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.
FAQs on Effect of Change of Pressure
1. What is the general effect of changing pressure on the states of matter?
Increasing pressure generally brings the particles of matter closer together. This effect is most significant in gases, which are highly compressible and can be turned into liquids. The effect on liquids and solids is minimal because their particles are already packed closely together with very little empty space between them.
2. How can a gas be turned into a liquid using pressure?
A gas can be converted into a liquid through a process called liquefaction by simultaneously applying high pressure and lowering the temperature. Applying pressure forces the gas particles closer, which increases the intermolecular forces of attraction. Lowering the temperature reduces the kinetic energy of the particles, making it easier for these forces to convert the gas into a liquid. A common example is the Liquefied Petroleum Gas (LPG) in cylinders.
3. How does increasing pressure affect the boiling point of a liquid?
Increasing the external pressure on a liquid raises its boiling point. For a liquid to boil, its internal vapour pressure must overcome the external atmospheric pressure. When the external pressure is increased, the liquid requires more energy, and thus a higher temperature, to reach the point of boiling. This principle is famously used in a pressure cooker to cook food faster.
4. How does a change in pressure affect a chemical reaction at equilibrium?
According to Le Chatelier's principle, if the pressure is changed for a gaseous reaction at equilibrium, the system will shift to counteract that change. This effect is only relevant when the number of gas molecules on the reactant and product sides are different.
- If pressure is increased, the equilibrium shifts to the side with fewer gas molecules.
- If pressure is decreased, the equilibrium shifts to the side with more gas molecules.
5. Why does pressure significantly affect gases but has a negligible effect on solids and liquids?
The large difference in the effect of pressure is due to the distance between particles in each state of matter. In gases, particles are far apart with weak intermolecular forces, leaving a lot of empty space, which makes them highly compressible. In liquids and solids, particles are already tightly packed with strong intermolecular forces, so applying pressure cannot significantly reduce their volume further.
6. What are some real-world examples of applying the effect of pressure change?
The effects of changing pressure are evident in many common applications and natural phenomena:
- Aerosol Deodorants: High-pressure gas inside the can forces the liquid product out as a spray when the valve is pressed.
- Hydraulic Brakes: Applying pressure to the brake fluid, which is a liquid, transmits force to stop a vehicle. This works because liquids are nearly incompressible.
- Deep-Sea Diving: Divers experience immense pressure from the water above, which can affect the gases dissolved in their blood.
- Cooking at High Altitudes: At high altitudes, the atmospheric pressure is lower. This causes water to boil at a temperature below 100°C, increasing cooking time.
7. How do pressure and temperature work together to change the state of matter?
Pressure and temperature are interdependent factors that control the state of a substance. Applying pressure pushes particles closer, favouring a denser state (solid or liquid). Increasing temperature gives particles more kinetic energy, causing them to move apart and favouring a less dense state (liquid or gas). To efficiently liquefy a gas, one must both increase the pressure and decrease the temperature, as one without the other may not be sufficient to cause a phase change.
8. Is it possible to liquefy any gas just by applying pressure, regardless of its temperature?
No, pressure alone is not always enough. Every gas has a specific critical temperature. Above this temperature, the gas particles possess too much kinetic energy, and no amount of pressure can force them to condense into a liquid. To liquefy a gas, it must first be cooled to a temperature below its critical point. Only then will applying pressure be effective in turning it into a liquid.
