Boiling Point Elevation: Videos & Practice Problems

Boiling point elevation occurs when a solute is added to a solvent, resulting in a higher boiling point for the solution compared to the pure solvent's normal boiling point. The change in boiling point, denoted as ΔTb, can be calculated using the formula:

ΔTb=i×Kb×m where 'i' represents the Van't Hoff factor, 'Kb' is the boiling point elevation constant of the solvent (in °C/m), and 'm' is the molality of the solution (moles of solute per kilogram of solvent). The boiling point of the solution is then found by adding ΔTb to the normal boiling point of the solvent. Common solvents like water, benzene, chloroform, and ethanol have specific Kb values provided for these calculations. Notably, covalent, non-volatile, or non-ionic compounds have a Van't Hoff factor of 1. Understanding these concepts is crucial for predicting the boiling point of a solution after solute addition.

Boiling Point Elevation

When a solute is added to a pure solvent, the boiling point of the solvent increases, a phenomenon known as boiling point elevation. The more solute that is added, the higher the boiling point becomes. The normal boiling point (denoted as bp) refers to the boiling point of the pure solvent before any solute is introduced, while the boiling point of the solution (denoted as bp solution) is the boiling point after the solute has been added.

The relationship between the amount of solute and the increase in boiling point can be quantified using the boiling point elevation formula:

$\Delta T_b = I \cdot k_b \cdot m$

In this equation, ΔT_b represents the change in boiling point, I is the van 't Hoff factor, which indicates the number of particles the solute breaks into in solution, k_b is the boiling point elevation constant of the pure solvent (expressed in degrees Celsius per molality), and m is the molality of the solution, defined as moles of solute per kilogram of solvent.

To find the boiling point of the solution, the following equation is used:

$bp solution = bp + \Delta T_b$

Common solvents used in boiling point elevation problems include water, benzene, chloroform, and ethanol, each with specific normal boiling points and k_b values: 0.51 for water, 2.53 for benzene, 3.60 for chloroform, and 1.20 for ethanol. While it is not necessary to memorize these values, they are useful for solving problems related to colligative properties.

It is also important to note that for covalent, non-volatile, or non-ionic compounds (which do not dissociate into ions), the van 't Hoff factor I is equal to 1. Understanding these concepts is crucial for accurately determining the boiling point of a solution when a solute is added.

Boiling Point Calculation Example

To calculate the boiling point of a 3.71 molal aqueous calcium bromide solution, we start with the fundamental concept of boiling point elevation. The boiling point of a solution can be determined using the formula:

T_b = T_b^0 + \Delta T_b

Here, T_b is the boiling point of the solution, T_b^0 is the boiling point of the pure solvent (water in this case), and \Delta T_b is the change in boiling point.

For pure water, the boiling point is 100 degrees Celsius. To find the change in boiling point, we use the equation:

\Delta T_b = i \cdot k_b \cdot m

In this equation:

• i is the van 't Hoff factor, which represents the number of particles the solute dissociates into. Calcium bromide (CaBr₂) dissociates into one calcium ion (Ca²⁺) and two bromide ions (Br^-), resulting in a total of three ions. Thus, i = 3.
• k_b is the ebullioscopic constant for water, which is 0.51 degrees Celsius per molal.
• m is the molality of the solution, given as 3.71 molal.

Now, substituting these values into the equation for \Delta T_b:

\Delta T_b = 3 \cdot 0.51 \cdot 3.71

Calculating this gives:

\Delta T_b = 5.68 degrees Celsius

Finally, we add this change to the boiling point of pure water:

T_b = 100 + 5.68 = 105.68 degrees Celsius

This phenomenon is known as boiling point elevation, which indicates that the boiling point of the solution increases as solute is added to the pure solvent. Therefore, the boiling point of the 3.71 molal calcium bromide solution is 105.68 degrees Celsius.

An ethylene glycol solution contains 25.2 g of ethylene glycol (C₂H₆O₂) in 99.5 mL of water. Determine the change in boiling point. Assume a density of 1.00 g/mL for water.

• A. 18.4°C
• B. 9.22°C
• C. 2.08°C
• D. 0.572°C

Answer: (C)

Pure water boils at 100°C. What is the new boiling point of water after the addition of 13.12 g aluminum chloride, AlCl₃, to 615 g water?  

• A. 100.25°C
• B. 100.08°C
• C. 100.33°C
• D. 100.16°C

Answer: (C)

What is the molality of glucose in an aqueous solution if the boiling point of the solution is 103.15°C?

• A. 0.15 m
• B. 0.29 m
• C. 6.18 m
• D. 1.6 m

Answer: (C)

Carbon dioxide is dissolved in 722 mL of benzene with a density of 1.59 g/mL. What mass of carbon dioxide would you add to make the boiling point of the solution 104.7°C?

• A. 11 g
• B. 500 g
• C. 22.9 g
• D. 491 g

Answer: (D)

Here’s what students ask on this topic:

What are some practical applications of boiling point elevation?

Boiling point elevation, a colligative property, has several practical applications across different industries and everyday life. In cooking, adding salt to water increases its boiling point, allowing food to cook at a higher temperature, which can alter cooking times and results. This principle is also used in candy making, where sugar syrups are boiled to specific temperatures to achieve different textures.

In colder climates, boiling point elevation is utilized for road safety. Salts or chemicals are spread on icy roads to lower the freezing point of water, effectively melting ice and snow, which prevents re-freezing and makes roads safer for travel.

In the automotive industry, antifreeze, a solution with a higher boiling point than pure water, is used in car radiators. This prevents the coolant from boiling over in hot weather and from freezing in cold temperatures, ensuring the engine operates within an optimal temperature range.

In industrial processes, boiling point elevation is important in the design of heat exchangers and other equipment where control over phase changes of liquids is crucial for efficiency and safety.

How do you calculate boiling point elevation?

To calculate boiling point elevation, you'll use the formula:

ΔT b = i ⋅ K b ⋅ m where ΔT_b is the boiling point elevation, i is the van't Hoff factor (which represents the number of particles the solute splits into or generates in solution), K_b is the ebullioscopic constant (a property specific to the solvent), and m is the molal concentration of the solute in the solution.

Here's a step-by-step process:

1. Determine the molal concentration (m) of the solute in the solution, which is the number of moles of solute per kilogram of solvent.
2. Find the ebullioscopic constant (K_b) for the solvent, which is usually provided in textbooks or reference materials.
3. Calculate the van't Hoff factor (i) for the solute, which is the number of ions into which a compound dissociates in solution. For non-electrolytes, i is typically 1.
4. Multiply these three values together to find the boiling point elevation (ΔT_b).

Finally, add the boiling point elevation (ΔT_b) to the pure solvent's normal boiling point to get the new boiling point of the solution.

What is boiling point elevation?

Boiling point elevation is a phenomenon where the boiling point of a liquid (a solvent) increases when a solute is dissolved in it. This occurs because the presence of the solute particles makes it more difficult for the solvent molecules to escape into the gas phase, which is necessary for boiling to occur. The added particles disrupt the solvent's natural tendency to vaporize by creating additional intermolecular forces that the molecules must overcome to transition into the gas phase.

To boil the solvent, you now need to apply more heat to give the molecules enough energy to break these interactions. The amount of boiling point elevation is directly proportional to the molal concentration of the solute particles in the solution. This relationship is described by the boiling point elevation equation:

ΔT_b = iK_bm

where ΔT_b is the change in boiling point, K_b is the ebullioscopic constant (a property of the solvent), m is the molal concentration of the solution, and i is the van 't Hoff factor, which accounts for the number of particles the solute breaks into or forms in the solution.