Rate Equations

Question 1

Define the term rate of reaction

Answer 1

Change in concentration of the reactant or product per unit time

Question 2

At a given instant how do you calculate rate if reaction

Answer 2

Change in Conc/ Change in time

Gradient of a concentration-time graph

Y value of a rate-concentration graph

Question 3

How would you measure rate if reaction experimentally? (include different methods) 3

Answer 3

Use a calibrated colourimeter at suitable intervals, if there is a notable colour change- to test for the concentration of a specific ion

If a gas is expelled use a gas syringe to measure the volume of gas at intervals

Measure the mass of the reactants at suitable intervals

Question 4

How can you determine the rate constant and rate expression for a reaction

Answer 4

Can only be done experimentally, theoretical data cannot be calculate prior

Question 5

What affects the value of rate constant for a given reaction

Answer 5

Only Temeprature

Question 6

Write a generic rate expression and state what each term means

Answer 6

Question 7

Why are some chemicals in the chemical equation but not the rate equation

Answer 7

Rate Equation includes any species’ that affect rate

Chemicals included after the rate determining step may not be included

Catalysts may be included as they affect the rate of reaction

Question 8

Define the term :

“Order of a reaction with Respect to a given product”

Answer 8

The power to which a specific concentration is raised to in the rate equation

Question 9

Define the term:

“Overall order of reaction”

Answer 9

The Sum of the orders of reaction of all species in the rate expression

ie (X+Y)
where a^x b^y = rate

Question 10

How do you calculate the units of the rate constant

Answer 10

Rate units = moldm^-3 s^-1
Conc units = moldm^-3

rearrange rate equation to get k

sub in units and cancel out

Question 11

How would you calculate the order of a reaction with only concentration and time data

Answer 11

Plot [A] against time , draw tangents at different values

Draw a secondary graph of rate against concentration [A]

gradient of 0 = Zero order

y= mx + c = 1st order

X^2 = 2nd rder

Question 12

Draw a rate-concentration for a zero order reactant

Answer 12

Question 13

Draw a rate-concentration for a 1st order reactant

Answer 13

Question 14

Draw a rate-concentration for a 2nd order reactant

Answer 14

Question 15

How would you confirm an order of reaction greater then 2

Answer 15

Rate against conc^x graph would be a straight line through the origin when the order of reaction is x

Question 16

Explain how you would find the rate expression using the initial rates method

Answer 16

Do a series of experiments during - vary the concentrations - the concentration of just one reactant changes over time

Plot a graph of concentration against time for each reactant

Draw a tangent at t=0 to find the initial rate of reaction

Compare rates and concentrations between each experiment to find order of reactants and the overall rate equation

Question 17

Write the ionic equation for the iodine clock reaction

What are the reactants

Explain what happens

Answer 17

Question 18

When does the starch tuen a blue-black colour in an iodine clock reaction and why?

Answer 18

When all of the Sodium thiosulphate has been used up so I2 is produced

I2 reacts with the starch leading to Blue-Black Colour

Question 19

How do you calculate the rate of reaction from the data from an iodine clock reaction

Answer 19

Record time taken for colour change to occur

Initial rate is roughly equal to 1/ the time taken

Question 20

What temheprature increase roughly doubles the rate of reaction

Answer 20

10K

Question 21

Explain what each of the Components from the Arrhenius equation mean

Answer 21

Question 22

How can you convert the Arrhenius equation into y= mx + c format

Answer 22

ln(k) = -Ea/RT + ln(A)

for a graph of ln(k) against 1/T

Gradient = -Ea/R and the y intercept is ln(A)

Question 23

What is the rate determining step?

Answer 23

The slowest step in a reaction mechanism which determines the overall rate of reaction

Question 24

How does the Rate determining step link to the species involved in the rate expression.

Answer 24

Any Species involved in the rate determining step appear in the rate expression

Species only involved after, do not appear in the rate expression.

Question 25

Answer 25

1) Sodium Thiosulphate neutralises the acid catalyst in the aliquot, stopping the reaction (to observe the rate at that time)

2) propanone quantity is much larger then the quantity of iodine

Question 26

In the iodine clock reaction, an experiment takes aliquots of the reaction mixture every 20 seconds in order to determine the rate of which concentration of [H+] ions decrease

Describe what must be done after each aliquot is taken.

Answer 26

Add a reagent that reacts with H2O2 or I- in order to prevent further reaction of H+

So that a concentration of H+ stays constant

Called Quenching

Question 27

Is rate of reaction a vector or a scalar?

Answer 27

Scalar

Allways positive