Reversible Reactions Flashcards
Yield definition
How much product is actually made in a chemical reaction
Reasons for not 100% yield (5)
- Impure reactants
- Unexpected reactants producing unexpected products
- Reaction is reversible
- Product is lost when transferring between pieces of apparatus
- Product is lost during separation processes
How to calculate % yield
Write balanced equation
Use moles moles mass
Use % yield equation: product produced/ maximum product produced x100
Benefits of high yield
Maximises profit (by making as much product as possible in the quickest amount of time)
More environmentally friendly (less energy is required to make products so more crude oil is reserved)
What is a reversible reaction
A chemical reaction in which the products can be converted back into the original reactants
Example of reversible reaction (2)
Thermal decomposition of ammonium chloride
White fumes turn back into white solid
Carboxylic acid+alcohol 🔁 ester+water
Equilibrium definition
At equilibrium there is no change in the concentrations of the products and the reactants because the rate of the forwards and backwards reaction are equal. The reaction has not stopped.
What is Le Chatelier’s principle
The position of equilibrium shifts to try and oppose and cancel out changes imposed on the system
Factors affecting equilibrium (3)
Temperature
Concentration
Pressure
How does changing the temperature affect the equilibrium position
If the forward reaction is exothermic the temperature needs to decrease to increase the yield of the products.
Equilibrium shifts to the right to heat up as the system opposes the decrease.
How does changing the concentration affect the equilibrium position
Increasing concentration of one of the reactants shifts the equilibrium to the right (in the forwards direction) and increase the yield of the products.
System opposes the increase and tries to decrease the concentration by reacting more reactants together
Decreasing concentration of the product also shifts the equilibrium to the right
How does changing pressure affect the equilibrium position
If there are more moles of the products that the reactants, pressure needs to decrease to increase the yield of the products.
Equilibrium shifts to the right to increase the pressure and the system opposes the decrease.
Rate of the forwards reaction is…
Fastest at the start and slows down as the number of reactants particles decreases
Rate of backwards reaction…
Speeds up as more products are formed
How does adding a catalyst affect things
Does not affect the yield or the position of equilibrium as it affects the rate of forwards and backwards reactions equally
Speeds up the rate of reaction by providing an alternative pathway with a lower activation energy
Why is ammonia important
Source of nitrogen for plants so is put into fertilisers and also used for household cleaners
Conditions for production of ammonia
450°C
200atm
Iron catalyst
Describe the Haber process
H2 from methane at high temperatures, N2 from air by fractional distillation
Gas stream contains purified H2 and N2
Converter - H2 and N2 is heated to 450°C and compressed to 200atm
Reaction vessel - gases pass over iron catalyst
Separator - gases are cooled, ammonia is removed as a liquid (condenses)
Unreacted H2 and N2 are recycled back for the reaction vessel
Why is this pressure chosen for production of ammonia
200atm
More moles of the reactants than products so high pressure
High pressure is expensive and risk of explosion so compromise to intermediate pressure
Why is this temperature chosen for production of ammonia
450°C
Exothermic so low temperature
Low temperature means slow rate of reaction so compromise with intermediate temperature
Iron catalyst can help speed up the rate of reaction
Equation for making ammonia
N2(g) + 3H2(g) -> 2NH3(g)
