Unit 4- How fast?

Question 1

Define the rate of reaction

Answer 1

The change in concentration over time with the units moldm^-3s^-1

Question 2

Define the rate equation

Answer 2

An equation where the relationship between the concentration and rate is shown

Question 3

Define the rate determining step

Answer 3

The slowest step of a reaction which controls the rate of reaction

Question 4

Define heterogenous catalyst

Answer 4

catalyst which are in a different state to the reactant

Question 5

define homogenous catalyst

Answer 5

catalyst which are in the same state as the reactants

Question 6

How do catalyst work?

Answer 6

adsorbs the molecules on the surface to increase the rate of reaction. Normally a high surface area and a transitional metal used. Can become poisoned through impurities

Question 7

What are the methods that can be used to measure the rate of reaction?

Answer 7

• Colorimetry
• conductimetry
• change in mass
• Volume produced
• Titrimetric analysis

Question 8

How can measuring the volume of the gas produced be used to measure the rate of reaction?

Answer 8

• Simple but inaccurate method is to count the number of bubbles in a given time
• Measure the volume of the gas given through water displacement in a gas syrine or test tube. A graph can be produced of volume against time can show the change in rate

Question 9

How to calculate the rate of reaction by measuring the change in mass?

Answer 9

• Measure the decrease in mass
• As the reaction proceeds

Question 10

How to use colorimetry to calculate the rate of reaction?

Answer 10

• Involved when colour changes are used
• Can use simple observation or a colorimeter
• Appearance of colour or loss of colour is used
• Can change the reaction to measure the impact on rate

Question 11

How can titrimetric analysis be used to measure the rate of reaction?

Answer 11

• remove aliquots of the reactive mixture at regular intervals
• Aliquots added to another reagent to quench the reaction or leave it in cold water to prevent the concentration changing
• Quenched aliquots titrated to find the concentration of known compounds
• If an acid is used, titrate can be carried out with sodium hydroxide

Question 12

How is conductimetric analysis used to measure the rate of reaction?

Answer 12

• Number and type of ions can affect the elecrtrical conductivity
• A change in the ions will be reflected by a change in the conducitivty
• This is seen by changes in the ion

Question 13

What are the reactants in the iodine clock reaction?

Answer 13

• hydrogen peroxide ( H₂O₂)
• I^-
• H⁺
• I₂
• S₂O₃^2-

Question 14

What are the products in the iodine clock reactions?

Answer 14

• H₂O
• I₂
• I^-
• S₄O₆^2-

Question 15

How does the iodine clock reaction work?

Answer 15

• Potassium iodide is added to hydrogen peroxide
• Acidified dilute sulfuric acid, starch and sodium thiosulfate added
• Iodine produced which reacts with sodium thiosulphate to produce iodide ions
• When thiosulfate used up, the iodide reacts with starch to form a blue complex
• Excess can be used

Question 16

What is the symbol equation for the iodine clock reaction?

Answer 16

• Step 1 - H₂O₂ + 2I^- + 2H⁺ –> I₂ + 2H₂O
• Step 2- I₂ + 2S₂O₃^2- –> 2I^- + S₄O₆^2-
• Step 1 is the rate determining step

Question 17

What is the rate of reaction proportional to?

Answer 17

e^Ea/RT

Question 18

What is the rate of reaction equal to when referring to activation energy?

Answer 18

k=Ae^Ea/RT

Where A is the arhennius equation, R is the gas constant, T is the absolute temperature

Question 19

What is the equation when logarithm is used?

Answer 19

lnK = -Ea/RT

Question 20

When an activation energy graph is drawn, what do the axis represent?

Answer 20

• Y axis is lnK
• x axis is 1/T
• Gradient is -Ea/R
• Y intercept is the arrhenius constant

Question 21

When an S_N1 reaction occurs, what does the rate equation look like?

Answer 21

• Only one substance involved in the rate determining step
• R = k[X] where X is a substance
• nucleophile not involved in the rate determining

Question 22

What does the rate equation look like for an S_N2 reaction look like?

Answer 22

• Both substances in the rate equation
• R=k[X][C] where X and C are two substances, the nucleophile generally involved in the rate determining step

Question 23

If the order of reaction is a) 1 and b) 2 , what does the graph for half life like?

Answer 23

a) constant curve
b) as half life halves, the concentration doubles

Question 24

if the order is a)0 b)1 c)2 what does the graph for concentration against time look life? what does the graph for rate against concentration look like?

Answer 24

a) for concentration against time - a y=x graph

for rate against concentration - straight line in x axis

b) for concentration against time- a curved graph

for rate against time - a y=x graph

c) for concentration against time - a steeper curved graph for rate against time- a curved graph

Question 25

Explain the iodine-propanone reaction

Answer 25

• small aliquots removed
• sodiom hydrogencarbonate qunches the reaction
• aliquots titrated with standard solution of sodium thiosulfate -starch is indicator at end point
• rate=k[H][CH3COCH3]
• Iodine is 0 order therefore is in the reaction after RDS
• Excess proponone therefore concentration stays constant
• If experiment is repeated , use twice as much proponone, the slope is found to be twice as steep

Question 26

Suggest the possible mechanism for the iodine- proanone reaction

Answer 26

• Protonation of Oxygen in propanone
• Electron in C=O shifts into carbon to form carbocation
• Carbocation loses proton
• CH bond broken
• iodine molecule acts as an electrophile
• electrophilic adddition occurs

Question 27

what is the effect of temperature on rate?

Answer 27

increase in temperature increases the rate of reaction as the rate constant increases with temperature. Therefore smaller Ea means faster reaction