FCAT SCA 1.4.4 Factors that Affect Rate of Reactions

Chemical reactions occur when atoms rearrange.

Methane (CH4) and oxygen (O2) (reactants) react to form carbon dioxide (CO2) and water (H2O) (products)

Notice how atoms have switched partners or rearranged.

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What is a Chemical Reaction?

Collision Model For reactions to occur particles must collide.

In order for collisions to be effective, the molecules must collide with sufficient energy called Activation Energy

Molecules must also collide with proper orientation

The rate of reaction is effected by the number of collisions of reactant molecules

Applet showing collision of particles with different energy (<EA

and > or = EA) and orientation.

Rate of Reaction Rate of reaction = Quantity of product

produced Time Required

Chemical reactions occur at different rates

Today we will consider some of the key factors that influence the rate of reaction

Collision Model Factors we will examine

1. Concentration2. Pressure3. Catalyst & pH4. Temperature5. Surface area (particle size)

These influence the reaction by: ◦ Increasing the number of collisions OR ◦ Increasing the fraction of collisions that will be

successful

Roles:1. Timer (one per class)2. Recorder (one per class)3. Molecule A both arms on hips4. Molecule B one arm on hip, other arm straight

on your side

5. Enzymes Materials

◦ Radio w/ music or CD◦ Rope◦ 4 chairs or stools

Concentration Concentration: the amount of a substance

(solute) present in a given volume of solution

“Packing” in more reactant molecules increases the number of collisions

Therefore a more concentrated solution = increased rate of reaction (makes more products)

A B

Pressure: reactant molecules are compressed in a smaller area.

Again reactant molecules are forced together

Therefore the greater the pressure = more collisions = increased reaction rate

Pressure

A B

What is a Catalyst ? Substance that enhances the rate of

chemical reaction. They decrease the amount of collision

energy needed to break bonds and form new ones making the fraction of successful collisions more effective, thus….

Catalysts lower the activation energy (collision energy)

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Effect of a Catalyst

The blue line represents a reaction without a catalyst.

The red line represents a reaction where a catalyst was added.

The activation energy is lower so the reaction will go faster.

A catalyst = enzyme = protein

►Catalysts ARE NOT being consumed itself but may be denatured by temperature or pH.

►Therefore presence of catalyst @ optimal temperature and pH= increased rate of reaction

Can catalyst be reused? Are they consumed/ used up once a reaction occurs?

Lets look at the decomposition of hydrogen peroxide (H2O2) into water & oxygen. H2O2(l) H2O(l) + O2(g)

Other Factors that influence Reaction

RatesTemperature &Surface Area

In groups of three obtain materials. Select trial group:

Ice water: to 20 mL of water add 7 to 8 ice cubes Warm water: heat about 20 mL of water in the microwave

Lab: Effects of Temperature & Surface Area on Reaction Rates

Room Temp Water Ice Water Warm Water

Trial Tablet Time (s)

Trial Tablet Time (s)

Trial Tablet Time (s)

1 Whole 3 Whole 5 Whole

2 powder 4 powder 6 powder

Temperature Increases energy of particles due to

increase of temperature Increase speed of particles and there are

more successful collisions because molecules to contact one another more readily and with harder force

Therefore increased temperature = increased rate of reaction

Surface Area If particles are in the same phase

(liquid/liquid or gas/gas) then it is easy for them to mix with each other

This gives particles maximum opportunity to collide

But if one of the reactants is a solid, the reaction can only take place on the SURFACE of the solid

The smaller the size of the particles, the greater the surface area that the reaction can take place in

Therefore, smaller surface area = increased rate of reaction

Concentration: more reactants means faster reaction rate

Pressure: more collisions means faster reaction rate

Catalyst: lower activation energy means faster reaction rate

pH Optimal pH means faster rate of reaction

Temperature: more collisions means faster reaction rate

Surface AreaGreater surface area means more collisions for a

faster reaction rate16

Factors that Affect Reaction Rate Summary

Multiple Choice (MC) FCAT QuestionCatalysts may reduce the amount of activation energy required for a chemical reaction to occur. Platinum (Pt) is a catalyst that is used in the catalytic converters in automobiles. In the graphs below, pathway x is a solid line representing the uncatalyzed reaction. The dotted line shows the catalyzed reaction. Which graph best illustrates the changes in a reaction when the catalyst reduces the amount of energy required?

Short-Response (SR) FCAT Released Question

REPORT ON THE 2006 FCAT SCIENCE RELEASED ITEMS

Student Responses SCORE: 1

Catalysts are not used up in a reaction!

To produce sulfuric acid (H2SO4), one industrial plant uses the “contact process,” which consists of several reactions. The initial reaction in this process uses sulfur dioxide (SO2 ) and oxygen (O2 ) in the presence of vanadium oxide (V2 O5) pellets to produce sulfur trioxide (SO3) as shown below. V2O5

Pellets 2S02 (g) + O2 (g) ---------------- 2SO3 (g) Sulfur Oxygen Sulfur Dioxide trioxide

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Short Response SC.A.1.4.4

Part A In the reaction between SO2 and O2, what is the role of the V2O5

pellets?

Part B Explain what the industrial chemist could do to increase the efficiency of the reaction.

The vanadium oxide pellets serve as a catalyst in the reaction between SO2 and O2.

To increase the reaction’s efficiency, the chemists could increase the temperature at which the reaction occurs.