Problem 101 - Analytical Chemistry

Question

Three molecules of similar size are bearing one, two, and three negative charges, respectively. (a) Which molecule has the highest electrophoretic mobility? (b) Is the electrophoretic mobility a positive or negative value?

Step-by-Step Solution

Answer

(a) The molecule with three negative charges has the highest mobility. (b) The electrophoretic mobility is negative.

Step 1: Understanding Electrophoretic Mobility

Electrophoretic mobility refers to the ability of charged particles to move through a medium under the influence of an electric field. It depends on two main factors: the charge of the molecule and the frictional resistance (often related to the size or shape of the molecule).

Step 2: Analyzing Charge Influence

Since the molecules are of similar size, their electrophoretic mobilities will primarily depend on their charges. More negatively charged molecules will experience a stronger force in the electric field, thus having higher electrophoretic mobility.

Step 3: Comparing Molecule Charges

The molecule with one negative charge has the least electrophoretic mobility, the molecule with two negative charges has intermediate mobility, and the molecule with three negative charges has the highest mobility due to the greater force exerted by the electric field on more charged particles.

Step 4: Determining Electrophoretic Mobility Sign

Electrophoretic mobility is defined as moving towards the opposite charge. Since the molecules bear negative charges, they will move towards the positive electrode. The actual value of electrophoretic mobility in calculations can be described as negative due to this direction (opposite the direction considered positive by convention).

Key Concepts

Charge of Molecules

Molecules can possess different charges based on their chemical structure. A charge is essentially an excess or deficiency of electrons compared to protons. In the context of electrophoresis, it is crucial to understand that the charge impacts how a molecule behaves in an electric field. Negative charges, often indicated by an excess of electrons, are common in biological molecules.

When we talk about electrophoretic mobility, molecules with more negative charges have a tendency to move more swiftly. This is because they experience a larger electric force when placed in an electric field. The larger the charge, the greater the force acting on the molecule.

• Molecules with a higher negative charge move faster.
• Size being constant, charge dictates the movement speed of the molecule in an electric field.

Electric Field Influence

The influence of an electric field on molecules is akin to the way a magnet influences metal filings. An electric field exerts a directional force on charged particles, causing them to move.

In electrophoresis, when molecules are placed in an electric field, they experience this directed force. The force is proportional to the charge of the molecules and dictates both direction and speed of movement.

• Positive charges move towards the negative electrode.
• Negative charges move towards the positive electrode.

The strength of the electric field, alongside the molecule's charge, determines the resulting motion. For larger charges, the effect of the electric field is more pronounced, making these molecules accelerate faster in comparison to those with lesser charge.

Electrophoresis

Electrophoresis is a technique widely used in laboratory settings to separate charged molecules. The principle behind this technique is quite straightforward: separating molecules based on their different rates of migration through a medium, under the influence of an electric field.

The process starts with an electric field applied across a gel or fluid medium. As the electric field penetrates this medium, it compels molecules to move towards the electrode opposite their charge type.

• Negative molecules migrate towards the positive electrode.
• The rate of migration is termed electrophoretic mobility.
• Larger charges result in higher electrophoretic mobility.

The movement of the molecules allows scientists to determine characteristics like the size, charge, and even the purity of the sample being analyzed. Notably, in terms of sign convention, mobility values can be negative for negatively charged molecules as they travel towards a positive electrode.