REACTION RATES AND EQUILIBRIUM

1. REACTION RATES AND EQUILIBRIUM Spontaneous processis 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. Chemistry 21A Dr. Dragan Marinkovic

2. REACTION RATES AND EQUILIBRIUM Endergonicmeans absorbing energy in the form of work. Endergonic processis one wherein the system absorbs energy from the surroundings. Exergonic means to release energy in the form of work. Exergonic processis one that releases energy from the system, of which it is a part, to the surroundings. Entropyis 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

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

4. REACTION RATES AND EQUILIBRIUM REACTION RATE is the speed of a reaction. A + B → C ∆C Ct - C0 REACTION RATE ---------- = -------------- ∆t∆t ∆C change in concentration Ctconcentration at the end of reaction C0 concentration at the beginning of reaction ∆ttime change Reaction rate tends to increase with concentration - a phenomenon explained by collision theory Chemistry 21A Dr. Dragan Marinkovic

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

6. REACTION RATES AND EQUILIBRIUM Particles withunpaired electrons react spontaneously and rapidly with each other. (FREE RADICALS) 2O → O2 The energy level of products is much lower than the energy of reactants. Chemistry 21A Dr. Dragan Marinkovic

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

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

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

10. REACTION RATES AND EQUILIBRIUM H2 + Cl2 -> 2HCl 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. Chemistry 21A Dr. Dragan Marinkovic

11. REACTION RATES AND EQUILIBRIUM NO + NO3 → 2NO2 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. importance of molecular orientations during collisions Chemistry 21A Dr. Dragan Marinkovic

12. REACTION RATES AND EQUILIBRIUM High activation energy. Low activation energy. 4P(s) + 5O2(g) → P4O10(s) S(s) + O2(g) → SO2(g) EXOTHERMIC REACTIONS Chemistry 21A Dr. Dragan Marinkovic

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

14. REACTION RATES AND EQUILIBRIUM While ionic reactions in solutions are instantaneous, such as e.g.: Ba2+(aq) + SO42-(aq) → BaSO4(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 RATE • depends on: • The nature of reactants • The concentration of reactants • The temperature of reactants • The presence of catalyst Chemistry 21A Dr. Dragan Marinkovic

15. REACTION RATES AND EQUILIBRIUM While ionic reactions in solutions are instantaneous, such as e.g.: Ba2+(aq) + SO42-(aq) → BaSO4(s) Reactions involving covalent bonds take time – for bonds of reactants to be broken and for the new bonds in products to be formed. NO + NO3 → 2NO2 • REACTION RATE • depends on: • The nature of reactants • The concentration of reactants • The temperature of reactants • The presence of catalyst importance of molecular orientations during collisions Effective collision A collision that causes a reaction to occur between the colliding molecules. Chemistry 21A Dr. Dragan Marinkovic

16. REACTION RATES AND EQUILIBRIUM NO + NO3 → 2NO2 • REACTION RATE • depends on: • The nature of reactants • The concentration of reactants • The temperature of reactants • The presence of catalyst The reaction rate generally doubles for every 10oC increase in temperature. importance of molecular orientations during collisions Effective collision A collision that causes a reaction to occur between the colliding molecules. Chemistry 21A Dr. Dragan Marinkovic

17. REACTION RATES AND EQUILIBRIUM REACTION RATE depends on: The presence of catalyst Solid heterogeneous catalysts such as in automobile catalytic converters are plated on structures designed to maximize their surface area 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. Chemistry 21A Dr. Dragan Marinkovic

18. REACTION RATES AND EQUILIBRIUM 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. 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. 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 → XC (1) • Y + XC → XYC (2) • XYC → CZ (3) • CZ → 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 Chemistry 21A Dr. Dragan Marinkovic

19. REACTION RATES AND EQUILIBRIUM CHEMICAL EQUILIBRIUM the creation of products is called the forward reaction the creation of reactants is called the reverse reaction The equilibrium state is independent of the direction from which it is approached. Whether we start with an equimolar mixture of H2 and I2 (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. Chemistry 21A Dr. Dragan Marinkovic

20. REACTION RATES AND EQUILIBRIUM CHEMICAL EQUILIBRIUM 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) NO2 N2O4 2 NO2 D N2O4 + ↓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. D Chemistry 21A Dr. Dragan Marinkovic

21. REACTION RATES AND EQUILIBRIUM CHEMICAL EQUILIBRIUM aA + bB  → cC + dD D Chemistry 21A Dr. Dragan Marinkovic

22. REACTION RATES AND EQUILIBRIUM 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. N2 + 3 H2 ⇌ 2 NH3    ΔH = −92kJ mol-1 Henry Louis Le Chatelier, an influential French/Italian chemist and engineer 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 N2 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. Chemistry 21A Dr. Dragan Marinkovic

23. REACTION RATES AND EQUILIBRIUM CHEMICAL EQUILIBRIUM H2(g) + I2(g) 2HI(g) D spontaneous spontaneous equilibrium Chemistry 21A Dr. Dragan Marinkovic

24. REACTION RATES AND EQUILIBRIUM CHEMICAL EQUILIBRIUM H2(g) + I2(g) 2HI(g) D When a catalyst is used activation energy for both forward and reverse reactions is lowered. Chemistry 21A Dr. Dragan Marinkovic

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