In reactions where the number of gaseous product moles is less than reactant moles, high pressure increases both the yield and the rate of reaction.
While high pressure is theoretically ideal, it is limited by economic and safety factors: the cost of energy to compress gases is high, and the specialized thick-walled equipment required to contain high pressure is extremely expensive to build and maintain.
A compromise pressure (e.g., 200 atm) is used to maximize yield without incurring the prohibitive costs or safety risks of extreme pressures.
| Feature | Increasing Temperature | Increasing Pressure |
|---|---|---|
| Effect on Rate | Always increases (more frequent/energetic collisions) | Always increases (more frequent collisions) |
| Effect on Yield | Decreases for exothermic; Increases for endothermic | Increases if product moles < reactant moles |
| Industrial Constraint | Catalyst degradation and energy costs | Equipment cost and safety risks |
A catalyst is the most effective way to manage the rate-yield compromise because it increases the rate of reaction without affecting the position of equilibrium.
By providing an alternative pathway with a lower activation energy (), catalysts allow industrial processes to operate at lower temperatures, which saves energy and favors the yield of exothermic reactions.
Identify the Enthalpy: Always check if the forward reaction is exothermic or endothermic before discussing temperature shifts.
Count the Moles: When discussing pressure, explicitly state the number of gaseous moles on each side of the equation to justify the shift.
The 'Why' of Compromise: If asked why a specific condition is used, mention both the kinetic reason (rate) and the thermodynamic reason (yield/Le Chatelier).
Safety and Cost: Don't forget to mention that 'high pressure' isn't just about chemistry; it's about the engineering cost of the plant and the safety of the workers.
