2 Video: World of Chemistry Chemical Kinetics Use this video as an introduction to this concept.
3 Reaction Rate Not all reactions occur at the same speed. Some reactions are very slow while others are fast.
4 Reaction RateThe reaction rate of a chemical reaction is a measurement of the increase in the concentration of a product or the decrease in the concentration of a reactant as the reaction proceeds over time.
6 N2 + 3H2 2NH3The units generally used to express reaction rate are mol/L . sec.What does it mean if the rate of the reaction above was given as 4.5 x 10-2 mol N2/L . sec?
7 C(s) + O2(g) → CO2(g) Occurs slowly at room T° and normal [O2]. Can speed up the reaction by increasing the T° or increasing [O2].We can explain these changes in reaction rate using the “collision theory”.
8 Collision TheoryAll substances are comprised of millions of tiny particles in constant motion. These particles are colliding with each other constantly in any substance.All collisions between particles do not result in a reaction.There are two factors that determine whether or not a reaction will occur between two particles that are colliding.
9 Collision TheorySubstances most come into contact, (collide) with enough energy.Activation Energy has been supplied by the force of the collision.
10 Activation EnergyThe activation energy is the amount of energy that must be available in order for a reaction to occur.
11 The sparks generated by striking steel against a flint provide the activation energy to initiate combustion in this Bunsen burner.The blue flame will sustain itself after the sparks are extinguished because the continued combustion of the flame is now providing the necessary energy through an exothermic reaction.
13 Collision TheorySubstances most come into contact, (collide) in the correct orientation (facing the correct way).
14 Collision TheoryThe collision theory states that reacting substances most come into contact, (collide) with enough activation energy, and in the correct orientation (facing the correct way), so that their electron shells can rearrange to form the products of the reaction.Therefore any factor which changes the frequency (how often), or energy of the collisions will change the rate of the reaction.
15 Five Factors Affecting Reaction Rate nature of the reacting substancesconcentrationsurface areatemperaturecatalysts
16 Nature of the reacting substances The type, strength, and number of chemical bonds or attractions between atoms differ from one substance to another.These differences determine the energy and orientation of the reacting particles that is necessary to create an effective collision resulting in a reaction.
17 TNT vs GunpowderExplosive materials which react very violently are known as high explosives. In contrast, there are some materials that react more slowly. These are known as low explosives. They release a large amount of energy, but due to the relatively slow rate of reaction.
18 TNT C6H2(NO2)3CH3 is a high explosive. TNT vs GunpowderGunpowder is a mixture of potassium nitrate (KNO3), sulphur (S8) and charcoal (C). It is a high explosive.TNT C6H2(NO2)3CH3 is a high explosive.
19 As the concentration of the reactants increases, the reaction rate increases. Why?
20 ConcentrationConcentration of the reactant refers to the number of reactant particles within a given volume.If the concentration of the reactants increases there will be a greater number of collisions.The greater the number of total collisions, the greater the number of “effective” collisions (collisions that will form product) and the greater the rate.
22 Concentration and Reaction Rate Steel wool burning in air Steel wool burning in pure oxygen A (21% oxygen) (100% oxygen)
23 As the surface area of the reactants increases, the reaction rate increases. Why?
24 Surface AreaIncreasing the surface area of the reactants results in a higher number of reaction sites.Reaction sites - specific sites on molecules at which reactions occur.Increasing the number of reaction sites increases the number of total collisions.The greater the number of total collisions, the greater the number of “effective” collisions (collisions that will form product) and the greater the rate.
26 Reaction Rate and Surface Area – Lycopodium Powder Exit slideshow and play using Windows Media Player. There is a delay if we try to show this file within the slide show.
27 As the temperature of the reactants increases, the reaction rate increases. Why?
28 TemperatureIncreasing the temperature increases the kinetic energy of the particles.This results in more frequent collisions and more energetic collisions.Therefore not only are there more collisions but also a greater percentage of the collisions have the needed activation energy.
29 Both light sticks have been activated however the one on the left was placed in ice water and the one on the right in boiling water.
32 CatalystsCatalysts lower the activation energy by providing an alternate pathway by which the reaction can occur at a lower energy.This results in a greater percentage of the collisions having the necessary energy to be effective resulting in an increase in reaction rate.Catalysts are remain unchanged at the end of a reaction.
35 Left: Partially caramelized cube sugar, Right: burning cube sugar with ash as catalyst
36 EnzymesEnzymes act as catalysts that lower the activation energy of a chemical reaction within a living organism.Enzymes carry out their function of lowering activation energy by temporarily combining with the chemicals involved in the reaction. These chemicals that the enzyme combines with are called the substrate.When the enzyme and substrate combine, the substrate is changed to a different chemical called the product. The enzyme is not consumed or altered by the reaction.
38 EnzymesEnzymes are specific for their substrate: A particular substrate molecule will combine temporarily with one enzyme type, and the active site of a particular enzyme will fit only one kind of substrate. For example, the enzyme sucrase will attach only to the substrate sucrose.
42 This shows the forward and reverse reaction. Reversible ReactionsN2 + 3H2 → 2NH3Reactions can normally be reversed.2NH3 → N2 + 3H2Reversible reactions are often indicated by a double arrow (↔).N2 + 3H2 ↔ 2NH3This shows the forward and reverse reaction.
43 For a reversible reaction such as A + B ↔ C Dthere are actually two reactionsForward reaction: A + B → C + DBackward reaction: C D → A + B43
44 A + B ↔ C + D Forward reaction: A + B → C + D A and B are used up C and D are formedBackward reactionC D → A BC and D are used upA and B are formed
45 At beginning of the reaction A + B ↔ C DThere is only A and B in the reaction container (no C and D formed yet)Forward reaction is very fastNo backward reaction occurs yet
46 A little laterA + B ↔ C DForward reaction is still fast (since the container still has mainly A and B)Some C and D have been formedThe backward reaction startsBackward reaction is very slow (since there is only a small amount of C and D)
47 Still some time later A + B ↔ C + D More A and B have been used up The forward reaction slows downMore C and D have been formedThe backward reaction speed up
48 The system is then said to be in Eventually…A + B ↔ C Dthe point is reached where the speed (rate) of the two reactions become equal.The system is then said to be inEQUILIBRIUM
49 Chemical Equilibrium At equilibrium The rate of the forward reaction becomes equal to the rate of the reverse reaction.How could this graph be adjusted and still show equilibrium?
50 Chemical Equilibrium At equilibrium The forward and reverse reactions continue at equal rates in both directions.For this reason we often refer to a “dynamic equilibrium”50
54 Chemical Equilibrium When equilibrium is Reached: There is no further change in the amounts (concentrations) of reactant and product.Concentrations at equilibrium are constant (not equal).
55 Chemical Equilibrium N2 + 3H2 ↔ 2NH3 A chemical reaction is at equilibrium when the forward and reverse reactions are occurring at the same rate.A reaction that has reached equilibrium is assigned an equilibrium constant (Keq or just K).
56 The equilibrium constant expression N2 + 3H2 ↔ 2NH3All we need to write an equilibrium expression is a balanced equation.
57 Write the equilibrium expression for: 4HCl+ O2↔2Cl2+ 2H2O If we know the concentrations (molarities) we can calculate a numerical value for K.
58 Given [CO] = 0. 200, [H2O] = 0. 500, [H2] = 0. 32 and [CO2] = 0 Given [CO] = 0.200, [H2O] = 0.500, [H2] = 0.32 and [CO2] = Find K for: CO + H2O ↔ H2 + CO2
59 Given [H2S] = 0. 706, [H2] = 0. 222 and [S2] = 0 Given [H2S] = 0.706, [H2] = and [S2] = Find K for: 2H2S ↔ 2H2 + S2K =
60 Given K = 0.0875 and [N2O4] = 0.0172M. Find [NO2] for: N2O4 ↔ 2NO2
61 Given K = 0. 0140 and [H2] and [I2] are each 2. 00 x 10-4M Given K = and [H2] and [I2] are each 2.00 x 10-4M. Find [HI] for: 2HI ↔ H2 + I2[HI] = M
62 LeChâtelier’s Principle states that when a stress is applied to a system at equilibrium, the system will respond (shift) in a manner that attempts to undo the stress.62
63 Stresses are… Change in concentration ([ ]) (adding or removing substances)Change in temperature(heating or cooling the system)Change in pressure(increasing or decreasing pressure)63
64 CONCENTRATION CHANGE Increase concentration of a reactant (add more nitrogen)N2 + 3H2 ↔ 2NH3 + heatEquilibrium shifts to the right(FORWARD reaction is favored because it will use up the nitrogen)64
65 CONCENTRATION CHANGE Increase concentration of a product (add more ammonia)N2 + 3H2 ↔ 2NH3 + heatEquilibrium shifts to the left(REVERSE reaction is favored because it will use up the ammonia)65
66 CONCENTRATION CHANGE Decrease concentration of a reactant (remove some nitrogen)N2 + 3H2 ↔ 2NH3 + heatEquilibrium shifts to the left(REVERSE reaction is favored because it will replace the nitrogen)66
67 CONCENTRATION CHANGE Decrease concentration of a product (remove the ammonia)N2 + 3H2 ↔ 2NH3 + heatEquilibrium shifts to the right(FORWARD reaction is favored because it will replace the ammonia)67
68 TEMPERATURE CHANGE Increase the temperature. (heat is added) N2 + 3H2 ↔ 2NH3 + heatEquilibrium shifts to the left(REVERSE reaction is favored because it will use up the added heat)68
69 TEMPERATURE CHANGE Decrease the temperature. (heat is removed) N2 + 3H2 ↔ 2NH3 + heatEquilibrium shifts to the right(FORWARD reaction is favored because it will replace the heat that was removed)69
70 Pressure ChangePressure can change by adjusting the volume.
71 Pressure ChangesThe side of the reaction with the greater number of moles of gas will create higher pressure.The side of the reaction with the lesser number of moles of gas will create lower pressure.
72 N2(g) + 3H2(g) ↔ 2NH3(g) + heat PRESSURE CHANGEIncrease the pressure.(volume of the container is decreased)N2(g) + 3H2(g) ↔ 2NH3(g) + heatEquilibrium shifts to the right(FORWARD reaction is favored because it will change 4 moles of gas into 2 moles of gas therefore returning to a lower pressure)72
73 N2(g) + 3H2(g) ↔ 2NH3(g) + heat PRESSURE CHANGEDecrease the pressure.(volume of the container is increased)N2(g) + 3H2(g) ↔ 2NH3(g) + heatEquilibrium shifts to the left(REVERSE reaction is favored because it will change 2 moles of gas into 4 moles of gas therefore returning to a higher pressure)73
74 Le Chatelier’s Principle 2 NO2(g) N2O4(g) Ho = kJDisturbance Equilibrium ShiftAdd more NO2………………Add more N2O4…………….Remove NO2………………Add a catalyst……………..no shiftDecrease pressure…………Decrease temperature….…