Unit 7: Kinetics and Equilibrium What is kinetics?

deals with the rates of chemical reactions. Kinetics: deals with the rates of chemical reactions. Rates: speed of any reaction Collision Theory: There can be no reaction unless there is an effective collision of ions, atoms or molecules. -In order for the collision to be effective, the particles have to have 2 things correct orientation correct amount of kinetic energy. The greater the number of effective collisions, the faster the reaction.

Factors of Kinetics Nature of the reactants: Compounds that are ionic or very polar (acids) AND aqueous react fast Temperature: The higher the temperature, the frequency and effectiveness of collisions increase Concentration: The higher the concentration ([ ]),the faster the rxn. You can change [ ] by adding more of a reactant or decreasing the volume. Surface area: The more surface area, the faster the rxn. Catalyst: A substance that increases the rate of a reaction by creating a different pathway. It is not used up in the rxn and so it is not a reactant or product, it appears above the rxn arrow. (ex) Which will have a higher rate of reaction; a cube of sugar or the same amount of sugar crushed into a powder?

Kinetics of Bonding Rates: * a precipitate (insoluble solid) Rates can refer to how fast the forward reaction occurs or how fast the reverse reaction occurs. (ex) NaCl(aq) ↔ Na+(aq) + Cl-(aq) Not all reactions can exist at equilibrium (go forward and backward) Some reactions go to completion which means all of the reactants get used up and it DOES NOT go backwards. A reaction will go to completion if it is not a closed system. Another reason if may not have a reverse rxn is if it forms: * a precipitate (insoluble solid) (ex) Pb(NO3)2(aq) + 2KI(aq)  PbI2(s) + 2K(NO3)(aq) This is a rxn that goes to completion because it forms a solid (precipitate).

Potential Energy Diagram Potential Energy of the Reactants: the amount of energy the reactants have Activation Energy: the amount of energy it takes to break existing bonds and get the rxn started. Activated Complex: The reactants that have broken apart. They are very unstable and exist only momentarily. PE of the Products: the amount of energy the products have Heat of rxn (ΔH): the net change in energy for the reaction (difference between reactants and products) (ΔH ↑- endo, ΔH ↓- exo) Activation Energy of Reverse Rxn: the amount of energy it takes to get the reverse reaction going ΔH of Reverse Rxn: the net change in energy for the reverse rxn

Exothermic Reactions Release more heat than they absorb There is a net loss of energy (-ΔH) The energy released from making bonds is greater than the energy absorbed from breaking bonds The energy of the products is less than the energy of the reactants

Endothermic Reactions: Absorb more heat than they release There is a net gain in energy (+ΔH) The energy absorbed from breaking bonds is greater than the energy released from making bonds The energy of the reactants is less than the energy of the products In most reactions there is both bond making and bond breaking occurring.

Catalysts Catalysts lower activation energy (forward and reverse) by allowing a different activated complex to be formed. Does not change ΔH

rate of the forward rxn = rate of the reverse rxn Equilibrium: Occurs when the rate of the forward rxn = rate of the reverse rxn **Many reactions are reversible  the rxns in both directions occur simultaneously. (ex) 2SO2 (g) + O2 (g)  2SO3 (g) Forward Rxn: 2SO2 (g) + O2 (g)  2SO3 (g) Reverse Rxn: 2SO3 (g)  2SO2 (g) + O2 (g) **!!!** Equilibrium DOES NOT MEAN that there is the same amount of reactants and products. A dynamic equilibrium does mean that the amounts of reactant and products are staying constant even though the reaction is still going because the rate of change is equal in both directions.

Equilibrium Example Products How many products? How many reactants? Are they the same? Do the have to be the same for the system to be at equilibrium? How many are going up? How many are going down? Is the rate of change the same in both directions? Is this a dynamic equilibrium? Why? 7 2 no no 2 2 yes yes There is change still occurring, however the reactants and products remain constant.

3 Equilibrium Types Phase Equilibrium Chemical Equilibrium Solution Equilibrium

Phase Equilibrium Phase Equilibrium: occurs when the rate of endothermic phase change = rate exothermic phase change. Temperature stays the same (ex) for water, at a constant 00C rate of freezing = rate of melting. If the temperature remains constant, there is a slush mixture of water and ice (liquid and solid). H2O (s)  H2O (l)

Phase Equilibrium cont’d Phase equilibrium occur only during phase changes at constant temperature. This happens because at 00C there is only enough energy to melt some of an ice cube, not all of it

Chemical Equilibrium Chemical Equilibrium: the rate of the forward rxn = rate of the reverse rxn. There is no net change in the amount of reactants and products. ** It means that the rxn has a balance between forward and reverse rxns. **If a reaction at equilibrium (balance) is disturbed, the reaction must adjust to find a NEW equilibrium. This adjustment is called an equilibrium shift.

LeChaltelier’s Principle LeChatelier’s Principle: if a stress is applied to a system at equilibrium, the system must shift to relieve that stress. What if something does change??? Change in the amount of reactants or products (concentration or temperature): The rxn will always shift away from something you add and toward something you take away.

LeChatelier’s Principle (ex) N2 (g) + 3H2 (g)  2NH3 (g) If the [N2] increases, _________[H2] _________[NH3] If the [NH3] increases, _________[N2] _________[H2]

LeChatelier’s Principle (ex) N2 (g) + 3H2 (g)  2NH3 (g) If the [H2] decreases, _________[N2] _________[NH3] If the [NH3] decreases, _________[N2] _________[H2]

LeChatelier’s Principle Adding heat: When heat is added, the endothermic rxn is always favored. (ex) PCl5 (g) + heat  PCl3 + Cl2 _________[PCl5] _________[Cl2] _________[PCl3]

LeChatelier’s Principle Removing heat (cooling): When heat is removed the exothermic rxn is always favored. (ex) 2SO2 (g) + O2 (g)  2SO3 (g) + heat _________[SO2] _________[O2] _________[SO3]

LeChatelier’s Principle Pressure or volume change: If pressure is increased or volume is decreased, it squeezes the molecules together. Therefore the rxn will shift towards the side that has less gas molecules. Increase in pressure/decrease in volume: (ex) 2SO2 (g) + O2 (g)  2SO3 (g) _________[SO2] _________[O2] _________[SO3]

LeChatelier’s Principle Pressure or volume change: If pressure is decreased or volume is increased, it spreads the molecules apart. Therefore the rxn will shift towards the side that has more gas molecules. decrease in pressure/increase in volume: (ex) 2SO2 (g) + O2 (g)  2SO3 (g) _________[SO2] _________[O2] _________[SO3]

LeChatelier’s Principle Catalyst: Increases the rate of the forward and reverse rxns equally by lowering activation energy in both directions. Therefore there is NO SHIFT in the rxn.

Equilibrium Constants Equilibrium Constants (Keq): A number that describes a system in equilibrium by comparing the [products] to the [reactants] at a specific temp. **Remember: there does not have to be equal amounts of reactants and products for there to be equilibrium. Keq describes the likelihood of the forward rxn occurring. Keq= [C]3[D]2 [A]2[B] The larger the Keq, the more products there are, we say the rxn favors the forward direction. 2A(g) + B(g)  3C(g) + 2D(g)

Writing Keq Only include reactants and products that are gases or aqueous. Use the coefficients as exponents. (ex) N2 (g) + 3H2 (g)  2NH3 (g) Keq= [NH3]2 [N2][H2]3 (ex) CaCl2 (s) + H2O (l)  Ca+2 (aq) + 2Cl- (aq) Keq= [Ca+2][Cl-]2 The only factor that can effect the Keq of a rxn is temp

Types of Keq Keq: for all chemical rxns Ka: ionization constant for acids dissolving in water Kb: dissociation constant for bases dissolving in water Ksp: solubility product for any solid dissolving in water Kw: ionization constant for water

Meaning of Keq If Keq>1 this means that the forward reaction is favored and there is more products than reactants If Keq<1 this means that the reverse reaction is favored and there is more reactants than products

Ex 1) When 0. 40 moles of PCl5 is heated in a 10 Ex 1) When 0.40 moles of PCl5 is heated in a 10.0 L container, an equilibrium is established is which 0.25 moles of Cl2 is present. PCl5(g)  PCl3(g) + Cl2(g) a) What are the equilibrium concentrations of all three components? b) What is the number of moles of PCl5 and PCl3 at equilibrium?  

Ex 2) When 1. 00 mole of NH3 gas and 0 Ex 2) When 1.00 mole of NH3 gas and 0.40 moles of N2 gas are placed in a 5.0 L container and allowed to reach an equilibrium at a certain temperature, it is found that 0.78 moles of NH3 is present. The reaction is: 2 NH3(g)  3 H2(g) + N2(g) a) What is the concentration in moles/L of each species? b) What is the number of moles of H2 and N2 at equilibrium?

Ex 3) A mixture of H2 and I2 is allowed to react at 448oC Ex 3) A mixture of H2 and I2 is allowed to react at 448oC. When equilibrium is established, the concentrations of the participants are found to be [H2] = 0.46 mol/L, [I2] = 0.39 mol/L, and [HI] = 3.0 mol/L. Calculate the value of the Keq at 448oC.             H2(g) + I2(g)  2 HI(g)

Ex 4) Assume that in the analysis of another equilibrium mixture of the same reaction as above, at the same temperature of 448oC, the equilibrium concentrations of I2 and H2 are both 0.50 mol/L. What is the equilibrium concentration of HI?

Ex 5) The equilibrium constant for the reaction represented below is 50. at 448oC H2(g) + I2(g)  2 HI(g) a) How many moles of HI are present at equilibrium when 1.0 moles of H2 is mixed with 1.0 moles of I2 in a 0.50 L container and allowed to react at 448oC? b) How many moles of H2 and I2 are left unreacted? c) If the conversion of H2 and I2 to HI is essentially complete, how many moles of HI would be present? d) What is the percent yield of the equilibrium mixture?

Ex 6) How many moles of HI are present at equilibrium when 2 Ex 6) How many moles of HI are present at equilibrium when 2.0 moles of H2 is mixed with 1.0 moles of I2 in a 0.50 L container and allowed to react at 448oC. At this temperature Keq = 50.

Q vs K Ex 7) When 3.0 moles of HI, 20 moles of H2, and 1.5 moles of I2 are placed in a 1.0 L container at 448oC, will a reaction occur?   If so, which reaction will take place?

Solution Equilibrium: Definitions the rate of dissolving = rate of crystallizing Solution: mixture of 2 or more substances that are not always present in a definite ratio. (all sol’ns are homogeneous mixtures) Solute: the substance being dissolved (usually present in the lesser amount) Solvent: the substance doing the dissolving (usually present in the greater amount) Crystalization: “undissolving” solute coming out of solution to form a ppt

Types of Solutions Saturated solution: Solutions that are holding the maximum amount of solute for a given amount of solvent at constant conditions. Saturated solutions exist at equilibrium NaCl(s) + H2O(l)  Na+(aq) + Cl-(aq)

Dissolving When something dissolves it gets completely surrounded by water. There is only enough water to surround so much solute therefore only a certain amount of solute can dissolve in a given amount of water. For ionic compounds, the polar ends of the water pull the ions apart during the dissolving process. This is called dissociation.

Table G Saturated solutions are ON THE LINE of solubility. When reading Table G, make sure the question is talking about 100 g of water. Density of water is 1.0 g/mL so 100 mL and 100 g of water is the same thing.

Saturated Sol’n practice (ex) How many grams of solute (substance X) is necessary to prepare a saturated solution at each of the following temperatures? 200C 450C 650C 15 grams 42 grams 80 grams

Unsaturated Solutions Unsaturated Solutions contain less solute than they can normally hold at a certain temperature. Represented by any point UNDER THE LINE of solubility. Additional solute can be added to an unsaturated solution until it is saturated.

Supersaturated Solutions Supersaturated solutions contain more solute than the solution can normally hold at that temperature. (Achieved by heating, adding solute, then cooling to original temp) Represented by any point ABOVE THE LINE of solubility. Supersaturated solutions are not stable. Any additional solute can cause all excess solute to ppt out of solution (Rock Candy)

Table G practice unsaturated supersaturated (ex) How many grams of solute are required to saturate the solution at 250C? If less than 20 grams of solute are used to make the solution, then the solution is _______________. If more than 20 grams of solute are used to make the solution, then the solution is _______________. unsaturated supersaturated

Table G practice (ex) Are the following solutions saturated, unsaturated or supersaturated? 20 grams of salt is dissolved in 100 grams of water at 400C. 35 grams of salt is dissolved in 100 grams of water at 400C. 10 grams of salt is dissolved in 100 grams of water at 50C. 45 grams of salt is dissolved in 100 grams of water at 650C. 70 grams of salt is dissolved in 100 grams of water at 600C. 5 grams of salt is dissolved in 100 grams of water at 200C. unsaturated saturated supersaturated unsaturated saturated unsaturated

Table G practice (ex) The following solutions are prepared. How many more grams of solute need to be added to saturate it? 5 grams of salt is dissolved in 100 grams of water at 150C. 15 grams of salt is dissolved in 100 grams of water at 350C. 25 grams of salt is dissolved in 100 grams of water at 650C. 35 grams of salt is dissolved in 100 grams of water at 450C. 8 grams 15 grams 55 grams 7 grams

Table G practice (ex) A saturated solution is prepared at the temperature given. Then the solution is cooled to 200C, how many gram of solute will precipitate out? 250C. 400C. 450C. 5 grams 20 grams 27 grams

Table G practice (ex) How many grams of salt are required to make the following saturated solutions? 50 grams of water at 200C. 300 grams of water at 250C. 800 grams of water at 300C. 50 grams of water at 350C. 2000 grams of water at 400C. 7.5 grams 60 grams 200 grams 15 grams 700 grams

Solubility of Gases Solubility of Gases: Temp: gases are the only substances that decrease in solubility with increased temperature (due to their low density) Press: they increase in solubility with increased pressure (ex) A liquid exists in a closed flask in with its vapor. What kind of equilibrium exists in this container? Phase equilibrium

Concentration Concentration: A number that tells how much solute is dissolved in a given amount of solvent. **Concentration always refers to a mixture of 2 things. You cannot have a concentration of any 1 substance. **We always measure the [ ] of homogeneous mixtures.

Molarity Molarity: Measures moles of solute per liters of solution. M = mol L (ex) Calculate the molarity of a 2000. milliliter solution containing 3.0 moles of HCl. (ex) Calculate the molarity of 2.0 moles of HCl dissolved in a 500. mL solution. (ex) How many moles of NaCl are dissolved in 500. mL of a 0.2 M solution?

Molarity from grams If the problem gives you grams instead of moles, you will have to convert grams to moles first by using the mole conversion. (ex) A 1500. mL solution has 100. g of KOH dissolved in it. Calculate the molarity of the solution. (ex) There is a 0.250 L solution with 53 g of Na2CO3 completely dissolved. What is the molarity of the solution?

Molality: molality (m) = moles solute kg of water ex) Calculate the molality of a solution that contains 34.0 grams of C6H12O6 dissolved in 2000.mL of water at STP.

Molality: molality (m) = moles solute kg of water ex) Calculate the molality of a solution that contains 34.0 grams of C6H12O6 dissolved in 2000.mL of water at 850C. The density of water at 850C is 0.96862 g/mL. ex) Calculate the molality of a solution that contains 189.0 grams of KNO3 dissolved in 1400.mL of water at 550C. The density of water at 550C is 0.98570 g/mL.

Parts per Million (ppm) ppm also measures concentration but in different units- grams. ppm = grams of solute x 1,000,000 grams of sol’n (ex) A certain gas has a concentration in water of 0.002 grams in 100 grams of water. What is the concentration in ppm? (ex) 2.5 grams of a groundwater solution is found to contain 5.4 x 10-6 grams of the Cu+2 ion. What is the concentration of the copper ion in ppm?

Colligative properties: any property of a solvent that changes when a solute is added. (specifically bp and fp) Depends on the: # of moles of solute particles formed from dissolving(van 't Hoff factor). Acids break up into pieces in water Ionic cmpds break into pieces in water Molecular and organic cmpds DO NOT break up in water. The nature of the solvent The concentration of the solute

How many moles of particles? (ex) HCl (aq)  __________ moles of product particles. (ex) MgCl2 (s) + H2O (l)  (ex) C6H12O6 (s) + H2O (l)  H+(aq) + Cl-(aq) 2 Mg+2(aq) + 2Cl-(aq) 3 C6H12O6(aq) 1

Boiling Point Elevation When any particles are dissolved in water, it takes more energy for the solution to boil ( bp) **water normally boils at what temperature? 1000C (373 K) When anything is dissolved in water, it disrupts the boiling process and therefore increases the amount of energy it takes to boil the water. Therefore the boiling point increases by 0.520C for every mole of product particles in 1000mL of water at standard temperature.

Use the equation:   New bp = 100 + iKbm where i = van 't Hoff factor (number of moles of product particles) Kb = molal boiling point elevation constant in °C kg/mol (0.52 °C kg/mol ) m = molality of the solute   ex) 31.65 g of sodium chloride is added to 220.0 mL of water at 35 °C. How will this affect the boiling point of the water? Assume the sodium chloride completely dissociates in the water. Given: density of water at 35 °C = 0.994 g/mL

Freezing Point Depression When any particles are dissolved in water, it requires a lower temperature to freeze ( fp) **water normally freezes at what temperature? 00C (273 K) When a substance freezes, the particles of the liquid must align in an orderly pattern. The presence of a solute in water disrupts the formation of this pattern because the particles are different sizes. This means that the solution has to get even colder before freezing. The freezing point decreases by 1.860C for every mole of product particles in 1 L of water at standard temperature.

Use the equation: new fp = 0 - iKfm where i = van 't Hoff factor Kf = molal freezing point depression constant or cryoscopic constant (1.86°C kg/mol) m = molality of the solute ex) 31.65 g of sodium chloride is added to 220.0 mL of water at 35 °C. How will this affect the freezing point of the water? Assume the sodium chloride completely dissociates in the water. Given: density of water at 35 °C = 0.994 g/mL