Ch.16: Reaction Rates Sec. 16.1: A Model for Reaction Rates

Ch.16: Reaction Rates

Sec. 16.1: A Model for Reaction Rates

Objectives

• Calculate average rates of chemical reactions from experimental data.

• Relate reaction rates to collisions between reacting particles.

Reaction Rate

• Reactions take place at different rates or speeds. Some products form quickly, some form slowly.

• Reaction rate is defined as the amount of change in the concentration of a reactant or product per unit of time. How fast a reactant disappears or how fast a product appears is what is measured.

• Reaction rates are very important in many industries - the faster the rate, the more product that can be made in a given amount of time.

Expressing Reaction Rates

When reaction rates are determined experimentally, usually the changing amount of a reactant is measured.

Since the amount of a reactant would normally decrease, a negative sign is used so that the reaction rate is a positive number.

Expressing Reaction Rates (cont.)

SECTION16.1

Reaction Rate• For any reaction to take place, reactant

particles have to collide in order to react. This is called Collision Theory.

• The reacting particles have to collide with a certain amount of force for a reaction to start.

• The activation energy is the amount of energy reactant particles must have to form the activated complex and lead to a reaction.

An activated complex is a temporary, unstable arrangement of atoms in which old bonds are breaking and new bonds are forming.

SECTION16.1

Collision Theory (cont.)

If a reaction has a high activation energy, only a small number of collisions will have the required energy to form the activated complex. The reaction rate, therefore, will be slow.

SECTION16.1

Reaction Rate

• In an exothermic reaction, the energy of the reactants is greater than the products because energy is released. The reaction STILL requires some input of energy to start it.

Reaction Rate

In endothermic reactions, the reactants have less

energy than the products because the reaction

absorbs energy. More energy is required than is

stored in the products because some of the energy

is used as activation energy.

SECTION16.1

16.2: Factors Affecting Reaction Rates

Objectives• Identify factors that affect the rates of

chemical reactions.• Explain the role of a catalyst.

Reaction Rate

Five factors affect reaction rate:1. Nature of Reactants

2. Concentration of reactants

3. Surface Area of reactants

4. Temperature of system

5. Presence of catalyst/inhibitor

Some substances react more readily than others. This is simply due to their structure.

Sodium, for example, is more reactive than magnesium because it has to only give up 1 electron while magnesium must lose 2.

1. The Nature of Reactants or “reactivity”

Factors That Influence Reaction Rate


2- Concentration: Increasing the concentration of a reactant will speed up a reaction. This is because, with more reactant particles, more collisions between them are likely. (Concentration is defined as the amount of a substance present in a given volume.)

What will decreasing concentration do?


•Keep in mind that, if there is more than one reactant in a reaction, it will not matter what the concentrations of the other reactants are if one of the reactants is used up.•If I want to make s’mores, for example, it will not matter how many marshmallows I have if I only have one piece of chocolate. Chocolate is the limiting reactant.


3 – Surface Area

Increasing the surface area of the reactants will increase the rate of a reaction. This is because increasing the surface area of a reactant would allow more collisions to occur.

Which will dissolve faster – a sugar cube or sugar granules?


4- Temperature: If the temperature is increased, most reactions go faster. This is because increasing the temperature will increase the speed & force of collisions between reactant particles.

What happens if the temperature is decreased?

SECTION16.2

High-energy collisions are more frequent at a higher temperature.


5- Catalysts: Adding or removing a catalyst will change a reaction rate. A catalyst is a substance that speeds up the rate of a reaction without being permanently changed or used up itself.

• Biological catalysts are called enzymes.

Catalysts

Catalysts lower activation energy.

A lower activation energy means more of the collisions between particles will have enough energy to bring about a reaction.

A heterogeneous catalyst exists in a physical state different than that of the reaction it catalyzes.

A homogeneous catalyst exists in the same physical state as the reaction it catalyzes.

SECTION16.2

Catalysts

Inhibitors

•An inhibitor is a substance that slows down or prevents a reaction. •These substances are used to slow down spoilage of food and in medications to increase their shelf life. Many medicines will decompose under normal circumstances, limiting their effectiveness.

Chapter 17: Chemical Equilibrium

Sec. 17.1: A State of Dynamic Balance

Objectives

• List the characteristics of chemical equilibrium

• Write equations for systems at equilibrium• Calculate equilibrium constants from

concentration data.

Reversible Reactions

• Some reactions “go to completion” because one of the reactants is completely used up (called the limiting reactant) and the reaction stops.

• However, many reactions can change direction. They are called reversible reactions.


• When a reaction takes place, reactants combine to form products. If the products formed begin to react with each other to reform the reactants, the reaction is reversible. The reactants are never used up!

• To indicate that the reaction is reversible, we use a double arrow in the chemical equation: A + B C + D


• In reversible reactions, two separate reactions are actually taking place.– A forward reaction: A + B C + D – A reverse reaction: A + B C + D

or C + D A + B


• At first, the forward reaction occurs (reactants produce products) but no the reverse reaction occurs. This is because the amount of products is not high enough to start a reverse reaction.

• Soon, though, when the amount of products is high enough, the products will react to reform the reactants.

Equilibrium

• Eventually, the amount of products formed no longer changes. The amount of each product remains constant.

• The amount of reactants re-formed also no longer changes. The amount of each reactant remains constant.

• The “system” has reached equilibrium.

Equilibrium

• If you were to graph data concerning the changes in concentration of reactants and products over time, a graph like this would result. Note that eventually, the concentrations of both level off and remain constant. This is when equilibrium is reached.

Reactants

Products

Equilibrium

Forward: N2(g) + 3H2(g) 2NH3(g)Reverse: N2(g) + 3H2(g) 2NH3(g)

Equilibrium

• A system is in equilibrium when no net change occurs in the amounts of reactants and products.

• The forward and reverse reactions are still occurring, but reactants and products are forming at the same rate. It is often called a dynamic equilibrium because reactions are still taking place.

• CaCO3 CaO + CO2

• PCl5 PCl3 + Cl2

• Each reaction has reached equilibrium - the amounts of reactants and products don’t change.

• This does not mean the actual amounts of reactants and products are equal. It means the RATES at which they form are equal.



• PCl5 PCl3 + Cl2

• Actual measurements in the lab show that in this reaction, at equilibrium, there is more PCl5 than PCl3 and Cl2.

• Even in reversible reactions at equilibrium, stability is favored and PCl5 is more stable. There is a balance that favors stability.

Equilibrium Expressions and Constants

• A majority of reactions reach a state of equilibrium.

• By definition, in these reactions, not all reactants are consumed and not all of the product predicted by the equation will be produced.

• How then can chemists predict the yield of a reaction?


• The law of chemical equilibrium states that at a given temperature, a system will reach a state in which a ratio of reactant and product concentrations will have a constant value.

• For the reaction

aA + bB cC + dD,

Keq = [C]c[D]d

[A]a[B]b


• The ratio is called the equilibrium constant expression.

• Brackets indicate concentrations in mol/L.• Keq is called the equilibrium constant. It is

constant at a specified temperature. The Keq will always be the same, regardless of initial concentrations of reactants and products, at a given temperature. See Table 1, p. 604.


• To interpret the meaning of Keq values, recall that fractions with large numerators are larger than fractions with large denominators.

• Therefore Keq > 1 means the equilibrium mixture contains more products than reactants.

• A Keq < 1 means the equilibrium mixture contains more reactants than products.

Homogeneous equilibria

• In a homogeneous equilibrium, all the reactants and products are in the same physical state.

• For H2 (g) + I2 (g) 2HI (g),

Keq = [HI]2

[H2][I2]• At 731 K, Keq = 49.7 (Note: there are no

units). This equilibrium favors . . .?

Heterogeneous equilibria

• When the reactants and products are present in more than one physical state, it is a heterogeneous equilibrium.

• Liquids and solids are pure substances whose density (concentration) at a given temperature does not change.

• Therefore, their concentration is constant and is combined with the K value.

Heterogeneous equilibria

Examples:• I2 (s) I2 (g)

Keq = [I2(g)]

• C2H5OH (l) C2H5OH (g) Keq = [C2H5OH (g)]

Practice Problems

• Calculate Keq for N2 (g) + 3H2 (g) 2NH3 (g) if [NH3] = 0.933 mol/L, [N2] = 0.533 mol/L, and [H2] = 1.600 mol/L.

• Calculate Keq for N2O4 (g) 2NO2 (g) if [N2O4] = 0.0185 mol/L and [NO2] = 0.0627 mol/L.

17.2: Factors Affecting Chemical Equilibrium

Objectives• Describe how various factors affect

chemical equilibrium.• Explain how Le Châtelier’s Principle

applies to equilibrium systems.

Le Chatelier’s Principle

• Once an equilibrium was established, Le Chatelier proposed that it was possible to disturb the equilibrium.

• He believed that a disruption to the equilibrium would make the system readjust in a way that would reduce the disturbance and regain equilibrium.

• In other words, a system under “stress” will act so as to relieve that stress.

• A stress is any change that upsets an equilibrium.

An Analogy: Le Chatelier’s Principle

Before drinking, the water level in the bowl is at equilibrium. Once

the dog drinks, the water level is “stressed”. As a result, water will come from the bottle into the

bowl to reestablish equilibrium.

Le Chatelier’s Principle

• Industrial chemists use this principle to increase the amount of product that will form in a reaction.

• There are 3 ways to stress a chemical reaction. – To change the concentration of a reactant or

product– To add or remove energy (by changing

temperature)– To change the pressure (or volume) of gases that

are in a reaction

Changing the concentration of a reactant or product

1A- Adding more of a reactant will cause the forward reaction to speed up to get rid of the “excess” reactant. We say the reaction “shifts” to the right.

3C + 2Al2O3 4Al + 3CO2

As a result, the concentration of products increases and the concentration of other reactants decreases.


1B- Removing a reactant will cause the reverse reaction to speed up to replace the “lost” reactant. We say the reaction “shifts” to the left.

3C + 2Al2O3 4Al + 3CO2

As a result, the concentration of products decreases and the concentration of other reactants increases.


1C- Adding more of a product will cause the reverse reaction to speed up to get rid of the “excess” product. We say the reaction “shifts” to the left.

3C + 2Al2O3 4Al + 3CO2

As a result, the concentration of reactants increases and the concentration of other products decreases.


1D- Removing a product will cause the forward reaction to speed up to replace the “lost” product. We say the reaction “shifts” to the right.

3C + 2Al2O3 4Al + 3CO2

As a result, the concentration of reactants decreases and the concentration of other products increases.


Note:• Products that are gases can be easily

removed from the reaction.• A product that is insoluble (does not

dissolve in a liquid) is also easily removed. It will form a precipitate and will remove itself from the reaction.

Adding or Removing Energy

• Energy is part of every reaction. It can be thought of as a reactant or product.– Endothermic reactions require energy so energy

is a reactant.

A + B + energy C + D– Exothermic reactions release energy so energy

is a product.

A + B C + D + energy


3C + 2Al2O3 + energy 4Al + 3CO2

If energy is added to an endothermic reaction (by increasing the temperature), the reaction shifts to the right.

If energy is removed from the reaction (by decreasing the temperature), the reaction shifts to the left.


N2 + 3H2 2NH3 + energy

If energy is added to an exothermic reaction (by increasing the temperature), the reaction shifts to the left.

If energy is removed from the reaction (by decreasing the temperature), the reaction shifts to right.

Changes in volume or pressure

• If you decrease the space a gas occupies (decrease its volume), the pressure of the gas increases.

• If you increase the space a gas occupies (increase its volume), the pressure of the gas decreases.

• A change in volume or pressure is a stress on a reaction at equilibrium.


• At a given volume, the number of particles in a sample of gas also affects the pressure.If the number of particles increases, the

pressure increases.If the number of particles decreases, the

pressure decreases.


CO (g) + 3H2 (g) CH4 (g) + H2O (g)• In this reaction, in the reaction chamber, there will

be 4 molecules of reactants for every 2 molecules of products.

• If the volume on the reaction chamber is decreased, the pressure in the reaction chamber increases and the equilibrium is stressed. To relieve the stress of increased pressure, a lower number of particles is favored, so the equilibrium shifts to the right.

• If the volume is increased???


• Changes in volume or pressure of a system at equilibrium will only shift the equilibrium if – there are gaseous reactants or products in the

reaction, AND– the number of particles of gaseous reactant is

different from the number of particles of gaseous product.

Practice Problems

2CO + O2 2CO2

3H2 + N2 2NH3Assume all reactants & products are gases.

1. How would decreasing the volume affect each equilibrium?

2. How would decreasing the pressure affect each equilibrium?

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Ch.16: Reaction Rates Sec. 16.1: A Model for Reaction Rates