This article explores the principles behind vapour pressure, including key laws such as Raoult’s Law and the Antoine Equation.

What Is Vapour Pressure?

Vapour pressure is the pressure exerted by the vapour of a substance in thermodynamic equilibrium with its condensed phases (solid or liquid) in a closed system. At a given temperature, the vapour pressure represents the tendency of molecules to escape from the liquid or solid phase into the gaseous phase.

As temperature increases, more molecules have the energy to escape into the vapour phase, leading to an increase in vapour pressure.

This property does not depend upon quantity. It can be calculated by using the Antoine equation which expresses vapour pressure as a function of temperature.

What Is The Antoine Equation?

The Antoine Equation is an empirical relationship that describes the variation of vapour pressure with temperature. It is widely used because of its simplicity and accuracy for a broad range of substances.

Where:

T – Temperature of the liquid or substance

P – Vapour Pressure of a liquid or substance

A, B & C – are liquid or substance specific constants/ coefficients

The equation can be rearranged to calculate temperature as follows:

In fractional distillation, this property plays an important role as the design of the column depends upon vapour pressure differences.

A Brief History of The Antoine Equation

The Antoine Equation was developed by French engineer and chemist Louis Charles Antoine in 1888. Antoine’s work was pivotal in providing a practical tool for engineers and scientists to calculate vapour pressures at different temperatures, especially in the design and operation of distillation columns and other separation processes.

What Are The Units of Vapour Pressure?

In general value of V_p is measured in the same units of pressure. As we know that there are different units available for a measure of pressure like:

• kg/cm²
• PSI
• N/m²
• kPa
• Bar
• Pascal

Factors That Affect Vapour Pressure

Some of the key factors which affect the vapour pressure are:

• Temperature
• Solute concentration and nature
• Boiling point

How Does Temperature Affect Vapour Pressure?

As you increase the temperature of the solid or liquid in a system then its V_p will also increase and vice versa for when it decreases.

How Does Solute Concentration And Nature Affect Vapour Pressure?

If you add more non-volatile solute to dissolve into a volatile solvent then the vapour pressure of the solvent will reduce, hence, in this example the more solute you add the lower the vapour pressure of the solute gets. 

Factors That Do Not Affect Vapour Pressure

Volume

V_p does not increase or decrease with respect to the volume of its system.

Surface Area

The surface area of the solid or liquid in contact with the gas will have no effect on the vapour pressure of the system.

What is Raoult’s Law? 

Raoult’s Law is a principle that relates the vapour pressure of an ideal solution to the vapour pressures of its individual components and their mole fractions. It states that the partial vapour pressure of each component in a solution is proportional to its mole fraction in the solution and its vapour pressure when at the same temperature. It can be represented in the formula below:

Where:

P_solution – Vapour Pressure of the solution

X_solvent – Mole fraction of the solvent

P_solvent – Vapour Pressure of the pure solvent

Raoult’s law can be used to estimate the contribution of individual components of a liquid or solid mixture to the total pressure exerted by the system.

We can use Raoult’s Law to calculate the vapour pressure of a given liquid. So Raoult’s Law is very helpful in the design of distillation columns. Using Raoult’s Law we can calculate the required temperature under a given vacuum in a distillation system.

A Brief History of Raoult’s Law

Raoult’s Law was first discovered by French chemist François-Marie Raoult in 1887. Raoult’s work on the connecting properties of solutions, such as freezing point depression and boiling point elevation, led to the development of this law. Although Raoult’s Law applies strictly to ideal solutions, it laid the groundwork for understanding the behaviour of real solutions, particularly in chemical engineering and thermodynamics.

Vapour Pressure of Water

At 25 degrees Celsius, the vapour pressure of water is 23.8 mmHg. At 100 degrees Celsius, water reaches its boiling point, and the vapour pressure becomes equal to one atmosphere (which is equivalent to 760 mmHg).

Hassan Ahmed

Hassan graduated with a Master’s degree in Chemical Engineering from the University of Chester (UK). He currently works as a design engineering consultant for one of the largest engineering firms in the world along with being an associate member of the Institute of Chemical Engineers (IChemE).

The post A Comprehensive Guide to Vapour Pressure | Understanding Key Laws and Their Applications appeared first on Engineeringness.

Aluminium Chloride and Sodium Hydroxide Example

The best way to show the factors affecting a chemical reaction is to use an example. The chemical reaction we will look at is the double displacement reaction between Aluminium Chloride (AlCl₃) and Sodium Hydroxide (NaOH) that produces Aluminium Hydroxide (Al(OH)₃) and Sodium Chloride (NaCl) (equation 1).

AlCl3(aq) + 3NaOH(aq)  → Al(OH)3(s)  + 3NaCl(aq)

Equation 1: Double displacement reaction between Aluminium Chloride and Sodium Hydroxide

Factors Affecting The Rate Of A Chemical Reaction:

Concentration

The concentration of reactants in a chemical reaction is related to the rate of a chemical reaction. If we increase reactant concentration, then the rate of the reaction will also increase. The rate of the reaction is dependent on the concentration of both the reactants. Because the AlCl₃ solution is a strong electrolyte, it dissociates completely into two ions, Al³⁺ and Cl^–. Therefore, it means that AlCl₃ and Al³⁺ ions are present in aqueous solution. In this reaction, AlCl₃ is the limiting reagent, as the rate of the reaction is dependent on it.

While sodium hydroxide solution is not an electrolyte, but it dissociates into Na⁺ and OH^– ions. Thus, NaOH and OH^–ions are present in aqueous solution. In this reaction, NaOH is the limiting reagent, in terms of concentration. This is because the rate of the reaction is dependent on it. So, this reaction is an equilibrium reaction, as it depends on the product.

The pH of the reaction must be in the range required by the reaction. If it is outside this range, then the reaction will be slow. For instance, in the case of the reaction between AlCl₃ and NaOH, the concentration of both the reactants are equal. However, in this case, the highest concentration of the product is achieved when the reaction is carried out at pH 3, and the highest concentration of the reactant is at pH 9. So, the reaction requires the pH to be in the range of 3-9.

Physical State

The physical state of the reactants affects the rate of a chemical reaction. For instance, in the case of the reaction between AlCl₃ and NaOH, if both the reactants are in the gaseous state, then it will be difficult to achieve the equilibrium. This is because gaseous reactants move away from each other, and the distance between them has a bearing on the diffusion rate. Moreover, in the gaseous state, the molecules have high kinetic energy, which slows down the rate of reaction.

While if both the reactants are in the solid-state, then it is easier to reach the equilibrium, in this case. Solid reactants do not have great kinetic energy. Moreover, in the solid-state, the molecules are close to each other, so it is easier to achieve equilibrium.

If either of the reactants is in a liquid state, then it is easy to achieve the equilibrium in the liquid state also. But, if both the reactants are in the liquid state, then it will be easier to achieve equilibrium again, as the molecules are closer to each other. Thus, the liquid state is always favourable for a reaction, the reason being that it ensures a greater number of collisions. On the other hand, free gas is less reactive as compared to a gas dissolved in a solution.

Catalyst

 The presence of a catalyst has an impact on the rate of the reaction. For instance, in the case of a reaction between hydrogen and oxygen, a catalyst is not required. This is because neither reaction is extremely fast, nor reactive. However, if we want a faster and more productive reaction, we can use the catalyst in this case, as well. The catalyst’s role, in this case, is to speed up the rate of reaction. But this also depends on the type of catalyst used. For instance, different catalysts react differently under different conditions.

Temperature

The temperature of the reaction has a big impact on the rate of the chemical reaction. If we increase the temperature of the reaction, hence the temperature of both the reactants, then the reaction rate will increase. Hence, the rate of chemical reactions increases with temperature.

In the reaction between AlCl₃ and NaOH, if the temperature of the two reactants is doubled, then the rate of reaction, hence production will also increase. For instance, if we carry out the reaction at a temperature of 98°C, then the rate of reaction will increase by a factor of four, as the rate of reaction doubles with every 10° rise in temperature.

Surface Area

 The surface area of both the reactants has a significant impact on the rate of reaction. It is because the surface area of reactants has a direct relation with the number of collisions per unit time. Therefore, if we want to increase the rate of a chemical reaction, then we must increase the surface area of both the reactants.

In the case of a reaction between AlCl₃ and NaOH, there is no significant change in the reaction rate, whether the number of reactants is increased or the reactants change in shape or size. However, if we increase the surface area of a reactant, then the reaction rate also increases. For instance, if AlCl₃ and NaOH are powdered, then the reaction rate will decrease by a factor of two, as only a small number of ions come in contact with each other. So, we can state that the rate of reaction is dependent on the surface area of the reactants. The maximum size of reactant particles in which a chemical reaction takes place at the maximum rate is called the critical size.

Thus, in the case of the reaction between AlCl₃ and NaOH, both these reactants are powdered, hence the change in the rate of reaction is not significant. Therefore, due to the powdered nature of the reactants, the reaction rate is dependent on the size of the reactants. As a result, the reactants keep on colliding with each other, in case of powdered reactants. Hence, the rate of the reaction is higher in this scenario.

Dr. Adam Zaidi

Dr. Adam Zaidi, PhD, is a researcher at The University of Manchester (UK). His doctoral research focuses on reducing carbon dioxide emissions in hydrogen production processes. Adam’s expertise includes process scale-up and material development.’

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