Reaction Rate Notes CP Chemistry Chapter 16

Reaction Rates  Speed at which chemical reactions occur can vary greatly  Space rocket = fast  Concrete hardening = slow  Fossil fuels created = very, very slow

Reaction Rate Defined  Definition: the change in concentration of a reactant or product per unit of time  OR: the amount of change in a reactant or product measured over time  Examples of rates:  Sprinter: meters/second  Car: miles/hour  Reaction rate: concentration/time (Molarity/second, moles/L/s)

Reaction Rate Equation time must be measured in seconds!

Reaction Rate Example Problem  Determine the rate of reaction if a 2 M concentration of NaCl increases to an 8 M concentration after 10 seconds of the reaction has passed.

Collision Theory  Theory = atoms, ions, and molecules MUST collide in order to react  NOTE: not all collisions of particles produce reactions – must meet three requirements  Requirements for a Reaction to Occur 1.Particles must COLLIDE 2.Particles must have CORRECT ORIENTATION 3.Particles must have SUFFICIENT ENERGY (ACTIVATION ENERGY)

Correct Orientation  Atoms, molecules, or ions without the proper orientation will NOT react  If proper orientation occurs – forms activated complex (transition state)  Active complex = temporary, unstable arrangement of atoms that may form reactants or products

In order for something to be produced, what must the molecules do? Crash Course What are the two criteria that must be met when particles collide? For example, how is water produced? What must collide? 1. 2.

Activation Energy  Definition: the minimum amount of energy required by the reacting particles in order to form the activated complex and lead to a reaction  Activation energy (E a ) determines if the activated complex will form reactants or products

Products or Reactants?  If enough energy is available to overcome activation energy, products will form  If NOT enough energy is available to overcome activation energy, reactants will form  Like trying to push a boulder over a hill – it takes energy to get to top, but at the top it rolls easily down other side

Mini Lab Activity – I’m Feeling Dizzy To see how important energy is to controlling a reaction, you are going to design an experiment using marbles and a petri dish. 1. You must find the relationship between temperature, the frequency of collision, and the energy needed to cause a change from reactants to products. Results:

What you have just modeled from the activity was called the activation energy, E a, of a reaction. What is activation energy (E a )? What can we compare activation energy to? What is an activated complex and where does it form?

Energy Diagrams  Exothermic Reactions  energy is released  products will have lower energy than reactants  Endothermic Reactions  energy is absorbed  reactants will have lower energy than products

What is the difference between the diagrams? Label them as endothermic and exothermic. How does E a affect a reaction?

Factors Affecting Reaction Rate  Five factors can influence how fast a reaction will occur: 1.Nature of the reactants 2.Concentration 3.Surface Area 4.Temperature 5.Catalysts (Inhibitors)

Nature of Reactants  Depending on their positions in the periodic table, some elements release their electrons easier than others  Example: Elements in families 1A & 2A are more reactive than those in 3A & 4A  Also, as move down a family, elements may react faster or slower – elements in families 1A & 2A react faster as you move down the family

Concentration  More particles  more collisions  more activated complexes  more product formed  Lower concentration = lower reaction rate  Higher concentration = higher reaction rate

Surface Area  Increasing the surface area of the reactants increases the sites available for collisions  more collisions  increased reaction rate  Less surface area = lower reaction rate  Higher surface area = higher reaction rate

Temperature  Increasing the temperature of the reactants causes them to move faster – have more kinetic energy  causes more collisions  have more energy – enough to overcome activation energy  Lower temperature = lower reaction rate  Higher temperature = higher reaction rate

Catalysts  Definition: a substance that increases the rate of a chemical reaction by lowering the activation energy but is not itself consumed in the reaction  NOT a product or a reactant

Inhibitors  Definition: type of catalyst that slows down the reaction rate of a chemical reaction or prevents a reaction from occurring

Factors That Affect Reaction Rates Name the five factors that affect the reaction rate: 1. What happens to group 1A elements when they are placed in water? 2. How does concentration affect the rate of a reaction? 3. What is surface area and how does it affect reaction rate? 4. Popping the Top activity – how does temperature affect the rate of reaction? 5. Mini Lab Activity – Jeanie in a Bottle – How does a catalyst affect the rate of a reaction? Factors That Affect Reaction Rates

This graph represents the energy of particles at two different temperatures. T 1 is at a lower temperature than T 2. A. What do you notice about the height of the hills? Which one is higher? B. Notice that curve T 2 is shifted relative to T 1. What happened to the number of particles as the temperature increased relative to the activation energy? C. If you wanted to increase the rate of reaction, how would you do it according to the chart? D. Based on what you wrote, why would your guess work?

What is the difference between a homogeneous catalyst and a heterogeneous catalyst? homogeneous catalyst Example heterogeneous catalyst Example Look at the chart below. Can you figure out how a catalyst works? Answer: What can we use to slow down the rate of reaction? ________________

Test Tube 1Test Tube 2Test Tube 3Test Tube 4 10 mL HCl 0 mL water Total = 10 ml 6 mL HCl 4 mL water Total = 10ml 3 mL HCl 7 mL water Total = 10ml 1 mL HCl 9 mL water Total = 10ml Before = after M 1 V 1 =M 2 V 2 3 (10) = x (10) X = 3 M Before = after M 1 V 1 =M 2 V 2 3 (6) = x (10) X= 1.8 M Before = after M 1 V 1 =M 2 V 2 X = ? Before = after M 1 V 1 =M 2 V 2 X= ? 1.Add water to test tubes Time how long Mg reacts and record data 3.Clean test tubes and return to front table