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1.3.1 - Relative Lowering of Vapour Pressure

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Introduction to Solutions

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Teacher
Teacher

Today we will start by understanding what solutions are. Can anyone tell me how we define a solution?

Student 1
Student 1

A solution is a mixture where the composition is uniform throughout.

Teacher
Teacher

Excellent! Solutions can be classified into solid, liquid, and gas forms. But today we will focus more on liquid solutions. What do you think happens to the vapour pressure when we add a solute to a volatile solvent?

Student 2
Student 2

I think the vapour pressure would decrease.

Teacher
Teacher

Correct! This decrease is due to the presence of solute particles, which interfere with the escape of solvent molecules into the vapour phase.

Raoult’s Law

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Teacher
Teacher

Let’s delve deeper into how we quantify this vapour pressure lowering. What principle do we use to describe this relationship?

Student 3
Student 3

Is it Raoult’s Law?

Teacher
Teacher

Exactly! Raoult’s Law states that the partial vapour pressure of each component in a solution is proportional to its mole fraction. Can anyone summarize how this is mathematically expressed?

Student 4
Student 4

p = p₀ * x, where p₀ is the vapour pressure of the pure solvent.

Teacher
Teacher

Good job! This formula is essential in calculating how the concentration of solute affects the vapour pressure.

Colligative Properties

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Teacher
Teacher

Now, let's connect vapour pressure lowering to the concept of colligative properties. Who can tell me what colligative properties are?

Student 1
Student 1

They are properties that depend on the number of solute particles in a solution, regardless of their identity.

Teacher
Teacher

Exactly! Examples include boiling point elevation, freezing point depression, and osmotic pressure. Can anyone think of how relative lowering of vapour pressure ties into these properties?

Student 2
Student 2

Since all these properties depend on the amount of solute, the more solute you have, the greater the change in these properties.

Teacher
Teacher

Perfectly stated! The relationship shows us just how vital solute concentration is in any solution.

Practical Applications

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Teacher
Teacher

Let’s discuss some real-world applications of these concepts. Can anyone give me an example where the lowering of vapour pressure plays a role?

Student 3
Student 3

How about antifreeze in cars? It lowers the freezing point of water.

Teacher
Teacher

Absolutely! And it also raises the boiling point, making it effective in extreme temperatures. What other examples can you think of?

Student 4
Student 4

In food preservation, salt is used to lower the thawing point of ice and keep the food fresh.

Teacher
Teacher

Great examples! Understanding these concepts helps us appreciate their applications in daily life.

Introduction & Overview

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Quick Overview

This section discusses the concept of relative lowering of vapour pressure in solutions and the principles underlying colligative properties.

Standard

The relative lowering of vapour pressure is a key concept in understanding how solute concentration affects the properties of solutions. This section details the relationship between vapour pressure and solute concentration, highlights Raoult’s Law, and introduces colligative properties that depend on the number of solute particles in a solution rather than their identities.

Detailed

Relative Lowering of Vapour Pressure

In this section, we explore the phenomenon of relative lowering of vapour pressure in solutions. A solution is defined as a homogeneous mixture of a solvent and one or more solutes. When a non-volatile solute is added to a volatile solvent, the solution’s vapour pressure decreases compared to that of the pure solvent.

Raoult’s Law establishes that the vapour pressure of a solvent is directly proportional to the mole fraction of the solvent in the solution. The lowering of vapour pressure (p) is given by the equation 0p = p₀ - p. Here, p₀ is the vapour pressure of the pure solvent and p is the vapour pressure of the solution. The relative lowering of vapour pressure is expressed as:

Relative Lowering

dp/p₀ = x₂

Where x₂ represents the mole fraction of the solute. This concept is crucial as it underpins other colligative properties such as boiling point elevation, freezing point depression, and osmotic pressure, which all depend solely on the quantity of solute particles.

In the practical sense, the lowering of vapour pressure is significant in various fields, including biology and environmental science, as it affects the phase behavior of solutes in solvents.

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Audio Book

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Understanding Vapour Pressure

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The vapour pressure of a solvent in a solution is less than that of the pure solvent. Raoult established that the lowering of vapour pressure depends only on the concentration of the solute particles and it is independent of their identity. The equation (1.20) given in Section 1.4.3 establishes a relation between vapour pressure of the solution, mole fraction and vapour pressure of the solvent, i.e.,
p = x p₀ (1.22)

In a solution containing several non-volatile solutes, the lowering of the vapour pressure depends on the sum of the mole fraction of different solutes.

Detailed Explanation

Vapour pressure is the pressure exerted by the vapour of a liquid when it is in equilibrium with its liquid phase. When a solute is dissolved in a solvent, this equilibrium is affected. Specifically, the presence of non-volatile solute particles reduces the number of solvent molecules that can escape into the vapour phase, thereby reducing the overall vapour pressure of the solution compared to the pure solvent.

Raoult's law states that the lowering of vapour pressure is proportional to the mole fraction of the solute. This means that if you have a solution with more solute particles relative to the solvent particles, the vapour pressure will be lower. The equation p = x p₀ links the observed vapour pressure (p) of the solution to the mole fraction of solvent (x) and the vapour pressure of the pure solvent (p₀). It shows that as the mole fraction of the solute increases, the overall pressure decreases.

Examples & Analogies

Think of vapour pressure like a crowded room where people represent solvent molecules. In a situation where only solvent molecules are in the room (pure solvent), there can be a lot of people who can leave by going out the door (escaping to the vapour phase). However, if you add a large number of people carrying heavy bags (solute particles), they take up space and block some of the exits, making it harder for people to leave. Consequently, fewer people can escape and the 'pressure' inside the room (the vapour pressure of the solvent) will be less.

Mathematical Representation of Vapour Pressure Lowering

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The reduction in the vapour pressure of solvent (∆p) is given as:

∆p = p₀ – p = p₀ – p₀ x₁ = p₀ (1 – x₁) (1.23)

Knowing that x = 1 – x₁, equation (1.23) reduces to:

∆p = x₂ p₀ (1.24)

Detailed Explanation

This chunk explains how to mathematically express the reduction in vapour pressure caused by the addition of a solute. The change in vapour pressure (∆p) is calculated by taking the difference between the vapour pressure of the pure solvent (p₀) and that of the solution (p).

The equation ∆p = p₀ – p establishes that the reduction (or lowering) of vapour pressure is a function of the concentration of solute expressed in mole fraction. As the concentration of the solute increases, it directly affects the overall vapour pressure observed. The equation can further be simplified using mole fractions, showing that the lowering of vapour pressure is also proportional to the mole fraction of the solute (x₂), confirming its direct relationship.

Examples & Analogies

Imagine a hyperloop station that has a certain air pressure (p₀) when it's empty. Now, if you start adding people (solute) into the station, the air pressure (vapour pressure) inside drops because there's less space for air to move freely. When we calculate the difference in air pressure before and after adding people, we can see how much it drops based on how many people have been added. This is akin to how the lower density of solvent molecules (due to the presence of solute) leads to reduced vapour pressure.

Colligative Properties Overview

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In a solution containing several non-volatile solutes, the lowering of the vapour pressure depends on the sum of the mole fractions of different solutes. Equation (1.24) can be written as:

∆p/p₀ = x₂ (1.25)

The expression on the left-hand side of the equation as mentioned earlier is called relative lowering of vapour pressure and is equal to the mole fraction of the solute.

Detailed Explanation

We recognize that solutions can contain multiple solutes, and the lowering of vapour pressure can be summed across all solutes present. The relationship indicated by the equation ∆p/p₀ = x₂ shows that the proportionality is maintained for multiple solutes, meaning that the total effect on vapour pressure can be assessed by considering the combined effect of each solute. Here, the relative lowering of vapour pressure is directly proportional to the mole fraction of the solute present in the solution.

Examples & Analogies

Picture a busy coffee shop where each added friend (solute) reduces the overall 'atmospheric pressure' of conversation. Each friend you bring has their own energy and presence, contributing to the dynamic of the coffee shop. The collective contribution of all friends to the overall buzz of conversation reduces the silence you’d enjoy on your own. The same principle applies when multiple solutes affect the vapour pressure of the solvent — their collective presence lowers it further.

Definitions & Key Concepts

Learn essential terms and foundational ideas that form the basis of the topic.

Key Concepts

  • Relative Lowering of Vapour Pressure: The decrease in vapour pressure of a solvent due to the presence of a non-volatile solute.

  • Raoult’s Law: The principle that describes the relationship between vapour pressure and mole fraction of a solvent.

  • Colligative Properties: Properties that depend on the total number of solute particles in a solution, including boiling point elevation and freezing point depression.

Examples & Real-Life Applications

See how the concepts apply in real-world scenarios to understand their practical implications.

Examples

  • When a solute such as salt is dissolved in water, the vapour pressure of the resulting solution is lower than that of pure water.

  • In automotive antifreeze solutions, the presence of antifreeze agents lowers the freezing point of water.

Memory Aids

Use mnemonics, acronyms, or visual cues to help remember key information more easily.

🎵 Rhymes Time

  • If you want to rise the boiling line, add a solute to make it shine.

📖 Fascinating Stories

  • Imagine a thirsty bird, it could only drink in the rain. Just like in solutions, more drink means a lower pressure!

🧠 Other Memory Gems

  • Use the acronym 'CRAMP' to remember: Colligative properties Relate to Amount of solute Mass of solvent Present.

🎯 Super Acronyms

Use 'RELAX' to remember

  • Relative lowering of vapour pressure Equals the Loss in pressure At eXample time.

Flash Cards

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Glossary of Terms

Review the Definitions for terms.

  • Term: Solution

    Definition:

    A homogeneous mixture of two or more substances.

  • Term: Vapour Pressure

    Definition:

    The pressure exerted by a vapor in equilibrium with its liquid or solid phase.

  • Term: Raoult’s Law

    Definition:

    A law stating that the vapour pressure of a solvent in a solution is proportional to its mole fraction.

  • Term: Colligative Properties

    Definition:

    Properties that depend on the number of solute particles in a solution rather than their identity.

  • Term: Nonvolatile Solute

    Definition:

    A substance that does not vaporize easily and does not exert vapour pressure.