unit 4 section 2 rate equations and Kp

Question 1

how do you work out the rate of reaction of a straight line graph

Answer 1

gradient = change in y /change in x

Question 2

how do you work out the rate of reaction of a curve

Answer 2

you draw a tangent to the curve and then work out the gradient of the tangent

Question 3

what is the rate reaction for this general equation A + B = C + D

Answer 3

rate = K[A]^m[B]^n
[A]- concentration of A
[B]- concentration of B
K- rate constant
m&n - the order of reaction

Question 4

what is m & n restricted to

Answer 4

it is restricted to 0,1 and 2

Question 5

what is the arrhenius equation

Answer 5

k = Ae^(-Ea/RT)
k = rate constant
A= arrhenius constant
Ea= activation energy (j mol-1)
R= gas constant (8.31)
T= temperature (K)

Question 6

what does higher Ea mean in the arrhenius equation

Answer 6

the higher the Ea the smaller the value of K therefore the slower the reaction.

Question 7

what is the other way the arrhenius equation can be written

Answer 7

lnk = -Ea / RT + ln A

Question 8

what is partial pressure

Answer 8

each individual gas exerts its own pressure

Question 9

what is a mole fraction

Answer 9

it is the proportion of gas mixture that is made up of a particular gas

Question 10

what is the equation for mole fraction

Answer 10

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

Question 11

what is the equation for the partial pressure of gas in a mixture

Answer 11

mole fraction of gas x total pressure of the mixture

Question 12

what is Kp

Answer 12

it is the equilibrium constant for a reversible reaction where some, or all , products and reactants are gases

Question 13

give the equation for Kp using this general equation aA(g) + bB(g) = cC(g) + dD(g)

Answer 13

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

Question 14

what happens when you change the temp on equilibrium in gases

Answer 14

-if increase -the equilibrium will shift in the endothermic direction
-if decrease - the equilibrium will shift in the exothermic direction

Question 15

what happens when you change the pressure of equilibrium in gases

Answer 15

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.

Question 16

what effect does a catalyst have on the Kp

Answer 16

it has no effect as it speeds up the rate of reaction of both the forward reaction and the backward reaction

Question 17

what is the arrhenius equation used for

Answer 17

it is used to determine the effect of a temperature change on the rate constant

Question 18

how is a zero order reaction presented on a graph

Answer 18

it is presented as a horizontal straight line.
it has the equation: rate = k
the graph would look like a y=1 graph

Question 19

how is a first order reaction presented on a graph

Answer 19

it is present as a straight line graph
it has the equation: rate = k[A]
it would look like the graph y=x

Question 20

how is a second order reaction represent on a graph

Answer 20

it is presented as curved line
it has the equation: rate = k[A]^2
it would look like the graph y = x^2

Question 21

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)

Answer 21

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.

Question 22

State how the initial rate is obtained from a graph of the concentration of the product
against time
(2)

Answer 22

-(Calculate) gradient (of tangent/curve/graph)
-at t=0 or at start of graph/curve

Question 23

how do you convert kpa to pa

Answer 23

x1000

Question 24

what is the iodine clock experiment

Answer 24

it is a reaction where you can time how long it takes for a colour change

Question 25

what is the iodine clock reaction

Answer 25

H2O2 + 2H^+ + 2I^- = 2H20 + I2

Question 26

how is the iodine clock experiment carried out

Answer 26

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 .

Question 27

how can the iodine clock experiment be used to determine the order of the reaction

Answer 27

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

Question 28

how can you make the reaction more visible in the iodine clock experiment

Answer 28

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 )

Question 29

what are all the different methods of measuring for rate of reaction

Answer 29

-iodine clock
-change in PH
-amount of mass lost
-volume of gas produced
-colorimetry

Question 30

explain how a change in PH test is done in order to work out the rate of reaction

Answer 30

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.

Question 31

how is the amount of mass lost test is done for working out the rate of reaction

Answer 31

• 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.

Question 32

explain how the volume of gas produces can be done for working out the rate of reaction

Answer 32

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

Question 33

what is a common reaction in the calorimeter test

Answer 33

-propanone and iodine
-the solution goes from brown to colourless

I2 + CH3COCH3 = CH3COCH2I + I- + H+

Question 34

what does a colorimeter do

Answer 34

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.

Question 35

what type of graph is plotted when doing colorimetry and how is it made and what are the axis’

Answer 35

• 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)

Question 36

what is the rate determining step

Answer 36

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.

Question 37

how can you speed up the rate determining step

Answer 37

• catalyst
• temperature

Question 38

how can the rate equation be found from a mult-step reaction

Answer 38

• 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.

Question 39

what can the Arrhenius plot be used to find

Answer 39

the activation energy and the Arrhenius constant.

Question 40

how is the arrhenius plot made

Answer 40

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.