unit 4 section 2 rate equations and Kp Flashcards
how do you work out the rate of reaction of a straight line graph
gradient = change in y /change in x
how do you work out the rate of reaction of a curve
you draw a tangent to the curve and then work out the gradient of the tangent
what is the rate reaction for this general equation A + B = C + D
rate = K[A]^m[B]^n
[A]- concentration of A
[B]- concentration of B
K- rate constant
m&n - the order of reaction
what is m & n restricted to
it is restricted to 0,1 and 2
what is the arrhenius equation
k = Ae^(-Ea/RT)
k = rate constant
A= arrhenius constant
Ea= activation energy (j mol-1)
R= gas constant (8.31)
T= temperature (K)
what does higher Ea mean in the arrhenius equation
the higher the Ea the smaller the value of K therefore the slower the reaction.
what is the other way the arrhenius equation can be written
lnk = -Ea / RT + ln A
what is partial pressure
each individual gas exerts its own pressure
what is a mole fraction
it is the proportion of gas mixture that is made up of a particular gas
what is the equation for mole fraction
mole fraction of a gas in a mixture = number of moles of gas in a mixture / total number of moles of gas in a mixture
what is the equation for the partial pressure of gas in a mixture
mole fraction of gas x total pressure of the mixture
what is Kp
it is the equilibrium constant for a reversible reaction where some, or all , products and reactants are gases
give the equation for Kp using this general equation aA(g) + bB(g) = cC(g) + dD(g)
it is the same equation as Kc however you use the partial pressure instead of the concentration (this is done by adding pp in front of the molecule) and the brackets are normal brackets and not square brackets
what happens when you change the temp on equilibrium in gases
-if increase -the equilibrium will shift in the endothermic direction
-if decrease - the equilibrium will shift in the exothermic direction
what happens when you change the pressure of equilibrium in gases
the position of equilibrium shifts in such a way the partial pressure of the reactants and products at the new equilibrium position keeping Kp constant.
what effect does a catalyst have on the Kp
it has no effect as it speeds up the rate of reaction of both the forward reaction and the backward reaction
what is the arrhenius equation used for
it is used to determine the effect of a temperature change on the rate constant
how is a zero order reaction presented on a graph
it is presented as a horizontal straight line.
it has the equation: rate = k
the graph would look like a y=1 graph
how is a first order reaction presented on a graph
it is present as a straight line graph
it has the equation: rate = k[A]
it would look like the graph y=x
how is a second order reaction represent on a graph
it is presented as curved line
it has the equation: rate = k[A]^2
it would look like the graph y = x^2
Rate = k [E]
Explain qualitatively why doubling the temperature has a much greater effect on the rate of
the reaction than doubling the concentration of E
(3)
reactions occur when the molecules have the required activation energy. by doubling the temperature it causes many more molecules to have the required activation energy. whereas doubling the concentration of E only doubles the amount of particles with the activation energy.
State how the initial rate is obtained from a graph of the concentration of the product
against time
(2)
-(Calculate) gradient (of tangent/curve/graph)
-at t=0 or at start of graph/curve
how do you convert kpa to pa
x1000
what is the iodine clock experiment
it is a reaction where you can time how long it takes for a colour change
what is the iodine clock reaction
H2O2 + 2H^+ + 2I^- = 2H20 + I2
how is the iodine clock experiment carried out
react hydrogen peroxide(H2O2) with iodide ions in acid to form iodine(I2) then add sodium thiosulfate and starch.
- the sodium thiosulfate reacts with the iodine immediately to form a colourless solution
- end point - when all thiosulfate ions used up and remaining iodine stays in solution and reacts with the starch and the solution turns dark blue .
how can the iodine clock experiment be used to determine the order of the reaction
varying the concentration of iodine/and or hydrogen peroxide and keeping everything constant will change the time taken for the dark blue solution to appear. this can be used to work out the order
how can you make the reaction more visible in the iodine clock experiment
you place the reaction vessel on a sheet of paper with a black X on it. and when the X is nolonger visible the reaction is over. ( time until the cross disappears )
what are all the different methods of measuring for rate of reaction
-iodine clock
-change in PH
-amount of mass lost
-volume of gas produced
-colorimetry
explain how a change in PH test is done in order to work out the rate of reaction
the PH of a reaction may change over time if the H+ ions are used up or produced.
A PH meter is used to measure the PH at regular intervals, this can then be used to calculate the concentration of H+ ions.
how is the amount of mass lost test is done for working out the rate of reaction
- this is done for reactions that produce a gas
- place the reaction on a balance and measure the mass lost as gas is lost.
- calculate the number of moles of gas lost and hence calculate the amount of reactants left.
explain how the volume of gas produces can be done for working out the rate of reaction
this is done by measuring the amount of gas produced using a gas syringe over a specific period of time. use the ideal gas equation to work out the gas produced. then molar ratio to work out the reactant concentrations
what is a common reaction in the calorimeter test
-propanone and iodine
-the solution goes from brown to colourless
I2 + CH3COCH3 = CH3COCH2I + I- + H+
what does a colorimeter do
it measures the absorbance of light by a coloured sample. the more concentrated the sample is the darker the color and hence more light absobance.
what type of graph is plotted when doing colorimetry and how is it made and what are the axis’
- calibration graph
- done by making up a the range of known different concentration of iodine. the absorption is measured for each one and the results are plotted.
- absoption ( x-axis )
- concentration of iodine I2 ( y-axis)
what is the rate determining step
it is the slowest step in a mult-istep reaction.
- the whole rate of the reaction depends on how fast the rate determining step is.
how can you speed up the rate determining step
- catalyst
- temperature
how can the rate equation be found from a mult-step reaction
- determine the rate determining step ( slowest step). the number in front of the reactant tells you the order of that reactant e.g 2H - is 2nd order
- the reactants that was used to make the reactant in the rate determining step must be in the rate equation. that is how you work out the rate equation.
what can the Arrhenius plot be used to find
the activation energy and the Arrhenius constant.
how is the arrhenius plot made
use the equation lnk = lnA - Ea/RT
- plot the graph of lnk against 1/T and then draw a line of best fit
-the gradient of the graph represents the -Ea/R. this can be used to work out the activation energy and the arrhenius constant.
