17 — Rate Of Reactions

Question 1

Particle size

Answer 1

• particle size -> ^ SA -> frequency of effective collision ^, rate of reaction ^

Question 2

Catalyst effect on rate of reaction

Answer 2

+ catalyst -> lowers activation energy by providing an alternative pathway -> more particles HV energy >/ activation energy -> frequency of effective collision ^ -> rate of reaction ^

Question 3

Temperature

Answer 3

+ tempt -> ^ AVE. Ke of particles, particles move faster -> more particles HV energy >/ activation energy -> frequency of effective collision ^ -> rate of reaction ^

Question 4

Concentration

Answer 4

+ Concentration -> no. Of particles per unit volume ^ -> frequency of effective collision ^, rate of reaction ^

Question 5

Pressure

Answer 5

+ pressure -> no. Of particles per unit volume ^ -> frequency of effective collision ^, rate of reaction ^

Only for gases, if reactants r not gases -> no effect on reaction

Question 6

Catalyst definition

Answer 6

A catalyst is a substance which increases the rate of a chemical reaction and remains chemically unchanged at the end of the reaction.

Question 7

Characteristics of a catalyst

Answer 7

Chemically unchanged
-> can be reused

Not used up in the reaction, only a small amt is needed
->can be reused

Highly selective

Provides an alternative pathway w lower Ae -> lower operating tempt, reducing energy required for reaction to tk place.

Increase rate of reaction but not yield (depends on moles of reactants)

Impurities can poison them.

Question 8

Examples of catalyst

Answer 8

Iron in Haber process
Platinum palladium and rhodium in catalytic converters in cars
Nickel in manufacture of margarine from vege oils
Aluminium oxide or silicon dioxide in cracking of hydrocarbons

Question 9

Biological catalysts: enzymes characteristics

Answer 9

Made of proteins

Tempt sensitive
-> below optimum tempt, enzyme dormant, reducing catalytic activity/denatured, unable to catalyse reactions

pH sensitive

Specific catalytic action

Question 10

Measuring rate of chemical reactions

Answer 10

Volume of gas produced at regular time intervals
-> recorded variable until there is no more change in volume -> spd of reaction measured

Change in mass of a reactant or product at regular time intervals
-> recorded variable until there is no more change in mass

Graphs:
Gradient steep: spd of reaction fast
Gradient decreasing: spd of reaction decreasing

Question 11

Initial gradient and plateau

Answer 11

Initial gradient: rate of reaction
Segment of graph that plateau: when reaction stops n vol of gas or mass change

To tk note:
Limiting reactant? Mole ratio the same? -> same vol or mass of product?
Acids R monobasic or dibasic? -> graph steeper?

Question 12

In terms of collision theory, explain the graphs drawn.

Answer 12

Experiment using Sulfuric acid has the steepest gradient while ethanoic acid has the gentlest gradient. Sulfuric acid is a dibasic strong acid while HCl is a weak monobasic acid. Thus, H2SO4 fully dissociates in water to give higher conc of H^+ ions than CH3COOH.

H2SO4 is a dibasic acid but HCl is a monobasic acid. Thus, number of reacting particles per unit volume of H2SO4 is higher than HCl. Frequency of effective collision is the highest, rate of reaction is hence the highest, producing the steepest gradient.

Furthermore, final volume of CO2 produced in HCL and ethanoic acid is half of H2SO4 as limiting reactant for the experiments is the acid. No. Of moles of H^+ in H2SO4 is twice the no. Of moles of H^+ in HCl and ethanoic acid. Thus, volume of CO2 produces in H2SO4 experiment is twice of HCl and ethanoic acid experiment.

Question 13

Graph of ethanoic acid vs HCl

Answer 13

Both HCl and CH3COOH r monobasic acids butHCl is a strong acid and CH3COOH is a weak acid. Hence, lower conc of H^+ ions results in slower rate of reaction. Since same volume and concentration r used, volume of gs produced is the same, the acid is the limiting reactant and gradient of HCl is steeper; gradient of CH3COOH is gentler.

Question 14

H2SO4 vs HCl graph

Answer 14

H2SO4 has a steeper gradient & higher volume of gas produced than HCl. H2SO4 is a dibasic acid hence has twice the conc of H^+ ions -> faster rate of reaction. Unlike monobasic acids like HCl, although same conc n vol were used, H2SO4 produces larger volume of gas due to its mole ratio of 1:1. HCl:gas = 2:1

Question 15

Why catalysts reduce costs in the long run

Answer 15

Remains chemically unchanged at the end of the reaction -> can be reused.
Catalysts r only needed in small amounts to increase rate of chemical reaction -> completed in shorter time -> reduce cost

Question 16

Explain why smth atoms is a catalyst.

Answer 16

Smth atoms r present at the beginning of reaction n r regenerated at the end of the reaction -> hence it did not tk part in the reaction (characteristic of catalyst) -> can be reused to function:[link to qs]

Question 17

Explain how coating on a catalyst affects reaction

Answer 17

Coating prevents availability of catalyst surface for alternative reaction pathway hence catalyst is unable to carry out function effectively.

Question 18

How lower pressure affects rate of reaction

Answer 18

Lower pressure -> gas particles spread further apart -> less particles per unit volume -> decreased frequency of effective collision -> rate of reaction slower

Question 19

Effect of lower pressure on yield

Answer 19

No change in final volume of yield from amt of reactants, however rate of reaction is slower. Unreacted reactants r recycled and reacted eventually to form products.

Question 20

Describe the experiments that the student should conduct to confirm if manganese(IV) oxide acts as a catalyst for the decomposition of hydrogen peroxide. Your Answ should include the experiments he should conduct, measurement to be taken and how the results can be used to confirm if manganese(IV) oxide acts as a catalyst.

Answer 20

2H2O2 (aq) -> 2H2O (l) + O2(g)

1. Students should measure the time taken for a fixed volume of oxygen gas to evolve.
2. Repeat the experiment with a small amount of Manganese (IV) oxide.
3. If he is correct, the experiment with Manganese (IV) oxide should take a shorter time.

OR by plotting a graph of…the graph with a steeper gradient contains the catalyst.

Question 21

Rate of reaction is directly proportionate to concentration of sodium thiosulfate. Show that this relationship is correct.

Answer 21

Rate of reaction = k x conc of sodium thiosulfate.

Since for all 3 experiments, rate of reaction/conc of sodium thiosulfate is a constant value, rate is directly proportional to concentration of sodium thiosulfate.