When a catalyst is used in a reaction,

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BlackTom AI · Accepted Answer

When a catalyst is used in a reaction, it provides an alternative pathway with a lower activation energy, speeding up both the forward and reverse reactions equally without shifting the position of equilibrium. 
Option 1: 'the equilibrium concentrations of reactants and products are not affected.' This is true because catalysts change rates, not the thermodynamic balance; they leave the equilibrium composition unchanged.
Option 2: 'the temperature at which the reaction proceeds spontaneously is decreased.' Catalysts do not alter the spontaneity temperature or the thermodynamic favorability; they merely accelerate the rate to reach equilibrium faster.
Option 3: 'the enthalpy change for the reaction becomes more exothermic.' A catalyst does not change the overall enthalpy change (ΔH) of the reaction; it only provides a different reaction pathway with a lower activation barrier.
Option 4: 'the equilibrium constant (K) increases.' The equilibrium constant is determined by thermodynamics (ΔG, ΔH, ΔS) and remains the same with or without a catalyst; a catalyst does not shift K.
Option 5: 'the activation energy for the forward and reverse reaction do not change.' This is incorrect; a catalyst lowers the activation energy for both directions, though not equally in all cases, but generally it lowers the barriers, which speeds up both directions.

When a catalyst is used in a reaction,

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