Chapter 17 - Rate of Reactions

Question 1

How do you measure the rate of a reaction?

Answer 1

OBSERVE
- How much product appears
- How much reactant disappears

Question 2

How do you physically measure the appearance or disappearance of substances?

Answer 2

MEASURE
- volume of gas produced in reaction
- change in mass of a reactant or product

Question 3

How do you describe a graph when measuring volume?

Answer 3

1) - Describe where the gradient is steepest (e.g. the gradient is steepest at the start of the reaction)
- Hence ROR is fastest (at the start)
- Volume of gas produced is greatest (at the start)
2) - Describe the middle of the gradient/where it starts to change
- As reaction progresses, the gradient becomes gentler
- ROR decreases with time
3) - Describe the end of the gradient
- At x mins, the gradient approaches 0 and the graph levels off
- No more gas is produced and reaction stopped
- Total amt of gas produced is y

Question 4

What happens to mass of reactants as the reaction progresses?

Answer 4

Mass decrease, reactants are used up. Mass of products increase

Question 5

How do you describe a graph when measuring volume?

Answer 5

1) - Describe where the gradient is steepest (e.g. the gradient is steepest at the start of the reaction)
- Hence ROR is fastest (at the start)
2) - Describe the middle of the gradient/where it starts to change
- As reaction progresses, the gradient becomes gentler
- ROR decreases with time
3) 3) - Describe the end of the gradient
- At x mins, the gradient approaches 0 and the graph levels off
- Mass of the reaction mixture has stopped decreasing and reaction stopped
- Reaction mixture decreased by y g

Question 6

The gradient of the graph can be positive or negative. True or false? Why?

Answer 6

True, it depends on whether the mass of the reactant or products are being measured
Products: gradient (+) & mass of products increases with time
Reactants: gradient (-) & mass of reactants decreases with time

Question 7

What is an effective collision?

Answer 7

When a chemical reaction occurs between particles

Question 8

What has to happen for an effective collision to occur?

Answer 8

Reactant particles must collide with energy that is greater than or equal to the activation energy

Question 9

What are the 5 factors which affect the ROR?

Answer 9

1) Concentration of reactants
2) Pressure of system (gases only)
3) Particle size of reactants
4) Temperature of reaction mixture
5) Addition of catalyst

Question 10

Describe the effect of concentration

Answer 10

• conc shows no. of particles in a given volume
• as conc increases, no. of reacting particles per unit volume increases
• collides more frequently
• frequency of effective collisions increase
• ROR increase

Question 11

Describe the effect of pressure

Answer 11

• the particle spacing in gases is larger than in liquids/solids
• when gases are compressed to a smaller volume at high pressure, spacing between particles decreases
• particles are closer tgt, collide more frequently
• increases frequency of effective collision and ROR

Question 12

Describe the effect of particle size

Answer 12

• fine sugar dissolves faster than same mass of sugar cubes due to smaller particle size
• break large into small, SA:V increases while total mass and volume remains the same
• in reactions, solids that are finely divided have more surface area in contact with other reactants
• increased SA:V allows for increased frequency of effective collisions to occur
• increased ROR

Question 13

How do you describe a graph for particle size? Lumps v/s powder

Answer 13

1)Describe the initial gradients
- The different gradients show the initial reaction rates due to the difference in particle size of x and y
2) Describe which has steeper gradient and why
- A has a steeper initial gradient than B, ROR increases with decreasing particle size
3) Describe the total decrease in mass
Total decrease in mass of reactants is same for A and B, cuz mass of limiting reactant used is the same for both A&B and HCl is in excess

Question 14

What is a data logger?

Answer 14

• used when reaction is too vigorous for data to be collected manually
• measures changes in conditions such as mass, temp and pressure using external sensors

Question 15

Describe the effect of temperature (both ways)

Answer 15

1. At higher temperatures, reactant particles have more kinetic energy, move and vibrate faster OR
   At higher temperatures, more reactant particles possess energy that is greater than or equal to activation energy
2. Both increasing frequency of effective collisions, increasing the ROR

Question 16

How do you describe a graph for temperature

Answer 16

• time taken for cross to disappear decreases as temperature increases
• shorter time taken would mean a faster ROR
• hence ROR increase

Question 17

What is a catalyst?

Answer 17

A catalyst increases the rate of chemical reactions without being chemically changed at the end of the reaction

Question 18

Name 2 characteristics of catalyst

Answer 18

1) Increases ROR but not yield
2) Not used up in reaction
3) Selective
4) Impurities can poison them
5) Physical appearance may change but remain chemically the same
6) Provide an alternative pathway with a lower activation energy for the reaction

Question 19

How does a catalyst work?

Answer 19

Catalyst provide an alternative pathway of lower activation energy for the reaction to occur. This speeds up the ROR

Question 20

How do catalyst increases ROR?

Answer 20

Due to the alternative pathway with lower activation energy, more reactant particles will have energy greater than r equal to activation energy. This increases the frequency of effective collisions. Hence ROR increases

Question 21

Why are catalysts usually made of transitional metals or transition metal compounds?

Answer 21

Due to their ability to have variable oxidation states

Question 22

What are some common industrial catalysts?

Answer 22

Iron, aluminium oxide/silicon dioxide, nickel, platinum/palladium/rhodium

Question 23

What is the use of iron catalyst?

Answer 23

Manufacture of ammonia in the Haber process

Question 24

What is the use of aluminium oxide/silicon dioxide catalyst?

Answer 24

Cracking of hydrocarbons

Question 25

What is the use of nickel catalyst?

Answer 25

Manufacture of margarine from vegetable oils

Question 26

What is the use of platinum, palladium, rhodium catalyst?

Answer 26

Catalytic converters

Question 27

Describe the industrial manufacture of ammonia by the Haber process is catalysed by finely divided iron, allowing good yields at lower pressures

Answer 27

1) N2 and H2 molecules bind onto the surface of the catalyst 2) N≡N and H-H bonds are weakened 3) N and H atoms rearrange to form NH3 4) NH3 molecules leave the catalyst surface. New N2 and H2 molecules bind onto the surface of the catalyst

Question 28

What are biological catalysts?

Answer 28

Enzymes - aid in energy production and detoxification

Question 29

What are some characteristics of enzymes?

Answer 29

1) Made of proteins 2) Temperature sensitive - operate best at body temp - Below temp, enzymes denature --> less catalytic activity - High temp, enzymes denature unable to catalyse 3) pH sensitive - operate best at optimal range (varies) - inactive above/below range 4) Specific catalytic action - Active site of each enzyme is specific - Active site can only fit certain reactant molecules in a specific orientation

Question 30

Describe the stages of an enzyme catalysing a reaction

Answer 30

1) Reactant enters the active site of enzyme 2) Enzyme changes shape to bind to the reactant 3) Enzyme catalyses the reaction, changing the reactant to the products 4) Products leave the active site of enzyme