1. A chemical change must occur. You start with one compound and turn it into another. That's an example of a chemical change. A steel garbage can rusting is a chemical reaction. That rusting happens because the iron (Fe) in the metal combines with oxygen (O2) in the atmosphere. When a refrigerator or air conditioner cools the air, there is no reaction. That change in temperature is a physical change. Nevertheless, a chemical reaction can happen inside of the air conditioner.
2. A reaction could include ions, molecules, or pure atoms. We said molecules in the previous paragraph, but a reaction can happen with anything, just as long as a chemical change occurs (not a physical one). If you put pure hydrogen gas (H2) and pure oxygen gas in a room, they can be involved in a reaction. The slow rate of reaction will have the atoms bonding to form water very slowly. If you were to add a spark, those gases would create a reaction that would result in a huge explosion. Chemists would call that spark a catalyst.
3. Single reactions often happen as part of a larger series of reactions. Take something as simple as moving your arm. The contraction of that muscle requires sugars for energy. Those sugars need to be metabolized. You'll find that proteins need to move in a certain way to make the muscle contract. A whole series (hundreds actually) of different reactions are needed to make that simple movement happen.
The rate of a reaction is the speed at which a reaction happens. If a reaction has a low rate, that means the molecules combine at a slower speed than a reaction with a high rate. Some reactions take hundreds, maybe even thousands of years while other can happen in less than one second. The rate of reaction depends on the type of molecules that are combining.
Example of slow chemical reactions are
i) Photosynthesis process
- During the photosythesis process, carbon dioxide reacts with water in the presence of chlorophyll and sunlight to produce glucose and oxygen
- The reaction will take from several hours to a day to complete
ii) Iron rusing process
- Iron rusting takes place with the present of oxygen and water. It takes few days for the iron to rust.
- Iron takes a long time to product ferum (III) oxide.
iii) Sulphur precipitate
- When dilute hydrochloric acid is added to a sodium thiosulphate solution, no changes are shown at the beginning of the reaction. However after a few seonds a yellow precipitate slowly appears.
- The formation of solid yellow sulphur is a slow reaction.
On the other hand some chemical reactions happen so suddenly that they are spontaneous.
Examples
i) Reaction between sodium carbonate and diluted hydrochloric acid
- When a small amount of sodium carbonae is added to 5 cms cube of diluted hydrochloric acid, bubbles appear immediately.
- Carbon dioxide gas is given out and the reaction stops in just a few seconds
ii) Reaction of burning magnesium in air
- When a ribbon of magnesium is burnt in air, it burns vigorously with a white flame.
- In just a few seconds the combustion is completed and a white solid (magnesium oxide) is formed.
2Mg + 02 -> 2Mg0
iii) Reaction of reactive metal with water
- Metal such as sodium and potassium are very reactive. When a small piece of sodium is put into a basin of water, it turns into a slivery ball and moves vigorously and randomly on the surface of the water while producing a hissing sound.
- Within seconds he reactions ends, and small pieces of solid sodium hydroxide is formed and sinks to the bottom of the basins
There is another big idea for rates of reaction called collision theory. The collision theory says that the more collisions in a system, the more likely combinations of molecules will happen. If there are a higher number of collisions in a system, more combinations of molecules will occur. The reaction will go faster, and the rate of that reaction will be higher.
Reactions happen, no matter what. Chemicals are always combining or breaking down. The reactions happen over and over but not always at the same speed.
EQUILIBRIUM BASICS
Equilibrium is a pretty easy topic. Big name, but easy idea. First, when you have a system made up of a bunch of molecules, those molecules sometimes combine. That's the idea of a chemical reaction. Second, a chemical reaction sometimes starts at one point and moves to another. Now imagine the reaction finished and you have a pile of new chemicals. Guess what? Those chemicals want to go through a reverse chemical reaction and become the original molecules. We don't know why. Sometimes they just do.
Put those two ideas together and you have equilibrium.
1. Two reactants combine to make a product.
2. Products like to break apart and turn back into the reactants.
3. There is a point where those two reactions happen and you can't tell that any reactions are occurring. That point is when the overall reaction is happy. There is no pressure to do more of one thing or another.
There are some other traits of equilibrium. Equilibrium always happens at the same point in the reaction no matter where you start. So if you start with all of substance A, it will break up and become B and C. Eventually, B and C will start combining to become A. Those reactions happen until they reach equilibrium. They reach equilibrium at the same point if you start with all B and C or half A and half B/C. It doesn't matter. There is one special point where the two reactions cancel each other out.
IT HAPPENS ON ITS OWN
Another idea is that equilibrium is reached by itself with no outside forces acting on the system. If you put two substances in a mixture, they will combine and react by themselves. Eventually, they will reach equilibrium. Scientists say equilibrium happens through spontaneous processes. They happen on their own.
There is one last idea. Do you remember that some atoms and molecules have charges? A system "at equilibrium" appears to have no charge (neutral). All the pluses and minuses cancel each other out and give a total charge of "0". Scientists use the letter "K" to add up all of the actions and conditions in a reaction. That "K" is the equilibrium constant.
What factors influence the rate of a chemical reaction?
Temperature
Concentrations of reactants
Catalysts
Surface area of a solid reactant
Pressure of gaseous reactants or products
If you are planning an investigation, I suggest that you investigate the effects of temperature or the effects of the concentration of the reactants because these will allow you to choose a suitable range of values for the controlled or independent variable. The dependent variable will be the rate of the reaction. Keep all the other variables fixed.
To make a prediction for your investigation you will have to ask yourself the question: What will happen to the rate of the reaction when I increase the temperature? or What will happen to the rate of the reaction if I increase the concentration of one of the reactants? The answer to that question is your prediction. The next thing to do is to explain your prediction. You will have to answer the question: Why will the reaction go faster if I increase the temperature? or Why will the reaction go faster if I increase the concentration of one of the reactions? The answer to this question is your explanation, and to get the highest possible marks, you will have to provide a full scientific explanation.
Once you have written your hypothesis (prediction with explanation) you will decide how to do the experiments, i.e. write the proposed method.
How does temperature affect the rate of a chemical reaction?
When two chemicals react, their molecules have to collide with each other with sufficient energy for the reaction to take place. This is collision theory. The two molecules will only react if they have enough energy. By heating the mixture, you will raise the energy levels of the molecules involved in the reaction. Increasing temperature means the molecules move faster. This is kinetic theory. If your reaction is between atoms rather than molecules you just substitute "atom" for "molecule" in your explanation.
How do catalysts affect the rate of a reaction?
Catalysts speed up chemical reactions. Only very minute quantities of the catalyst are required to produce a dramatic change in the rate of the reaction. This is really because the reaction proceeds by a different pathway when the catalyst is present. Adding extra catalyst will make absolutely no difference. There is a whole page on this site devoted to catalysts.
How does concentration affect the rate of a reaction?
Increasing the concentration of the reactants will increase the frequency of collisions between the two reactants. So this is collision theory again. You also need to discuss kinetic theory in an experiment where you vary the concentration. Although you keep the temperature constant, kinetic theory is relevant. This is because the molecules in the reaction mixture have a range of energy levels. When collisions occur, they do not always result in a reaction. If the two colliding molecules have sufficient energy they will react.
If reaction is between a substance in solution and a solid, you just vary the concentration of the solution. The experiment is straightforward. If the reaction is between two solutions, you have a slight problem. Do you vary the concentration of one of the reactants or vary the concentration of both? You might find that the rate of reaction is limited by the concentration of the weaker solution, and increasing the concentration of the other makes no difference. What you need to do is fix the concentration of one of the reactants to excess. Now you can increase the concentration of the other solution to produce an increase in the rate of the reaction.
How does surface area affect a chemical reaction?
If one of the reactants is a solid, the surface area of the solid will affect how fast the reaction goes. This is because the two types of molecule can only bump into each other at the liquid solid interface, i.e. on the surface of the solid. So the larger the surface area of the solid, the faster the reaction will be.
Smaller particles have a bigger surface area than larger particle for the same mass of solid. There is a simple way to visualize this. Take a loaf of bread and cut it into slices. Each time you cut a new slice, you get an extra surface onto which you can spread butter and jam. The thinner you cut the slices, the more slices you get and so the more butter and jam you can put on them. This is "Bread and Butter Theory". You should have come across the idea in your biology lessons. By chewing your food you increase the surface area so that digestion can go faster.
What affect does pressure have on the reaction between two gasses?
You should already know that the atoms or molecules in a gas are very spread out. For the two chemicals to react, there must be collisions between their molecules. By increasing the pressure, you squeeze the molecules together so you will increase the frequency of collisions between them. This is collision theory again.
In a diesel engine, compressing the gaseous mixture of air and diesel also increases the temperature enough to produce combustion. Increasing pressure also results in raising the temperature. It is not enough in a petrol engine to produce combustion, so petrol engines need a spark plug. When the petrol air mixture has been compressed, a spark from the plug ignites the mixture. In both cases the reaction (combustion) is very fast. This is because once the reaction has started, heat is produced and this will make it go even faster.
CATALYSTS SPEED IT UP
A catalyst is like adding a bit of magic to a reaction. Reactions need a certain amount of energy to happen. If they don't have it, oh well, the reaction probably can't happen. A catalyst lowers the amount of energy needed so that a reaction can happen easier. A catalyst is about energy; it doesn't have to be another molecule. If you fill a room with hydrogen gas and oxygen gas, very little will happen. If you light a match in that room (or just a spark), all of the hydrogen and oxygen will combine to create water molecules. It is an explosive reaction.
The energy needed to make a reaction happen is called the activation energy. As everything moves around, energy is needed. The energy a reaction needs is usually in the form of heat. When a catalyst is added, something special happens. Maybe a molecule shifts it's structure. Maybe that catalyst makes two molecules combine and they release a ton of energy. That extra energy might help another reaction to occur. In our earlier example, the spark added the activation energy.
Catalysts are also used in the human body, not to cause explosions but to make very difficult reactions happen. They help very large molecules combine. There is another interesting fact about catalysts. Catalysts lower the activation energy required for a reaction to occur. With the activation energy lower, the products can also combine more easily. Therefore, the forward and reverse reactions are both accelerated. It helps both reactions.
INHIBITORS SLOW IT DOWN
There is also something called an inhibitor that works exactly the opposite of catalysts. Inhibitors slow the rate of reaction. Sometimes they even stop the reaction completely. You might be asking, "Why would anyone need those?" You could use an inhibitor to make the reaction slower and more controllable. Without them, some reactions could keep going and going and going. If they did, all of the molecules would be used up. That would be bad, especially in your body.
Application of factors affecting the rate of reaction in our daily life
a) Combustion of charcoal to produce heat and electricity
b) Storing food in a refrigerator.
c) Cooking food in a pressure cooker
d) Production of amonia
e) Production of nitric acid
f) Production of sulphuric acid
Haber Process
The Haber Process combines nitrogen from the air with hydrogen derived mainly from natural gas (methane) into ammonia. The reaction is reversible and the production of ammonia is exothermic.
A flow scheme for the Haber Process looks like this:
The optimum conditions for maximum oxidation of hydrogen and nitrogen into amonia in the Haber process
a) Temperature between 450 to 550 Celcius
b) Pressure between 250 to 500 ATM
c) Catalyst powdered Ferum
The catalyst
The catalyst is actually slightly more complicated than pure iron. It has potassium hydroxide added to it as a promoter - a substance that increases its efficiency.
The pressure
The pressure varies from one manufacturing plant to another, but is always high. You can't go far wrong in an exam quoting 200 atmospheres.
Recycling
At each pass of the gases through the reactor, only about 15% of the nitrogen and hydrogen converts to ammonia. (This figure also varies from plant to plant.) By continual recycling of the unreacted nitrogen and hydrogen, the overall conversion is about 98%.
Oswald Process
Ammonia is converted to nitric acid in two stages.
Stage 1
It is oxidized (in a sense "burnt") by heating with oxygen in the presence of a catalyst such as platinum with 10% rhodium, to form nitric oxide and water. This step is strongly exothermic, making it a useful heat source once initiated (ΔH = -908 kJ):
4NH3(g) + 5O2(g) → 4NO(g) + 6H2O(g)
Stage Two
Stage two (combining two reaction steps) is carried out in the presence of water in an absorption apparatus. Initially nitric oxide is oxidized again to yield nitrogen dioxide:
2NO(g) + O2(g) → 2NO2(g)
This gas is then readily absorbed by the water, yielding the desired product (nitric acid, albeit in a dilute form), while reducing a portion of it back to nitric oxide:
3NO2(g) + H2O(l) → 2HNO3(aq) + NO(g)
The NO is recycled, and the acid is concentrated to the required strength by distillation.
Alternatively, if the last step is carried out in air:
4NO2(g) + O2(g) + 2H2O(l) → 4HNO3(aq)
Typical conditions for the first stage, which contribute to an overall yield of about 96%, are:
pressure between 4 and 10 atmospheres (approx. 400-1010 kPa or 60-145 psig) and
temperature is about 1173 K (approx. 900 °C or 1652 °F.).
Contact Proces
The main 3 stages for production of sulphuric acid
1) Makes sulphur dioxide;
2) Convers the sulphur dioxide into sulphur trioxide (the reversible reaction at the heart of the process);
3) Converts the sulphur trioxide into concentrated sulphuric acid.
Making the sulphur dioxide
This can either be made by burning sulphur in an excess of air:
Heating sulphide ores like pyrite in an excess of air:
In either case, an excess of air is used so that the sulphur dioxide produced is already mixed with oxygen for the next stage.
Stage 2
Converting the sulphur dioxide into sulphur trioxide
This is a reversible reaction, and the formation of the sulphur trioxide is exothermic.
Stage 3
This can't be done by simply adding water to the sulphur trioxide - the reaction is so uncontrollable that it creates a fog of sulphuric acid. Instead, the sulphur trioxide is first dissolved in concentrated sulphuric acid:
The product is known as fuming sulphuric acid or oleum.
This can then be reacted safely with water to produce concentrated sulphuric acid - twice as much as you originally used to make the fuming sulphuric acid.
THE COLLISION THEORY
The Collision theory, proposed by Max Trautz[1] and William Lewis in 1916 and 1918, qualitatively explains how chemical reactions occur and why reaction rates differ for different reactions.[2] This theory is based on the idea that reactant particles must collide for a reaction to occur, but only a certain fraction of the total collisions have the energy to connect effectively and cause the reactants to transform into products. This is because only a portion of the molecules have enough energy and the right orientation (or "angle") at the moment of impact to break any existing bonds and form new ones. The minimal amount of energy needed for this to occur is known as activation energy. Particles from different elements react with each other by releasing activation energy as they hit each other. If the elements react with each other, the collision is called successful, but if the concentration of at least one of the elements is too low, there will be fewer particles for the other elements to react with and the reaction will happen much more slowly.
The theory of matter states that matter is made up of a large number of tiny and discrete particles.
The kinetic theroy of matter, state that the tiny and discrete particles are either atoms or molecules whcih are in continuous motion and collide with one another from time to time
Before any reaction can happen, the must first get together (transfer or sharing of electrons). The particle must not be too far apart from one another. They must collide to form and break the bonds in between them.
THe particles must come together in the proper orientation unless the particles involved are single atoms or small, symmetrical molecules.
The collision must achieve a certain minimum energy called the activation energy in order for an effective reaction to take place
Collision
According to the collision theory, chemical reactions occur as a result of collisions between reacting molecules. However not all collisions results in a reaction
Based on the kinetic theory of gases, it is possible to calculate how many collisions occurs between gas molecules at any given time. Only small fraction of collisions resulted in reaction
Effective collision
A collision that results in reaction is called effective collision. So they must be properly oriented so that that specific bonds can be broken and formed. Orientation is the position of the particles relative at the time of collision. The molecules must also have sufficient energy.Activation energy
The activation energy in chemistry is the energy needed by a system to initiate a particular process. activation energy is often used to denote the minimum energy needed for a specific chemical reaction to occur. For a reaction to occur between two colliding molecules they must collide in the correct orientation and possess a certain minimum amount of energy. As the molecules approach their electron clouds repel. This requires energy - activation energy - and comes from the heat of the system, i.e. the translational, vibrational, and rotational energy of each molecule. If there is enough energy available, this repulsion is overcome and the molecules get close enough for attractions between the molecules to cause a rearrangement of bonds.
Generally , reactions with larger activation energies have lower reaction rates compared to reactions with a smaller activation energy.
Collision Frequency
The Collision frequeny is controlled by concentration and temperature.
To increase the collision frequency, we can increase the concentration of molecules. The more concentrated the reactants are, the most frequent the particles will collide, simple because there are more of them in a given volume.
Another way to increases the frequency of collision is to increase the temperature, the molecules have a higher kinetic energy and velocity.
Effective collision frequency
Even though molecules collide, they may not react. For a reaction to occur, the molecules have to collide with sufficient eneryg so that bonds are broken and new bonds are formed.
When temperature and concentration increases the particle will gain enery from them to move faster. The collisions are more energetic and the collision frequency will increase.
Energy Profile Diagram
The activation energy may be shown in diagrams called energy profile diagram or a reaction profile.
The enthalpy change (H) is the amount of heat released or absorbed when a chemical reaction occurs at constant pressure.
delta H = H(products) - H(reactants)
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