Question

consider the reaction below
c₂h₄(g) + h₂(g) → c₂h₆(g)
which change would likely cause the greatest increase in the rate of the reaction?
○ decrease temperature and decrease pressure
○ increase temperature and decrease pressure
○ decrease temperature and increase pressure
○ increase temperature and increase pressure

Explanation:

To determine the change that increases the reaction rate, we use collision theory:

• Temperature: Increasing temperature increases particle kinetic energy, so they collide more frequently and with greater energy (more effective collisions), speeding up the reaction. Decreasing temperature slows it.
• Pressure (for gases): Increasing pressure (e.g., by decreasing volume) increases the concentration of gaseous reactants, leading to more frequent collisions. Decreasing pressure reduces concentration and collision frequency.

Now analyze each option:

1. Decrease temperature and decrease pressure: Both changes slow the reaction (lower temp = less energy; lower pressure = fewer collisions). Eliminate.
2. Increase temperature and decrease pressure: Increasing temp speeds up, but decreasing pressure slows down (due to lower concentration). The net effect is not the greatest increase. Eliminate.
3. Decrease temperature and increase pressure: Decreasing temp slows the reaction, while increasing pressure speeds it. The slowdown from lower temp outweighs the pressure increase. Eliminate.
4. Increase temperature and increase pressure: Increasing temp boosts collision energy/frequency, and increasing pressure boosts collision frequency (higher concentration). Both changes act to increase the reaction rate, leading to the greatest increase.

Answer:

D. increase temperature and increase pressure (Note: Assuming the options are labeled A - D as per typical multiple - choice, with the last option being D. If the original options had different labels, adjust accordingly. The key is the combination of increasing temperature and increasing pressure.)