why would lowering the temp and decreasing the pressure not increase the rate of the reaction: carbon monoxide gas is combusted in the presence of oxygen gas into carbon dioxide. (DeltaH for this reaction is -285kJ.)
CO + O2 ==> CO2 + heat
decreasing P drives the reaction to the left (2 mols gas on left and only 1 right; therefore, increasing P drives reaction to right). Raising T drives the reaction to the left.
Better to use a balanced equation, it doesn't affect the argument in this case.
2CO + O2 ==> 2CO2 + heat
3 molescules gives 2 molecules but the argument is the same.
oops!. Dr Russ is right. I ALWAYS check to see if the equation is balanced (but not this time so I can't say always anymore.)
Lowering the temperature and decreasing the pressure would actually decrease the rate of the carbon monoxide combustion reaction, rather than increasing it.
To understand why this happens, let's consider the factors that affect the rate of a chemical reaction. The rate of a reaction is determined by the collision frequency and the energy of the collisions between reactant molecules.
Lowering the temperature decreases the average kinetic energy of the reactant molecules. As a result, the reactant molecules move slower and collide less frequently. With fewer collisions, the rate of the reaction decreases.
Similarly, decreasing the pressure of the reaction system reduces the number of gas molecules per unit volume. This reduction in the number of collisions also decreases the rate of the reaction.
Moreover, the balanced equation for the combustion of carbon monoxide shows that the reaction produces more gas molecules on the product side than on the reactant side. This means that increasing the pressure would favor the formation of products (CO2), as per Le Chatelier's principle, which states that if a stress is applied to a system at equilibrium, the system will adjust to relieve that stress. In this case, increasing the pressure relieves the stress by shifting the equilibrium towards the side with fewer gas molecules.
Hence, lowering the temperature and decreasing the pressure both have a negative impact on the rate of the reaction, rather than increasing it.