Rate Of Reactions Flashcards
Equations for the rate of a reaction
Rate of reaction = amount of reactant used / time
Or
Rate of reaction = amount of product produced / time
Rate of reaction graphs
- x axis: time
- y axis: amount of product formed
Analysing the graph:
- the steeper the line, the faster the rate of reaction
Over time the line gets less steep as the reactants are used up
- the quickest reactions have the steepest lines and become flat in the least time
How do chemical reactions take place
- chemical reactions can only take place when reacting particles collide with each other and with sufficient energy
(Activation energy should be reached) - the more collisions between particles in the reaction, the faster the reaction it
Factors that affect rate of reaction
temperature:
- when temperature is increased, particles move faster so they collide more frequently and energetically
- this increases rate of reaction
concentration of reactants in solution:
- increase conc means there are more particles to collide with each other in same volume of solvent, this increases rate of reaction
pressure of reacting gas:
- when pressure of gas is increased, same number of particle occupy less space meaning more frequent collisions. So increase rate of reaction
surface area:
- increasing the sa of solid reactants increased frequency of collisions, increasing rate of reaction
presence of a catalyst:
- catalysts speed up reactions without being used up in the process
- they decrease the activation energy needed to start a reaction by proving and alternative reaction pathway with a lower activation energy
How to calculate mean reaction rate from a graph (when given 2 different points)
- the graph will give you to points of time
- find the difference in the y axis of both points and divide it by the difference in time (x axis)
How to find rate of reaction at a particular point using a graph
- draw a tangent at the particular point
- pick 2 point on the tangent and use them to find the gradient
What are reversible reactions
Reactions with a double arrow
- have a forwards and backwards reaction
- as the reactants react, their concentration fall slowing down the forwards reaction
- but as more products are made their concentration will increase, speeding up the backwards reaction
* if the forwards reaction if exothermic the backwards one will always be endothermic vice versa
What does it mean when a reversible reaction is at equilibrium
- when the rate of the forwards and backwards reaction are equal
- the conc of the reactants and products will no longer change
- can only take place in a closed system so no reactants or products escape, and nothing can get in either
Factors affecting equilibrium
Le chateliers principle: the system will counteract any change to keep balance
Conc:
- when the conc of products is higher than reactants the equilibrium lies on the right, vice versa
- if more reactants are added the forwards reaction will produce more products
- if more products are added the backwards reaction will produce more reactants
Temperature:
- If temp is raised, the yield from the endothermic reaction increases to decrease temp
- but the yield from the exothermic reaction decreases
I temp is lowered, yield from the exothermic reaction increases to increase temp
Pressure (only in gaseous reactions):
- increased pressure will favour the reaction that produces the least number of molecules (shown by symbol equation)
- decreased pressure will favour the reaction producing the greater number of molecules
What is the haber process
- the haber process is used to produce ammonia
- one of the main uses of ammonia is to make nitrogen based fertilizers for farming
Nitrogen + hydrogen <=> ammonia
(Forwards reaction is exothermic)
1) the raw materials for the haber process are nitrogen and hydrogen
* Nitrogen can be extracted from the air and hydrogen from a natural gas
2) the purified nitrogen and hydrogen are passed over an iron catalyst at high temps (450C) and high pressure (200 atmospheres)
3) This causes some of the nitrogen and hydrogen molecules to react to form ammonia.
* because the reaction is reversible some of the ammonia breaks back down into nitrogen and hydrogen
4) to increase the yield we cool the ammonia. On cooling, the ammonia liquifies and is removed.
5) the unreacted nitrogen and hydrogen are recycled
Why are there specific conditions used in the haber process
Temperature:
-the forwards reaction to produce ammonia if exothermic
- therefore, increasing temperatures lowers the yield of ammonia however it will increase the rate of reaction so it is used as a compromise temperature
- it can’t be higher than 450C as this is costly and will require lots of energy
Pressure:
- because there are less molecules of ammonia produced in the forwards reaction, high pressure will allow more ammonia to be produced so a larger yield it will also increase the rate of reaction
- a pressure higher than 200 atmospheres would be too expensive and costly so we settle for a compromise
Iron catalyst:
- speeds up reaction/increases rate by decreasing activation energy needed
What is the contact process
- the contact process is used to produce sulfuric acid
1) sulfur is burned in air to produce sulfur dioxide
S + O2 —> SO2
2) sulfur dioxide reacts with more oxygen to make sulfur trioxide
2SO2 + O2 <=> 2SO3
- this exothermic reaction is reversible and requires a vanadium oxide catalyst (V2O5), a temperature of 450C and atmospheric pressure
3) sulfur trioxide reacts with water to make sulfuric acid
H2O + SO3 —> H2SO4
Why are the conditions of stage 2 in the contact process used
Stage 2 is an exothermic reaction:
2SO2 + O2 <=> 2SO3
Temperature:
- 450C is needed to provide sufficient kinetic energy to keep a good rate of reaction
- if higher than 450C the yield of SO3 would be too low as the reaction would become more endothermic
Pressure:
- there are fewer molecules produced in the forwards reaction so pressure would increase the yield of sulfur trioxide produced
- a pressure higher than 2 atmospheres would produce too much sulfur dioxide as the equilibrium is already to the right without high pressures
