C5.2 - Controlling reactions

Question 1

Rate of reaction

Answer 1

how quickly a reaction happens

Question 2

Rate of reaction equation

Answer 2

rate of reaction = amount of reactants used or amount of products formed ÷ time

Question 3

3 examples of measuring the rate of reaction

Answer 3

Precipitation
Change in mass (usually gas given off)
The volume of gas given off

Question 4

How do measure rate of reaction in a precipitation reaction?

Answer 4

method works for any reaction where mixing two colourless solutions produces a precipitate which makes the solution cloudy

pour one solution into a flask with the other solution
place flask on a piece of paper that has a cross on it
observe the cross through the mixture
measure how long it takes for the mark to be obscured
faster it disappears - faster the reaction

result is subjective - different people may not agree on exactly when the mark disappears

Question 5

How do you measure rate of reaction using change in mass?

Answer 5

can measure the rate of a reaction that produces a gas using mass balance

as gas is released the lost mass is easily measured on the balance
as the reading on the balance drops - mass is lost
use the results to plot a graph of change in mass against time

method released gas produced into the room - if gas is harmful need to take safety precautions (do experiment in fume cupboard)

Question 6

How do you measure rate of reaction using volume of gas given off?

Answer 6

involves the use of gas syringe to measure the volume of gas given off

more gas given off during a set time interval - faster the rate of reaction
can use your results to plot a graph of gas volume against time elapsed

ensure you use right size gas syringe - if reaction is too vigorous, plunger can blow out of the end of syringe

Question 7

Suggest an experiment which can be used to show how surface area affects reaction rate?

Answer 7

reaction of hydrochloric acid and marble chips

Question 8

Method for reaction of hydrochloric acid and marble chips

Answer 8

in a flask place dilute HCl and marble chips
attach a gas syringe to the flask
measure volume of CO2 gas produced using gas syringe
take readings at regular time intervals and record results in a table
plot a graph of results - time on x-axis, volume on y-axis
repeat experiment with same volume and concentration of acid and same mass of marble chips but they are smaller in size
repeat experiment with powdered chalk

Question 9

What are marble chips and chalk made out of?

Answer 9

calcium carbonate

CaCO3

Question 10

Finer particles mean marbles have…

Answer 10

larger surface area

Question 11

Rate of reaction of finer particles

Answer 11

finer particles - means greater the surface area of the solid reactants
reaction goes faster

Question 12

Rate of reaction if a greater mass of marble chips is added

Answer 12

extra surface area gives a faster reaction

also more gas made overall

Question 13

Suggest an experiment which can be used to measure the effects of changing the concentration on the rate of reaction

Answer 13

reaction of Magnesium metal and dilute HCl

reaction gives off hydrogen gas - measure mass loss of gas using mass balance

Question 14

higher concentration gives…

Answer 14

a faster reaction

reaction finishes sooner

Question 15

Calculating rate of reaction from a graph

Answer 15

If graph is amount of products formed (or reactants used up) against time - gradient of graph is rate of reaction

gradient = change in y ÷ change in x

Question 16

Relation between rate and time

Answer 16

rate is inversely proportional to time

1/time = rate of reaction

Question 17

What does rate of reaction depend on?

Answer 17

temperature
concentration (pressure for gasses)
size of particles

Question 18

Collision theory

Answer 18

rate of chemical reaction depends on -
collision frequency of reacting particles - more successful collisions, the faster the reaction is
energy transferred during collisions - particles have to collide with enough energy for collision to be successful

Question 19

Increasing the temperature…

Answer 19

particles move faster - have more collisions
increases energy of collisions since particles are moving faster
so more successful collisions - more particles collide with enough energy to react
so reaction rate increases

Question 20

Increasing concentration (or pressure) …

Answer 20

if solution is made more concentrated - more particles of reactants in same volume
collisions are more likely - reaction rate increases

increasing pressure of a gas - particles are more crowded
frequency of collisions between particles will increase - reaction rate increases

Question 21

Smaller solid particles (or more surface area)…

Answer 21

if one reactant is a solid - breaking it into smaller pieces will increase its surface area to vol ratio
particles around it will have more area to work on - frequency of collisions increases
reaction rate increases for solids with a larger surface area to volume ratio

Question 22

Catalyst

Answer 22

substance which increases the rate of reaction without being chemically changed or used up in the reaction

Question 23

How do catalysts work?

Answer 23

decrease the activation energy needed for a reaction occur

they do this by providing an alternative reaction pathway that has a lower activation energy

Question 24

How do you find out if a substance is a catalyst for a reaction?

Answer 24

measure the reaction rate between two solutions without the solid you think may be a catalyst present
and measure again with a know mass of the solid
if rate increases but solid appears to be unchanged it could be a catalyst
to check that none of the solid has been used up - filter it out at the end of the experiment, dry it and measure the mass to check if its still all there

Question 25

Biological catalyst

Answer 25

catalyse (speed up) the chemical reactions in living cells

enzymes are biological catalysts

Question 26

Examples of reactions catalysed by enzymes

Answer 26

respiration
photosynthesis
protein synthesis