Rates and equilibrium Flashcards
what is the rate of reaction equation?
change in concentration(mol/dm cubed)/time(s)
describe the relationship between rate of reaction and concentration?
directly proportional
Describe the concentration time graphs?
- rate is faster at the start (steepest) as reactant is at its highest concentration
- rate decreases as concentration of reactants decrease as they are being used up
- once the reactants are completely used up the rate is zero (horizontal line)
what factors change the rate of reaction?
- concentration (pressure when gases)
- catalyst
- surface area of solid reactants
- temperature
what are the conditions needed for a reaction to occur?
- sufficient energy to overcome the activation energy
- collision with the correct orientation
how does concentration affect rate?
increasing concentration, increases rate of reaction as there are more molecules in a given volume so molecules are closer, increasing the frequency of successful collisions.
how does increasing pressure in a gas affect rate?
increasing the pressure of a gaseous reaction increases the number of gaseous molecules in a given volume(smaller), so molecules are closer together. This leads to more frequent successful collisions and an increase in the rate of reaction
what methods measure the progress of a reaction?
- monitoring the removal (decrease in concentration) of a reactant
- following the formation (increase in formation) of a product
if the reaction product is a gas wet methods can be used to measure the rate?
- monitor the volume of gas in regular time intervals using an inverted measuring cylinder of gas syringe
- monitoring the mass loss of reactants using a balance
what is a catalyst?
a substance that changes the rate of chemical reaction without undergoing any permanent change itself
how does a catalyst affect the rate of reaction?
provides an alternative pathway of lower activation energy increasing the frequency of successful collisions, increasing the rate
what is a homogenous catalyst?
it has the same physical state as the reactants. The catalyst reacts to form an intermediate. The intermediate then breaks down to give the product and regenerates the catalyst
what is a heterogeneous catalyst?
A heterogeneous catalyst has a different physical state from the reactants. They are usually solids. Reactant molecules are adsorbed (weakly bonded) onto the surface of the catalyst, where the reaction takes place. After the reaction, the product molecules leave the surface of catalyst by desorption.
what are the economic advantages using a catalyst?
less cost is associated with developing the process (reduced temperature and energy requirements)
how are catalysts more sustainable?
high atom economy and fewer pollutants (using less fossil fuels will cut emissions of carbon dioxide linked to global warming)
when is a chemical reaction said to have undergone autocatalyst?
if a reaction product acts as a catalyst for that reaction. Typically the reaction starts slowly, then seeds up as products are formed.
what is the Boltzmann distribution?
shows the spread of molecular energies in gases. Some molecules will have high energy and some will have low energy depending on the speed of the molecules
In the Boltzmann distribution, how do you calculate the total number of molecules?
the area under the curve
In the Boltzmann distribution, why does the curve start at the origin?
no molecules have zero energy
In the Boltzmann distribution, is there a maximum energy for a molecule?
no, the curve doesn’t meet the x-axis at high energy only if the energy is infinite
In the Boltzmann distribution, what does the peak curve represent?
the most probable energy that a molecule will have
In the Boltzmann distribution, how do you find the average energy?
It is the at the right of (slightly greater) the most probable energy
How does the Boltzmann distribution change, when temperature increases?
- average energy increases
- more molecules have higher energy greater than activation energy (increased frequency of successful collisions)
- graph is stretched out
- peak of graph is lower on y-axis but further on x-axis (peak is at higher energy)
- area (total number of molecules) is the same
What is the hater process equation for manufacturing ammonia?
N2(g) + 3H2(g) -reversible arrow- 2NH3(g)
In an equilibrium system…
- rate of forward reaction is equal to rate of reverse direction
- concentration of reactants and products do not change
What does DYNAMIC equilibrium mean?
both forward and reverse reaction is taking place at the same rate
Why must a reaction remain in equilibrium of a closed system?
so temperature, pressure and concentration of reactants and products are not affected.
What does le chatlier’s principle state?
when a system in equilibrium is subjected to an external change, the system counteracts it to minimise its effects
How does concentration changes affect equilibrium?
- if there are more products formed, equilibrium shifts to the right
- if there are more reactants formed, equilibrium shifts to the left
What does the position of equilibrium show?
compares how many reactants there are compared to products, the extent of the reaction. Position of equilibrium can be detected through colour change
How does temperature affect equilibrium?
- increase in temp, shifts equilibrium in the endothermic direction
- decrease in temp, shift equilibrium in the exothermic direction
How do you answer a question on factors affecting equilibrium?
- include what direction is favoured
- how position of equilibrium has shifted
- do products or reactants increase?
what condition are needed for pressure to affect equilibrium?
- must be gases
- uneven number of molecules
How does increasing pressure affect equilibrium?
-equilibrium shifts to the side with fewer molecules, reducing pressure. Decreasing pressure shifts equilibrium to side of more molecules
How does a catalyst affect equilibrium?
doesn’t affect equilibrium position as it speed rate of both forward and backward reaction equally. Catalyst increases the rate at which equilibrium was established.
What conditions are needed for manufacturing ammonia through the hater process?
350 - 500 degrees
100 - 200 atm (pressure)
iron catalyst
Why are low temps and high pressures not used in the hater process?
- even though low temps increase production of product, it will cause the reaction to be too slow and equilibrium may not be established
- high temps increase yield however a strong container is needed and lots of energy and money
- however unreacted reactants are recycled and eventually convert into ammonia
what does an equilibrium constant provide?
the actual position of equilibrium. The magnitude of an equilibrium constant indicates whether there are more reactants or more products in an equilibrium system.
How do you calculate exact position of equilibrium?
using equilibrium law
What is the equilibrium law which defines equilibrium constant (Kc) in terms of concentrations? Using
aA + bB (reversible arrows) cC +dD
Kc= [C] to the power of c [D] to the power of d / [A] to the power of a [B] to the power of b
products/ reactants
what does the value of Kc tell us?
- Kc value of 1 indicates equilibrium position halfway between reactants and products
- Kc value> 1 indicates position of equilibrium towards products
- Kc value<1 indicates equilibrium position towards reactants
