Presentation on theme: "Ch. 19 Reaction Rates and Equilibrium. Reaction Rates Objective: Describe what is meant by the rate of a chemical reaction. Some chemical reactions."— Presentation transcript:
Ch. 19 Reaction Rates and Equilibrium
Reaction Rates Objective: Describe what is meant by the rate of a chemical reaction. Some chemical reactions occur very rapidly, such as an explosion. Other chemical reactions take years to come to completion, such as decomposition of organic material. – –Coal is produced by the decomposition of plants under pressure which takes millions of years. This chapter deals with reaction rates of chemical reactions.
Reaction Rates Fast reaction rates: – –Burning a candle – –Explosions Medium reaction rates: – –Rusting – –Aging (human) – –Decomposition of organic material (rotten food) Slow reaction rates: – –Formation of coal, diamond
Collision Theory According to collision theory, atoms, ions and molecules can react to form products when they collide, provided that the particles have enough kinetic energy. If they don’t have enough energy, they may just bounce off each other.
Collision Model Collisions must have enough energy to produce the reaction (must equal or exceed the activation energy). Reactants must have proper orientation to allow the formation of new bonds.
Activation Energy How do you know if the colliding particles have enough energy? The minimum amount of energy that they must have in order to react is the activation energy. Activation energy is like a barrier that they have to cross to for reactants to be converted to products. Otherwise the reaction won’t happen. During a reaction there may be some intermediate products, called the “activated complex” that form momentarily ( seconds) and then turn into the products. Another name for activated complex is “transition state”.
Activation Energy Activated Complex or transition state
Activation Energy The minimum energy required to transform reactants into the activated complex (also known as the transition state) (The minimum energy required to produce an effective collision) Flame, spark, high temperature, radiation are all sources of activation energy
Exothermic Processes Processes in which energy is released as it proceeds, and surroundings become warmer Reactants Products + energy
Endothermic Processes Processes in which energy is absorbed as it proceeds, and surroundings become colder Reactants + energy Products
2NO 2 (g) 2NO(g) + O 2 (g) Reaction Rates: 2. Can measure appearance of products 1. Can measure disappearance of reactants 3. Are proportional stoichiometrically
The Reaction Mechanism The reaction mechanism is the series of steps by which a chemical reaction occurs. A chemical equation does not tell us how reactants become products; it is a summary of the overall process. The sign represents the reaction mechanism, but gives no indication of the steps in the mechanism Reactants Products
The Rate-Determining Step In a multi-step reaction, the slowest step is the rate- determining step. It therefore determines the rate of reaction.
Factors Affecting Rate – Collision Theory Demos Temperature Increasing temperature usually increases the rate of a reaction, because it raises the kinetic energy of the particles when they collide. Surface Area Increasing surface area increases the rate of a reaction because it increases the surface area that is exposed to participate in the reaction. Concentration Increasing concentration USUALLY increases the rate of a reaction, because it increases the frequency of collisions. Presence of Catalysts and/or Inhibitors. A catalyst serves to lower the activation energy and allow the reaction to proceed more easily. An inhibitor interferes with the action of the catalyst.
Catalysis Catalyst: A substance that speeds up a reaction by lowering activation energy. A catalyst is not actually consumed in the reaction, it just serves as an intermediate, so it is neither a reactant nor a product. Remember to list catalysts on top of the arrow! (Pt) Pt 2H 2 + O 2 → 2H 2 O Enzyme: A large molecule (usually a protein) that catalyzes biological reactions (makes them occur at lower temperatures like your body temperature!) Homogeneous catalyst: Present in the same phase as the reacting molecules. Heterogeneous catalyst: Present in a different phase than the reacting molecules.
Endothermic Reaction w/Catalyst
Exothermic Reaction w/Catalyst
Reaction Rates Simulation reactions-and-rates_en.jarreactions-and-rates_en.jar file on hard drivereactions-and-rates_en.jar Web based address (if you are running this at home)
Section 19.2 Reaction Equilibrium
Chemical Equilibrium Reversible Reactions: A chemical reaction in which the products can react to re-form the reactants. In a reversible reaction, the reactions occur simultaneously in both directions. Chemical Equilibrium: When the rate of the forward reaction equals the rate of the reverse reaction and the concentration of products and reactants remains unchanged 2HgO(s) 2Hg(l) + O 2 (g) Arrows going both directions ( ) indicates equilibrium in a chemical equation
Reversible Reactions 2 SO 2 + O 2 2SO 3
Which substance(s) have the highest concentration at equilibrium? If you start out with an excess of SO 3, which way does the reaction proceed? Chemical equilibrium is shown at the right hand side of both graphs – when the rate at which the forward and reverse reactions take place is equal. NOTE: rates are equal, but concentrations are not.
Equilibrium position – indicates favored direction Imagine this reaction: A B 1% 99% Here the formation of B is favored (note length of arrows). Now what about this one: A B 99% 1% In this reaction, the formation of A is favored. In principle every reaction is reversible, but if it is very unbalanced like this, it can be considered irreversible (reversible part is negligible).
Catalysts – forward and reverse reactions Does a catalyst speed up the reaction in only one direction or both? A catalyst speeds up the forward and reverse reactions exactly the same because the reverse reaction is exactly the opposite of the forward reaction. In other words, the catalyst lowers the activation energy barrier when looked at from either side. Does a catalyst shift the location of the equilibrium position (amount reactants/products)? No! Catalysts do not affect the amounts of reactants and products present at equilibrium, just the time it takes to establish equilibrium.
Le Chatelier’s Principle Henry Le Chatelier
When you take something away from a system at equilibrium, the system shifts in such a way as to replace what you’ve taken away.When you take something away from a system at equilibrium, the system shifts in such a way as to replace what you’ve taken away. Le Chatelier Translated: When you add something to a system at equilibrium, the system shifts in such a way as to use up what you’ve added. When you add something to a system at equilibrium, the system shifts in such a way as to use up what you’ve added. Some things that could change include concentrations of reactant or product, changes in temperature and changes in pressure. Some things that could change include concentrations of reactant or product, changes in temperature and changes in pressure.
Le Chatelier Example #1a A closed container of ice and water at equilibrium. The temperature is raised. Ice + Heat Energy Water The equilibrium of the system shifts to the _______ to use up the added energy. *** ENDOTHERMIC EXAMPLE *** right
Le Chatelier Example #1b The reaction below is at equilibrium. Then heat is added. This reaction as shown is EXOTHERMIC, so heat can be considered to be a product here. If you add more heat (products) then the reaction is driven backwards towards reactants to try to restore equilibrium. If you remove heat, then reaction shifts to the right. 2SO 2 (g) + O 2 (g) 2SO 3 (g) + heat Add heat ← direction of shift Remove heat (cool) direction of shift →
Le Chatelier Example #2 A closed container of N 2 O 4 and NO 2 at equilibrium. NO 2 is added to the container. N 2 O 4 (g) + Energy 2 NO 2 (g) The equilibrium of the system shifts to the _______ to use up the added NO 2. left
LeChatelier Example #3 A closed container of water and its vapor at equilibrium. Vapor is removed from the system. water + Energy vapor The equilibrium of the system shifts to the _______ to replace the vapor. right
Pressure and Le Chatelier’s If pressure is increased, it drives the reaction to the side that has fewer moles, to reduce the number of molecules in order to offset the pressure increase.
LeChatelier Example #4 A closed container of N 2 O 4 and NO 2 at equilibrium. The pressure is increased. N 2 O 4 (g) + Energy 2 NO 2 (g) The equilibrium of the system shifts to the _______ to lower the pressure, because there are fewer moles of gas on that side of the equation. left
CoCl 2 LeChatelier’s video Reaction is as follows:Reaction is as follows: CoCl 4 -2 (aq) + 6H 2 O(l) ↔ Co(H 2 0) Cl - (aq) + heatCoCl 4 -2 (aq) + 6H 2 O(l) ↔ Co(H 2 0) Cl - (aq) + heat BLUE PINK BLUE PINK So the reaction is exothermic in the forward direction as shown.So the reaction is exothermic in the forward direction as shown. If we write that reaction backwards:If we write that reaction backwards: Co(H 2 0) Cl - (aq) + heat ↔ CoCl 2 -2 (aq) + 6H 2 O(l)Co(H 2 0) Cl - (aq) + heat ↔ CoCl 2 -2 (aq) + 6H 2 O(l) PINK BLUE PINK BLUE Now can you see that the reaction is endothermic in that direction?Now can you see that the reaction is endothermic in that direction? Video using CoCl2 to demonstrate LeChatelier’sVideo using CoCl2 to demonstrate LeChatelier’s
Sample problem 19-1 What effect do each of the following changes have on the equilibrium position for this reversible reaction? PCl 5 + heat PCl 3 + Cl 2 a) a)addition of Cl 2 Shifts the equilibrium to the left, forming more PCl 5 b) b)increase in pressure Shifts equil. to left (fewer moles) to decrease P. a) a)removal of heat Shifts equil. to left to produce more heat. a) a)removal of PCl 3 as it forms Shifts equil. to right to produce more PCl 3.
Sample Problem 19-2
Sample problem 19-3 (more difficult)
Sample problem 19-4
Equilibrium Constant with Heterogeneous Reactions
Heterogeneous reactions Example: PCl 5 (s) ↔ PCl 3 (l) + Cl 2 (g) Keq = [products]/[reactants] Keq = [PCl 3 ] [Cl 2 ] / [PCl 5 ] Pure liquids and solids are omitted So Keq = [Cl 2 ]
Heterogeneous Equilibrium Now you try these ones: 2H 2 O (l) ↔ 2H 2 (g) + O 2 (g) K eq = 2H 2 O (g) ↔ 2H 2 (g) + O 2 (g) K eq =
Related Videos to help you Equilibrium – Crash Course Chemistry-10’ videoEquilibrium – Crash Course Chemistry-10’ video Le Chatelier’s PrincipleLe Chatelier’s Principle https://www.youtube.com/watch?v=g5wNg_dKsYYhttps://www.youtube.com/watch?v=g5wNg_dKsYYhttps://www.youtube.com/watch?v=g5wNg_dKsYY Equilibrium Equations – Crash Course ChemistryEquilibrium Equations – Crash Course Chemistry 9:30 video 9:30 video Keq and how to do RICE tables Keq and how to do RICE tables https://www.youtube.com/watch?v=DP-vWN1yXrY https://www.youtube.com/watch?v=DP-vWN1yXrYhttps://www.youtube.com/watch?v=DP-vWN1yXrY
RICE tables RICE stands forRICE stands for R = reaction - write the balanced equation I = initial concentrations of reactant and product product C = change in concentrations of reactants and products and products E = equilibrium concentrations of reactants and products and products These are often called “ICE” tables as well because they assume you have the good sense to write down the reaction first anyway.
RICE example 1 from video Let’s use the reaction H 2 (g) + F 2 (g) ↔ 2HF(g) Note that we will have 2x the amount of HF than either H 2 or F 2 R H 2 (g) + F 2 (g) ↔ 2HF(g) I C E
RICE example 1 from video Let’s use the reaction H 2 (g) + F 2 (g) ↔ 2HF(g) Let’s say we start with 3.00 mol H 2 (g) and 6.00 mol of F 2 (g) in a 3.0 L container. The concentration of H 2 (g) =3.00 mol/3L = 1.00M. The concentration of F 2 (g) =6.00 mol/3L = 2.00M. The concentration of HF is zero because reaction hasn’t started yet. R H 2 (g) + F 2 (g) ↔ 2HF(g) I1.00 M2.00 M0.00 M C E
RICE example 1 from video We don’t know the change in concentration of the reactants, but we are being asked to find the final concentration. So for now, let’s say the concentration of H 2 and F 2 both decrease by x, because the coefficients in the balanced equation are both 1. Because the coefficient for HF is 2, the amount of product made is 2x R H 2 (g) + F 2 (g) ↔ 2HF(g) I1.00 M2.00 M0.00 M C-x +2x E
RICE example from video R H 2 (g) + F 2 (g) ↔ 2HF(g) I1.00 M2.00 M0.00 M C-x +2x E1.00-x2.00-x2x
RICE example 1 from video
RICE example from video
RICE example 1 from video So x = M Therefore [H 2 ] eq = = 0.032M And [F 2 ] = = 1.03 M And [HF] = 2 x = 1.94 M R H 2 (g) + F 2 (g) ↔ 2HF(g) I1.00 M2.00 M0.00 M C-x +2x E1.00-x2.00-x2x
Here’s a more complicated example of how to set up an ICE table when the values of how much it changed (the C) are given to you. Let’s say moles of SO 2 and moles of O 2 are placed in a 1 L container (so that’s 0.6M of each). At equilibrium, [SO 3 ] = 0.250M RICE/ICE Ex. 2 R 2SO 2 (g) + O 2 (g) ↔ 2SO 3 (g) I M 0.00 M C ?? M E M
How do you know what to put for the change in the reactants? Use stoichiometry! If 0.250M of SO 3 was created, which is 2SO 3 then if 2SO 2 was consumed, because the mole ratio of 2SO 2 / 2SO 3 is a ratio of 1, it must be that 0.250M goes in the box for SO 2 RICE/ICE Ex. 2 R 2SO 2 (g) + O 2 (g) ↔ 2SO 3 (g) I M 0.00 M C M ?? M E M
If 0.250M of SO 2 was consumed, then it must be that ½ that amount of O 2 was consumed because the mole ratio of O 2 to SO 2 is ½. ½ of 0.250M is M, as shown in the table below. Now it is a simple matter to subtract to get the equilibrium concentrations of the reactants, in green. RICE/ICE Ex. 2 R 2SO 2 (g) + O 2 (g) ↔ 2SO 3 (g) I M 0.00 M C M M M E M M M
RICE/ICE Tables RICE/ICE tables are a major part of this unit because you need the ability to solve for equilibrium concentrations of reactants and products, given initial concentrations and an equilibrium constant. Pay attention to the POGIL called Equilibrium – it helps teach you the concept. Watch the video that is embedded in this power point (go to my website, open the power point file, go to those slides on slideshow, click on the links and it will go to the correct youtube file). You may need to watch it more than once. Mostly, you will learn ICE tables by really working through Worksheet B Eq. Calcs using Ice, and Worksheet C Equilibrium Calculations. YOU NEED TO BE ABLE TO DO THESE KINDS OF PROBLEMS FOR THE TEST.
Section 19.3 Spontaneity Free energy is energy that is available to do work. Energy that is used to do work is typically not used very efficiently. For example, an internal combustion engine in a car maybe only about 30% efficient. That means that of the energy produced by burning gasoline, only 30% goes into making the car go forward. The rest is lost as heat or friction. Also, energy can only be obtained from a reaction if the reaction actually occurs. Reactions may not always be spontaneous (they may not occur on their own). A really great efficiency for energy use would be 70%.
19.3 Spontaneity This reaction normally would occur (it is spontaneous, given a spark): C(s) + O 2 (g) → CO 2 (the burning of something organic) Spontaneous means the reaction favors the formation of products at equilibrium. Spontaneous reactions release free energy. This reaction normally would not occur: CO 2 → C(s) + O 2 (g) We do not normally see carbon dioxide falling apart to become solid carbon plus oxygen gas. Since the second reaction is non-spontaneous, this reaction does not give substantial amounts of product at equilibrium.
19.3 Spontaneity In nearly all reversible reactions, one direction is favored over the other. It is important to note that the terms spontaneous and non-spontaneous do not refer to HOW FAST the reactants go to products. Some spontaneous reactions go so slowly that they appear to be nonspontaneous. The reaction of C 6 H 12 O 6 + O 2 ↔ CO 2 + H 2 O, however a bowl of sugar in air does nothing, so you might suspect that the reaction favors the reactants. The reaction actually does favor the products, but the reaction rate is really slow. If you supply energy in the form of heat, then the reaction goes quite quickly towards the products (burning sugar).
19.3 Spontaneity Some reactions that are nonspontaneous at one set of conditions are spontaneous at other conditions. For example, photosynthesis: 6CO 2 + 6H 2 O → C 6 H 12 O 6 + 6O 2 This nonspontaneous reaction can be driven to completion by the addition of sunlight as energy. Sometimes if a nonspontaneous reaction is coupled with a spontaneous reaction (that releases free energy), the free energy released by the spontaneous reaction can enable the non- spontaneous reaction to go forward.
Spontaneity and Entropy You might expect that based on heat being released for spontaneous reactions, only exothermic reactions would be spontaneous. But some other reactions are spontaneous even though they absorb heat! Consider: ice + heat → water Ice melts spontaneously. That seems to violate the rule that in spontaneous processes, the direction of the change in energy is from higher to lower energy (free energy is released). Some factor other than heat must help determine whether a physical or chemical process is spontaneous.
Entropy The other factor is related to order. The disorder of a system is measured as entropy Increasing entropy
A more advanced definition of entropy Entropy is a measure of the “disorder” of a system. What disorder refers to is really the number of different microscopic states a system can be in, given that the system has a particular fixed composition, volume, energy, pressure and temperature. By “microscopic states” we mean the exact states of all the molecules making up the system. Suppose you put a marble in a large box, and shook it up, and didn’t look inside afterwards. Then the marble could be anywhere inside the box. Because the box is large, there are many places inside the box that the marble could be, so the marble in the box has a high entropy. Now suppose you put the marble into a tiny box, and shook it. You have better knowledge of where the marble is because the box is small, so the box has low entropy.
Entropy If entropy is considered, then it makes sense why the melting of ice to form liquid water is spontaneous, because it raises the entropy of the system.
4 Entropy Guidelines (p. 552/553) 1. 1.For a given substance, the entropy of the gas is greater than the entropy of the liquid or solid. So entropy increases in reactions in which solid reactants form liquid or gaseous products. Entropy increases when liquid reactants form gaseous products too Entropy increases when a substance is divided into parts, for example when a crystalline solid like NaCl dissolves in water, because solute particles are more separated than in the crystalline solid form Entropy tends to increase when the total number of product molecules is > total number reactant molecules Entropy increases when temperature increases because disorder increases as molecules move faster.
More entropy guidelines (advanced) 5. 5.Entropies of large, complicated molecules are greater than those of smaller, simpler molecules. Examples for S o - CH 4 : C 2 H 6 : C 3 H 8 : C 4 H 10 : Entropies of ionic solids are larger when the bonds within them are weaker: Examples for S o – NaF: NaCl:72.13 NaBr:86.82 NaI: 98.53
Example: ice going to water
Example: water going to ice (doesn’t happen!)
Section Calculating Entropy and Free Energy
Table 19.2 gives standard entropies for a variety of compounds.
Example 19-6 What is the standard change in entropy for the following reaction at 25 o C and kPa: 2NO(g) + O 2 (g) → 2NO 2 (g) From Table 19.2 NO(g) S o = J/K mol NO 2 (g) S o = J/K mol O 2 (g) S o = J/K mol First balance the reaction (I’ve done that above) Then multiply the per-mole values by the number of moles and subtract [products – reactants] Δ S o = 2(240.5) – [1(205.0) + 2(210.6)] = J/K
Free-Energy Calculations In every spontaneous process, some energy becomes available to do work. This energy, the Gibbs free-energy change (ΔG) is the maximum amount of energy that can be coupled to another process to do useful work. The change in Gibbs free energy is related to the change in entropy (ΔS) and the change in enthalpy (ΔH) of the system by the following equation: ΔG = ΔH – TΔS The temperature in this equation must be in Kelvin. ΔG o = ΔH o – TΔS o You can calculate the standard free energy change using this equation
Free energy calculations If ΔH and ΔS are unknown, you can use ΔG f o, the standard free energy change for the formation of substances from their elements, to calculate ΔG o for a given reaction. ΔG o = ΔG f o (products) - ΔG f o (reactants) This is similar to how we calculate standard heats of formation ΔH f o.
Note that ΔG f o = 0 for elemental substances in their standard state. Note that this is at 25 o C and kPa only. Why do you think Br 2 gas has more free energy than Br 2 liquid? (Liquid is the std. state for bromine)
Free Energy Consider the reaction CaCO 3 (solid, formula unit) → CaO (solid, formula unit) + CO 2 (gas, molecule) In this reaction, entropy increases because one of the products formed from the solid reactant is a gas. This entropy increase is not sufficient for the reaction to be spontaneous at ordinary temperatures because the reaction is endothermic. The enthalpy of the reactants is lower than that of the products. The effect of an entropy increase is magnified as the temperature increases. At temperatures above 850 o C, the TΔS term outweighs the unfavorable enthalpy term ΔH o, and the reaction becomes spontaneous.
Sample problem 19-8 Using the ΔG f o for the reactants and products, determine whether the reaction from the last example is spontaneous: C(s, graphite) + O 2 (g) → CO 2 (g)C(s, graphite) + O 2 (g) → CO 2 (g) C(s, graphite) ΔG f o = 0 kJ/mol O 2 (g) ΔG f o = 0 kJ/mol CO 2 (g) ΔG f o = kJ/mol ΔG o = ΔG f o (products) - ΔG f o (reactants) ΔG o = kJ/mol – (0 + 0 ) ΔG o = kJ/mol Of course these results are identical to the last problem, nd the reaction is spontaneous, with a large release of free energy, the ΔG o is negative.
19.5 Rate Laws and Reaction Mechanisms
How about a more general reaction, like a double replacement? aA + bB → cC + dD Ro a one-step reaction of A with B, the rate of reaction is dependent on the concentrations of both A and B: Rate = k[A] a [B] b When each of the exponents in the reaction above are 1, then reaction is said to be first order in A, first order in B, and second order overall. The overall order of a reaction is the sum of the exponents for the individual reactants. For “ideal” one step reactions, the coefficents a and b are also the exponents, but in “real” reactions, these are experimentally determined because it may not be a simple one-step reaction.
Initial concentration of A (mol/L) Initial rate (mol/L sec) x x x Rate Laws and Reaction Mechanisms What can we observe about what order this rate equation should be? When we 2x [A] the rate is multiplied by 4 When we 4x [A] the rate is multiplied by 16 What does that suggest about the order? The rate must be 2 nd order in [A] given the factors above ( 2 2 = 4, 4 2 = 16)
If you were to graph all the energy changes that occur as reactants are converted to products in a chemical reaction, that would be a reaction progress curve. An elementary reaction is one where reactants are converted to products in a one step reaction. – –This reaction would have one activated complex between reactants and products, and therefore only one activation energy peak. Most chemical reactions, however, consist of a number of elementary reactions. For a complex reaction, the reaction progress curve resembles a series of hills and valleys. The hills correspond to the energy of the activated complexes, and the valleys represent intermediate products (the reactants of the next part of the reaction) Rate Laws and Reaction Mechanisms
The three peaks in this energy diagram correspond to activation energies for the intermediate steps of the reaction. The middle hump represents the highest energy barrier to overcome; therefore, the reaction involving N2O2 + 2H2 is the rate determining step. Example: