Topic 6 - Rate of Reaction

Question 1

how does the rate change over time
1) start(steep line)
2) middle(curving line)
3) end (flat line on graph)

Answer 1

1) rate is fastest at very start because concentration of reactants is highest
2) rate slows down because concentration of reactants decreases as they get used up
3) rate is 0 because all reactants are used up so reaction has stopped

Question 2

what is reversible reaction

Answer 2

when products can react to form original reactants

Question 3

what is the symbol for a reversible reaction

Question 4

what happens as reactants react in a reversible reaction

Answer 4

as they react, their concentration falls- so forward reaction slows down BUT as more products are formed their concentration rises so backwards reaction speeds up

Question 5

what happens when a reversible reaction is at equilibrium

Answer 5

soon forward and backwards reaction will go at the same rate

Question 6

is a reversible reaction endo or exothermic

Answer 6

if the reaction is endothermic one way, it will be exothermic the other way

Question 7

what happens to the energy in a reversible reaction

Answer 7

the energy transferred from surroundings by endothermic is equal to energy transferred to surroundings by exothermic

Question 8

what explains the rate of reaction

Answer 8

the Collison theory- in terms of particles of reactants colliding with each other

Question 9

what does the rate of reaction depend on

Answer 9

• the collision frequency
• the energy transferred during a collision as particles have to collide with enough energy for it to be succesful

Question 10

what is activation energy

Answer 10

the minimum amount of energy needed to start a reaction

Question 11

what is the link between rate of reaction and collisions

Answer 11

more collisions increases rate

Question 12

what do catalysts do

Answer 12

increases rate of reaction (speed it up)

Question 13

how do catalysts work

Answer 13

increases rate by providing a different pathway for the reaction that has a lower activation energy
- not used up in reaction

Question 14

what do enzymes work as

Answer 14

biological catalysts

Question 15

what does using a catalyst look like on a reaction profile

Answer 15

• a lower curved line to show the less activation energy

Question 16

how does pressure affect rate

Answer 16

increases pressure of gas, increases the number of particles in a certain volume. this means particles collide, more often increasing rate

Question 17

how does concentration affect rate

Answer 17

if a solution is made more concentrated, there are more particles in the same volume
this makes collisions more frequent

Question 18

how does surface area affect rate

Answer 18

breaking up a solid into smaller pieces increase SA: V
for the same volume of solid, the particles will have more area to hit - so collisions will be more frequent
- the larger the SA the faster the rate

Question 19

how does temperature affect rate

Answer 19

the higher the temperature the faster the rate
- particles have more energy and move faster so collisions are more frequent
also more particles have the activation energy so more collisions are successful

Question 20

how do you calculate the mean ate of reaction from a graph (2)

Answer 20

1. change in y/ change in x
   2.draw a tangent at the point and find its gradient

Question 21

what is the rate of a reaction

Answer 21

how fast the reactant are changed into products

Question 22

what is an example of a slow reaction

Answer 22

rusting iron

Question 23

what is an example of a moderate reaction

Answer 23

magnesium reacting with acid

Question 24

what id an example of a fast reaction

Answer 24

explosions

Question 25

how do you calculate the mean rate of reaction

Answer 25

quantity of reactants used or products formed/ time

Question 26

what happens when the line on the graph is flat

Answer 26

reaction has stopped

Question 27

why does the line on the graph get less steep (rate of reactions)

Answer 27

line gets less steep as the reactants are used up

Question 28

what does the steepness of the line mean

Answer 28

the steeper the line on the graph, the faster the rate of reaction

Question 29

how can you measure the rate of reaction (2)

Answer 29

1. colour change
2. gas produced

Question 30

how can you use the colour change to measure the rate

Answer 30

1. add a set volume of dilute sodium thiosulfate to a conical flask
2. place a flask on top of a piece of paper with a black cross on
3. add dilute HCl to flask and start stopwatch
   4.time how long it takes for the black cross to disappear through the cloudy sulphur
4. repeat the experiment but increases the concentration of HCl (must have same depth each time)

Question 31

what is the link between concentration and time it takes for cross to disappear

Answer 31

the higher the conc. the quicker the reaction and leass time it takes for mark to disappear

Question 32

how can you use gas produced to measure rate of reaction

Answer 32

1. add a set volume of dilute HCL to a conical flask
2. ass some magnesium ribbon to acid and quickly attach gas syringe to the flask
3. start the stopwatch, take readings of the volume of gas at regular intervals
4. put results in a table
5. repeat experiment with different conc. of acid