When we mix chemicals together for a reaction, they are referred to as reactants. What we get is referred to as products. reactants will react at a certain fixed rate. At a given period of time it will reach equilibrium. There is no reaction where the reactants will be completely changed to the products. At equilibrium the rate at which the products are formed will equal the rate at which the products will react to produce the reactants.
Let us take the neutralisation reaction of hydrochloric acid and sodium hydroxide as an illustration. The reaction can be represented by;
NaCl + H2OThe first equation wants to portrait a dynamic equilibrium where the reactants react to form the products and the products in turn react to give the reactants. Both the forward and reverse rates are equal. (Reaction rates are concentration dependent).
The second equation wants to say that no material must be lost during a reaction. All atoms and electrons must be accounted for. This is known as the Law of the Conservation of Matter. A universal law that says; "nothing is created nor destroyed" in a chemical reaction, they only get transformed.
Although no reaction is complete, there are many reactions that can proceed to almost 100 percent. This neutralisation reaction is one of them. To represent this we sometimes write it as ;
NaCl + H2OWhat we are saying is that the trace amount of the reactants present at equilibrium is negligible.
However in some cases we do have to bother with the trace amount of reactants present even though numerically it is of no consequence. A good example is the reaction of vinyl chloride monomer (VCM) to give poly(vinyl chloride), or PVC. PVC was widely used to make clear plastic bottles for drinks. However it is now deemed to be a health risk because of the trace amount of VCM in the final product. Next time you hold a clear plastic bottle in your hand examine it. You might find PETE (or PET) imprinted on it. This says that the material used to manufacture the bottle is polyester. Would you be surprised if I say that polyester is also used to make the cloth for your dress or shirt? The magic of chemistry.
On the other hand some reactions, like autoprotolysis, will only proceed to a limited extent. This reaction is better represented by ;
{H3O}+ + OH ‾That is, only a small amount of the water will react to give the products. In chemistry we say the reaction is towards the left.
EQUILIBRIUM CONSTANTS
Every reaction will finally reach equilibrium, and so possess an equilibrium constant. Many of the constants would be given a meaningful name. For example, for the dissociation of compounds in water to give the hydronium ion the equilibrium constant is known as the acidity constant. With a meaningful name we can relate to the equilibrium better.
THE STOICHIOMETRY OF COMPOUNDS
| Let us consider a simple reaction: | 2 H2 + 2 O2 | � | H2O + O2 |
| We can also write it as: | H2 + O2 | � | H2O + � O2 |
Some important facts you must know
As we have mentioned before matters react because all universal system prefer stability (or lower energy). This is the driving force for all reactions.
Matters can only react if the atoms or molecules collide into each other. Two solids placed side by side will not react. The two systems that allow for collision of atoms and molecules are gases or solutions. Gaseous reactions are not important because they are too dilute and so the frequency of collision is rather low. So they have to be conducted at high pressure. Most chemical reactions are conducted by dissolving the matter in a common solvent.
Since reactants must be soluble for reaction, products that are insoluble will find it hard to revert back to the reactants. So for reactions where a product precipitates out it has a greater tendency to proceed to complete reaction.
The collision must produce the minimum energy required for a reaction to take place. Every reaction has its own energy requirement. That is why sometimes we heat the reaction mixture. This will make the atoms and molecules move at greater velocity and impact each other with greater force.
Since many reactions of interest are conducted in water, there is a greater focus on the solubility of compounds in water.
Balancing chemical equations, Dr Ng's way
When you want to write a chemical equation you must first know your chemistry. You cannot just write the reactants, put an arrow in between, and make up story on the products side. You must know what products you will get, before you can start writing the equation.
Once you have written the reactants and the products, the next step is to balance the materials on both sides of the equation. Below are some techniques to help you balance the equation. I will walk you through the method. I am very sure you will have no problem using this technique. But if you do, get back to me.
Rule 1: Break up the equation into individual components, and make sure that each step is balanced. Then when you sum them up they have to be balanced.
Example: Fe+� + NH3 � Fe2O3 + NH4+
2 Fe+� + 3H2O � Fe2O3 + 6 H+
6 NH3 + 6 H2O � 6 NH4+ + 9 OH ‾
SUM: 2 Fe+� + 6 NH3 + 9 H2O � Fe2O3 + 6 NH4+ + 6 H2O
OR: 2 Fe+� + 6 NH3 + 3 H2O � Fe2O3 + 6 NH4+
Rule 2: For reactions in water, if you need an oxide use, O-�. If you need a hydride use H+. Balance the charge with the help of e (electron). All O ‾� introduced must be removed from the final equation. The illustrations below will show you how.
Example I: Al + NO3 ‾ + OH ‾ � AlO2- + NH3
Al + 2 O ‾� � AlO2 ‾ + 3e (e for electron)
NO3 ‾ + 3 H+ + 8e � NH3 + 3 O ‾�
Remove e 8 Al + 3 NO3 ‾ + 9 H+ + 7 O ‾� � 8 AlO2 ‾ + 3 NH3 Remove O ‾� using H+ + O ‾� � OH ‾ Final: 8 Al + 3 NO3- + 2 H2O + 5 OH- � 8 AlO2 ‾ + 3 NH3
Example II: As2O3 + I2 (in alkaline) � AsO4 ‾� + HI
As2O3 + 5 O ‾� � 2 AsO4 ‾� + 6e
I2 + 2 H+ + 2e � 2 HI
Remove e As2O3 + 3 I2 + 3 H2O + 2 O ‾� � 2 AsO4 ‾� + 6 HI Remove O ‾� using 2H+ + 2OH ‾ � 2 H2O Final: As2O3 + 3 I2 + H2O + 4 OH ‾ � 2 AsO4 ‾� + 6 HI
Example III: KIO3 + KI (in acid) � I2 + K+
2 KIO3 + 10e � 2 K+ + I2 + 6 O ‾�
2 KI � 2 K+ + I2 + 2e
Remove e 2 KIO3 + 10 KI � 12 K+ + 6 I2 + 6 O ‾� Remove O ‾� using 12 H+ � 12 H+ Final: 2 KIO3 + 10 KI + 12 H+ � 12 K+ + 6 I2 + 6 H2O
Rule 3: If the reaction is in an acid medium make sure that no base like OH ‾ is left as a product. If the reaction is in a basic medium make sure that no H+ is left as a product.
Balance the equations of the following reactions in water
Na2HPO4 + CaCl2 � Ca3(PO4)2 + NaCl + HCl
Na2S2O3 + I2 � Na2S4O6 + NaI
CaC2O4 + KMnO4 (in H2SO4 acid) � CaSO4 + K2SO4 + MnSO4 + CO2
As2O3 + KMnO4 (in H2SO4 acid) � As2O5 + MnSO4
K2Cr2O7 + Fe2SO4 (in H2SO4 acid) � Cr2(SO4)3 + Fe2(SO4)3
Ce(SO4)2 + NaNO2 � Ce2(SO4)3 + NaNO3
MnSO4 + KIO4 (in H2SO4 acid) � HMnO4 + KIO3
KMnO4 + FeSO4 (in H2SO4 acid) � K2SO4 + MnSO4 + Fe2(SO4)3
