A change, in which the composition of a substance is altered, is called a chemical change. As a result, the original properties are changed and one or more new substances are formed. Iron is a greyish white metal which conducts electricity. It is attracted by a magnet and reacts with dilute acids to yield hydrogen.
Sulfur is a non-metal, and is yellow in color. It dissolves in carbon disulphide. When powdered iron and sulfur are heated together, a completely new substance, iron sulphide is formed. The properties of iron sulphide are entirely different from those of iron and sulfur. It is black in color, does not get attracted by a magnet and does not allow electric current to pass through it. It reacts with dilute acids to from hydrogen sulphide gas. In short, the properties of neither iron nor sulfur are exhibited by iron sulphide.
Change is an important part of our lives and we cannot ignore change. Personally, change is associated with growth, attitude and perception. In chemistry, change can be classified into two categories, namely physical change and chemical change.
In a physical change, the chemical composition of the substance does not change, whereas, in a chemical change, the composition of the substance changes.
Definition of chemical change
"Chemical changes are changes in which new substances are formed, and theses changes cannot be reversed by reversing the conditions. Generally, energy is either absorbed or evolved during a chemical change."
Chemical changes and rearrangement of atoms
We have seen that matter is made up of atoms and a new substance is formed during a chemical change. Where does this new substance come from?
According to the law of conservation of mass, matter can neither be created nor destroyed, so the total quantity of matter, i.e., the total mass, before and after a change, remains the same. So, the mass of the substances, taken together, before and after a chemical change, will also remain same. From this, it can be concluded that any new substance formed during a chemical change is due to the rearrangement of the atoms of the original substance. The number of atoms of each kind remains the same, before and after the chemical change.
Let us take an example. When sodium hydroxide reacts with hydrogen chloride, then sodium chloride and water are formed. Here, the atoms of sodium, oxygen, chlorine and hydrogen, present in the molecules of NaOH and HCl, rearrange themselves. After rearrangement of these atoms, new molecules of NaCl and H2O are formed.
NaOH + HCl → NaCl + H2O
The criterion for a chemical change is the production of one or more new substances. Therefore, certain conditions should be met with in order to bring about a chemical change.
The necessary conditions are :
- A minimum amount of energy, needed to initiate a reaction (activation energy) should be supplied in the form of heat, light or electric current. In a chemical change, the reactants combine to form new products. For this process, it is necessary to break the old bonds of the reactants and form fresh bonds, in order to give new products. This requires a certain amount of energy.
- For the occurrence of any reaction, the molecules or atoms of the reactants must collide with one another, in order to break old bonds and form new bonds.
- The speed with which the chemical reaction takes place is called the rate of the chemical reaction and it should be appreciable to bring about the change.
The rate of a reaction depends on following factors :
- Presence of light
- Presence of catalyst
Certain chemical reactions do not take place at room temperature but occur readily at a higher temperature. Thus, heat is required to start the reaction. Foe e.g, fuels like coal and wood only start burning when heated to a certain temperature (ignition temperature).
2. Presence of Light
Some reactions take place only in the presence of light and do not take place in the dark. For e.g., photosynthesis in green plants or reaction between H2 and Cl2 to form HCl.
3. Presence of a Catalyst
A catalyst is a substance that increases the rate of chemical reaction without undergoing any change itself.
Examples: Hydrogenation of oils to form fats takes place faster in the presence of nickel.
Decomposition of potassium chlorate (KClO3) is greatly accelerated in the presence of MnO2 (Manganese dioxide).
Certain reactions take place with the help of an electric current.
Some chemical reactions need very high pressure to proceed. For example, in the manufacture of ammonia from nitrogen and hydrogen (Haber's process), a pressure of over 200 atmosphere is required, in the presence of the catalyst iron, and a temperature of 450oC to 500oC.
A group of chemical laws, developed during the late 18th and early 19th centuries, arose from the recognition of the importance of quantitative study of chemical reactions. The relative proportion, in which the different elements unite, is regulated by fixed laws. These important laws, which are three in number, regulate the mode of combination of every known chemical compound. They are usually termed as the laws of chemical combinations.
The Laws are :
- Law of conservation of mass. (matter)
- Law of constant (definite) proportion
- Law of multiple proportion
- Law of equivalent (or reciprocal) proportion
Law of Conservation of Mass (matter)
If the total mass involved in a chemical reaction is precisely measured before and after the reaction takes place, the most sensitive balances cannot detect any change. This generalization is known as the law of conservation of mass. No detectable change in the total mass occurs during a chemical reaction.
Early chemists' studies of the burning of wood concluded that because the ash remaining was so much lighter than the object burned, something called phlogiston was lost in the reaction. However, their reasoning was faulty. Lavoisier realized that oxygen in the air, which could not be seen, was a reactant and that carbon dioxide and water vapor, also invisible, were products. If we take into account this factor of gases, we find that :
Total mass of reactant = Total mass of products
This equation is the Law of Conservation of Mass. In a non-nuclear change, mass is conserved; it is neither created nor destroyed.
The fact that matter can be conserved to energy and vice versa does not necessarily repeal the Law of Conservation of Mass. For all nonnuclear changes the law remains valid within our ability to measure such changes. If we include nuclear changes, the law must be modified by stating that the total of all mass and energy in a change is conserved.
In chemical science, the law of definite proportions, often named Proust's Law, states that a chemical compound always holds exactly the same ratio of chemical elements by mass. The equivalent statement is the law of invariant composition, which in turn states that all samples of a given chemical compound have the same elementary constitution. This law may be stated in very few words "In every chemical compound, the nature and the properties of its constituent elements are fixed, definite and invariable". Different samples of a pure compound always contain the same elements in the same proportion by mass. an>For example,
O is composed of 8g of oxygen and 1g of hydrogen
O is composed of 16g of oxygen and 2g of hydrogen
O is composed of 32g of oxygen and 4g of hydrogen
In each case, the ratio by mass of oxygen to hydrogen is 8 to 1. Because in any given compound, the elements are present in a specific proportion by mass, as each of these elements exists in a specific percentage. The general equation for calculating the percentage of any component X in a total is
This equation can be used to find the percentage by mass of an element, if the total mass of the compound and the mass contribution of the element are known. The percentage by mass of an element X in a compound can be calculated from the compounds formula by using the following equation.
In 1803, the British chemist Dalton,
proceeding from the atomistic conception of the structure of substances, came to the following conclusion, known by the rather inappropriate name of the Law of multiple proportions.The law of multiple proportion stated as, "If two elements A and B unite together in more proportions than one, on comparing together quantities of the different compounds each of which contains the same amount of A, the quantities of B will bear a very simple relation to each other".
Many elements are capable of combining with each other in several different proportions by weight, yielding, of course, different substances in each case.
For example, copper forms two compounds with oxygen; one of them is black (cupric oxide) and contains 79.9 percent copper and 20.1 percent oxygen, the other is red and contains 88.8 percent copper and 11.2 percent oxygen. Similarly, mercury and oxygen, tin and chlorine, form two compounds, each of different compositions.Two very important conclusions follow the Law of multiple proportions :
- Compounds, formed of the same elements, usually differ sharply in composition by weight. The individual compounds of a series, consisting of the same elements, differ in composition jump-wise.
- Any change in the quantitative ration of combining elements leads to the appearance of a new quality. Although, the oxides of nitrogen exhibit combination between nitrogen and oxygen, the difference in element ratio number gives rise to different chemical substances, e.g., NO (nitrogen oxide), NO2 (nitrogen di-oxide) etc. This difference is a striking illustration of one of the universal laws of nature - the law of the transition of quantity into quality.
This law, which was recognized in essence in 1791 by the German chemist Jeremias Benjamin Richter, is called the law of equivalent proportions.
This may be considered as the most important law in chemical philosophy and its discovery and application have been the great cause of the rapid advance of modern chemistry. Chemical analysis has shown that the body oxygen can form one definite compound or more than one, with every element yet discovered except with fluorine.
Law of equivalent proportion is considered as the third law and it may be stated as follows "An elementary substance, in combining with other elements, does so in a fixed proportion which may be represented numerically".
For example, if two elements A and B each form one compound with a third element C, then the mass ratio in compound consisting of A and B is never totally unrelated, but is a combination of two numbers which are multiples of the mass ratios in the first two compounds.
Elements always combine with one another to form chemical compounds in a ratio of definite combining masses (equivalent masses) or integral multiples of these masses. This law goes beyond the scope of both the preceding laws, and includes them as well.
Effect of Heat on Lead Nitrate
Lead nitrate is a white, crystalline solid. When heated, it starts decomposing with a crackling sound, producing a reddish brown gas called nitrogen dioxide, and a colorless gas, oxygen. A yellow residue of lead monoxide is left behind in the test tube.
Action of Concentrated Sulfuric Acid on Sugar
When concentrated sulfuric acid is added to sugar, it becomes a black mass of sugar charcoal. The acid removes all the hydrogen and oxygen (as water) from the sugar and absorbs it, leaving a residue of spongy carbon.
Action of Sodium on Cold Water
When a small piece (pea size) of sodium is placed in cold water, it darts about on the surface of the water with a hissing sound and produces hydrogen. The water left behind, acquires the property of turning red litmus blue. This shows the presence of a basic substance, which is sodium hydroxide.
Evidence of a Chemical Change
The following can indicate that a chemical change has taken place.
1. Change of Color Example:
Black copper (II) oxide on reaction with hydrogen converts to red colored copper.
CuO + H2 → Cu + H2O
2. Formation of GasExample:
Solid calcium carbonate reacts with hydrochloric acid to form calcium chloride and carbon dioxide gas.
CaCO3(s) + 2HCl(aq) → CaCl2(aq) + CO2(g)
3. Change in Temperature or Energy
Chemical reactions can be divided into two categories depending upon the absorption or evolution of heat.
A reaction in which energy is released or temperature is increased is called exothermic reaction.
Example: C + O2(g) → CO2(g) + Heat
A reaction in which energy is absorbed or temperature is decreased is called endothermic reaction.
Example: N2(g) + O2(g) → 2NO(g) - Heat
4. Formation of Precipitate
Example: Silver nitrate solution forms white precipitate of silver chloride on reaction with sodium chloride solution.
AgNO3(aq) + NaCl(aq) → NaNO3(aq) + AgCl(s)
5. Change of Odor
Example: Iron(II) sulphide reacts with hydrochloric acid to form hydrogen sulphide gas, which has the smell of a rotten egg.
FeS(s) + 2HCl(aq) → FeCl2(aq) + H2S(g)
List of Chemical Changes
Chemical changes are of different types. These changes depend on the reaction conditions and the behavior of reactants under different conditions.
1. Combination reactions
Some examples of combination reactions are as follows :
2Mg + O2 → 2MgO C + O2
2H2 + O2
- Combination of hydrogen and oxygen
Fe + S
- Combination of iron and sulfur
4NO2 + 2H2O + O2
- Nitrogen dioxide dissolves in water in presence of oxygen to form nitric acid
2. Decomposition reaction
Some examples of decomposition reactions are as follows :
- Decomposition of potassium chlorate
→ 2KCl + 3O2
- Decomposition of copper(II) carbonate
→ CuO + CO2
- Decomposition of ammonium dichromate
→ Cr2O3 + N2 + 4H2O
- Decomposition of trilead tetroxide
→ 6PbO + O2
- Decomposition lead nitrate
→ 2PbO + 4NO2 + O2
3. Precipitation reaction
Some examples of precipitation reactions are as follows :
- BaCl2(aq) + H2SO4(aq) → BaSO4(s) + 2HCl(aq)
- AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)
- FeCl3(aq) + 3NaOH(aq) → Fe(OH)3(s) + 3NaCl(aq)
- AlCl3(aq) + 3NaOH(aq) → Al(OH)3(s) + 3NaCl(aq)
- CuSO4(aq) + 2NaOH(aq) → Cu(OH)2(s) + Na2SO4(aq)
4. Displacement reaction
Some examples of displacement reactions are as follows :
- Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
- Cl2(g) + 2KI(aq) → 2KCl(aq) + I2(s)
- 2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
- Fe(s) + 2AgNO3(aq) → Fe(NO3)2(aq) + 2Ag(s)
- Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
We can say beyond any doubt or reason that chemistry makes our lives a bit more worth living, from toothpaste to toiletries, snacks to meals, cloths to machines; at every step we feel the tremendous influence of chemistry. Some examples are listed below.
- Photosynthesis – Plants make their food from carbon dioxide, water vapor and sunlight.
- Respiration – Oxidation of food to release energy.
- Cooking – Chemical composition of raw food changes.
- Burning of wood, coal, gas, oil to form mainly carbon dioxide and water vapor and lots of energy, used for different useful purposes.
- Fermentation of sugars to form alcohol in beer and wine.
- Digestion of food.
- Redox reaction in battery that changes the chemical energy to useful electrical energy.
- The hardening of glues due to the formation of new chemical compounds.
- Bleaching action of bleaching powder is an example of oxidation reaction.
- Cleansing action of soaps and detergents.
Chemical Change Experiment
In the laboratory, you can make plastic in the following way :
Pour 100 mL of saturated solution of aniline hydrochloride and 100 mL of 40% of formaldehyde solution into a large beaker. Heat gently while constantly stirring with a glass rod. After sometime you will observe plastic is formed in the beaker.