CHEM - Reactions 2 Flashcards
(38 cards)
Define dynamic equilibrium
- chemical equilibrium is described as dynamic because, at equilibrium, the forward and back reactions are still occurring, but at equal rates. The forward and back reactions haven’t stopped.
- prior to the equilibrium position being reached, the rate if the forward reaction is greater than the rate of the back reaction. At equilibrium: rate of forward reaction = rate of back reaction
A system is said to be at equilibrium if…
- a reversible chemical reaction is set up in a closed system
- it is at constant temperature
- the concentration of all reactants and products remain constant with time
- the macroscopic (observable) properties are constant - pH and colour intensity
- for gases a sealed container at constant temperature is required
- for reactions in aqueous solution there is limited exchange of reactants and/or products with the surroundings
What does the magnitude of the equilibrium constant, Kc, show?
The magnitude of Kc provides an indication of the yield of products. It is an indication of the extent of the reaction.
Large Kc values indicate a high yield. Kc»_space;1
Small Kc values indicate the low yield of products. Kc «_space;1
According to Le Chatelier’s principle, what may shift the position of equilibrium
A change in:
- concentration of reactants or products
- the pressure (if any gases present)
- the temperature
State Le Chatelier’s principle
If an external change is made to the reaction conditions of a system at equilibrium, a net reaction will occur (if possible) in the direction that counteracts the change.
How do you determine if a concentration change has occurred on a graph?
For a concentration change, one of the quantities will show a sudden increase or decrease.
Eg. Sudden spike
If yet pressure is increased by decreasing the volume of the reaction vessel, what is the system response?
Equilibrium position is shifted to the side with the least molecules of gas.
Opposite side if pressure decreased.
How do you determine a pressure change on a graph?
For a pressure change, all quantities show a sudden increase or decrease in concentration as the volume is decreased or increased.
What is the system response to an increase in temperature?
Increase = equilibrium shifts in endothermic direction Decrease = exothermic direction
How do you determine a temperature change on a graph?
For a temperature change, the quantities of reactants and products change gradually.
Does the addition of a catalyst alter the position of the equilibrium?
No it does not alter the position of the equilibrium. Instead, the rate of both the forward and back reaction is increased so that equilibrium is achieved in a shorter period of time.
Production of ammonia
Haber process
Explain the reaction conditions that will maximise yield according to Le Chatelier
- high concentration of N2 and H2 should be maintained
- Nh3 should be removed as it forms to lower the concentration
- a low temperature would be chosen to favour the forward exothermic reaction
- a high pressure would be chosen as there are less molecules of gas on the right side of the equation
Haber process
Actual conditions
- a moderately high temp of 400 degrees is chosen for a faster rate of reaction and a reasonable yield
- pressure of 250 atmospheres. Too high pressure is expensive and can create a dangerous environment for workers for little increase in yield
- an iron (Fe) catalyst is used to increase the rate of the reaction
- these provide most economical yield (time and cost)
- uses: fertilisers and explosives
Sulfuric acid production
The contact process
Le Chatelier’s predicted reaction conditions
- high concentration of SO2 and O2 should be maintained
- SO3 should be removed as it is formed to lower the concentration
- low temperature to favour exothermic reaction
- high pressure
Sulfuric acid production
The contact process
Actual conditions
- moderately high temp of 450degrees, faster rate of reaction and reasonable yield
- atmospheric pressure. High pressure expensive and dangerous
- vanadium pentoxide (V2O5) catalyst used
- most economical yield of sulfur trioxide
- uses: fertilisers
Equation for production of ammonia (Haber process)
N2 + 3H2 >< 2NH3
Delta H = -46 kj mol
Equation for production of sulfur trioxide
The contact process
2SO2 + O2 >< 2SO3
Delta H = -99kJ mol
Industrial flow charts
Raw Materials
- unrefined substances that may be converted through a range of processes into products.
- on flow diagrams, seen as arrows entering process that aren’t conditions
Industrial flow charts
By-products
- products of a chemical process that aren’t intended but have some economic value or may be reused in the process.
- flow diagram seen as arrows exiting process for economic gain or arrows being recycled on the flow chart
Industrial flow charts
Waste products
- products of a process with no economic value or use
- on flow Diagram: arrows exiting the process which don’t lead to any other industry or cause
Energy considerations
Transfer of heat energy
- heat energy released by an exothermic reaction can be transferred from nor part of the process to another using heat exchangers.
- can be used to create electricity for the plant
- once exothermic reactions have been started, often, the heat produced can maintain the reaction
Industrial flow charts
Lowering operating temperatures with a catalyst
- creates alternative energy pathway with lowered activation energy.
- in this way, less energy needs to be supplied to maintain the reaction
- catalysts don’t get used up so can use lots of times
Energy considerations
Running a process continuously
- shift workers can be used to ensure a process runs 24hrs a day
- less heat energy is consumed compared to frequently starting and stopping as don’t have to regenerate energy requires to get to operating temp
Metal reactivity
What metals are found naturally in their un combined state
Unreactive metals found at the bottom of the reactivity series are found in the uncombined state, eg. Gold, silver, platinum
A more reactive metal will displace the ions of a less reactive metal from solution. Therefore, not found in uncombined state
