Rates of reaction

Question 1

Name 4methods use tostudy therate of reaction

Answer 1

1. Collection of a gas ina measuring cylinder by downward displacement of water2. Collection of a gas in a gas syringe3. Mass lossusing a balance4. Disappearing cross by formation of a precipitate

Question 2

What are the possible units for therate of reaction?

Answer 2

1/s - per secondg/s - grams lost per secondcm3/ second - volume of gas produced per second

Question 3

What is meant by rate of reaction?

Answer 3

Rate of reaction is how fast a reaction occurs.

Question 4

What assumptions does kinetic theory make about chemical reactions?

Answer 4

1. For a reaction to take place, the particles must collide with one another.2. The colliding particles must have sufficient energy to react. This energy is known as the activation energy.3. To increase the rate of reaction it is necessary to increase the frequency of successful collisions.

Question 5

How does increasing the concentration of a reactant affect the rate of reaction?

Answer 5

• more particles of reactant in a given volume- more frequent collisionsincreased rate of reaction

Question 6

How does increasing the temperature affect the rate of reaction?

Answer 6

• reactant particles have more kinetic energy-there are more particles with the required activation energy- there are more successful collisions- PLUS- more frequent collisions-increased rate of reaction

Question 7

How does decreasing the concentration of a reactant affect the rate of reaction?

Answer 7

• lessparticles of reactant in a given volume- lessfrequent collisions-decreased rate of reaction

Question 8

How does increasing the concentration of a reactant affect the rate of reaction?

Answer 8

• more particles of reactant in a given volume- more frequent collisionsincreased rate of reaction

Question 9

How does doublingthe concentration of a reactant affect the rate of reaction?(be quantitative!)

Answer 9

• double the number ofparticles of reactant in a given volume- double thefrequency ofcollisions-double therate of reaction

Question 10

How does decreasing the concentration of a reactant affect the rate of reaction?

Answer 10

• lessparticles of reactant in a given volume- lessfrequent collisions-decreased rate of reaction

Question 11

How does decreasing the temperature affect the rate of reaction?

Answer 11

• reactant particles have lesskinetic energy-there are lessparticles with the required activation energy- there are fewersuccessful collisions- PLUS- lessfrequent collisions-decreased rate of reaction

Question 12

How does increasing the temperature affect the rate of reaction?

Answer 12

• reactant particles have more kinetic energy-there are more particles with the required activation energy- there are more successful collisions- PLUS- more frequent collisions-increased rate of reaction

Question 13

What is the general rule for how the rate of reaction is affected in the temperature increases by 10 degrees?

Answer 13

A 10 degree increase roughly doubles the rate of reaction

Question 14

What is a catalyst?

Answer 14

A catalyst is a substance the increases the rate of a chemical reaction but is chemically unchanged at the end of the reaction

Question 15

How does doubling thesurface area of a solid reactant affect the rate of reaction?

Answer 15

• double the surface area- double theparticles exposed to the liquid reactant-double the number ofcollisions per unit time-double therate of reaction

Question 16

How does a catalyst work?

Answer 16

A catalyst provides an alternate route with a lower activation energy than the original- more the collisions which take place will have the necesssary activiation energy- more successful collisions per second-increased rate of reaction

Question 17

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

Answer 17

• lowersurface area- fewerparticles exposed to the liquid reactant-lessfrequent collisions-decreased rate of reaction

Question 18

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

Answer 18

• higher surface area- more particles exposed to the liquid reactant-more frequent collisions-increased rate of reaction

Question 19

What happens to the rate of reaction if the surface area of a catalyst is increased?

Answer 19

The rate of reaction would increase-surface area of catalyst in contact with reactant has increased- more collsions per unit second

Question 20

If the reaction between calcium carbonate and hydrochloric acid was being measured. What would the rate be if 50cm3 of carbon dioxide was collected in 100 seconds.

Answer 20

rate = volume of gas collected / timerate = 50 cm3 / 100srate = 0.5 cm3/s

Question 21

In a reaction where silver iodide precipitate was formed, the cross at the bottom of the flask disappeared in 50 seconds. Calculate the rate of reaction.

Answer 21

rate = 1 / timerate = 1/50s = 0.02 s-1

Question 22

Use ideas of reaction rate to help explain why a spark in a flour mill with fine flour dust can be very dangerous.

Answer 22

The fine flour dust has a very large surface area.A very large amount of flour is exposed to oxygenThere is a very large number of collisions per unit timeIncreased rate of reaction means a spark can cause an explosion.

Question 23

Why does a graph of amount of CO2 gas produced against time eventually level off?

Answer 23

Rate of reaction decreases over time.Number of reactant particles decreases over time of reaction because they are being used up.Less collisions per unit time results in a slower reaction. (gradient of graph decreases- less gas produced per time)Eventually the limiting reactant runs out and no more CO2 will be produced. (graph has levelled off)

Question 24

In the reaction of Magnesium metal and hydrochloric acid, hydrogen gas is released. If the concentration of the acid is doubled, the rate of reaction should double. Why isn’t this seen when this is done experimentally?How can the method be improved?

Answer 24

The reaction is exothermic. It is difficult to keep the other control variable constant. Increased temperature speeds up the reaction so that the reaction rate ismore than doubled.You would get better resultsifthe reaction was carried out in a water bath- and temperature controlled.