1. Kinetics and equilibrium

2. Before we can consider equilibrium, we have to look at the kinetics of reactions.
Chemical kinetics is the study and application of methods that can be used to speed up or slow down chemical reactions.
A knowledge of chemical kinetics is important in the chemical industry. Modern pharmaceuticals, for example, would not be possible without a thorough understanding of the kinetics involved in the various reactions.
We study what happens during reactions, and we are not concerned about the why and the how.
In chemistry, we define reaction rates as a change in concentration over time.
This can be shown as: reaction rate = ∆ c / ∆ T
= delta c divided by delta t
where c is concentration and t is time and
delta (∆ ) is change.

3. An example: What is the overall rate of production of nitrogen dioxide in the system
2 Ns(g) + O2(g) → 2 NO2(g) if the concentration of nitrogen dioxide changes from 0.32mol/L to 0.80 mol/L in 3 min?
First calculate the change in concentration of nitrogen dioxide in 3 minutes.
∆C = ∆[no2]
= (0.80 mol/l – 0.32 mol/l)
= 0.48 mol/l
2. We then apply the rate equation (make note of the units)
rate no2 = [0.48 mol/l] ÷ 3 minutes
= [0.16 mol/l min (the dot means per)
3. The overall rate of production of no2(g) is expressed as [0.16 mol/l min

4. What is the average rate of production of ammonia for the system between 1.0 and 4.0 minutes?
n2(g) + 3 H2(g)  2 NH3(g) this means 1 mole of nitrogen gas reacts with 3 moles of hydrogen gas to produce 2 moles of ammonia gas.
Answer: if the concentration of ammonia is 3.5 mol/L after 1.0 min and 6.2 mol/L after 4.0 min?
Δc = Δ[NH3] = ( ) mol/L
Δc = 2.7 mol/L
Δt = (4.0 min. – 1.0 min.) = 3.0 min.
We use the rate equation Δr = Δc / Δt Δ[NH3] = 2.7 mol/l divided by 3.0 min.
Δ[NH3] = 0.90mol/l . Min.
ammonia is produced at a rate of 0.90 mol/l per minute.

5. Practise questions
In the reaction between solid phosphorus and oxygen gas to form p4010(s), what is the overall rate of reaction if the concentration of oxygen gas changes from mol/l to mol/l over a 40 second time frame?
answer: mol/(L•s)
One way to remove rust stains from ceramic bathroom fixtures is to apply a solution of oxalate ion:
c2o iron 3+  2 co iron 2+
answer: 0.12 mol(L•s)
what is the average rate of reaction for this system over the specified period if the concentration of the oxalate ion changes from 0.80 mol/l at 3.0 minutes to 0.20 mol/l at the 8 minute mark?
Use the reaction rate equation to solve these 2.

6. How do we measure reaction rates?
The techniques used depend on what is being measured.
For reactions involving gases, reaction rates can be determined by measuring the pressure of the system or by measuring the volume of gas collected. (volume and pressure are inversely related – the greater the pressure, the smaller the volume of gas.)
For reactions involving ions, for some reactions that occur in solution, there is a measurable change in conductivity.
For other reactions occurring in solution there is a change of color. This color shift can be measured by using a colorimeter.

7. Factors affecting reaction rates
There are 5 factors that affect the rate of chemical reactions
- chemical Nature of the reactants
- concentration of the reactants
- temperature
- the presence (or absence) of a catalyst
- surface area

8. Chemical nature of the reactants
Gold has been used for the manufacturing of jewellery because it is highly resistant to oxidation.
Sodium, on the other hand, is extremely reactive in the presence of oxygen and is never found in its elemental form in nature.
Both of these metals may undergo the same process of oxidation but the difference is time.
Elements found in the same group (like the alkali metals) all react in the same manner but they will have different reaction rates.
K is more reactive than sodium, with cesium being the most reactive (we don’t consider francium because it is highly radioactive and doesn’t stick around)

9. In homogenous chemical systems, like reactions in aqueous solutions, reactions involving monoatomic ions like k+ or F-, the reactions tend to be very quick.
Reactions involving molecules tend to be slower. As an example, the breakdown of complex carbohydrates within our bodies is a slow process because the large number of the covalent bonds from the carb. Have to be overcome.
The general rule of thumb is “the bigger the molecule, the slower the reaction.”

10. concentration
Concentrated hydrochloric and acetic acids are extremely dangerous and can cause very serious burns.
But our stomachs contain dilute hydrochloric acid and we put dilute acetic acid on fish and chips (as vinegar).
Both of these concentrated and dilute acids are capable of the same reactions but there will be a considerable difference in the time it takes.
The more concentrated an aqueous solution or gas, the more reactive they become.
Gases become more concentrated usually by increasing the pressure. Aqueous solutions become more concentrated by the addition of more moles..
You can say the same about the reactivity of certain solids in their elemental states (K or Cesium) but the majority of solids react very slowly or not at all.

11. temperature
A lot of chemical reactions are very sensitive to changes in temperature.
Poikilothermic animals like snakes have metabolic systems that are completely dependent on the ambient temperature of their surroundings.
This explains how a large snake only needs to eat every few days, or weeks, or months.
Another example of temperature sensitivity is the baking of a cake. You can mix everything together in a bowl, but it’s not going to turn into something edible by sitting on the counter. You need to raise the temperature by placing the batter into an oven.
It is a general rule of thumb that you can double the reaction rate of most systems by increasing the temperature ten degrees Celsius ( conversely, you can slow a reaction down by half by dropping the temperature ten degrees.)

12. Practise questions
You are the plant chemist for the acme fertilizer company. At present, the plant is capable of producing 3 metric tonnes per minute at 12ºC. What temperature would you have to raise the system to in order to produce 9 metric tonnes per minute?
answer: you want to triple production so you would raise the temperature by 30ºC.
You are producing 30l of red wine every minute at the acme winery. The production temperature is 25ºC. You only want to produce 15l of wine per minute while you work on the line. What is the new temperature you need to set?
answer: you would need to drop the temperature to 15ºc

13. catalysts
a catalyst is defined as a material that is added into a chemical system to speed up a process without being used up in the process.
An example of a catalyst (in this case an enzyme) is what happens when you chew a soda cracker. Initially the cracker is bland, but the more you chew the sweeter it gets.
This is due to the presence of amylase in your saliva. Amylase is catalyst that breaks long-chain sugars into smaller simple sugars that are readily digested.
Another example is the catalyst chlorophyll found in green plants. Chlorophyll speeds up the production of sugars from the sunlight.
Chemists discovered early on that certain powered metals – like platinum – added to reactions will speed those reactions up significantly.

14. Surface area
Increasing the surface area of reactants is another way to increase reaction rates.
Powered sugar is much easier to dissolve in coffee than sugar cubes and it is almost impossible to burn a block of wood with a match.
You stand a much better chance of starting a fire if you increase the surface area of the wood by turning it into kindling.
When a chemical system is heterogeneous (a solid in a liquid, for example) the reaction takes place at the interface between the 2 components.
Increasing the point of contact between the reactants by increasing the surface area of the solid will speed up the reaction.

15. Assignment (due Thursday)
Identify five different factors that are likely to affect the rate of a reaction. Give a practical example of each.
Which of the five factors that affect the rate of reactions applies only to heterogeneous systems? Give an example of such a system.
What would happen to the rate of a reaction if the temperature were raised from 20°C to 40°C? Explain qualitatively and make a quantitative prediction.
A match can be applied to a lump of coal with little effect. However, the ignition of coal dust has caused many fatal mining explosions. Explain why.
Signs warn about the dangers of having sparks or open flames near oxygen tanks or near flammable fuels. Which of the five factors that affect reaction rate are involved in each of these warnings?