Using reaction rates to follow the progress of a reaction.

The progress of a chemical reaction can be followed by examining the reaction rate. There are several methods that can be used to follow a reaction rate.

• Change in colour of a reaction
• Change in concentration of reactants or products
• Change in volume of any gases produced
• Change in mass of reaction mixture if gases are given off or taken in.
• etc.

As these changes happen over a period of time, the reaction rate is expressed as a change in some measurable quantity over a period of time.

For example, the reaction rate could be measured by examining the change in concentration of a product over a certain time - in other words, the difference between the final concentration of a product and the initial concentration of the product taken over a measured period of time.

Mathematically, this would be written as :

The units of the rate of a reaction depend on the method used to calculate the rate.
The simplest way to determine the units is to put them into the calculation used to find the reaction rate.

For example, if we measured the change in concentration over time in order to obtain the rate of the reaction, the units would be as follows :

The unit of concentration is moles per litre, which is written as mol l^-1.
The unit of time is seconds, written as s

Putting these units into the above equation gives :

In the same way, a reaction rate measured by the change in volume of a gas produced (measured in cm³) over a period of time (measured in s), would have the units cm³s^-1.
A reaction rate measured by the change in mass of a reaction would have the units g s^-1.

The progress of a chemical reaction can be illustrated by the use of a graph. These graphs show that reaction rates are usually highest at the beginning of the reaction (shown by a steep slope). This is because as a reaction proceeds, the reactants are used up and the reaction begins to slow down.

If we measure the change in concentration of a reaction by the concentration (or volume, or mass) of products, this value increases as the reaction proceeds.
If we measure the change in concentration of a reaction by the concentration (or volume, or mass) of reactants, this value decreases as the reaction proceeds.

The average reaction rate is proportional to 1/time.

• When a reaction is quick (small time), the reaction rate is high.
• When a reaction is slow (large time), the reaction rate is low.

You can quickly test your knowledge of the above information.

 
Collision Theory

For substances to react together, they first have to collide with one another.
This idea is called the Collision Theory.

This theory can be used to explain why certain factors can affect the rate of a reaction.

 
Factors affecting reaction rate - Concentration

The more particles there are in a given space, the more likely they are to collide with one another.

These animations illustrate that the more concentrated a reaction mixture, the more likely the reactants are to collide with one another and hence an increase in the rate of the reaction.

Not all collisions result in a chemical reaction taking place - the collisions must take place with sufficient energy to break any bonds within the reactants. For example nitrogen and oxygen particles collide in the air very frequently, but do not normally undergo a chemical reaction as there is insufficient energy in the collisions.

 
Factors affecting reaction rate - Surface area

If a large piece of material (1 particle) has a size of 2cm x 2cm x 2cm, the surface area of the reactant will be 24cm². This means that any particles that are trying to react with this material have to collide somewhere on this surface of 24cm².

If this large piece of material was broken up into eight pieces of 1cm x 1cm x 1cm, there would now be a surface area of 48cm². This means that for this reaction, any particles that are trying to react with this material have a much larger surface area (or target area) to hit - they are more likely to collide and react with one another.

Also, as can be seen in the animation for the effect of concentration, one particle is less likely to collide with another particle than 4, or even 8 particles are.

 
Factors affecting reaction rate - Temperature

As reactants are heated up, they begin to move more rapidly. As can be seen in the animations below, the faster the particles move, the more frequently they collide, meaning they are more likely to react with one another.

Also, as the particles are now moving faster, the collisions between them have more kinetic energy and as a result, they are more likely to be successful collisions that cause a chemical reaction.

You can quickly test your knowledge of the above information.

 
Catalysts

Catalysts are used to speed up a chemical reaction. They are not used up in the reaction and can be recovered chemically unchanged at the end of the reaction.

Many catalysts are elements (or compounds of) found between groups 2 and 3 of the Periodic Table - The Transition metals (This was mentioned in Unit 1 : Substances).

Hydrogen peroxide decomposes (breaks up) slowly releasing oxygen gas. If manganese dioxide is added the reaction is much faster and oxygen gas is given off quickly. Manganese dioxide is a catalyst for this reaction. At the end of the reaction all of the manganese dioxide can be recovered (by filtration) and used again.

Catalysts help to increase the rate of a reaction by increasing the percentage of successful collisions between reactants without increasing the surface area, concentration or temperature.

How does a catalyst work ?

For a chemical reaction to occur, the bonds within the reactants have to be broken before new ones can been formed with other reactants. When reactants simply collide with one another, there is not always enough energy in the collision to break the bonds.
When a catalyst is used, the reactants adsorb onto the surface of the catalyst. Because the reactants use some of the energy from their existing bonds to form a new bond with the catalyst, the original bonding in the reactants is slightly weakened. This reduces the energy needed during a collision to break these bonds and for a reaction to take place.

After the reaction has taken place, the new product breaks its bond with the catalyst and leaves the surface.

You can quickly test your knowledge of the above information.

 
What types of catalyst can be used ?

• A very common source of catalysts are the Transition metals.
• When the reactants are gases, a solid catalyst is usually used.
• A catalyst that is in a different physical state (solid, liquid or gas) than the reactants is called at Heterogeneous Catalyst - e.g. a platinum gauze being used as a catalyst in the formation of nitric acid from ammonia and oxygen in the Ostwald Process.
• A catalyst that is in the same physical state (solid, liquid or gas) as the reactants is called a Homogeneous Catalyst.

 
Uses of catalyst.

As we have seen, reactions are more likely to take place when high concentrations, large surface areas and high temperatures are used. These factors increase the likelihood of collisions of the reactants, and the more energy that these collision have, the more likely it will be that these collisions are successful and cause a chemical reaction to take place.

When chemical processes are performed on a large scale in industry, the costs can be extremely high if the reactions require a large amount of energy (usually in the form of heat). Also some reactants and products actually begin to decompose or react in different ways if the temperature is too high; so although the temperature gives the collisions enough energy to cause a chemical reaction, the product may decompose before it can be isolated.

Catalysts reduce the energy required for a reaction to proceed and as a result usually require much lower temperatures (cheaper), and more collisions are successful (efficient).

Catalysts are also used in car exhausts to convert harmful gases such as carbon monoxide to carbon dioxide and oxides of nitrogen to nitrogen.

Occasionally Catalyst Poisoning can take place if impurities are adsorbed onto the surface of a catalyst. When this happens, the impurities block the sites on the catalyst that the reactants adsorb onto. This reduces the efficiency of the catalyst, and can stop it working completely if the entire surface is coated with impurities.

Some poisoned catalysts can be Regenerated by removing the impurities from the surface, but other catalysts have to be Renewed if the impurities cannot be removed or if the surface has been damaged.

An example of catalyst regeneration is the removal of carbon (by burning it off in air) from the aluminium oxide catalyst used in the catalytic cracking of long chain hydrocarbons.

In a car exhaust, lead can poison a catalytic convertor and this is why only lead-free fuels should be used.

 
Enzymes

Another type of catalyst is a biological catalyst, or Enzyme.

Enzymes are very specific in the type of reaction that they will catalyse, and these reactions take place in living material - e.g. the human body, animals and plants. They are examples of homogeneous catalysts as they are found in a solution with the reactants.

Some examples of enzymes are :

• Saliva contains an enzyme called amylase which helps the digestion of certain foods.
• During the baking and brewing process, yeast (which contains an enzyme called zymase) breaks down sugars into carbon dioxide (gas) and alcohol.

You can quickly test your knowledge of the above information.

New words and their meanings

Reaction rate - The speed at which a reaction takes place.

Collision theory - Reactant particle need to collide to react with one another.

Kinetic energy - The energy a moving particle has. Fast, heavy particles have higher kinetic energies than slow, light particles.

Catalyst - A substance used to speed up a chemical reaction. Catalysts can be recovered chemically unchanged at the end of a reaction.

Heterogeneous catalyst - A catalyst that is in a different physical state than the reactants.

Homogeneous catalyst - A catalyst that is in the same physical state as the reactants.

Catalyst poisoning - The surface of a catalyst can be poisoned, or smothered by an impurity, rendering it useless.

Catalyst regeneration - Poisoned catalyst can be cleaned to remove impurities, allowing it to be used again.

Catalyst renewal - Catalyst that cannot be cleaned are discarded and replace with new ones.

Enzyme - A biological catalyst.