Topic 6 - The rate and extent of chemical change

Question 1

what is the rate of a reaction

Answer 1

how fast the reactants are changed into products

Question 2

what are examples of slow reactions

Answer 2

rusting of iron
chemical weathering

Question 3

what is an example of a moderate speed reaction

Answer 3

magnesium reating with an acid to produce a gentle stream of bubbles

Question 4

give examples of fast reactiond

Answer 4

burning
explosions

Question 5

describe a graph for rates of reaction

Answer 5

the steeper the line, the faster the rate of reaction
the line becomes less steep as the reactants are used up
the quickest reactions have the stepest lines and become flat in the least amount of time
the flat line shows the reaction has finished

Question 6

what does collision theory state

Answer 6

the rate of a chemical reaction depends on :
The collision frequency of reacting particles (how often they collide ) . The more collisions there are , the faster the reaction is .
The energy transfered during a collision . particles have to collide with enough energy for the collision to be succesfull .

Question 7

what are the four factors that affect rates of reactions

Answer 7

temperature :
when the temperature increases, the particles move faster
if they move faster, they are going to collide more frequently
also the faster they move, the more energy they have so more of the collisions will have enough energy to make the reaction happen

Concentration of solution or pressure of gas :
if a solution is made more concentrated, there are more particles
similarly when pressure in gas is increases, same particles occupy a smaller space.
this makes collisions between the reactant particles more frequent

surface area:
if one of the reatants is a solid, then breaking it up into smaller pieces will increase its surface area to volume ratio
this means that for the same volume of the solid, the particles arounf it will have more area to work on , so there will be collisions more frequently.

the presence of a catalyst :
a catalyst is a substance that speeds up a reaction, without being used up in the reaction itself . this means it is not part of the overall reaction equation .
different catalysts are needed for different reactions but they all work by decreasing the activation energy required. they do this by providing an alternative pathway with a lower activation energy .
enzymes are biological catalysts - they catalyse reactions in living things .

Question 8

what is the equation for finding the rate of a reaction

Answer 8

rate of reaction = amount of reactant used or amount of product formed /time

Question 9

what is the unit for rate of reaction in : gas, solid

Answer 9

gas - cm^3 /s
solig - g/s

Question 10

what are the two methods for investigating rates of reactions

Answer 10

sodium thiosulfate and HCL produces a cloudy precipitate :

in this reaction one of the products is sulfur/ a solid
makes it clooudy / turbidity
we use this to see how long the reaction takes to finish

1. use a measuring cylinder to put 10cm^3 of sodium thiosulfate solution into a conical flask
2. place the conical flask onto a printed black cross
3. add 10 cm ^3 of hydrochloric acid into the conical flask
4. swirl the solution and start a stopwatch
5. look down through the top of the flask , after a certain time the solution will turn cloudy
6. stop the stopwatch when you can no longer see the cross
7. repeat using lower concentrations of sodium thiosulfate
8. repeat whole experiment and calculate mean values for each concentration

magnesium and HCL react to produce H2 gas :
this allows us to measure the volume of hyrdrogen gas produced

1. use a measuring cyclinder place 50 cm ^3 of hydrochloric acid to a conical flask
2. attach the conical flask to a bung and delivery tube
3. place the delivery tube in a container filled with water .
4. place an upturned measuring cylinder also filled with water over the delivery tube .
5. add a 3cm strip of magnesium to the hyrdrochloric acid and start a stopwatch
6. the reaction produces hydrogen gas which is trapped in the measuring cylinder
7. every ten seconds measure the volume of hydrogen gas untill no more hydrogen is given off
8. repeat using different concentrations of hydrochloric acid
   plot the results on a graph

amout of magnesium and volume of acid should be kept the same

Question 11

what do both methods for finding rates of reaction tell us

Answer 11

the greater the concentration of chemical, the faster the reaction
this means the finding is reproducible

Question 12

what are the issues with measuring rates of reaction by looking at colour change

Answer 12

subjective results , people may not agree over the exact point the black cross dissapeared .
you cannot plot a graph

Question 13

explain why recording mass change is the most accurate for rates of reaction

Answer 13

the mas balance gives an accurate reading

Question 14

what is the risk of measuring volume of gas being given off

Answer 14

if the reaction is too vigorous you can easily blow the plunger out the end of the syringe

Question 15

how do you calculate the mean reaction rate from a graph

Answer 15

work out the overall change in the y value and divide it by the total time taken for the reaction .

Question 16

explain how reversible reactions work

Answer 16

as the reactants react , their concetrations fall so the forward reaction will slow down .
as more and more products are made and their concentrations rise,the backward reaction will speed up .
after a while , the forward reaction will be going at exactly the same rate as the backward one , the system is at equilibrium

Question 17

what is dynamic equilibrium

Answer 17

both reactions are happening but there is no overall effect .
this means the concentrtions of reactants and products have reached a balance and wont change .

Question 18

how can equilibrium be reached

Answer 18

only if the reversible reaction takes place in a closed system . (one where nothing can escapce

Question 19

what does it mean if the posistion of equilibrium lies on the right or left

Answer 19

if it lies to the right, the concentration of products is greater than that of the reactants
if it lies to the left , the concentration of reactants is greater than that of the products .

Question 20

in a reversible reaction , if the forward reaction is endothermic what is the backward reaction

Answer 20

exothermic

Question 21

what is a good example of a reversible reaction

Answer 21

thermal decomposition of hydrated copper sufate

hydrated copper sulfate (reversible reaction symbol ) anhydrous copper sulfate and water
forward reaction endo
backward exo

if you heat blue hydrated copper sulfate crystals , it drives the water off and leaves white anhydrous copper sulfate powder. this is endothermic.
if you add a couple drops of water to the white powder, you get the blue crystals back again .this is exothermic

Question 22

what is le chatelier’s principle

Answer 22

the idea that if you change the conditions of a reversible reaction at equilibrium. the system will try to counteract that change .
it can be used to predict the effect of any changes you make to a reaction system

Question 23

what effect does changing the temperature have on a reversible reaction

Answer 23

if you decrease the temperature . the equilibrium will move in the exothermic direction to produce more heat.
this means you will get moer products for the exothermic reaction and fewer for the endothermic reaction.
if you raise the temperature the equilibrium will move in the endothermic direction to try and decrease it . You will now get more products for the endothermic reaction and fewer products for the exothermic reaction .

Question 24

what effect does changing the pressure have on a reversible reaction

Answer 24

changing the pressure only affects an equilibrium involving gases
if you increase the pressure , the equilibrium tries to reduce it- it moves in the direcition where there are fewer molecules of gas.
if you decrease the pressure , the equilibrium tries to increase it - it moves in the direction hwere there are more molecules of gas .
you can use the balanced symbol equation for a reaction to see which sife has more molecules of gas

Question 25

what effect does changing concentration have on a reversible reaction

Answer 25

if you change the concentration of the reactants or products , the system will no longer be at equilibrium .
so the system responds to bring itself back to equilibrium again.
if you increase the concentration of the reactants, the system tries to decrease it by making more products .
if you decrease the concentration of the products the system tries to increase it again by reducing the amount of reactants