Rates of Reaction

Question 1

What factors will affect the chemical weathering of rocks and limestone buildings?

Answer 1

• Speeded up by higher temperatures and higher concentration fo acid in rainwater

Question 2

How can the rate of a chemical reaction be defined (both ways)?

Answer 2

• Change in amount of reactant per unit time
• Change in amount of product per unit time

Question 3

What two methods could be used to measure the volume of gas produced in a reaction?

Answer 3

• Collecting it in a gas syringe, measuring cylinder or burette over water
• Allow a gas product to escape and follow the reusltant loss of mass of the reaction vessel

Question 4

What happens to the rate of reaction as the reactants are used up?

Answer 4

• The reaction will become progressively slower until one of the reactants is completely used up at which point the reaction will stop

Question 5

How do you work out the average rate of reaction over a period?

Answer 5

• average rate = total change in amount of reactant or product / time taken

Question 6

How do you find the exact rate at any instant of any plotted reaction?

Answer 6

• Draw a tangent to the curve at the relevant point nad divide the vertical change by the horizontal change for the tangent (i.e. gradient of the curve)

Question 7

What four factors could have an effect on the rate of a chemical reaction?

Answer 7

• Concentration of liquid reactions (or pressure of gaseous reactants)
• Surface area of solid reactants
• Temperature of reactants
• Use of a catalyst

Question 8

What is collision theory?

Answer 8

• In order to react with each other, the rectant particles must collide, and collide with sufficient energy to react successfullly.

Question 9

What does the rate of reaction depend on?

Answer 9

• The number of successful collisions per second

Question 10

What will increasing the concentration of a liquid reactant do?

Answer 10

• Increases the number of reactant particles per unit volume.

Question 11

What will increasing the number of reactant particles per unit volume do?

Answer 11

• There would be more reactant particles packed into a given volume, therefore they are more likely to collide, and so there will be more collisions per second hence a greater rate

Question 12

What happens under higher pressure?

Answer 12

• The same amount of gas is compressed into a smaller volume, so there are moe gas particles per cm³.
• Therefore there will be more collisions per unit time

Question 13

What willl doubling the concentration of a particular reactant?

Answer 13

• It will double the numbe rof particles per unit volume, this will generally result in a doubling of the reaction rate

Question 14

How can the surface area of a solid reactant be increased

Answer 14

• By breaking it up into smaller lumps, i.e. into a power

Question 15

How does increasing the surface area of a solid reactant affect the reaction?

Answer 15

• It will create a greater ‘target area’ in contact with the other reactnat , so the greater the area over which collisions may take place
• This results in more collisions per second and hence a greater rate

Question 16

How does an increase of 10°C affect a reaction?

Answer 16

• Cause the rate of many reactions to double

Question 17

How generally does the rate vary with temperature?

Answer 17

• They exponentially vary
• They are not directly proportional

Question 18

What happens when the temperature of the reactants is increased?

Answer 18

• Increases the average kinetic energy of the particles.
• This means a greater proportion of collisons will occur with an energy in excess of the activation energy,
• This means a greater proportion of collisions will be successfull, resulting in more successful collisions per second and hence a much higher rate

Question 19

What is this minimum energy needed for a collision to be successful called?

Answer 19

• The activation energy, E_a.

Question 20

What is a catalyst?

Answer 20

• A substance which increases the rate of a chemical reaction, without being used up in the overall reaction

Question 21

How does a solid catalyst work?

Answer 21

• By bringing together the reactant molcules on its surface.
• The bonds in the reactant molvecules are weakened when this happenes which means less energy is required to break them and hence a lower activation energy is possible.

Question 22

What does a catalyst provide?

Answer 22

• Provides an alternative pathway for a reaction, with lower activation energy

Question 23

What is the catalyst in the decompostion of hydrogen peroxide?

Answer 23

• Manganese (IV) oxide

Question 24

What is the catalyst in the haber process (ammonia manufacture)?

Answer 24

• Iron

Question 25

What is the catalyst in the contact process (sulfuric acid manufacture)?

Answer 25

• Vanadium pentoxide

Question 26

How are enzymes used in biological washing powders?

Answer 26

• Enzumed added to the degertent help to break down protein and make stains easier to remove.

Question 27

How are enzymes used in making soft centred chocolates?

Answer 27

• Invertase, an enzyme, is injected into the centres of chocolates.
• THis breaks down long sugar molecules into shorter molecules, which are less prone to forming hard crystals

Question 28

How are enzymes used in brewing?

Answer 28

• Enzymes produced by yeast catalyse the breakdown of glucose into ehtnaol and carbon dioxide

Question 29

How does freezing help?

Answer 29

• The -30°C temperature immobilise enzyme molecules.
• This reduces the rate of harmful reactions virtually to zero.

Question 30

How does refigerating food help preserve it?

Answer 30

• The 5°C temperatures help slow down enzyme-catalysed reactions, allowing the food to be kept for much longer

Question 31

How does cooking help?

Answer 31

• It kills bacteria and denatures enzymes
• This prevents them from causing the harmful breakdown of chemicals in the food