**Change in Pressure and Reduction Potential:**

The reduction potential of a half cell is a measure of the tendency of a species to gain electrons and undergo reduction. It is generally expressed as the standard reduction potential, which is the reduction potential under standard conditions (1 atm pressure, 1 M concentration, and 25 degrees Celsius temperature).

When the pressure of a gas is increased, the concentration of the gas increases as well, assuming the volume and temperature remain constant. In the case of hydrogen gas (H2), an increase in pressure affects the concentration of H+ ions in the solution, which influences the reduction potential of the hydrogen half cell.

**Le Chatelier's Principle:**

Le Chatelier's principle states that when a system at equilibrium is subjected to a change in conditions (such as pressure), it will adjust to counteract the change and restore equilibrium. In this case, an increase in pressure will cause the system to shift in a direction that reduces the total number of gas molecules.

**Effect on Reduction Potential:**

An increase in pressure will cause the system to shift in the direction that decreases the concentration of gas molecules. In the case of the hydrogen half cell, an increase in pressure will cause the system to shift towards the side with fewer gas molecules, which is the side with H+ ions.

Since the reduction potential of the hydrogen half cell is determined by the concentration of H+ ions, an increase in pressure will increase the concentration of H+ ions and consequently increase the reduction potential.

**Calculation:**

To calculate the exact change in reduction potential, we need to use the Nernst equation, which relates the reduction potential to the concentration of species involved. The Nernst equation is as follows:

E = E° - (RT/nF) * ln(Q)

Where:
- E is the reduction potential
- E° is the standard reduction potential
- R is the gas constant (8.314 J/(mol·K))
- T is the temperature in Kelvin (25 degrees Celsius = 298 K)
- n is the number of electrons involved in the reaction (for hydrogen half cell n = 2)
- F is Faraday's constant (96485 C/mol)
- Q is the reaction quotient, which is the ratio of product concentrations to reactant concentrations

Using the Nernst equation, we can calculate the change in reduction potential by comparing the reduction potential under the initial and final conditions.

**Conclusion:**

In summary, when the pressure of H2 gas is increased from 1 atm to 100 atm while keeping the H+ concentration constant at 1 M, the reduction potential of the hydrogen half cell at 25 degrees Celsius will increase. This is because an increase in pressure results in an increase in the concentration of H+ ions, leading to a higher reduction potential. The change in reduction potential can be calculated using the Nernst equation, which takes into account the temperature, standard reduction potential, and concentration of species involved in the half cell reaction.