4.3 Rates

Question 1

Define activation energy

Answer 1

The amount of energy the particles need to react

Question 2

What does the overall rate of a reaction involving millions of particles depend on?

Answer 2

• Collision frequency: how often do the particles collide with each other?
• Percentage success: what percentage of the collisions have enough energy

Question 3

A chemical reaction can only happen if…?

Answer 3

If the reactant particles collide with enough energy.

Question 4

What happens when particles collide with less energy than the activation energy?

Answer 4

They rebound and remain unchanged

Question 5

Higher solid surface area increases rate because:

Answer 5

-More of the solid is exposed to the other particles
-So the collision frequency is higher
-So there are more successful collisions per unit time

Question 6

Higher solution concentration / gas pressure increases rate because:
MPPUV
CFH
MSCPUT

Answer 6

-There are more particles per unit volume
-So the collision frequency is higher
-So there are more successful collisions per unit time

Question 7

Higher temperature increases rate because:
PHMKE
POCWEEH
MSCPUT

Answer 7

-The particles have more kinetic energy
-So the percentage of collisions with enough energy is higher
-So there are more successful collisions per unit time

Question 8

Catalysts increase rate because:
PARWLAE
POCWEEH
TMSCPUT

Answer 8

-They provide an alternative route with a lower activation energy
-So the percentage of collisions with enough energy is higher
-So there are more successful collisions per unit time

Question 9

How can increased concentration be accomplished?

Answer 9

-by dissolving more solute particles

Question 10

How can increased pressure be accomplished?

Answer 10

-by making the reaction vessel smaller

Question 11

What variables affect the rate of reaction between sodium thiosulphate and hydrochloric acid? (disappearing cross)

Answer 11

IV: Concentration of Na2S2O3(aq)
DV: Time taken for the cross to disappear
Control variables:
-Total volume of solution
-Concentration of HCl(aq)
-Temperature

Question 12

How is the rate of reaction calculated in this experiment? (disappearing cross/magnesium and acid)

Answer 12

Rate (s^-1) = 1 / Time taken (s)

Question 13

How is the effect of concentration on rate demonstrated in this experiment? (disappearing cross)

Answer 13

-A solid (Sulphur) is slowly produced
-After a while, you can’t see through the mixture any longer.
-Measured by observing a black cross under the mixture.

Question 14

What do the results show about the relationship between concentration and rate? (disappearing cross)

Answer 14

-The graph shows a straight-line through the origin.
-This is because rate is proportional to concentration:
* Double the concentration = double the rate
* Triple the concentration = triple the rate
* Etc

Question 15

How is increased surface area accomplished?

Answer 15

By cutting or grinding up the solid

Question 16

What variables affect the rate of reaction between calcium carbonate (marble chips) and hydrochloric acid?

Answer 16

IV: the sizes of the CaCO3(s) chips
DV: the volume of CO2(g)
Control:
-Mass of CaCO3(s)
-Volume and concentration of HCl (aq)
-Temperature

Question 17

How is volume of CO2 measured in the marble chip experiment?

Answer 17

-using a gas syringe or inverted measuring cylinder in water

Question 18

What changes occur when comparing the higher surface area line (red) to the original (black) line?

Answer 18

-Steeper start:
Due to increased rate, CO2 volume increases faster.
-Same final volume:
As reactant amounts remain unchanged, the reaction eventually produces the same amount of CO2.

Question 19

What variables affect the rate of reaction between magnesium and hydrochloric acid?

Answer 19

IV: the temperature of the HCl (aq)
DV: the time taken for Mg (s) to disappear
Control:
-Mass and surface area of Mg (s)
-Volume and concentration of HCl (aq)

Question 20

What do the results show about the relationship between temperature and rate? (magnesium and acid)

Answer 20

-Graph is not a straight line (because rate is not proportional to temp)
-Graph doesn’t go through the origin (because rate isn’t zero at 0*c)

Question 21

What are catalysts?

Answer 21

They are substances (usually solids) which:
* Increase rate
* Remain chemically unchanged

Question 22

What variables affect the rate of reaction demonstrated using the decomposition of hydrogen peroxide?

Answer 22

IV: which catalyst is being used
DV: volume of O2(g)
Control:
-Volume and concentration of H2O2(aq)
-Temperature
-Mass and surface area of catalyst

Question 23

What catalyst accelerates the reaction that is normally slow and what colour is it? (hydrogen peroxide)

Answer 23

Manganese oxide (MnO2)- black solid

Question 24

How long does the reaction take without and with the catalyst?

Answer 24

Without- days
With- only a few seconds

Question 25

What effect do more effective catalysts have on reaction rate?

Answer 25

More effective catalysts increase the reaction rate more effectively.

Question 26

What do the results show about the relationship between catalysts and rate?(hydrogen
peroxide)

Answer 26

• Effective catalysts increase reaction rate more efficiently.
• Steeper graph line indicates enhanced rate with catalysts.
• Initial reactant amounts are same, leading to eventual flattening of lines at same O2 volume.
• MnO2 catalysed decomposition of H202 used to produce O2 in the lab