products and the reactants. According to Le Chatlier we can manipulate the reactions by manipulating the conditions.  Equilibrium is achieved when the forward rate of a reaction is equal to the reverse rate of a reaction. A reversible reaction is one where the reactants and products exist in a state of equilibrium. Although we learned in stoichiometry that the amount of product can be predicted by the reactants, a reaction that goes to completion is very rare. Most reactions exist in a state of equilibrium. in equilibrium the rates are equal (forward = reverse) NO 2 N2O4N2O4

REACTION RATES AND EQUILIBRIUM Chemistry 21A Dr. Dragan Marinkovic CHEMICAL EQUILIBRIUM a A + b B → c C + d D 

REACTION RATES AND EQUILIBRIUM Chemistry 21A Dr. Dragan Marinkovic CHEMICAL EQUILIBRIUM The principle is named after Henry Louis Le Chatelier Le Chatelier’s principle If a chemical system at equilibrium experiences a change in concentration, temperature, volume, or total pressure, then the equilibrium shifts to counter-act the imposed change. Henry Louis Le Chatelier, an influential French/Italian chemist and engineer N H 2 ⇌ 2 NH 3 ΔH = −92kJ mol -1 This is an exothermic reaction when producing ammonia. If we were to lower the temperature, the equilibrium would shift in such a way as to produce heat. An increase in pressure due to decreasing volume causes the reaction to shift to the side with the fewer moles of gas. As the concentration of N 2 is increased, the frequency of collisions (that are successful) of that reactant would increase also, allowing for an increase in forward reaction, and generation of the product.

REACTION RATES AND EQUILIBRIUM Chemistry 21A Dr. Dragan Marinkovic CHEMICAL EQUILIBRIUM H 2(g) + I 2(g) 2HI (g)  H 2 + I 2 → 2 HI "synthesis of hydrogen iodide“ → ← 2 HI → H 2 + I 2 "dissociation of hydrogen iodide" equilibrium spontaneous

REACTION RATES AND EQUILIBRIUM Chemistry 21A Dr. Dragan Marinkovic CHEMICAL EQUILIBRIUM When a catalyst is used activation energy for both forward and reverse reactions is lowered. H 2(g) + I 2(g) 2HI (g) 

REACTION RATES AND EQUILIBRIUM Chemistry 21A Dr. Dragan Marinkovic