KINETICS - HOW FAR - HOW FAST
Rate of reactions:
- Chemical reactions occur at a definite rate (speed) determined by the reaction conditions.
- Some reactions occur very fast. Explosions are a good example of very high rates.
- Some other reactions occur at a very slow rate. Rusting normally takes a long time.
- The rate of reaction is directly affected by the amount of collisions in between the reactants, so studying how different conditions affect the collisions, we can infer how the rate can be changed.
Rates can be changed by:
- Increasing temperature:
- Increasing the temperature increases the rate of collisions between reactants, since the particles have more kinetic energy, speeding up the reaction.
- Increasing concentration of reactants
- Increasing the concentration of the reactants increase the possibility of collisions between reactants since they are closer, so, if there is more concentration, the rate also increases.
- Increasing surface area
- If we try to burn a log or a bunch wood splints, we know that it is easier, to burn smaller the particles. By chopping the wood in smaller pieces, increases surface area (or in other words, the surface of contact between the reactants increase) and the rate of the reaction increase.of wooks.
To follow the rate of a reaction, one must either measure the decrease in concentration of a reactant or the increase in concentration of a product with time.
- Describe a practical method for investigating the rate (speed) of a reaction involving gas evolution
Some techniques for doing this are:
- Measure the volumes of gases evolved (gas syringe).
- Volumetric analysis – samples are removed at regular intervals, the reaction stopped by cooling, and mixture analysed by titration.
- Measuring changes in pressure (for gas reactions)
- Colorimetry may be used if one of the constituents is coloured. The colorimeter follows the change in intensity of colour.
- A conductivity meter may be used if there is a change in conductivity during the reaction i.e. if the number of ions present is changing. A pH meter is a special type of meter which will follow changes in H+.
Consider the reaction:
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
This may be followed by measuring the volume of H2 collected in the graduated cylinder at intervals of time:
The rate of reaction at a particular time is given by the gradient of the tangent.
At time t: rate = change of volume / time or a / b
Rates are usually expressed by the calculus notation.
The units are cm-3 s-1 or mol-1 s-1
The rate of a reaction is fastest at t = 0 and decreases steadily as the reactants are used up. Hence, the gradient of the graph decreases with time.
When examining the effect of changing conditions (e.g. concentration) on the rate of reaction, we usually take the initial gradient, at t = 0 as a measure of rate. We can then compare the initial rates of reaction under the different conditions and determine any effect.
- Describe the application of the above factors to the danger of explosive combustion with fine powders (e.g. flour mills) and gases (e.g. mines) Note: Candidates should be encouraged to use the term rate rather than speed.
When working in mines where there is a lot of powder (coal for example), or flower mills, gases mix with oxygen and they can explode since the particles size are very small, so the rate of reaction increase very much.
- Devise a suitable method for investigating the effect of a given variable on the rate (speed) of a reaction
Following the progress of a reaction:
- We can follow the change in mass by putting some chips in HCl solution and record how the mass changes with the time. (We need a digital balance). Why is a cotton wool used to cover the opening of the flask?
Observing the graph we can infer that the mass is lost faster at the beginning of the reaction. As the reactants are used up, the reaction slows down and we have no mass loss after a while. Why the mass is not going down to zero?
- We can follow the amount of gas delivered in a reaction by placing HCl + Mg, to measure the amount of H2 delivered. In this case, we have two curves, one that corresponds to the cold acid, (blue) and the other curve that corresponds to the hot acid, (red)
- We can follow the amount of solute produced by precipitation in the reaction of sodium thiosulfate with HCl by measuring the time it takes to cover the mark underneath the flask.
- Interpret data obtained from experiments concerned with rate (speed) of reaction
Since the speed of the reaction changes, so does the slope of the curve , at the beginning we have a fast reaction but as the reactants are used up, the rate decreases and so the slope of the curve. In the case of a PRODUCTION of gas, we will have a curve like the one below.
If the measurement is for example the mass DECREASE, like the example of the marble chips with hydrochloric acid, we will have the curve inverted, since the mass amount decrease with the progress of the reaction.
- Describe and explain the effects of temperature and concentration in terms of collisions between reacting particles
As the temperature increases, the particles move faster and the amount of collisions also increase, which increase the rate of reaction.
- The reactant particles move quicker
- They have more energy
- The particles collide more often, and more of the collisions are successful
- The rate of reaction increases
In the example below you can see that the increase in the temperature, increases the amount of gas delivered for a given reaction.
Compared to a reaction at a low temperature, the graph line for the same reaction but at a higher temperature:
- Has a steeper slope at the beginning
- Becomes horizontal sooner, showing that the reaction time is less
Increasing the concentration also increases the rate of reaction since there are more chances of collision.
The rate of a chemical reaction can be changed by altering the concentration of a reactant in solution, or the pressure of a gaseous reactant.
If the concentration or pressure is increased:
- The particles in the reactant are closer.
- There is a bigger chance of collisions.
- The rate of reaction increases
When we compare concentrations we normally talk about liquids. When increasing the pressure, we only refer to gases, since the pressure change does not affect the collisions when the reactans are in liquid state. Anyway, increasing the concentration of a liquid or the pressure in a gas reaction, we will notice the same consequences:
- The graph has a steeper slope at the start
- Becomes horizontal sooner, showing that the reaction finishes faster
- Describe the role of light in photochemical reactions and the effect of light on the rate (speed) of these reactions
All reactions require a minimum amount of energy to start. Many times we achieve this amount of energy through heat, we call Photochemical reactions those that light provides that “activation energy”
Photosynthesis is one of those examples. If sunlight is not present, photosynthesis does not take place.
Another reaction or set of reactions that take place in light and not in darkness are the ones involving free radicals. Free radicals are groups of atoms which have an extra UNPAIRED electron. This electron is very reactive and can produce chain reactions as the ones that produced the ozone depletion in the ’90s.
- Describe the use of silver salts in photography as a process of reduction of silver ions to silver; and photosynthesis as the reaction between carbon dioxide and water in the presence of chlorophyll and sunlight (energy) to produce glucose and oxygen
Silver salts decompose in light to produce a dark precipitate of silver metal. the reactions taking place are decomposition reactions where the silver halides decompose into Silver metal and the Halogen. Since there is a change in the oxidation state of the silver (and the halogen), this is considered a redox reaction
2AgBr ==> 2 Ag + Br2
2AgCl ==> 2 Ag + Cl2
Oxidation numbers of silver in both reactions: Ag +1 ==> Ag 0 The silver was reduced from ox #:+1 to 0
Is the reaction that take place in every leave of a plant where CO2 is taken from the air and with water it is combined to form Glucose and water in presence of Sunlight and chlorophyll.
6CO2 + 6 H2O Sunlight > C6H12O6 + 6 O2
There are certain substances that provide an alternate route with lower activation energy. These substances are called CATALYSTS.
Be careful, catalysts DO NOT LOWER the activation energy. They simply form an intermediate that needs a lower activation energy. They speed up the reaction but they can be retrieved at the end. they are NOT reactants.
Looking at the energy profiles, we will see something like this: