2X + Y₂ → X₂Y₂
A chemist is studying the reaction between the gaseous chemical species X and Y₂, represented by the equation above. Initial rates of reaction are measured at various concentrations of reactants. The results are recorded in the following table.

Experiment [X] [Y₂] Initial Rate of Appearance of X2 Y2 (M/s)
1 0.15 0.10 32
2 0.15 0.20 64
3 0.30 0.20 128

A second chemist repeated the three experiments and observed that the reaction rates were considerably greater than those measured by the first chemist even though the concentrations of the reactants and the temperature in the laboratory were the same as they were for the first chemist.
Which of the following is the best pairing of a claim about a most likely cause for the greater rates measured by the second chemist and a valid justification for that claim?

a) The pressures of the gases used by the second chemist must have been lower than those used by the first chemist, thus the collisions between reacting particles were less frequent than they were in the first chemist's experiments.
b) The pressures of the gases used by the second chemist must have been lower than those used by the first chemist, thus the number of collisions with sufficient B energy to cause reaction was lower than it was in the first chemist's experiments.
c) The second chemist must have added a catalyst for the reaction, thus providing a different reaction pathway for the reactant particles to react with an activation energy that was lower than that of the uncatalyzed reaction in the first chemist's experiments.

Catalyst increse the rate of reaction by decreasing the activation energy. Option C is correct.

The second chemist must have added a catalyst for the reaction, thus providing a different reaction pathway for the reactant particles to react with an activation energy that was lower than that of the uncatalyzed reaction in the first chemist's experiments.

Factors affecting Rate of reaction:

Reactants concentrationTemperature Pressure Presence of catalyst

All these factors are directly proportional to the rate of reaction.

Second chemist perform the experiment at same concentration and temperature. So the these cannot be the factors to increase the rate of reaction.

The pressure was less than the first chemist, hence it cannot increase the rate of reaction.

Therefore, the second chemist must have used a catalyst to increase the rate of reaction.

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c) The second chemist must have added a catalyst for the reaction, thus providing a different reaction pathway for the reactant particles to react with an activation energy that was lower than that of the uncatalyzed reaction in the first chemist's experiments.

Explanation:

The rate of reactions depend on:

TemperatureConcentration of the reactantsContact surfaceCatalysts

It is said that the second chemist repeated the three experiments with the same concentrations of the reactants and temperature. Then, they are not the reason of the difference in the meaured rates.

Since the reactants are gaseous, the contact surface is not a factor in this experiment:  contact surface is considered when the reactants are solid as the places where the reaction occurs is where the contact takes place.

Then, the only possible factor that can be the cause of the difference in the rates observed by the two chemist is the use of a catalyst. Since the second chemist observed that the rates of reaction were considerable greater, /she must have used a catalys, whose function is to speed up the reaction by providing a different reaction pathway for the reactant particles to react with a lower activation energy.

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