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 by providing an alternative pathway of lower activation energy for chemical reaction to occur, 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 ethanoic acid 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.

Explain the shape of your graph. [3]

Speed of reaction is slower/Graph is less steep than Expt 1 as ethanoic acid is a
weak acid (✓) that dissociates partially in water(✓) to form lower
concentration of H+ ions. (✓)
Frequency of effective collisions is lower (✓) and hence speed is slower.
4 (✓) – [2]; 2 – 3 (✓) – [1] , 1 (✓) – [0]
Volume of gas formed is the same as Expt 1 as the number of moles of acid
used or concentration and volume of acid remains unchanged [1].

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

Explain why smth atoms is a catalyst.

Answer 15

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 16

Explain how coating on a catalyst affects reaction

Answer 16

Coating reduce surface area of catalyst in contact with the reactant particles hence preventing the reaction from having a faster reaction.

Question 17

How lower pressure affects rate of reaction

Answer 17

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

Question 18

Effect of lower pressure on yield

Answer 18

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 19

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 19

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 20

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

Answer 20

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.

Question 21

Why stage 1 requires more energy than stage 2.

Answer 21

More energy needed in stage 1 as more heat is needed to maintain higher operating temperature of X than in stage 2, y.

Electrolysis process requires electrical energy thus using up more energy.

Stage 2 is an exothermic reaction hence releases thermal energy compared to stage 1 which is an endothermic reaction and gains thermal energy.

The activation energy for reaction B is lower than the activation energy needed for A.

Question 22

How catalysts save costs.

Answer 22

Catalyst lowers the activation energy of the reaction by providing an alternative pathway hence lowering the operating temperature required, thus lesser energy is required, reducing cost in the long run.

Catalysts remains chemically unchanged at the end of the reaction, and are regenerated at the end of the experiment. Hence can be reused, do not need to buy more catalysts, required in small quantity, reducing costs in the long run.

Catalysts increase the speed of reaction, decreasing time taken, more products can be produced within a shorter time, reducing cost in the long run.

Question 23

Why explain why significantly higher temperature and pressure is not used commercially.

Answer 23

Expensive to maintain high pressures as it requires expensive equipment and a large amount of electricity.
At high temperatures, ammonia will decompose back to N2 and H2 gas, reducing yield of ammonia.

Question 24

Why promote recycling of metals

Answer 24

Enables conservation of metal OREs as metal ORES are finite resources

Question 25

Explain how the equations show that chlorine atoms act as a catalyst for the breakdown of ozone. [1]

Answer 25

Cl atom participates in the reaction but remains chemically unchanged at the end of the reaction as they are not used up and the same amount is present at the beginning and at the end of the reaction.

Question 26

X claims that rate of reaction increases when the experiment involving aqueous reagents is under higher pressure. dya.

Answer 26

Incorrect. Pressure only increases speed of reaction when gaseous reactants are used.

Question 27

Explain which process is reversible [1]

Answer 27

B. DOUBLE HEADED ARROW shows that the process can go in both directions.
CO2 GAS DISSOLVING IN WATER TO FORM AQUEOUS CO2 IS THE FORWARD REACTION AND DISSOLVED CO2 ESCAPES FROM SEAWATER TO FORM GASEOUS CO2 IS THE BACKWARD REACTION.

Question 28

Cu is a catalyst for the reaction between Zn and dilute acid. Describe a reaction to prove that it is a catalyst in this reaction. [2]

Answer 28

1. Measure the same volume of HCl of the same concentration as that in experiment 2.
2. Add excess Zn and 0.5g of Cu.
3. Immediately start the stopwatch and stopper the conical flask with a rubber bung connected to a delivery tube linking to the gas syringe.
4. Measure the volume of gas produced in 30s. [1]
5. If Cu acts as a catalyst, rate of reaction would increase. Thus, volume of gas collected in 30s should be more than 29.0cm3. [1]

Question 29

Which one of the experiments is used as a reference to compare the effect of the variables? EYA [2]

Answer 29

Experiment 2 [1].

Experiment 2 can be compared to Experiment 1 with all factors remaining constant except concentration of lodide ions. Experiment 2 can also be compared to Experiment 3 with a change in temperature as the only changed variable. Lastly, Experiment 2 can be compared to Experiment 4 with the use of catalyst in Experiment 4 as the only changed variable. [1]

Question 30

Explain why changing the conditions changes the time taken for the blue-black colour to appear. [5]

Answer 30

As temperature increases from 20°C to 40°C from Experiment 2 to Experiment 3, reactant particles gain kinetic energy and move faster. More particles have energy more than or equal to activation energy, increasing frequency of effective collision. Thus, speed of reaction increases, decreasing the time taken for the blue-black colour to appear in Experiment 3.

As concentration increases from 0.1 mol/dm3 to 0.2 mol/dm3 from Experiment 1 to Experiment 2, there are more reactant particles per unit volume. This increases frequency of effective collision, increasing speed of reaction and hence, time taken for the blue-black colour to appear in Experiment 2 decreases.

With the addition of a catalyst in Experiment 4 compared to Experiment 3, the activation energy of the reaction is lowered as the catalyst provides an alternative pathway. Thus, more particles have energy more than or equal to activation energy, increasing frequency of effective collision. Thus, speed of reaction increases, decreasing the time taken for the blue-black colour to appear in Experiment 4.

Question 31

Popular: the rate of reaction is inversely proportional to the Mr of each gas. Do the data support this conclusion? EYA.[2]

Answer 31

As the Mr of the gas decreases, rate of diffusion increases.
Measurements do not support the conclusion as to be inversely proportional, both distance travelled and Mr of gas must be related by a constant value.
(Calculation)
Since they are not related by a constant value, rate of diffusion is not inversely proportional to the relative Mr of each gas.

Question 32

Explain how the magnesium protects the pipe from rusting [2]

Answer 32

Magnesium is a more reactive metal than iron, thus has a higher tendency to lose electrons to form positive ions, corroding in place of iron, acting as a sacrificial metal to prevent iron from coming into contact with oxygen and metal.

Question 33

State and explain expected observations when Mg is added to isomer X and Y. [2]

Answer 33

For both isomers, effervescence will be observed and solid magnesium will become smaller in size. [1]
Both isomers X and Y have pH of 3 and are carboxylic acids and will react MODERATELY with magnesium to produce hydrogen gas. Rate of reaction for both isomers will be slow as both are weak acids. [1]