4b: Equilibria - YK Flashcards
What does it mean for a reaction to ‘go to completion’?
The reaction continues until one of the reactants is completely used up (so the reaction stops), the reactions are irreversible since the products can’t be changed back into the reactants
What is an example of a reaction that goes to completion?
Hydrochloric acid + magnesium –> magnesium chloride + hydrogen
Mg (s) + 2HCl (aq) –> MgCl2 (aq) + H2 (g)
What is an example of a reversible reaction?
At 350 C mercury reacts with oxygen to form mercury oxide (forward reaction), at 1000 C mercury oxide breaks down to form mercury and oxygen (reverse reaction)
2Hg (l) + O2 (g) ⇌ 2HgO (s)
What are two examples of a reversible reaction?
CuSO4 (s) + 5H2O (l) ⇌ CuSO4.5H2O (s)
NH4Cl (s) ⇌ NH3 (g) + HCl (g)
What is a dynamic equilibrium?
A reaction where both the forward and reverse reactions occur at the same time (amount of reactant and product stay the same, rate of forward and reverse reaction equal), only occurs in a closed system where no loss of reactants or products can be lost, the equilibrium can be disturbed by change in temperature or substances in the reactions
How does an increase/decrease in temperature shift the equilibrium position?
Increase: endothermic direction (transferring energy from surroundings, cooling them)
Decrease: exothermic direction (transferring energy to surroundings, heating them)
How does an increase/decrease in gas pressure shift the equilibrium position?
Increase: direction that forms fewer gas molecules (reduces air pressure)
Decrease: direction that forms more gas molecules (increases pressure)
How does an increase/decrease in concentration shift the equilibrium position?
Increase: direction that uses up added substance
Decrease: direction that forms more of removed substance
What is the Haber Process?
The industrial reaction between hydrogen and nitrogen, catalysed by iron, to cheaply produce ammonia (and heat) for artificial fertilisers (and explosives)
What is the word equation and balanced equation for the Haber process?
Hydrogen + nitrogen ⇌ ammonia + heat
3H2 (g) + N2 (g) ⇌ 2NH3 (g) , ΔH = -92 kJ/mole
How is temperature and pressure controlled to maximise the products of the Haber process?
A low temperature is used because increasing temperature moves the equilibrium to the direction which produces less ammonia (however low temperature slows reaction so a compromise is made)
A high pressure is used because decreasing pressure moves equilibrium to the direction producing more gas (to increase pressure) which produces less ammonia
Why are compromises used which don’t give the maximum yield from a reaction?
High pressures are expensive (requires special, often fragile, pumps)
Higher temperatures make a reaction faster (time when product is available balanced with most efficient yield)
Catalysts can make a compromise less inefficient
What is the definition of a closed and open system?
Closed: matter can’t go in or out of the system
Open: matter can go in or out of the system
What is the definition of an endothermic reaction?
A reaction which transfers heat from the surroundings, cooling them down (exo is opposite)
