Reaction Rate and

Equilibrium

Reaction Rate • Not all reactions occur at the same speed. • Some reactions are very slow while others are

fast.

Reaction Rate

• The reaction rate of a chemical reaction is a measurement of the increase in the concentration of a product or the decrease in the concentration of a reactant as the reaction proceeds over time.

N2 + 3H2 2NH3

• The units generally used to express reaction rate are mol/L . sec.

• What does it mean if the rate of the reaction above was given as 4.5 x 10-2 mol N2/L . sec?

C(s) + O2(g) → CO2(g)

• Occurs slowly at room T° and normal [O2].

• Can speed up the reaction by increasing the T° or increasing [O2].

• We can explain these changes in reaction rate using the “collision theory”.

Collision Theory

• All substances are comprised of millions of tiny particles in constant motion. These particles are colliding with each other constantly in any substance.

• All collisions between particles do not result in a reaction.

• There are two factors that determine whether or not a reaction will occur between two particles that are colliding.

Collision Theory• Substances most come into contact, (collide)

with enough energy.

Activation Energy has been supplied by the force of the collision.

Activation Energy

• The activation energy is the amount of energy that must be available in order for a reaction to occur.

• The sparks generated by striking steel against a flint provide the activation energy to initiate combustion in this Bunsen burner.

• The blue flame will sustain itself after the sparks are extinguished because the continued combustion of the flame is now providing the necessary energy through an exothermic reaction.

Activation Energy

Collision Theory• Substances most come into contact, (collide) in

the correct orientation (facing the correct way).

Collision Theory

• The collision theory states that reacting substances most come into contact, (collide) with enough activation energy, and in the correct orientation (facing the correct way), so that their electron shells can rearrange to form the products of the reaction.

• Therefore any factor which changes the frequency (how often), or energy of the collisions will change the rate of the reaction.

Five Factors AffectingReaction Rate

• nature of the reacting substances• concentration• surface area• temperature• catalysts

Nature of the reacting substances

• The type, strength, and number of chemical bonds or attractions between atoms differ from one substance to another.

• These differences determine the energy and orientation of the reacting particles that is necessary to create an effective collision resulting in a reaction.

TNT vs Gunpowder

• Explosive materials which react very violently are known as high explosives. In contrast, there are some materials that react more slowly. These are known as low explosives. They release a large amount of energy, but due to the relatively slow rate of reaction.

TNT vs Gunpowder

TNT C6H2(NO2)3CH3 is a high explosive.

Gunpowder is a mixture of potassium nitrate (KNO3),

sulphur (S8) and charcoal (C). It is a high explosive.

As the concentration of the reactants increases, the reaction rate increases.

Why?

Concentration• Concentration of the reactant refers to the

number of reactant particles within a given volume.

• If the concentration of the reactants increases there will be a greater number of collisions.

• The greater the number of total collisions, the greater the number of “effective” collisions (collisions that will form product) and the greater the rate.

Concentration

Concentration and Reaction Rate

Steel wool burning in air Steel wool burning in pure oxygen A (21% oxygen) (100% oxygen)

As the surface area of the reactants increases, the reaction rate increases.

Why?

Surface Area

• Increasing the surface area of the reactants results in a higher number of reaction sites. • Reaction sites - specific sites on molecules at

which reactions occur. • Increasing the number of reaction sites

increases the number of total collisions. • The greater the number of total collisions, the

greater the number of “effective” collisions (collisions that will form product) and the greater the rate.

Surface Area

Reaction Rate and Surface Area – Lycopodium PowderExit slideshow and play using Windows Media Player. There is

a delay if we try to show this file within the slide show.

As the temperature of the reactants increases, the reaction rate increases.

Why?

Temperature

• Increasing the temperature increases the kinetic energy of the particles.

• This results in more frequent collisions and more energetic collisions.

• Therefore not only are there more collisions but also a greater percentage of the collisions have the needed activation energy.

Both light sticks have been activated however the one on the left was placed in ice water and

the one on the right in boiling water.

Temperature and Reaction Rate

Catalysts increase the rate of reactions.

Why?

Catalysts

• Catalysts lower the activation energy by providing an alternate pathway by which the reaction can occur at a lower energy.

• This results in a greater percentage of the collisions having the necessary energy to be effective resulting in an increase in reaction rate.

• Catalysts are remain unchanged at the end of a reaction.

Catalysts lower the activation

energy

Catalysts lower the activation energy

Left: Partially caramelized cube sugar, Right: burning cube sugar with ash as catalyst

Enzymes• Enzymes act as catalysts that lower the activation

energy of a chemical reaction within a living organism.

• Enzymes carry out their function of lowering activation energy by temporarily combining with the chemicals involved in the reaction. These chemicals that the enzyme combines with are called the substrate.

• When the enzyme and substrate combine, the substrate is changed to a different chemical called the product. The enzyme is not consumed or altered by the reaction.

Enzymes

Enzymes• Enzymes are specific for their substrate: A

particular substrate molecule will combine temporarily with one enzyme type, and the active site of a particular enzyme will fit only one kind of substrate. For example, the enzyme sucrase will attach only to the substrate sucrose.

Enzymes

Catalysts – Beakman’s World ≈ 6:25

Homework

• Worksheet: Reaction Rate

Reversible Reactions

N2 + 3H2 → 2NH3

• Reactions can normally be reversed.2NH3 → N2 + 3H2

• Reversible reactions are often indicated by a double arrow (↔).

N2 + 3H2 ↔ 2NH3

• This shows the forward and reverse reaction.

For a reversible reactionsuch as

A + B ↔ C + D

there are actually two reactions

Forward reaction: A + B → C + D

Backward reaction: C + D → A + B

A + B ↔ C + DForward reaction: A + B → C + D A and B are used up C and D are formed

Backward reaction C + D → A + B C and D are used up A and B are formed

At beginning of the reaction

A + B ↔ C + D

• There is only A and B in the reaction container (no C and D formed yet)

• Forward reaction is very fast • No backward reaction occurs yet

A little later

A + B ↔ C + D • Forward reaction is still fast (since the container still

has mainly A and B)• Some C and D have been formed

• The backward reaction starts• Backward reaction is very slow (since there is only

a small amount of C and D)

Still some time later

A + B ↔ C + D • More A and B have been used up• The forward reaction slows down

• More C and D have been formed• The backward reaction speed up

Eventually…A + B ↔ C + D

• the point is reached where the speed (rate) of the two reactions become equal.

The system is then said to be in

EQUILIBRIUM

Chemical EquilibriumAt equilibrium • The rate of the

forward reaction becomes equal to the rate of the reverse reaction.

• How could this graph be adjusted and still show equilibrium?

Chemical EquilibriumAt equilibrium • The forward and

reverse reactions continue at equal rates in both directions.

• For this reason we often refer to a “dynamic equilibrium”

Dynamic Equilibrium

Dynamic Equilibrium

• It often appears that a reaction at equilibrium has “stopped”. This however is only somewhat true.

• What would happen if the person stopped running?

Chemical Equilibrium

When equilibrium isReached:• There is no further

change in the amounts (concentrations) of reactant and product.

• Concentrations at equilibrium are constant (not equal).

Chemical Equilibrium

N2 + 3H2 ↔ 2NH3

• A chemical reaction is at equilibrium when the forward and reverse reactions are occurring at the same rate.

• A reaction that has reached equilibrium is assigned an equilibrium constant (Keq or just K).

The equilibrium constant expression

N2 + 3H2 ↔ 2NH3

• All we need to write an equilibrium expression is a balanced equation.

3

22

23

eq][H ][N

][NH =

][reactants

[products] = K

Write the equilibrium expression for:4HCl+ O2↔2Cl2+ 2H2O

•If we know the concentrations (molarities) we can calculate a numerical value for K.

Given [CO] = 0.200, [H2O] = 0.500, [H2] = 0.32 and [CO2] = 0.42

Find K for:CO + H2O ↔ H2 + CO2

Given [H2S] = 0.706, [H2] = 0.222 and [S2] = 0.111

Find K for:2H2S ↔ 2H2 + S2

K = 0.0110

Given K = 0.0875 and [N2O4] = 0.0172M. Find [NO2] for:N2O4 ↔ 2NO2

Given K = 0.0140 and [H2] and [I2] are each 2.00 x 10-4M.

Find [HI] for: 2HI ↔ H2 + I2

[HI] = 0.00169 M

LeChâtelier’s Principle

• states that when a stress is applied to a system at equilibrium, the system will respond (shift) in a manner that attempts to undo the stress.

Stresses are…

• Change in concentration ([ ]) (adding or removing substances)• Change in temperature (heating or cooling the system)• Change in pressure (increasing or decreasing pressure)

CONCENTRATION CHANGEncrease concentration of a reactantadd more nitrogen)

N2 + 3H2 ↔ 2NH3 + heat

Equilibrium shiftsto the right (FORWARD reaction is favored because it will

use up the nitrogen)

CONCENTRATION CHANGEncrease concentration of a productadd more ammonia)

N2 + 3H2 ↔ 2NH3 + heat

Equilibrium shiftsto the left (REVERSE reaction is favored because it will

use up the ammonia)

CONCENTRATION CHANGEDecrease concentration of a reactantremove some nitrogen)

N2 + 3H2 ↔ 2NH3 + heat

Equilibrium shiftsto the left (REVERSE reaction is favored because it will

replace the nitrogen)

CONCENTRATION CHANGEDecrease concentration of a productremove the ammonia)

N2 + 3H2 ↔ 2NH3 + heat

Equilibrium shiftsto the right (FORWARD reaction is favored because it will

replace the ammonia)

TEMPERATURE CHANGEIncrease the temperature.heat is added)

N2 + 3H2 ↔ 2NH3 + heat

Equilibrium shiftsto the left (REVERSE reaction is favored because it will

use up the added heat)

TEMPERATURE CHANGEDecrease the temperature.heat is removed)

N2 + 3H2 ↔ 2NH3 + heat

Equilibrium shiftsto the right (FORWARD reaction is favored because it will

replace the heat that was removed)

Pressure Change• Pressure can change by adjusting the volume.

Pressure Changes

• The side of the reaction with the greater number of moles of gas will create higher pressure.

• The side of the reaction with the lesser number of moles of gas will create lower pressure.

PRESSURE CHANGEIncrease the pressure.volume of the container is decreased)

N2(g) + 3H2(g) ↔ 2NH3(g) + heat

Equilibrium shiftsto the right (FORWARD reaction is favored because it will change

4 moles of gas into 2 moles of gas therefore returning to a lower pressure)

PRESSURE CHANGEDecrease the pressure.volume of the container is increased)

N2(g) + 3H2(g) ↔ 2NH3(g) + heat

Equilibrium shiftsto the left (REVERSE reaction is favored because it will change 2

moles of gas into 4 moles of gas therefore returning to a higher pressure)

Le Chatelier’s Principle2 NO2(g) N2O4(g) Ho = -57.20 kJ

Disturbance Equilibrium Shift Add more NO2………………

Add more N2O4…………….

Add a catalyst…………….. Remove NO2………………

Decrease pressure………… no shift

Decrease temperature….…

Homework

• Worksheet: Equilibrium