Reaction Rates and Equilibrium Chapter 17

17.1 Expressing Reaction Rates rates are expressed as a change in quantity (concentration) over a change in time –the rate of a chemical reaction is defined as the change in concentration of a reactant or product per unit time, expressed as mol/(Ls)

Expressing Reaction Rates Reaction rates must always be positive! –If you’re measuring the disappearance of a reactant then you use a negative sign to show that the concentration is decreasing.

The Collision Theory The collision theory states that atoms, ions, and molecules must collide in order to react. –Only a small fraction of collisions product reactions! How could this be? –Particles must have the right orientation and the right energy for a reaction to happen!

Collision Theory When the orientation of two colliding molecules is correct, a reaction occurs in which an intermediate substance is formed. –activated complex or transition state It is subsequently consumed and does not show up in the overall reaction.

Collision Theory Particles must have a minimum amount of energy to form the activated complex and to lead to a reaction. –A high activation energy leads to a low reaction rate. –A low activation energy means that more particles will have enough energy to react, so reaction rate is high (fast).

Summary of Collision Theory Reacting substances (atoms, ions, or molecules) must collide. Reacting substances must collide with the correct orientation. Reacting substances must collide with sufficient energy to form the activated complex.

17.2 Factors affecting rates Nature of Reactants - reactivity Temperature-Kinetic energy is affected Concentration-More frequent collisions Surface Area – size of particles Particle size- Greater surface area Catalysts- Lower the activation energy

Catalysts and Inhibitors Inhibitors slow down the reaction rate or prevent it from happening at all! Heterogeneous catalysts are in a different physical state than the reactions they catalyze. Homogeneous catalysts are in the same state.

17.3 Reaction Rate Laws The equation that expresses the relationship between the rate of a reaction and the concentration of reactants is called a rate law. –For the reaction A  B the rate law is rate = k[A] where k is the specific rate constant

Rate Law Constant Units for the rate constant may be: –L/(mol s), L 2 /(mol 2 s), or s -1 –k is unique for every reaction and depends on temperature! –k does not change with concentration

Reaction Order The reaction order for a reactant defines how the rate is affected by the concentration of that reactant. –First order: doubling the reactant concentration doubles the rate –Second order: doubling the reactant concentration quadruples the rate

Reaction Order For the reaction: aA + bB  cC –The general rate law is: rate = k[A] m [B] n rate = k[A] m [B] n where m and n are the orders for A and B where m and n are the orders for A and B –For example: 2NO + 2H 2  N 2 + 2H 2 O has the rate law: rate = k[NO] 2 [H 2 ] Which reactant is first order? Second order? What is the overall reaction order?

Determining Reaction Order Look at Trials 1 & 2: What happens when [A] is doubled? Look at Trials 2 & 3: What happens when [B] is doubled? What would be the rate law for this reaction?

Reversible Reactions These are reactions that can run forward and backward at the same time. A limited amount of materials will determine the rate A balance will occur between the two reactions at equilibrium. Won’t always be 50% each direction

Factors affecting Equilibrium ConcentrationTemperaturePressure It will rebalance according to Le Chatelier’s principle

Free Energy Spontaneous reactions will occur without help and release energy Nonspontaneous don’t occur readily or favor products Entropy- amount of disorder, an increase in entropy will drive a reaction (Law of disorder)

Reactions and possibilities Heat, Free Energy and Entropy all combine to determine if a reaction will occur Exothermic-Entropy Increase (Yes) Endothermic-Entropy decrease (No) Other combinations are a maybe

Entropy Calculations ΔS˚ = S˚(Products)- S˚(Reactants) S˚= standard entropy, page 558

Free Energy Calculations ΔG = ΔH – TΔS Δ G˚ = Δ G ˚f (Products)- Δ G˚ f (Reactants)