rate equations and gas equillibria Flashcards
how do we work out the rate of a reaction
if you want to find the rate of a reaction, you need to be able to follow the reaction as it is occurring
You can follow a reaction all the way through from start to end by recording the amount of product (or reactant) you have at a regular time intervals
This is called CONTINUOUS MONITORING
whar are the different methods of continuous monitoring
- gas volume
- change in pH
- colour change
- loss of mass
gas volume
if gas is given off, you could collect it in a gas syringe and record how much you have at regular intervals
- the rate of the equation is in terms of concentration per unit per time by drawing a concentration-time graph
you should know the concentration of the reactants at the start of the equation
ph change
if the reaction produces or uses up H+ ions, the pH of the solution will change
So you could measure the pH of the solution at regular intervals and calculate the concentration of H+
colour change
sometimes you can track the colour change of a reaction using a colourimeter
- plot a calibration curve of known concentrations against absorption
- During the experiment, take a small sample from your reaction solution at regular intervals and read absorbance
- use your calibration curve to convert absorbance at each time point into a concentration
the more concentrated a solution the higher the absorbance is
what is a colourimeter
a colourimeter measures absorbance (the amount of light of a particular wavelength absorbed) of the solution
loss of mass
if gas is given off, the system will lose mass
you can measure this at regular intervals with a balance
how do you work out the rate from a straight line graph
gradient = change in Y / change in X
order effect of 0
if the concentration of reactant changes and the rate remains the same, the order of reaction with respect to e.g. A is 0 ( or 0 order with respect to A)
So if A doubles, the rate will stay the same. If A triples, the rate will stay the same
order effect of 1
if the rate is proportional to [A], then order or reaction with respect to A is 1 (or first order with respect to A).
So if [A] doubles the rate will double and so on
order effect of 2
If the rate is proportional to [A]2 , then the order of reaction with respect to A is 2 ( or second order with respect to A)
What is the units of rate
mol dm-3 s-1
what is the rate constant
k is the rate constant - it is a constant that links the concentration of reactants to the rate of reaction
the bigger the rate constant, the faster the reaction
if you increase the temp, the rate constant increase too
what is the initial rates method
you can use a process called the initial rates method to work out the order of reaction for each reactant:
- carry out the reaction and monitor its progress - you need to collect enough data to work out an initial rate. Use this data to draw a concentration-time graph
- repeat the experiment using a different initial concentration of one of the reactants. Keep the concentrations of other reactants and other conditions the same. Draw another concentration-time graph
- now look at how the different initial concentrations affect the initial rate - use this to work out the order for the reactants
what are clock reactions
clock reactions are an example of an initial rates method. They involve measuring how long it takes for a certain amount of product to form as you vary the concentrations of one or the reactants
As part of a clock reaction, there will be a sudden increase in the concentration of a certain product as a limiting reactant is used up
what is an important feature of a clock reaction
there is usually an easily observable endpoint such as a colour change to tell when the desired amount of product is formed
the quicker the clock reaction finsihes
the faster the initial rate of the reaction
what are the following assumptions
- the concentration of each reactant does not change significantly over the time period of your clock reaction
- the temperature stays constant
- when endpoint is seen the reaction has not proceeded too far
what is the iodine clock reaction
in an iodine clock reaction the reaction that you are monitoring is this:
H2O+ 2I- + 2H+ →2H2O +I2
A small amount of sodium thiosulfate solution and starch are added to an excess of hydrogen peroxide and iodide ions in the acid solution
(starch is used as an indicator - it turns blue-black in the presence of iodine)
the sodium thiosulfate is added to the reaction mixture reacts instantaneously with the iodine that is formed
2S2O32- + I2→ 2I- + S4O62-
To begin with, the iodine that forms in the first reaction is used up straight away in the second reaction
But once all the sodium thiosulfate is used up, any more iodine that forms will stay in the solution, so the starch indicator will suddenly turn the solution blue black
This is the end of the clock reaction
Varying iodide or hydrogen peroxide concentrations while keeping the others constant will give different times for the colour change
describe the different rate graphs
the blue line represents the lines for concentration against time
what is Arrhenius equation
what does each letter in the Arrhenius equatoin stand for
k = rate
EA = activation energy (J mol-1)
T= temperature(K)
R = gas constant (8.31J K-1mol-1)
A= the Arrenhius constant (another constant)
what is the Arrhenius equation useful for
it is a useful tool for helping us to work out what happens to the rate constant if various conditions, such as the temperature or activation energy change
As the activation energy, EA gets bigger, k gets smaller
this therefore mean slower rate
how do we work out the activation energy from a graph
in this graph, the y-intercept is equal to A and the gradient is -EA/R
what happens in a mixture of gases
in a mixture of gases, each individual gas exerts its own pressure - this is called its partial pressure
the total pressure pf a gas mixture is the sum of all the partial pressures of the individual gases
what is the unit for partial pressure
kPa
what is a mol fraction
a mole fraction is just the proportion of a gas mixture that is made up of a particular gas.
mole fraction of a gas in a mixture = number of moles of gas/ total number of moles of gas in the mixture
how do we work out the partial pressure of a gas in a mixture
mole fraction of a gas x total pressure of the mixture
what is the gas equilibrium constant
Kp is the equilibrium constant for a reversible reaction where some or all of the reactants and products are gases
the expression for the constant is like the one for Kc but you use partial pressures instead of concentration
Kp= partial pressure of products/ partial pressure of rectants
what effect does temp have on Kp
temp can increase or decrease Kp depending on what side equilibrium is pushed (Kp is lower if fewer products is formed)
what effect does pressure have on Kp
if you increase the pressure, the equilibrium shifts to the side with fewer moles of gas, and if you decrease the pressure, equilibrium shifts to the side with most moles
However, these changes do not effect Kp.
Instead, if you change the pressure, the position of equilibrium shifts in such a way that the partial pressure of reactants and products at the new equilibrium position keep Kp constant
what effect do catalysts have on Kp
catalysts have no effect on the position of equilibrium it in the value of Kp
This is because a catalyst will increase the rate of both the forward and backwards reactions by the same amount
As a result, the equilibrium position will be the same as the uncatalysed reaction, but equilibrium will be reached faster
So catalysts can’t increase the amount of product formed, but they do increase the time taken to each equilibrium
