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Effect of Change of Pressure

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Effect of Change of Pressure: An Introduction

Exerted force on an object is called pressure. The force is perpendicular to the surface per unit area. Pressure is represented by the formula F/A, where F is force and A is per unit area. Pascal is the unit of pressure. Types of pressures are absolute, atmospheric, differential and gauge pressure. Pressure can change the shape of solid, liquid and gas molecules but the volume of liquid will be constant.


Effect of Pressure/Change in Pressure

A physical force applied to the per unit surface area of an object is called pressure. if the surface area decreases, the pressure increases; when the surface area decreases, the pressure decreases. Atmospheric pressure influences the boiling point of water. When atmospheric pressure increases, the boiling point becomes higher and when the atmospheric pressure decreases, the boiling point becomes lower.


Effect of Pressure on States of Matter

We can bring the particles of matter closer and closer by applying pressure on them. So, liquids can be converted into solids by applying pressure and further when we apply pressure on a gas enclosed in a cylinder, it starts compressing and converting into liquid. Basically, the volume of gas decreases with an increase in pressure, due to which gas converts into liquid and eventually into a solid state. The number of molecules in the gas is directly proportional to the pressure of gas and volume is inversely proportional to the gas.


As solids are non-compressible states of matter, pressure has less effect on them. Liquids can be converted into solids by exerting pressure and reducing temperature. Some factors affect the pressure in liquids like the density of the liquid, depth of liquid, acceleration due to gravity etc.


Effect of Pressure on Equilibrium

If the pressure of a gaseous reaction mixture is changed, the equilibrium will shift to minimise that change. The equilibrium will shift to favour a decrease in pressure if the pressure is increased. The equilibrium will shift to favour an increase in pressure if the pressure is decreased.


The pressure will increase when the volume of a system is decreased (and the temperature is constant). There are a greater number of collisions with the walls of the container. If there are fewer gas molecules, there will be less collisions and therefore lower pressure. The equilibrium will shift in a direction that decreases the number of gas molecules so that the pressure is also reduced. You need to look at the number of gas molecules in the balanced reactions, so, you can predict in which direction the equilibrium shifts to change pressure.


For example, the chemical reaction between nitrogen and hydrogen is shown below:

\[{N_2}\left( g \right) + 3{H_2}\left( g \right) \mathbin{\lower.3ex\hbox{$\buildrel\textstyle\leftharpoonup\over{\smash{\rightharpoondown}}$}} 2N{H_3}\left( g \right)\]

The ratio in the balanced equation is 1:3:2. That is, 1 molecule of N2 reacts with 3 molecules of H2 and forms the NH3 gas as a product (from the balanced equation). Therefore, the ratio is four molecules of reactant gas to yield two molecules of product gas.

  • An increase in pressure will favour the reaction that decreases the number of gas molecules. The forward reaction is favoured because there are fewer molecules of product gas. The yield of NH3 will increase because the equilibrium will shift to the right.

  • A decrease in pressure will favour the reaction that increases the number of gas molecules. The reverse reaction is favoured if there are more molecules of reactant gas. The yield of NH3 will decrease because the equilibrium will shift to the left.

Interesting Facts

  • Pressure is directly proportional to the temperature.

  • Pressure can easily apply to gases as compared to solids and liquids.

Key Features

  • Solid and liquid have less effect of pressure than gases.

  • The boiling point of water increases when atmospheric pressure is increased.

  • If the pressure of the gaseous mixture is changed, the equilibrium will shift to minimise that change.

Last updated date: 20th Sep 2023
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FAQs on Effect of Change of Pressure

1. Does changing the size of the container affect the pressure?

If the equilibrium reaction had not involved a change in the number of molecules in the gas phase, no shift in the concentrations could have made any difference in the pressure. Thus, changing the pressure by changing the size of the container would have no effect on the reaction. As the container size decreases or increases, the number of molecules remains the same and exerts the same amount of pressure.

2. How can the physical condition of the matter be altered?

By varying the pressure applied, the physical condition of matter can also be altered. Gases can be liquefied, by exerting pressure and lowering the temperature. A gas liquefies when it is compressed on high pressure applied to it, and when the temperature is lowered. As the pressure of the gas increases, the force of attraction increases and the gas is converted into liquid. For example, LPG (Liquefied Petroleum Gas).

3. How does pressure affect the boiling point of a liquid?

All liquids expand when they vapourise. The effect of pressure on the surface is the expansion and delay of vapourisation. Thus, the boiling point is increased on applying pressure. As the pressure increases, the temperature of the gas increases as the force of attraction increases between the atoms. More temperature is required to break the bond between atoms.