5.1 Reaction Rates

Chemical reactions involve the:

Conversion of reactants with a particular set of properties into products with a whole new set of properties

The investigation of the rate at which reactions occur and the factors that affect them

Chemical kinetics

For most reactions, the rate is ______ at the beginning of the reaction

Greatest

For most reaction, what happens to the rate as the reaction continues?

Decreases

Does the rate change as the reaction proceeds?

Yes

Reaction rates are generally expressed as:

Averages over a particular time period

Why are reaction rates generally expressed as averages over a particular time period?

Because the rate changes as the reaction proceeds

Formula for rate of A molecules:

-(change in [A]) / (change in t)

Formula for rate of B molecules:

(change in [B]) / (change in t)

Why do we have a negative sign in the formula for calculating the rate of A molecules?

Change in [A] is a negative number, but rate is always a positive value

Why is change in [A] a negative number?

Amount of A is decreasing

How to calculate average reaction rate?

Change in a measurable quantity of a chemical species over change in time

Thionyl chloride is a ______ compound

Reactive

Thionyl chloride is used in:

A variety of organic synthesis reactions

Is thionyl chloride controlled?

Yes

What is thionyl chloride controlled under?

Chemical Weapons Convention in the United States

Why is thionyl chloride controlled under the Chemical Weapons Convention in the United States?

It has potential to release dangerous gases explosively on contact with water

Can thionyl chloride be decomposed in solution?

Yes

How can thionyl chloride be decomposed in solution?

By using an organic solvent

Reaction for decomposition of thionyl chloride:

SO2Cl2(solvent) → SO2(g) + Cl2(g)

Can the rate of a compound be calculated relative to other compounds in the reaction?

Yes

How can the rate of a compound be calculated relative to other compounds in the reaction?

By using the stoichiometric equation for the reaction

In a reaction with equimolar relationships, the calculated reaction rates of all the compounds are:

The same

Are reaction rates absolute values?

No

Why are reaction rates not absolute values?

They depend on chemical chosen

The chemical chosen to calculate reaction rates is usually the one that is:

The easiest to measure

Can units vary depending on the measurements being made?

Yes

Fe(s) + 2 HCl(aq) → FeCl2 (aq) + H2 (g)

What are the units of measurements possible when measuring reaction rates in this reaction?

Change in [HCl] over time, change in pH over time, Change in volume of H2 over time, Change in pressure of H2 over time, change in mass of H2 over time

Fe(s) + 2 HCl(aq) → FeCl2 (aq) + H2 (g)

How to measure change in [HCl] over time?

By titrating it

Fe(s) + 2 HCl(aq) → FeCl2 (aq) + H2 (g)

Unit obtained when measuring change in [HCl] over time:

M/s

Fe(s) + 2 HCl(aq) → FeCl2 (aq) + H2 (g)

How to measure change in pH over time?

By using a pH meter

Fe(s) + 2 HCl(aq) → FeCl2 (aq) + H2 (g)

Unit obtained when measuring change in pH over time:

pH units/s

Fe(s) + 2 HCl(aq) → FeCl2 (aq) + H2 (g)

How to measure change in volume of H2 over time?

By using a eudiometer

Fe(s) + 2 HCl(aq) → FeCl2 (aq) + H2 (g)

Unit obtained when measuring change in volume of H2 over time:

mL/s

Fe(s) + 2 HCl(aq) → FeCl2 (aq) + H2 (g)

How to measure change in pressure of H2 over time?

By using a manometer

Fe(s) + 2 HCl(aq) → FeCl2 (aq) + H2 (g)

Unit obtained when measuring change in pressure of H2 over time:

kPa/s

Fe(s) + 2 HCl(aq) → FeCl2 (aq) + H2 (g)

How to measure change in mass of H2 over time?

By using a balance in an open system

Fe(s) + 2 HCl(aq) → FeCl2 (aq) + H2 (g)

Unit obtained when measuring change in mass of H2 over time:

g/s

Molecular bromine is ______ in color

Reddish-brown

Br2(aq) + HCOOH(aq) → 2Br-(aq) + 2H+(aq) + CO2(g)

What are the colors of the species in the reaction, other than molecular bromine?

Colorless

Br2(aq) + HCOOH(aq) → 2Br-(aq) + 2H+(aq) + CO2(g)

What happens to the concentration of Br2 as the reaction progresses?

Decreases

Br2(aq) + HCOOH(aq) → 2Br-(aq) + 2H+(aq) + CO2(g)

What happens to the color of Br2 as the reaction progresses?

Fades

Br2(aq) + HCOOH(aq) → 2Br-(aq) + 2H+(aq) + CO2(g)

The loss in color and concentration of Br2 as the reaction progresses can be monitored with a:

Spectrometer

A spectrometer registers the:

Amount of visible light absorbed by bromine

a A + b B → c C + d D

How can the average rate be calculated using each of the species?

-1/a x change in [A] over change in time

-1/b x change in [B] over change in time

1/c x change in [C] over change in time

1/d x change in [D] over change in time

Factors affecting reaction rate:

Increasing surface area of reactants, increasing concentration of reactants, increasing pressure of reaction, increasing temperature of reaction, using a catalyst in the reaction

Collision theory states that, for a reaction to occur, what must happen?

Particles must collide with the correct orientation and with sufficient energy

How do different factors affect the rate of reaction?

By affecting the frequency of particle collisions, and/or the proportion of collisions that have enough energy to react

How does increasing surface area change the frequency of collisions?

Increases

How does increasing surface area change the percent of successful collisions?

No change

Increasing surface area to increase reaction rate must be for what type of reactants?

Solid

How does increasing surface area of solid reactants increase frequency of particle collisions?

Number of particles exposed and available to react is increased

How does increasing concentration of reactants change frequency of collisions?

Increases

How does increasing concentration of reactants change percent of successful collisions?

No change

Increasing concentration of reactants to increase rate of reaction must only happen in:

Solutions

Why does increasing the concentration of reactants in solution increase rate of reaction?

There are a greater number of particles available to react

How does increasing pressure of reaction change frequency of collisions?

Increases

How does increasing pressure of reaction change percent of successful collisions?

No change

Increasing pressure of reaction to increase reaction rate must only happen in a reaction involving:

Gases

Why does increasing pressure of a reaction increase frequency of particle collisions?

Forces gas particles closer together

How does increasing temperature of reaction change frequency of collisions?

Increases

How does increasing temperature of reaction change percent of successful collisions?

Increases

Why does increasing temperature of reaction increase frequency of collisions and percent of successful collisions?

Kinetic energy of particles is increased

A catalyst provides an:

Alternate route for a reaction

The alternate route for a reaction provided by a catalyst has a:

Lower activation energy

Lower activation energy means that:

Particle collisions need less energy in order for a reaction to occur

Having a lower activation energy increases/decreases the rate of reaction

Increases