Reaction Rate Notes

CP Chemistry

Chapter 16

Reaction Rates

Speed at which chemical reactions occur can vary greatlySpace rocket = fastConcrete hardening = slowFossil fuels created = very, very slow

Reaction Rate DefinedDefinition: the change in concentration of a reactant or product per unit of timeOR: the amount of change in a reactant or

product measured over timeExamples of rates:Sprinter: meters/secondCar: miles/hourReaction rate: concentration/time

(Molarity/second, moles/L/s)

Reaction Rate Equation

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 TheoryTheory = atoms, ions, and molecules MUST collide

in order to reactNOTE: 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 OrientationAtoms, 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 (Ea) 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, Ea, of a reaction.What is activation energy (Ea)?       What can we compare activation energy to?     

 

What is an activated complex and where does it form?

 

Energy DiagramsExothermic Reactions

energy is releasedproducts will have lower

energy than reactants

Endothermic Reactionsenergy is absorbed reactants will have lower

energy than products

 

What is the difference between the diagrams? Label them as endothermic and exothermic.  How does Ea 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 rateHigher 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 energycauses more collisionshave more energy – enough to overcome

activation energyLower temperature = lower reaction rateHigher 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 reactionNOT a product or a reactant

InhibitorsDefinition: 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. T1 is at a lower temperature than T2. A. What do you notice about the height of the hills? Which one is higher? B. Notice that curve T2 is shifted relative to T1. 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 1 Test Tube 2 Test Tube 3 Test Tube 4

10 mL HCl0 mL waterTotal = 10 ml

6 mL HCl4 mL waterTotal = 10ml

3 mL HCl7 mL water Total = 10ml

1 mL HCl9 mL waterTotal = 10ml

Before = afterM1V1=M2V2

3 (10) = x (10)X = 3 M

Before = afterM1V1=M2V2

3 (6) = x (10)X= 1.8 M

Before = afterM1V1=M2V2

X = ?

Before = afterM1V1=M2V2

X= ?

1.Add water to test tubes 2-4

2.Time how long Mg reacts and record data

3.Clean test tubes and return to front table