- only for reactions involving at least 1 gaseous reactant
Understanding
- for gasses we cant use term concentration, thus we use the term pressure.
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Pressure → very little effect on reactants in solid, liquid, aqueous states → as the particles in these substances → are already very closely packed together
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Particles in gaseous substance → spaced far apart from each other.
- When pressure increases → the particles in this substance → forced much closer together
- thus there are more reacting particles per unit volume → causes the frequency of effective collisions between reacting particles to increase.
Sample
Rate of Reaction increases with increase in pressure
- Reacting particles are now closer together
- thus there are more reacting particles per unit volume
- resulting in more collisions between reacting particles per unit time
- thus frequency of effective collisions increases
- hence rate of reaction increases
Sample
Rate of Reaction decreases with decrease in pressure
- Reacting particles are further apart
- thus, there are lesser reacting particles per unit volume;
- resulting in lesser collisions between reacting particles of the reactants per unit time
- thus frequency of effective collision decreases
- hence rate of reaction decreases
Real Example: Explain why higher pressure will affect rate of reaction between nitrogen and hydrogen gas to produce ammonia gas.
- At higher pressure → reacting particles (of nitrogen and hydrogen gas or H2 and N2 molecules) are closer together
- thus there are more reacting particles (of nitrogen and hydrogen gas or H2 and N2 molecules) per unit volume
- this results in more collisions between reacting particles (of nitrogen and hydrogen gas or H2 and N2 molecules) per unit time
- Thus frequency of effective collisions increases and the rate of reaction increases.
Summary
Increase in pressure of gaseous reactants results in increase in rate of reaction