Reaction Rates and Activation Energy

Chemical Kinetics

Objectives

In this lesson you will

• Define chemical kinetics and collision theory.

• Identify ways to increase the rate of a chemical reaction.

• Use collision theory to explain how temperature, surface area, pressure, concentration and a catalyst can affect the rate of a reaction.

Chemical kinetics is the study of

• the rates at which chemical reactions occur and

• the factors that affect those rates.

Chemical kinetics

Multiple Choice

What is chemical kinetics?

• The study of how fast chemical reactions occur.

• The study of the kinds of chemical reactions.

• The study of the kinetic energy in an atom.

• The study of redox and acid base reactions.

Collision theory is a key concept in chemical kinetics that explains how chemical reactions occur at the molecular level.

Collision Theory

Collision theory states that for a reaction to be occur, the reacting molecules

• must collide with each other

• with sufficient energy (the activation energy) and

• in the proper orientation. 

​If all three occur, a collision will be successful (fruitful) and a chemical reaction will occur.

Drag and Drop

In order for a chemical reaction to occur, to particles must ​

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with sufficient ​

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and proper ​

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Drag these tiles and drop them in the correct blank above

collide

energy

orientation

temperature

splash

Factors Affecting Reaction Rate

The collision theory also helps to explain how factors such as temperature, pressure, concentration, and the presence of a catalyst can affect the rate of a chemical reaction by influencing the frequency and effectiveness of molecular collisions.

​On the following slides we will look at each of these factors.

Increasing the temperature of a reaction increases the rate of the reaction.

Remember that temperature is a measure in the kinetic energy of a substance. An increase in temperature results in an increase in the average kinetic energy of the particles in the reaction mixture. This leads to more energetic and frequent collisions between the reactant molecules.

Temperature

Increasing the temperature increases reaction rate because:

• Collisions happen more frequently.

• When they collide, more particles have sufficient energy to overcome the activation energy barrier.

Multiple Choice

Increase in temperature causes the particles to

• slow down

• move faster

• stay apart from each other

• move in the correct order

Multiple Choice

Why does a higher temperature increase the rate of a reaction?

• It increases both the frequency collision and energy of reactant particles

• It only increases the frequency of collision

• It only increases the energy of reactant particles

• It reduces the activation energy of the reaction

Multiple Choice

Increasing the temperature gives particles more __________.

• time

• energy

• space

• frequency

Surface Area

When a solid reactant is crushed into smaller particles, its total surface area increases. As a result, the increased surface area of the smaller particles allows more of the reactant to come into contact with the other reactants or the solvent (if present). This in turn increases the frequency of collisions between the reactant particles. As a result, the reaction occurs faster when the solid reactant is crushed into smaller particles.

Increasing Surface Area Increases Reaction Rate

• Decrease in particle size increases SA.

• More reactant can come into contact with other reactants.

​https://youtu.be/BWN8xVuzuFI

Multiple Choice

Grinding a seltzer tablet into powder increases the rate of reaction due to increased

• concentration

• surface area

• temperature

• reactants

Multiple Choice

Smaller particle size allows for a _________ surface area to be exposed for the reaction.

• larger

• smaller

Multiple Choice

Explosions can happen when there is a large amount of powdered substance because of a large

• temperature

• surface area

• concentration

• pressure

Volume

Remember that Boyles Law tells us that decreasing the volume increases the pressure of a gas. When we decrease the volume of a gaseous system the molecules are forced closer together and they collide with each other more frequently. This increase in pressure increases the frequency of collisions between the reacting molecules. Thus decreasing the volume of a gaseous system leads to an increase in the rate of the reaction.

Decrease Volume increases Rate:

• decrease volume increases pressure

• increases frequency of collisions

Multiple Choice

Increasing pressure causes the reacting particles

• to bond together

• to repel from each other

• gain more kinetic energy

• to move closer together

Multiple Choice

As the frequency of ________________ collision increases, the rate of reaction increases.

• time

• speed

• effective

• efficient

Multiple Choice

You make a space much smaller by increasing the ________________ hoping to increase the reaction rates.

• temperature

• concentration

• catalyst

• pressure

Concentration

In an aqueous system, increasing the concentration of a reactant increases the number of particles of that reactant in a given volume of the reaction mixture. Increasing the concentration of an aqueous reactant leads to a higher frequency of collisions between the reactant particles and the other reactants or products in the reaction mixture. As a result, the rate of the reaction increases.

Increased concentration

• increases particles in the space

• increases frequency of collisions.

Multiple Choice

Why does a higher concentration increase the rate of reaction?

• it increases the amount of reactants

• it lowers the activation energy

• it increases the energy of reactant particles

• it increases the frequency of collision

Multiple Choice

You add more bodies into a "mosh pit" to hope for more collisions. You have increased

• temperature

• concentration

• pressure

• catalyst

Multiple Choice

Why does a higher concentration increase the rate of reaction?

• it increases the amount of reactants

• it lowers the activation energy

• it increases the energy of particle collisions

• it increases the frequency of collisions

Catalyst

A catalyst increases the rate of a chemical reaction by lowering the activation energy needed for the reaction to occur. It can do this in two ways.

• May provide an alternative pathway with a lower activation energy than the original pathway, or

• A catalyst may physically orient the particles properly requiring less energy to react.

You may remember from biology that enzymes are important biological catalysts. However, catalysts also occur in daily life outside of living organisms. One example of such catalysts is the catalytic converter, which is used in the exhaust systems of vehicles to reduce harmful emissions.

Multiple Choice

Catalyst increase the rate of a reaction by

• increasing the amount of product obtained

• lowering the activation energy

• increasing the reaction temperature

• decreasing particle size.

Multiple Choice

What are enzymes

• Biological molecules which have large surface area

• Biological molecules which lower the activation energy of a reaction.

• Biological molecules which slow down reactions

• Biological molecules that have a lot of energy.

How to Speed up a Reaction

(and get a date)

Watch the video on the next slide which contains an analogy between rates of a reaction and getting a date. It will help you remember the factors affecting rates of reactions and why they increase the chance of a successful collision.

Match

Match the analogy descriptions from the video you just watched to its analogous factor which would increase the rate of a chemical reaction.

• Shrinking the hallways

• Adding more students

• Shortening the passing periods

• Students traveling alone by leaving the pack

• Hiring a matchmaker

decreasing the volume of the reaction container

adding more particles of reactants

increasing the temperature of the reaction

crushing the reactants into smaller pieces to increase surface area

adding a chemical catalyst

• decreasing the volume of the reaction container

• adding more particles of reactants

• increasing the temperature of the reaction

• crushing the reactants into smaller pieces to increase surface area

• adding a chemical catalyst