Laws of chemical combination and gas | Chemistry, Types, Definition, laws, | aurayne

During the quantitative studies of chemical changes, the scientists made some of generalisations, these are known as laws of chemical combination.

Law of Conservation of Mass

This law establish the relationship between the masses of reactants and products during a chemical reaction. This law was postulated by A. Lavoisier in 1750.

This law states that,

"During any physical or chaemical change, the total mass of the products is equal to the total mass of the reactants".

or

"Matter can neither be created nor destroyed during any physical or chemical change".

Example, 12g carbon combines with 32g oxygen to give 44g carbon dioxide. This law may be explained with the help of Landolt's experiments.

Law of Conservation of Mass in the Light of Modern Research

It is stated by modern research that mass can be converted into energy. There is some energy formed in each reaction, by which some mass has been lost. According to Einstein, mass and energy are related as,

E = mc²

Where, 

E = energy

m = the mass of substance, 

c = velocity of light (3 x 10⁸ m/s)

But chemical reactions those, we are completed in laboratory, energy released or absorbed in these reactions are too less, 

So, there is a slight change in mass (decrease or increase) takes place which is negligible. These changes can be clearly seen in nuclear reactions. We assumed the law of conservation of mass is true for normal chemical reactions.

This law does not hold good for nuclear reactions like, nuclear fission, nuclear fusion and radioactivity.

Laws of Constant Composition

This law deals with the composition of various elements present in a compound. This law was stated by French chemist Lewis Proust. This law states that, 

"A pure chemical compound always contains same elements combined together in the same definite proportions by weight."

The converse of this law is not true. 

Example, it is found by the analysis of water (taken from various places like river. Falls and wells) that in each sample of water the ratio of hydrogen and oxygen is 2 : 16 or 1 : 8.

Laws of Multiple Proportions

This law was proposed by Dalton (1803).

This law states that,

"When two elements combine to form two or more compounds then in two compounds the weights of one of the element which combine with a fixed weight of the other, bear a simple whole number ratio".

Example, carbon and oxygen react with each other to form carbon monoxide and carbon dioxide. Carbon monoxide (CO) contain 12 parts by weight of carbon and 16 parts by weight of oxygen and carbon dioxide (CO2) contains 12 parts by weight of carbon and 32 parts by weight of oxygen and ratio of the weights of oxygen which react with or fixed weight of carbon (12 parts) in these oxides is 16 : 32 which is a simple whole number ratio (1 : 2).

Other examples of law of multiple proportions,

(i) H2O and H2O2; 

(ii) CuO and Cu2O; 

(iii) N2O, NO, N2O3 and N2O5, 

which also verify the law of multiple proportions

Laws of Reciprocal Proportions

This law was proposed by Richter in 1792. This law is aiso called as the law of equivalent proportions or law of combining weights.

This law states that,

"When two different elements combine separately with the same weight of a third element the ratio in which they do so, wil be the same or some simple multiple of the ratio in which they combine with each other.

Example, carbon and sulphur react separately winth oxygen (third element) to give carbon dioxide (CO2) and sulphur dioxide (SO2). They also react together to form carbon disulphide (CS2). Now, in carbon dioxide 12 parts by weight of carbon are combined with 32 parts by weight of oxygen and in sulphur dioxide (SO2) 32 parts by weight of sulphur are combined with 32 parts by weight of oxygen. Ratio of weight of carbon and sulphur which combine with fixed weight (32 parts) of oxygen is

= 12 : 32 or 3 : 8       ........(i)

In carbon disulphide (CS2) 12 parts by weight of carbon are combined with 64 parts by weight of sulphur. 

So, ratio of weight of carbon and sulphur in which they react together,

= 12 : 64 or 3 : 16     ......(ii)

Now, the ratio (i) and (i) are related to each other 3 : 8 and 3 : 16 or 3 : 3 and 8 : 16 or 1 : 2

Gay-Lussac's Law of Gaseous Volumes

This law was proposed by Gay-Lussac. This law deals with the relation between volume of reactants and products during chemical reactions.

This law states that,

"Under the same conditions of temperature and pressure whenever gases react together the volume of the reacting gases as well as products bears a simple number ratio".

Example, one volume of hydrogen reacts with one volume of chlorine to give two volumes of hydrogen chloride (gas).

H2 + Cl2 → 2HCI

1vol    1vol       2 vol

Hence, volume ratio of H2 : Cl2 : 2HCI is 1:1:2.

Gaseous State

At STP

T = 273°K, p = 1atm = 101.35 Pa, 

V = 22.4 L / mol

Gas Laws

Boyle's Law

This law states that, "the volume of a certain mass of a gas is inversely proportional to its pressure at a constant temperature."

Mathematically,

V ∝ 1/p (temperature and mass constant)

or,              pV = Constant

Let V be the temperature T. The volume of a given mass of gas having pressure p1 at is. If pressure is changed to p2 at the same temperature, let the volume changes to V2. Then, according to Boyle's Law,

p1V1 = p2V2

Charle's Law

At constant pressure, volume of the fixed mass of a gas is directdy proportional to the temperature on Kelvin scale.

Mathematically,

V ∝ T (at constant pressure)

or,            V/T = Constant

Let V1 be the volume of a certain mass of a gas at temperature T1 and at pressure p. If temperature is changed to T2 keeping the pressure constant, volume changes to V2. Then,

V1/T1 = V2/T2

Absolute Temperature or Kehvin Scale

Termperature in Celsius scale is converted into Kelvin scale by addition of 273.

Absolute ternperature =t°C + 273. 

Temperature - Pressure Law

"At constant volume, the pressure of a fixed mass of a gas is proportional to its absolute temperature."

P ∝ T

or,                              P/T = Constant

Gas Equation

Combining Boyle's law, Charles' law and Avogadro's law, the relationship between temperature, pressure and volume can be obtained as follows,

V ∝ 1/p (Boyle's law)      .....(i)

    V ∝ T  (Charle's law)        ......(ii)

By combining Equations. (i) and (ii). we get,

V ∝ T/P

⇒                 pV ∝ T

⇒                pV = RT (where, R = gas constant)

Value af R is 8.314 JK-¹ mol-¹ or 2 cal K-¹ mol-¹. If temperature, volume and pressure of fix amount of a gas vary from T1, V1 and p1 to T2,V2 and p2, respectively.

Then,

p1V1/T1 = p2V2/T2

Dalton's Law of Partial Pressure

"At constant temperature, the pressure of a mixture of two or more non-reacting gases enclosed in a definite volume is equal to the sum of partial pressures of each gas."

If p is the total pressure of the mixture of non-reacting gases at temperature T and volume V

and partial pressure of the gases are p1, p2, p3,..... 

Then,

p = p1 + p2 + p3 + .....(T and V are constant)

Partial Pressure Individual pressure of a gas in the mixture is known as partial pressure of the gas.

Calculation of Partial Pressure If total pressure of the mixture is p, total number of gaseous molecules in the mixture is n and number of molecules of the gas is n1, then partial pressure of the gas is

p1 = p × n1 / n

Graham's Law of Diffusion

This law states that under the same conditions of temperature rate of diffusion of a gas is inversely proportional to the square root of its density or molar mass. If r is rate of diffusion and dis the density, then

r ∝ 1/√d

If r1 and r2 are the rates of diffusion of two gases, whose densities are d1 and d2, respectively. Then, according to Graham's law,

r1/r2 = √d2/√d1

⇒                        r1/r2 = √M2/√M1

Where, M1 and M2 are molar mass of gases.

Where M1 and M2 are molar masses of gases and molar mass (M) = 2 × Density (d1)

∵  r = V/T (Where, V = Volume and T =Time)

Then,

r1/r2 = V1T2/V2T1

Molecular Velocities of Gases

Root Mean Square Velocity Square root of the mean of squares of different velocities possessed by molecules of a gas at a given temperature is called root mean square velocity. It is related to the temperature and molar mass as follows,

vrms = √3RT/√M

If temperature of two gases are T1 and T2 and molar masses are M1 and M2, then root mean square velocity v1 and v2 are

v1/v2 = √T1M2/√T2M1

Most Probable Velocity Most probable velocity is the velocity possessed by maximum number of molecules of a gas at a given temperature. It is represented by ump. It is related to temperature and molar mass as follows

ump = √2RT/M

If temperature of two gases are T1 and T2 and molar masses are M1 and M2 then most probable velocity u1 and u2 are,

u1/u2 = √T1M2/√T2M1

Avogadro's Law

Under similar conditions of temperature and pressure, equal volume of all gases contain equal number of molecules. Avogadro's Number N. It is the number of formula unit present in 1 g-mol of a substance

N = 6.023 x10²³

Important TIPS 

• Law of conservation of mass is not valid for nuclear reactions. 

• Law of constant proportion is not valid for isotopes and metals which show various oxidation states. 

• Absolute temperature is at 0°C/273K and absolute pressure is at 760 mm or 76 cm. 

• For calculation purposes, temperature (°C) should be converted in absolute temperature by adding 273 in it. 

• At constant pressure, when the temperature of a gas having fixed volume, is raised from 0°C to 1°C, the initial volume increase by 1/273 part or vice versa

Rate of effusion ∝ 1/√density 

• Ideal gas is the gas which obey Boyle's law, Charle's law and strictly follow general gas equations. 

• Real gases are the gases which deviate from ideal behaviour at high pressure and laws temperature and do not obey laws. 

• A real gas approach ideal behaviour at high temperature and low pressure 

• Real gas equation (Van Der Waals' equation) is

[P + a/v²] (V - b) = RT

Related Questions : (Answer in comment box.)

Q1. Which one of the following is a correct relationship between mass and energy

(a) E = mc²

(b) E = m/c²

(c) m= Ec²

(d) c = √E/√m

Q2. Law of conservation of mass is not correct for

(a) radioactive change

(b) hydrolysis

(c) oxidation

(d) None of these

Q3. Radioactive change follows the law of

(a) conservation of mass

(b) conservation of mass-energy

(c) Both (a) and (b)

(d) None of the above

Q4. One part of an element A combines with two parts of another element B. Six parts of the element C combines with four parts of the element B. If A and C combined together the ratio of their weights will be governed by

(a) law of definite proportions

(b) law of multiple proportions

(c) law of reciprocal proportions

(d) law ot conservation of mass

Q5. Which of the following compounds do not conform to the law of multiple proportion?

(a) NaCl and BaCl2

(b) CaO and Na2O

(c) All of these

(d) H3PO4 and Ca3(PO4)

Q6. Law of combining volumes was given by

(a) Dalton

(b) Gay-Lussac

(c) Tswett

(d) Einstein

Q7. Real gas will approach the behaviour of ideal gas at

a) low temperature and high pressure

b) high temperature and low pressure

(C) low temperature and low pressure

(d high temperature and high pressure

Q8. Matter can be converted into energy. lt is represented by

(a) E = mc

(b) E = mc²

(c) E = m²c

(d) E = 1/2 mc² 

Q9. Which of the following is not a chemical reaction?

(a) Purification of milk

(b) Formation of water

(c) Burning of coal

(d) Vaporisation

Q10. Which is correct relation between rate of diffusion and their densities

(a) r1/r2 = √d1/√d2

(b) r1/r2 = d1/d2

(c) r1/r2 = √d2/√d1

(d) r1/r2 = d2/d1

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