Reaction Rates and Equilibrium Chapter 18

What do you already know? True or False: 1.The rate of a reaction describes the speed at which a reaction occurs. 2. Dynamic equilibrium involves two opposing processes occurring at the same rate. 3. In order for a reaction to occur, reactant particles must collide in a specific orientation. 4. As the number of effective collisions between reacting particles increases, the rate of reaction decreases 5. Increasing the temperature will not increase the rate of a reaction. jjajajajajajajajajj

What do you already know? 6. The energy needed to start a chemical reaction is called activation energy. 7. The energy stored in chemical bonds is called potential energy. 8. Products are more stable if the potential energy is higher than the reactants. 9. A catalyst is added to reaction to speed up the rate at which a reaction occurs. 10. If the potential energy of the products is greater than the reactants, the reaction is endothermic.

Speeds at Which Reactions Occur Kinetics: Study of factors that govern –How rapidly reactions occur. –How reactants change into products. Rate of Reaction: How quickly the number of moles of reactants disappears and form products in given period of time.

Important Questions that Kinetics Answers Practical value!! Chemical /pharmaceutical manufacturers –Is it practical to make drug? –Made on manageable time scale? –Can we adjust conditions to improve rate & yield? Mechanism of Reaction –Series of individual steps leading to overall observed reaction. –How reactants change into products? –Detailed sequence of events. –syrpp

Factors that Affect Reaction Rates 1.Chemical nature of reactants What elements, compounds, salts, ect.. are involved? What bonds must be formed, broken? The more bonds (like double or triple bonds) that need to be broken, the slower the reaction rate. –Ionic substances already have dissociate easily into ions and do not take long to react. –Molecular substances take longer to react because covalent bonds need to be broken.

Factors that Affect Reaction Rates 2. Concentration The greater the number of reacting particles: –The more collisions present. –The greater reaction rate.

Factors that Affect Reaction Rates 3. Temperature Increasing the temperature speeds up reactions. Decreasing the temperature slows down reactions. Why? –Faster molecules collide more often & collisions have more kinetic energy.

Factors that Affect Reaction Rates 4. Surface Area (Particle Size) An increase in surface area increases the amount of the reactant exposed, which increases the number of collisions and therefore the reaction rate.

Factors that Affect Reaction Rates 5. Catalyst Catalysts increase reaction rate by lowering the “activation energy” Catalysts DO NOT get used up during the reaction.

Theories about Reaction Rates Collision Theory The rate of reaction is proportional to the # of effective collisions per second among reactant molecules. Effective collision –A collision that gives rise to a product. –The GREATER the # of collisions greater reaction rate. Ex. At Room Temperature & Pressure –H 2 & I 2 molecules undergoing collisions/sec. –Yet reaction takes a long time. Only very small % of all collisions leads to a reaction. Why?

Molecular Orientation Molecules must be oriented correctly and have sufficient energy during collisions for reaction to occur. Ex. NO 2 Cl + Cl  NO 2 + Cl 2 Wrong Orientation  Correct Orientation

Activation Energy, E a Molecules must possess certain amount of kinetic energy (KE) in order to react. What is Activation Energy, E a ? –Minimum KE needed for reaction to occur –It is a barrier that reactants must cross to be converted to products. Get energy from collision with other molecules. If molecules move too slowly, too little KE, they will just bounce off each other. If they don’t have this minimum amount, the reaction will not occur even when correctly oriented.

Potential Energy Diagrams Used to visualize what actually happens during successful collisions. Relationship between E a & developing Total PE. Transition State Theory Used to explain details of reactions and… Describes what happens when reactant molecules collide.

Potential Energy Diagram (Exothermic) Reaction Coordinate (progress of reaction) Potential Energy Activation energy (E a ) = hill or barrier between reactants & products  H reaction = H products – H reactants heat of reaction (  H) =

Exothermic Reactions  H reaction less than 0 (–) PE of products is LOWER than PE of reactants –KE is lost in the process –T of system  Reaction gives off heat

Potential Energy Diagram (Endothermic) Potential Energy Reaction Coordinate Endothermic Reaction  H = + Activated Complex

Endothermic Reactions  H reaction is greater than 0 (+) PE of products is LARGER than PE of reactants –Temperature  Have to add energy to get reaction to go If  H rxn large & + –Activation energy must be high –Reaction is slow

Activation Complex (Transition State) Brief moment during successful collision when: –bond to be broken is partially broken & –bond to be formed is partially formed An activated complex is the arrangement of atoms at the peak of the activation energy barrier (VERY UNSTABLE). The activated complex is also called the transition state. Can you explain why?

Ex.NO 2 Cl + Cl  NO 2 + Cl 2

Enthalpy Changes & Spontaneity What does spontaneous mean? –Occurs by itself Many reactions which occur spontaneously are exothermic (-  H) –Iron rusting But some endothermic reactions occur spontaneously (+  H) –Ice melting

Criterion for Spontaneity Increase of randomness & disorder of system. Something that brings about randomness more likely to occur than something that brings order.

Entropy, S Entropy, (S) is a measure of the disorder or randomness of a system:  S = Change in Entropy The greater the disorder, the greater the entropy. Systems in nature tend to undergo changes toward LOWER ENERGY and MORE DISORDER

Entropy Entropy increases as a substance goes from: s → l → g Chemical systems go from conditions of greater order (less entropy) to conditions of greater disorder (higher entropy). Mixtures have greater entropy than pure substances (solids and liquids). Gases have the greatest entropy.

Entropy Increasing S solidliquidgas As temperature ↑, entropy increases ↑. Why?

Learning Check: Entropy Ex.1 Which series of physical changes represents an entropy increase during each change? 1)g → l → s 2)s → l → g 3)l → g → s 4)s → g → s Ex.2 As NaCl dissolves according to the equation NaCl(s) → Na + (aq) + Cl - (aq), the entropy of the system 1)increases 2) decreases 3) remains the same

Reversible Reactions Some reactions are reversible: –Indicated by a double arrow, or. Ex. 2SO 2 (g) + O 2 (g) 2SO 3 (g) –When the rates of the forward and reverse reactions are equal, the reaction has reached a dynamic equilibrium.

Dynamic Equilibrium in Chemical Systems Catalysts speeds up BOTH the forward and the reverse reactions equally. –Decreases the amount of time it takes to reach equilibrium. Chemical equilibrium exists when: –Rates of forward & reverse reactions are equal. –Concentrations of reactants and products DO NOT change over time.

Phase Equilibrium Same Substance Different Phase! Solid Liquid Liquid Vapor (gas) melting freezing evaporation condensation

Phase Equilibrium At 0 °C or 273 K, both ice and water exist at the same time, some is melting some if freezing: H 2 O(s) H 2 O(l) At 100 °C or 373 K, both liquid and vapor exist at the same time, some is evaporating and some is condensing: H 2 O(l) H 2 O(g)

Solution Equilibrium When the rate of dissolving and recrystallizing are equal, equilibrium exists, and the solution is saturated. In a closed bottle or can of soda there is equilibrium between the gaseous and dissolved state of carbon dioxide: CO 2 (g) CO 2 (aq)

Learning Check: Reversible Reactions Ex.1 Based on the following reaction, which of the following statements is true C 2 H 5 OH(l) C 2 H 5 OH(g) 1) The forward process proceeds faster than the reverse process. 2) The reverse process proceeds faster than the forward process. 3) The forward process proceeds slower than the reverse process. 4) The forward and reverse process proceeds at the same rate.

Learning Check: Reversible Reactions Ex.2 What factors must be equal in a reversible chemical reaction at equilibrium? 1) Activation energies for the forward and reverse reactions. 2) Concentrations of the reactants and products. 3) The rates of the forward and reverse reactions. 4) Potential energies of the reactants and products.

Learning Check: Equilibrium Ex.3 At what temperature will the following equilibrium reaction exist at kPa? H 2 O(s) H 2 O(l) 1)0 K 2)32 K 3)0 °C 4)273 °C

Learning Check: Equilibrium Ex.4 Which of the following represents a phase equilibrium reaction? 1)H 2 (g) + I 2 (g) 2HI(g) 2)Cl 2 (g) Cl 2 (l) 3)3O 2 (g) 2O 3 (g) 4)2NO 2 (g) N 2 O 4 (g)

Do Now: 1. Which of the following represents a phase equilibrium reaction? 1)H 2 (g) + I 2 (g) 2HI(g) 2)l 2 (s) l 2 (g) 3)3O 2 (g) 2O 3 (g) 4)2NO 2 (g) N 2 O 4 (g) 2.At what temperature does the following phase equilibrium exist at? H 2 O(s) H 2 O(l) 1)0 K 2)32 K 3)0 °C 4)273 °C

Do Now: 3. Which of the following substances has the greatest entropy? 1)KCl(s) 2)H 2 O (l) 3)H 2 (g)

Dynamic Equilibrium in Chemical Systems When the equilibrium of a system is disturbed, the system makes adjustments to restore equilibrium. Le Châtelier’s Principle: If a stress is applied to a system in dynamic equilibrium, the system changes in a way that relieves the stress.

Stresses that disturb equilibrium include: 1. Concentrations of reactants or products 2. Changes in temperature 3. Changes in pressure/volume (remember P and V are indirectly related) 4. Catalysts

1. Effect of Concentration: N 2 O 4 (g) 2NO 2 (g) Suppose we disturb the equilibrium by ↑ the concentration of NO 2. What happens? The system SHIFTS in a direction to minimize the disturbance. –The shift is caused by ↑ the concentration of NO 2 The rate of the reverse reaction ↑ (reaction shifts to the left) Add NO 2 Reaction shifts left

1. Effect of Concentration: N 2 O 4 (g) 2NO 2 (g) Suppose we disturb the equilibrium by ↑ the concentration of N 2 O 4. What happens? A shift is caused by ↑ the concentration of N 2 O 4. The rate of the forward reaction ↑ (reaction shifts to the right) Reaction shifts right Add N 2 O 4

Ex. 1: Effect of Concentration CH 4 (g) + H 2 O(g) 3H 2 (g) + CO(g) In which direction will the reaction shift if the concentration of CH 4 is increased? –To the right, to form more product. –The rate of the forward reaction ↑ until equilibrium is reached again. What happens to the concentration of H 2 O, H 2 and CO after adding more CH 4 ?

Ex. 2 Effect of Concentration CH 4 (g) + H 2 O(g) 3H 2 (g) + CO(g) In which direction will the reaction shift if the concentration of H 2 is increased? –To the left. –The rate of the reverse reaction ↑ until equilibrium is reached again. What happens to the concentration of CO, H 2 O and CH 4 after adding more H 2 ?

Ex. 3 Effect of Concentration 4NH 3 (g) + 5O 2 (g) 4NO(g) + 6H 2 O(g) + heat In which direction will the reaction shift if the concentration of NH 3 is decreased? –To the left, the reaction will produce that substance again (the reverse reaction is favored). –The rate of the reverse reaction ↑ until equilibrium is reached again. What happens to the concentration of O 2, NO, H 2 O and the amount of heat?

Ex. 4 Effect of Concentration CaCO 3 (s) CaO(s) + CO 2 (g) In which direction will the reaction shift if the concentration of CaCO 3 is increased? –Does not effect equilibrium because it is a solid. –Static equilibrium

2. Effect of Temperature: N 2 (g) + 3H 2 (g) 2NH 3 (g) + heat Suppose we disturb the equilibrium by adding MORE heat. What happens? The reaction will shift to the left since heat was absorbed, favors an endothermic reaction. Reaction shifts left Add heat

2. Effect of Temperature: N 2 (g) + 3H 2 (g) 2NH 3 (g) + heat Suppose we disturb the equilibrium by REMOVING heat. What happens? The reaction will shift to the right, to produce more heat, favors an exothermic reaction. Reaction shifts right Remove heat

3. Effect of Pressure: CO 2 (g) CO 2 (aq) Pressure effects the rate of a reaction only when gases are involved. When the pressure ↑, the concentration of CO 2 ↑. Le Châtelier’s principle predicts that the system will move away from the added stress and shift to the right. Reaction shifts right

3. Effect of Pressure/Volume: N 2 (g) + 3H 2 (g) 2NH 3 (g) How will a system react when there are gaseous molecules on both sides? An ↑ in pressure (decrease in volume) will favor the reaction toward the side with fewer gas molecules. In the reaction above, how many gas molecules in total are on the reactant side? Product side? 4 on the reactant side and 2 on the product. Reaction shifts to the right.

3. Effect of Pressure/Volume: N 2 (g) + 3H 2 (g) 2NH 3 (g) How will a system react if pressure is ↓ (increase in volume)? The system will respond by trying to bring the pressure back up. The reaction will shift to the left where there are MORE gas molecules, thus increasing the pressure again.

3. Effect of Pressure/Volume: What happens if there is the same number of gaseous molecules on both sides? H 2 (g) + Cl 2 (g) 2HCl(g) Pressure changes have NO EFFECT on the system.

4. Effect of a Catalyst: The addition of a catalyst changes the rate of the both forward and reverse reaction equally. IT DOES NOT CHANGE ANY OF THE EQUILBRIUM CONCENTRATIONS!! A catalyst ONLY changes how quickly the reaction proceeds.

Ex.1 What stress would cause the equilibrium to shift to the left: N 2 (g) + 3H 2 (g) 2NH 3 (g) + heat 1)Increasing the temperature 2)Increasing the pressure 3)Adding N 2 (g) 4)Adding H 2 (g)

Ex.2 2SO 2 (g) + O 2 (g) 2SO 3 (g) + heat For this reaction, what is the effect of 1)Removing SO 3 2)Removing O 2 3)Decreasing the pressure 4)Adding O 2 5)Removing heat

Ex.3 N 2 (g) + O 2 (g) + heat 2NO(g) For this reaction, what is the effect of 1)Decreasing O 2 2)Adding N 2 3)Increasing the pressure 4)Adding NO 5)Adding heat 6)Adding a catalyst

Ex.4 C(s) + O 2 (g) CO 2 (g) + heat For this reaction, what is the effect of 1)Decreasing O 2 2)Adding C(s) 3)Increasing the CO 2 4)Decreasing the CO 2 5)Adding heat 6)Removing heat 7)Adding a catalyst

Ex.4 C(s) + O 2 (g) CO 2 (g) + heat For this reaction, what is the effect of 1)Decreasing O 2 2)Adding C(s) 3)Increasing the CO 2 4)Decreasing the CO 2 5)Adding heat 6)Removing heat 7)Adding a catalyst

Equilibrium Constants The equilibrium constant (K eq ) is the ratio of product concentration to reactant concentrations at equilibrium. Solids ARE NOT included in an equlibrium expression Each concentration is raised to a power equal to the number of moles of that substance in the balanced equation. aA + bB cC + dD K eq = [C] c [D] d [A] a [B] b