UNIT 9 REACTION RATES

Reaction Rates Some chemical reactions go fast, some slowly rapidly, some

reactions go to completion, some seem to get stuck halfway, and still others do not seem to occur at all. This unit is about the factors that control the speed and outcome of a reaction.

Reaction Rates Take a look at the following exothermic rxns. :

P4 (s) + 5 O2 (g) → P4O10 (s)

4 Fe (s) + 3 O2 (g) → 2 Fe2O3 (s)

2 H2 (g) + O2 (g) → 2 H2O (l)

Reaction 1: Ignites spontaneously; phosphorus kept under water

Reaction 2: Rust process is slow; may not be noticeable for years

Reaction 3: BOOM!!! But, hydrogen & oxygen may be stored indefinitely until sparked.

What determines how a rxn. starts? How fast it proceeds? It’s all about…kinetics.

To determine the speed or rate of a reaction, you would measure the increase in product concentration over time, or the decrease in reactant concentration over time:

rate = decrease in reactant concentration

time interval

or rate = increase in product concentration

time interval

What you choose to measure depends mostly on what is easiest to measure. For this reaction: CO (g) + NO2 (g) → CO2 (g) + NO (g)

colorless red-brown colorless colorless

Factors that affect reaction rates: Concentration If you add HCl to a solution of sodium thiosulfate,

the thiosulfate ion decomposes to produce fine particles of solid sulfur, which makes the solution cloudy:

S2O32– (aq) + 2 H1+ (aq) → S (s) H2SO3 (aq)

If the S2O32– solution is twice as concentrated, the

mixture becomes cloudy twice as fast. The result is not surprising, but can we explain it in molecular terms?

Factors that affect reaction rates: Concentration Anything that would increase the number of

collisions would be likely to increase the rate of the reaction. This is called, collision theory of reaction rates. From collision theory, you predict that increasing

concentration increases reaction rate. (more collisions)

Factors that affect reaction rates: Concentration Anything that brings the components of a reaction into better

contact is likely to increase the number of successful collisions, and thus the rate or speed of the reaction:

• increasing the number of reactant molecules in the mixture (increasing concentration)

• increasing the surface area of a solid reacting with a liquid or gas

• stirring to increase contact between a solid and a liquid

Increasing concentration will not always affect reaction rate. Example: CO (g) + NO2 (g) → CO2 (g) + NO (g)

○ Adding more CO does not change the rate○ But, add more NO2 makes it go faster

○ Why?

The overall speed of the reaction is determined by the slowest step, called the rate determining step. For the reaction of NO2 and CO, the mechanism that has been suggested is (1) NO2 + NO2 → NO3 + NO slow

(2) NO3 + CO → NO + CO2 fast

Factors that affect reaction rates: Temperature

Collisions alone are not enough to cause a chemical change.Most collisions are ineffective and don’t form

a new product.

“To be effective, a collision must involve a certain amount of energy.”

“To be effective, a collision must involve a certain amount of energy.”

Can the activation energy be lowered? You bet! This is how a catalyst affects a

reaction.

Catalyst The catalyst is involved in the reaction mechanism

but is not changed in the overall reaction.

Reaction can proceed at a lower temperature than would be required for the uncatalyzed reaction mechanism.

Real World Application of Catalyst Enzymes are proteins that act as

biological catalyst. Photosynthesis

Equilibrium Colorless CO gas reacts with red-brown NO2 gas to produce

colorless CO2 and NO gases:

CO (g) + NO2 (g) → CO2 (g) + NO (g)

colorless red-brown colorless colorless

1) rxn. proceeds, color fades, stops fading, rxn. is stuck

2) Rxn. is at equilibrium

When is a system at equilibrium? When its macroscopic properties –– temperature,

concentration, color, pressure, or any other measurable property of the system –– are constant.

Equilibrium can only be achieved in a closed system. We recognize that any process is at equilibrium when its

observable properties become constant, and we explain equilibrium as the balance of two continuous opposing processes.

LeChatlier’s Principle In 1888 a French chemist, Henri LeChatelier, summarized

observations of how changing conditions affect equilibrium in a rule known as LeChatelier’s Principle: If a system at equilibrium is changed in a way that upsets

the equilibrium, processes occur that minimize the disturbance and return the system to equilibrium.

Consider if you will…

2 NO2 (g) ↔ N2O4 (g)

But...what if you add NO2 gas?

1) NO2 is consumed and more N2O4 will be produced 2) Two rates come back into balance, equilibrium is

restored. 3) NO2 you added has been consumed and more N2O4

has formed○ We say that this reaction “shifted right” ; more product

produced

Same reaction…different scenario 2 NO2 (g) ↔ N2O4 (g)

But… what if you remove NO2

1) Forward rate slows down2) New equilibrium, the concentration of N2O4 is

lower than before, some of the NO2 you removed has been replaced○ We say this reaction “shifted left”; more reactant

What else can we do to affect equilibrium? Change the temperature Change the volume.

What would this look like for temp? When you think of temp, think energy. 2 NO2 (g) ↔ N2O4 (g) + energy (exothermic rxn)

○ Cooling rxn. removes energy- LeChatlier’s predicts rxn. will shift right to replace energy

○ Heating rxn.- shifts rxn to the left using some of the added energy.

What would this look like for volume?2 NO2 (g) ↔ N2O4 (g)

○ Increase volume (decrease pressure)- shifts rxn. toward more gas molecules to restore pressure

○ Decrease volume (increase pressure)- shifts rxn. toward fewer gas molecules to relieve pressure

Hey! What about a catalyst? Adding a catalyst does not affect the

equilibrium. Lowers activation energy, allows rxn. to achieve equilibrium

sooner

The Rule of Thumb (LeChatlier’s Principle predicts…) equilibrium shifts away from whatever you add equilibrium shifts toward whatever you remove

Law of Mass Action We can describe any state of dynamic equilibrium

quantitatively with an expression known as the law of mass action.

Imagine for a moment the hypothetical reaction aA + bB ↔ cC + dD