Rate of Reaction and the Collision Theory. Factors that Affect the Rate of a Chemical Reaction

Transcription

1 Chemical Kinetics and Thermodynamics Chemical Kinetics- concerned with: 1. Rates of Chemical Reactions- # of moles of reactant used up or product formed Unit time Or 2. Reaction Mechanisms- Rate of Reaction and the Collision Theory Reactions take place at different rates Collision Theory Used to explain why reactions take place at different rates Effective Collision Factors that Affect the Rate of a Chemical Reaction 1. What bonds are being broken and formed? Fast = slight rearrangement of electrons AgNO 3 + NaCl AgCl ppct. + NaNO 3 ---FAST! In reactions in which ionic bonds are broken and formed occur quickly at room temperature Slow= many covalent bonds broken 2H 2 O 2 2H 2 O + O 2 ---Slow! Reactions that include covalent bonds being broken and formed occur very slowly at room temperature 2. An increase in concentration of any one of the reactants usually, but not always increases the rate of the reaction if the reaction is homogeneous i. Homogeneous reactions- all the reactants are in the same phase. ii. Heterogeneous reactions- reactants are in more than one phase. I.e.- rusting How can we increase the concentration of reactants in the following: i. Gasesii. Liquids

2 Collision Theory- Heterogeneous systems- solid and liquid- increase Surface Area Grinding, pulverizing, smashing, stirring, pureeing, blending, etc, etc, etc Reactions that incorporate two gases Partial pressure increase in one of the gases results in an increase in reaction rate: As a result of decreased volume or Increase in the number of molecules of gas 3. Temperature For many reactions- 10 o C increase in temperature = 2X rate Collision Theory- Temperature increase = Energy Distribution Curve 4. Catalyst- Pt metal Enzymes 2H 2 O 2 2H 2 O + O 2 ---Slow! 2H 2 O 2 2H 2 O + O 2 ---Fast of MnO 2 present as catalyst 5. Only for systems in which the reactants are in more than one phase Solid and a liquid Reaction Mechanisms 2C 2 H 2 (g) + 5O 2 (g) 4CO 2 (g) + 2H 2 O(l) What is the likelihood that 2 acetylene molecules will come into contact with 5 oxygen molecules with the correct orientation and energy? Most reactions proceed through a series of simple steps Each step- collision of two particles - 2 -

3 Reaction Mechanism- Consider: 2A + B A 2 B It is unlikely that this reaction occurs in one step. One proposed mechanism may be- OR Often, chemists don t know the mechanism. They may only know the end result. Why might it be difficult? It is difficult to determine the reaction mechanisms because the intermediate products have short lives Intermediate products have structures unlike the structure of either the products or the reactants Activated Complex: Reaction Mechanisms and Rates of Reactions Rate determining step- We stated earlier: An increase in concentration not always increases the rate of a homogeneous reaction WHY? - Potential Energy Diagrams A pictorial way to describe the energy involved in a reaction Activation Energy and Temperature, Concentration, and Catalysts Activated Complex: Short lived, unstable particle that will temporarily exist when reacting molecules collide at the proper angle with the proper amount of energy Rate Laws A rate law is an equation that can be used to calculate the reaction rate for any given concentration of reactants

4 The rate law can be determined by keeping the concentrations of all but one reactant constant while measuring the reaction rate for various concentrations of that reactant. The process is repeated for each reactant. [A] and [B] represent the molar concentrations of reactants A and B in moles / liter (M) exponents x and y are the powers of the concentrations of the reactants (determined experimentally) k = proportionality constant, rate constant, has a fixed value for a reaction at a particular temperature (determined experimentally) The actual form of the rate law varies from reaction to reaction NO 2 (g) + O 3 (g) NO 3 (g) + O 2 (g) Rate = k[no 2 ][O 3 ] In this reaction, the rate is directly proportional to the concentration of both NO 2 and O 3 Suppose the initial concentration of each reactant is 1.00M. If the concentration of either reactant is doubled to 2.00M, the rate increases by a factor of 2. Also, if the concentration of either reactant is multiplied by 4 to 4.00M, the rate is increased by a factor of 4. 2 NO(g) + O 2 (g) 2NO 2 (g) Rate = k[no] 2 [O 2 ] Notice the squaring of the NO concentration. If the concentration of NO is multiplied by 6, the rate increases by a factor of 36 (6) 2. If the concentration of O 2 increases by a factor of 6, the rate only increases by a factor of 6. Question: By how much would the rate increase if the concentration of both reactants is multiplied by 2? Important: the exponent in the rate law has no relationship to the coefficient in the balanced chemical equation. The coefficients are determined experimentally! 2N 2 O 5 (g) 4 NO 2 (g) + O 2 (g) Rate = k[n 2 O 5 ] Important: not necessarily all reactants appear in the rate law. If changing the concentration of a particular reactant does not change the rate, that reactant does not appear in the rate law. NO 2 (g) + CO(g) NO (g) + CO 2 (g) Rate = k[no 2 ] 2 Because changes in the concentration of carbon monoxide do not affect the reaction rate, [CO] does not appear in the rate law for this reaction

5 Questions: 1. The rate law for the reaction in which nitrogen monoxide reacts with oxygen to produce nitrogen dioxide was shown to be Rate = k[no] 2 [O 2 ] Suppose you measure the rate of the reaction with the concentration of each reactant at 1.00M. What will happen to the reaction rate if the concentration of NO is doubled? What will happen to the reaction rate if the concentration of O 2 is doubled instead? 2. The chemical equation and the rate law for the decomposition of hydrogen iodide are shown. What will the effect on the reaction rate if the concentration of HI is raised from 1.00M to 4.00M? 2HI (g) H 2 (g) + I 2 (g) Rate = k[hi] 2 Thermodynamics Enthalpy = 1 st Law of Thermodynamics heat energy of a system is constant as long as no energy enters or leaves the system In all chemical reactions, there is a change in enthalpy = heat of reaction Heat of Formation When one mole of a compound is formed from its elements Dependent on: Temperature of reaction Pressure of the reaction Phase of the product Standard Heat of Formation - H f Heat of formation at 25 o C and 760 mmhg Phases of the reactant and the products must be stated H 2 (g) + 1/2O 2 (g) H 2 O(l) kj When one mole of hydrogen gas reacts with ½ mole of oxygen gas, liquid water is formed and heat is given off

6 Exothermic reaction Energy is regarded as a product H 2 O(l) kj H 2 (g) + ½ O 2 (g) Heat must be absorbed by the water in order for one mole of liquid water decomposes into its elements reaction Energy is regarded as a **Notice that the standard heat of formation is written so that one mole of product is formed Stability of Compounds Unstable compounds- tend to break down into simpler substances or elements Which of the following compounds is most stable? Compound Carbon Monoxide Nitric Oxide Standard Heat of Formation H f -110 kj/mol kj/mol Compounds that give off large amounts of heat during their formation are considered stable Compounds that give off low amounts of heat during their formation or require heat to be formed are unstable Require little or no net input of energy to cause them to decompose Explosives!!! Hess Law of Constant Heat Summation When a reaction can be expressed as the algebraic sum of two or more other reactions, then the heat of the reaction is the algebraic sum of the heats of these other reactions CuO(s) + H 2 (g) Cu(s) + H 2 O(g) H =?kj Express as the sum of a series of other reactions CuO(s) Cu(s) + ½ O 2 (g) H=155kJ H 2 (g) + ½ O 2 (g) H 2 O(g) H = -242kJ CuO(s) + H 2 (g) Cu(s) + H 2 O(g) H=-87kJ Heat of Formation = Spontaneous Chemical Reactions - 6 -

7 Consider: A waterfall Skiing Cycling The bond strength of the products is greater than the bond strength of the reactants heat is give off in the reactions Energetically favorable, should occur spontaneously But H 2 O(l) H 2 O(s) kj This reaction should be spontaneous, however, we know that this occurs only at 0 o C Entropy A system has a large entropy if its is in a great state of disorder S = S f -S i Both the final and the initial entropies will be positive numbers, but the change between final and initial can be negative + S = in disorder - S = in disorder Substances in solid phase have fixed particle = low entropy As a solid changes to a liquid, entropy increases- particles that make up the liquid have greater freedom of movement Particles making up a gas have a great deal of random motion- highest entropy of all the phases What changes in entropy occur when substances react? Formation of a compound- The bonding of elements to form a compound creates a more orderly state for the atoms involved. Compound decomposition- order and organization breaks down- What effect does temperature have on Entropy? - 7 -

8 Entropy and Spontaneous Change An increase in entropy (+ S) favors spontaneous reactions Nature prefers a change from a more orderly system to a less orderly system When a system is observed to have a special order, you can assume that there is some reason for it. Some restraint on the system must prevent it from assuming a more random arrangement. In Summary: Two factors that determine if a reaction will occur spontaneously are Gibbs Free Energy Equation H = T = S = G = Spontaneous reactions G *Notice temperature is a factor in this equation Pb(s) + ½ O 2 (g) PbO(s) PbO(s) H f = -215kJ S f = kj/k Calculate DG at room temperature But What about when T = 3000K? ½ N 2 (g) + ½ O 2 (g) NO(g) H f = + 90kJ H f = kJ/K At room temp, T = 298K, will this reaction be spontaneous? - 8 -