C5.2 - controlling reactions

Question 1

define ‘rate of reactions’

Answer 1

a measure of how quickly reactants are used
or
a measure of how quickly products are made

Question 2

what is the equation to calculate rate of reactions? (reactants)

Answer 2

amount of reactant used
———————————
time taken

Question 3

what is the equation to calculate rate of reactions (product)?

Answer 3

amount of product formed
————————————
time taken

Question 4

what happens as a gas syringe fills?

Answer 4

• plunger moves outwards

- record the end place of plunger (with graduations marked on side)

Question 5

how do you measure the volume of gas produced?

Answer 5

1) place dilute hydrochloric acid in a conical flask connected to a gas syringe
2) add piece of magnesium to acid, stopper the flask and start stop watch
3) record amount of hydrogen produced (in gas syringe) at regular intervals

Question 6

state two things you must remember when measuring how volume of gas changes

Answer 6

• push the gas plunger all the way in (make sure reading starts at 0)
• make sure the flask is stoppered as soon as experiment begins (so all hydrogen is collected)

Question 7

what is the instantaneous rate of reaction?

Answer 7

the rate at a particular time (by drawing a tangent to the curve)

Question 8

how do you calculate the mean rate of reactions?

Answer 8

change in volume
———————
change in time

Question 9

why do reactions go faster at higher temperatures?

Answer 9

• particles gain energy
• collide more often
• greater PROPORTION of particles have enough activation energy

Question 10

what 2 conditions must be met for reactions to occur?

Answer 10

• reactant particles must collide

- particles must have enough energy (activation energy)

Question 11

define a successful collision

Answer 11

a collision that leads to a reaction

Question 12

how do you investigate the effect of temperature?

Answer 12

using the ‘disappearing cross experiment’
- mix sodium thiosulfate and hydrochloric acid

(time it takes for cross to disappear at different temperatures)

Question 13

how does the rate of reaction relate to reaction time?

Answer 13

it is inversely proportional

1/reaction time

Question 14

why do reactions go faster at higher concentrations?

Answer 14

• particles become more crowded
• collide more often
• higher rate of successful collisions

Question 15

when the concentration increases, does the energy stored in the particles change?

Answer 15

no

Question 16

does the limiting reactant affect the amount of product made?

Answer 16

yes, they will be proportional

Question 17

how does a smaller particle size affect rate of reactions?

Answer 17

• more reactant particles available for collisions (higher surface area to volume ratio)
• collisions more likely, so particles collide more often
• increased successful rate of collisions

Question 18

why do powders result in fast reactions?

Answer 18

• very large surface area to volume ratio

Question 19

define activation energy

Answer 19

minimum energy required for particles to react

Question 20

how do you investigate particle size on rate of reactions?

Answer 20

• use gas syringe to measure volume of co2 produced

- how it changes with powder vs lump

Question 21

describe characteristics of a catalyst (4)

Answer 21

• increases rate of reaction
• remains unchanged (mass)
• specific to particular reactions (like enzymes)
• high surface to volume ratio

Question 22

define enzymes

Answer 22

proteins that act as catalysts in biological systems

Question 23

how do catalysts work? - 4 mark

Answer 23

• provide alternate pathway
• lowers activation energy

therefore. ..
- greater PROPORTION of particles have the activation energy - or more
- rate of successful collisions increase

Question 24

do catalysts affect how much energy the particles have (increase/decrease)?

Answer 24

no, they lower the energy required for the reaction to occur

Question 25

Why do reactions go faster with powders, detailed.

Answer 25

The particles in a substance in the solid state can only vibrate about fixed positions. They cannot move from place to place. This means that only the particles at the surface can take part in collisions. The rate of reaction increases as the surface area increases because:
- more reactant particles are available for collisions
- collisions are more likely, so particles collide more often
The energy stored in the particles does not change, but because the rate of collisions increases, the rate of successful collisions increases.

Question 26

Why smaller lumps react faster than bigger lumps?

Answer 26

The total surface area available for collisions is larger

Question 27

Why is the fact that a small amount of catalyst will catalyse the reaction between a large amount of reactants useful?

Answer 27

Because catalytic converters use platinum, rhodium, and palladium.
These metals are very expensive, so they are coated onto an inert ceramic ‘honeycomb’ .
This uses only a few grams of catalyst, provides a large surface area for the reactions, and allows exhaust gasses through.

Question 28

Pb - define catalyst

Answer 28

A substance that speeds up a reaction, WITHOUT BEING USED UP ITSELF