The reaction between ethylene C2H4 and hydrogen H2 is a hydrogenation reaction, which is commonly catalyzed by a metal catalyst such as palladium or platinum. The mechanism for this reaction is a surface catalyzed reaction, where the ethylene and hydrogen molecules adsorb onto the catalyst surface, and then the hydrogen atoms are transferred to the ethylene molecule to form ethane C2H6 .The rate constant for this reaction at 298 K and 1 atm pressure can be determined using the Arrhenius equation:k = Ae^-Ea/RT where k is the rate constant, A is the pre-exponential factor, Ea is the activation energy, R is the gas constant 8.314 J/molK , and T is the temperature in Kelvin.Unfortunately, without specific values for the pre-exponential factor A and activation energy Ea for this reaction, it is not possible to calculate the exact rate constant. These values can be obtained from experimental data or from the literature.However, we can still discuss how the rate would change when the temperature is increased to 373 K. According to the Arrhenius equation, as the temperature increases, the rate constant k will also increase. This is because the exponential term e^-Ea/RT becomes larger as the temperature increases, leading to a higher rate constant and a faster reaction rate.In summary, the mechanism for the reaction between ethylene and hydrogen is a surface catalyzed reaction, and the rate constant will increase when the temperature is increased from 298 K to 373 K. To determine the exact rate constant values, you would need the pre-exponential factor and activation energy for this specific reaction.