Equilibrium and rate in the Haber process
Using resources • The Haber process and fertilisers
Flashcards
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Key concepts
What you'll likely be quizzed about
Dynamic equilibrium and the Haber reaction
The Haber reaction is N2(g) + 3H2(g) ⇌ 2NH3(g). Dynamic equilibrium occurs when forward and reverse reaction rates are equal and concentrations remain constant. The equilibrium constant (Kc or Kp) quantifies the position of equilibrium at a given temperature. Removing ammonia shifts the balance toward product formation because the forward reaction proceeds faster than the reverse until a new equilibrium is reached.
Le Chatelier’s principle applied to Haber
A change in concentration, pressure or temperature causes the system to shift to oppose that change. Increasing pressure favours the side with fewer gas moles (the product side: 4 mol reactants → 2 mol product), so higher pressure increases ammonia yield. Increasing temperature favours the endothermic direction. Because ammonia formation is exothermic, raising temperature shifts the equilibrium toward reactants and lowers yield. Lowering temperature shifts equilibrium toward ammonia but reduces reaction rate.
Rate versus position of equilibrium - the industrial trade-off
Low temperature favours high equilibrium yield of ammonia but reduces the rate because fewer collisions have sufficient energy to overcome activation energy. High temperature increases rate by increasing collision frequency and energy but reduces equilibrium yield. High pressure increases both collision frequency and equilibrium yield but requires stronger plant design and higher cost. A catalyst increases the rate without changing the equilibrium position, allowing operation at lower temperatures than would otherwise be practical. Industrial conditions represent a compromise between acceptable yield, high enough rate, and economic/engineering constraints.
Catalyst effect and limiting factors
A heterogeneous iron catalyst (often promoted with other metals) lowers activation energy and increases forward and reverse rates equally. The catalyst does not change the equilibrium constant or final yield. Limiting factors include availability and purity of reactant gases, surface area and activity of the catalyst, temperature-dependent equilibrium position, and capital/operating costs associated with very high pressures or very low temperatures.
HT only: interpreting graphs of reaction conditions versus rate
Graph of rate versus temperature shows an exponential increase in rate as temperature rises (Arrhenius behaviour), while equilibrium yield of ammonia decreases with temperature for the exothermic reaction. Graph of rate versus pressure shows increasing rate as pressure rises due to increased collision frequency; equilibrium yield also increases with pressure because of fewer gas moles on the product side. Graph of rate versus catalyst presence shows a sudden increase in rate when a catalyst is introduced and a higher plateau thereafter. Graph interpretation requires attention to axes (rate, yield or concentration), trends (direction and curvature), and indication whether curves represent kinetic rate or equilibrium composition.
Key notes
Important points to keep in mind