Average kinetic energy depends only on temperature Steps:

• Average kinetic energy of gas particles is given by KE = (3/2) kT, where T is temperature in Kelvin.
• Concentration changes affect particle number or reaction rate but not KE per particle.
• Catalysts speed up reactions by lowering activation energy but do not alter temperature or KE.
• Identify the option that modifies temperature to change KE of HCl gas.

Why B is correct:

• Cooling the reaction mixture lowers the temperature, directly reducing the average kinetic energy of HCl particles per kinetic molecular theory.

Why the others are wrong:

• A: Increasing hydrogen concentration affects rate but not KE; increasing temperature raises KE, not just changing it downward.
• C: Catalyst and chlorine concentration both impact rate only, leaving temperature and KE unchanged.
• D: Chlorine concentration affects rate; increasing temperature raises KE, not reducing it.

Final answer: B