Reversible Reactions Flashcards
What is a reversible reaction
- A chemical reaction where the products can react to reform the original reactants
- this means that the forward and reverse reactions happen simultaneously
Key characteristics of reversible reactions
- they don’t go to completion, instead they reach equilibrium in a closed system
- the reaction can be shifted in either direction by changing conditions (temperature, pressure, concentration)
- the same amount pf energy is transferred in both directions but one direction exothermic and the other is endothermic
Energy changes in reversible reactions
- every chemical reaction involves an energy change
- in reversible reactions: one direction is exothermic (releases energy) and the reverse direction is endothermic (absorbs energy)
- the same amount of energy is transferred in both directions
Hydrated and anhydrous copper sulfate
- forward reaction (heating hydrated copper sulfate): endothermic (absorbs heat)
- reverse reaction (adding water to anhydrous copper sulfate): exothermic
Common test for water:
- blue to white: water is removed (endothermic)
- white to blue: water is added (exothermic)
Ammonium chloride decomposition
- heating ammonium chloride - ammonia and hydrogen chloride gases are formed (endothermic)
- cooling the gases - solid ammonium chloride reforms (exothermic)
Dynamic equilibrium
- the rate of the forward reaction = the rate of the reverse reaction
- the concentrations of reactants an products remain constant (but are not necessarily equal)
- only happens in a closed system
Closed system
Where no reactants/products escape
Has reaction stopped or not at equilibrium
No it hasn’t stopped, both forward and reverse reactions still happen, but at the same rate
What is Le Chatelier’s Principle
- If a system at equilibrium experiences a change in conditions, the system will shift to oppose that changes
- this helps predict how equilibrium shifts when we change concentration, temperature, or pressure
Effect of changing concentration conditions
- increasing reactant concentration: equilibrium shifts right (more products)
- decreasing reactant concentration: equilibrium, shifts left (more reactants)
- increasing product concentration - equilibrium shifts left (more reactants)
- decreasing product concentration: equilibrium shifts right (more products)
Effect of changing temperature on equilibrium
- increasing temperature: shifts equilibrium towards the endothermic reaction
- decreasing temperature: shifts equilibrium towards the exothermic reaction
Effect of changing pressure on equilibrium
- applies only to gases
- increasing pressure: shifts equilibrium towards the side with fewer gas molecules
- decreasing pressure: shifts equilibrium towards the side with more gas molecules
What is the Haber process
An industrial method for making ammonia from nitrogen and hydrogen. Ammonia is a key product used in fertilisers explosives and cleaning products
Chemistry of Haber process
- reversible reaction - ammonia can break back into nitrogen and hydrogen
- exothermic in the forward direction
- endothermic in the reverse direction
Raw materials & sources from Haber process
- nitrogen, obtained from the air (78%)
- hydrogen, obtained from natural gas (extracting hydrogen from water is too expensive)
Haber process reaction conditions
- temp 450: lower temperature increases yield (exothermic) but makes the reaction too slow. It’s a compromise
- high pressure: favours ammonia production (fewer gas molecules). Too high pressure is dangerous and expensive
- iron catalyst: speeds up the reaction but doesn’t affect the equilibrium position
How le chatelier’s principle applies to Haber process (temperature)
- forward reaction is exothermic so: lower temperature shifts equilibrium right (more ammonia) but too slow = slow reaction rate
Effect of pressure on Haber process
- higher pressure shifts equilibrium right (towards ammonia which has fewer gas molecules)
- increasing pressure increases yield but also costs more due to stronger, thicker equipment
- 200 atm is a compromise - high enough to increase yield but not too costly or dangerous
Effect of catalyst on Haber process
- iron catalyst speeds up reaction by lowering activation energy
- does not change yield
