What is Positive Deviation from Raoult's Law?

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Grade Level:

Class 12

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Definition

What is it?

Positive deviation from Raoult's Law occurs when the vapor pressure of a solution is higher than what Raoult's Law predicts. This happens because the attractive forces between different molecules (A-B) are weaker than the attractive forces between similar molecules (A-A or B-B). As a result, molecules escape into the vapor phase more easily.

Simple Example

Quick Example

Imagine you have two friends, Rahul and Priya, who usually like to sit together in class. If you mix them with another friend, Rohan, and suddenly Rahul and Priya prefer sitting alone rather than with Rohan, they will escape to different benches more easily. Similarly, if molecules in a solution don't 'like' each other much, they escape into vapor more easily, increasing pressure beyond what's expected.

Worked Example

Step-by-Step

Let's say we have two liquids, A and B.---Step 1: Raoult's Law predicts that the partial vapor pressure of A in a solution (P_A_predicted) should be its pure vapor pressure (P_A_pure) multiplied by its mole fraction (X_A). So, P_A_predicted = P_A_pure * X_A.---Step 2: Similarly, for B, P_B_predicted = P_B_pure * X_B.---Step 3: The total predicted vapor pressure (P_total_predicted) would be P_A_predicted + P_B_predicted.---Step 4: Now, let's say after mixing A and B, we actually measure the vapor pressure of A (P_A_actual) and B (P_B_actual).---Step 5: If P_A_actual is greater than P_A_predicted, and P_B_actual is greater than P_B_predicted, then the solution shows positive deviation.---Step 6: For example, if P_A_pure = 100 mmHg, X_A = 0.5, then P_A_predicted = 100 * 0.5 = 50 mmHg. If we actually measure P_A_actual = 60 mmHg, it's a positive deviation.---Step 7: If P_B_pure = 200 mmHg, X_B = 0.5, then P_B_predicted = 200 * 0.5 = 100 mmHg. If we actually measure P_B_actual = 120 mmHg, it's also a positive deviation.---Step 8: So, the total actual vapor pressure (180 mmHg) is higher than the total predicted vapor pressure (150 mmHg), indicating positive deviation. Answer: The solution exhibits positive deviation from Raoult's Law.

Why It Matters

Understanding positive deviation is crucial in industries like chemical engineering and medicine. Chemical engineers use this knowledge to design distillation columns for separating mixtures efficiently, which is important for making medicines or fuels. It also helps biotechnologists understand how different solvents behave when processing biological samples.

Common Mistakes

MISTAKE: Thinking that positive deviation means the vapor pressure is zero or very low. | CORRECTION: Positive deviation means the vapor pressure is HIGHER than what Raoult's Law calculates, not lower.

MISTAKE: Confusing positive deviation with ideal solutions. | CORRECTION: Ideal solutions perfectly obey Raoult's Law. Positive deviation means the solution does NOT obey Raoult's Law perfectly; it deviates upwards.

MISTAKE: Believing that strong attractive forces between A and B cause positive deviation. | CORRECTION: Positive deviation occurs when the attractive forces BETWEEN A and B are WEAKER than the forces within A-A or B-B, making molecules escape more easily.

Practice Questions

Try It Yourself

QUESTION: If a solution of ethanol and water shows positive deviation, what can you say about the attractive forces between ethanol and water molecules compared to ethanol-ethanol and water-water molecules? | ANSWER: The attractive forces between ethanol and water molecules are weaker than the attractive forces between ethanol-ethanol molecules and water-water molecules.

QUESTION: A solution has a predicted vapor pressure of 150 mmHg based on Raoult's Law. If the actual measured vapor pressure of the solution is 170 mmHg, what kind of deviation is observed? Explain why. | ANSWER: Positive deviation. This is because the actual vapor pressure (170 mmHg) is higher than the predicted vapor pressure (150 mmHg).

QUESTION: For a binary solution of liquids A and B, the pure vapor pressures are P_A_pure = 80 kPa and P_B_pure = 120 kPa. If the mole fraction of A is 0.4 and the actual partial vapor pressure of A is 35 kPa, and B is 75 kPa, does the solution show positive deviation? Calculate the predicted total vapor pressure and compare it with the actual total vapor pressure. | ANSWER: Predicted P_A = 80 kPa * 0.4 = 32 kPa. Predicted P_B = 120 kPa * 0.6 = 72 kPa. Predicted Total P = 32 + 72 = 104 kPa. Actual Total P = 35 + 75 = 110 kPa. Yes, the solution shows positive deviation because the actual partial vapor pressures (35 kPa and 75 kPa) are higher than the predicted ones (32 kPa and 72 kPa), and the actual total vapor pressure (110 kPa) is higher than the predicted total vapor pressure (104 kPa).

MCQ

Quick Quiz

Which of the following is true for a solution showing positive deviation from Raoult's Law?

The attractive forces between A-B molecules are stronger than A-A or B-B.

The actual vapor pressure of the solution is lower than predicted.

The actual vapor pressure of the solution is higher than predicted.

The solution is an ideal solution.

The Correct Answer Is:

Positive deviation means the molecules escape into the vapor phase more easily, leading to a higher vapor pressure than what Raoult's Law predicts. Options A and B describe negative deviation or incorrect behavior, and D describes ideal solutions.

Real World Connection

In the Real World

In India, the pharmaceutical industry often deals with solutions that show positive deviation. When making certain cough syrups or antiseptic solutions, chemists need to know how different ingredients will mix and evaporate. Understanding positive deviation helps them choose the right solvents and predict how much of each component will be present in the vapor, ensuring the medicine is effective and safe.

Key Vocabulary

Key Terms

RAOULT'S LAW: A law stating that the partial vapor pressure of each component in an ideal mixture is equal to the vapor pressure of the pure component multiplied by its mole fraction in the mixture. | VAPOR PRESSURE: The pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. | MOLE FRACTION: The ratio of the number of moles of one component to the total number of moles of all components in a mixture. | IDEAL SOLUTION: A solution that obeys Raoult's Law perfectly over the entire range of concentrations and temperatures.

What's Next

What to Learn Next

Great job understanding positive deviation! Next, you should explore 'Negative Deviation from Raoult's Law'. This concept is the opposite of what you just learned, where vapor pressure is lower than predicted, and it will help you get a complete picture of how real solutions behave.