C6 - The Rate and Extent of Chemical Change

Question 1

How do you calculate mean rate of reaction?

Answer 1

amount of reactant used / time taken
or
amount of product formed / time taken

Question 2

Define rate of reaction

Answer 2

the amount of time is takes for the reactants to be used up or the products to form

Question 3

What is the mean rate of reaction (off a graph)?

Answer 3

overall rate until the reaction ends

Question 4

Describe how you calculate the mean rate of reaction from a graph

Answer 4

• work out the overall change in y (until the line goes flat)
• work out the overall change in x (until the line goes flat)
• change in y / change in x

Question 5

Describe how you calculate rate of reaction at a specific point (from a graph)?

Answer 5

• draw a tangent that touches the point on the graph
• turn the tangent into a triangle (as big as possible)
• find the change in y / change in x

Question 6

Name the 4 things that speed up chemical reactions

Answer 6

• increasing temperature
• increasing concentration/pressure
• increasing surface area
• using a catalyst

Question 7

Why does increasing the temperature increase the rate of a chemical reaction?

Answer 7

• when temp is increased, particles have more energy so move around faster
• this means the particles collide more frequently
• more collisions will have enough energy to make the reaction happen (activation energy)

Question 8

Why does increasing concentration/pressure increase the rate of a chemical reaction?

Answer 8

• there’s more particles in the same volume (conc)
• or there’s the same number of particles in a smaller space (pressure)
• either way, this leads to more frequent collisions

Question 9

Why does increasing the surface area increase the rate of a chemical reaction?

Answer 9

• more particles are exposed which means there’s more area for particles to collide with
• this means there’s more frequent collisions

Question 10

Why does adding a catalyst increase the rate of a chemical reaction?

Answer 10

• a catalyst decreases the activation energy needed for the reaction to occur by providing an alternative reaction pathway
• this increases the frequency of successful collisions

Question 11

Describe the disappearing cross practical

Answer 11

• put 10cm³ of hydrochloric acid in a beaker and place on top of a cross
• add 20cm³ of sodium thiosulfate - swirl and start a stopwatch
• stop the timer once you can’t see the cross anymore
• place the beaker in a fume cupboard once finished
• repeat and take a mean
• repeat with different concentrations

Question 12

Describe the practical - measuring the volume of gas collected

Answer 12

• half fill a bowl with water, fill the measuring cylinder with water and turn upside down
• pour 50cm³ of 2M hydrochloric acid into a conical flask
• add 2 x 3cm strips of magnesium into the flask and put bung into flask quickly
• start stopwatch and record volume of gas every 10 secs
• repeat with different concentrations

Question 13

Define dynamic equillibrium

Answer 13

when the forward and reverse reaction occur at the same rate

Question 14

How does temperature affect the position of equilibrium?

Answer 14

raise temp = moves in endo direction (to try decrease it)
lower temp = moves in exo direction (to try increase it)

Question 15

How does concentration affect the position of equilibrium?

Answer 15

increase conc = shifts in opposite direction
decrease conc = shifts in the same direction

Question 16

How does pressure affect the position of equilibrium?

Answer 16

increase pressure = moves to fewer molecules of gas (to reduce it)
decrease pressure = moves to more molecules of gas (to increase it)

Question 17

3 markers about reversible reactions should include:

Answer 17

• state the rule
• apply the rule (equilibrium shifts left/right)
• predict the effect on yield