C6 Measuring Rates of Reaction (pg 144)

Question 1

How can the rate of a reaction be observed by equation?

Answer 1

by how quickly the reactants are used up or how quickly the products are formed
Amount of reactant used or amount of product formed
Rate of reaction _______________________________________
Time

(This is the mean rate of reaction. To find out the rate of a reaction at a particular time, you’ll need to plot a graph and find the gradient at that time. (see page 146).

Question 2

When the product or reactant is a gas, how do you usually measure the amount in?

Answer 2

its usually measured in cm³

Question 3

When the product or reactant is a solid, how do you usually measure the amount in?

Answer 3

its measured in grams (g)

Question 4

When the product or reactant is measured in time, how do you usually measure the amount in?

Answer 4

its often measured in seconds (s)

Question 5

What do these units of measure mean?

cm³/s or in g/s

Answer 5

cm³/s measured product is gas with time in seconds

g/s measured product is a solid with grams with time in seconds

Question 6

you can also measure the amount of product or reactant in moles, what what would the units of rate be?

Answer 6

mol/s

(moles)

Question 7

What are the four ways to measure Rate of a Reaction in units?

Answer 7

cm³ gas
grams (g) solids
seconds (s) time

also mol/s (moles)

Question 8

you can only record the visual change in a reaction if….

Answer 8

You can record the visual change in a reaction if the initial solution is transparent and the product is a precipitate which clouds the solution ( it becomes opaque)

Question 9

You can observe a mark through the solution, and measure how long it takes for it to disappear, what do this mean?

Answer 9

The faster the mark disappears, the quicker the reaction

Question 10

If the reactants are coloured and the products are colourless, what can you observe about the solution?

Answer 10

how long it takes for the solution to lose (or gain) its colour.

The results are very subjective - different people might not agree over the exact point when the mark ‘disappears’ or the solution changes colour. Also, if you use this method, you can’t plot a rate of reaction graph from the results.

Question 11

What is the ‘posh’ way of saying that the cloudiness of a solution changes?

Answer 11

the posh answer is to say that its ‘turbidity’ changes

Question 12

What do precipitation mean in chemistry?

Answer 12

in an aqueous solution, precipitation is the process of transforming a dissolved substance into an insoluble solid from a super saturated solution. The solid formed is called the precipitate

Question 13

What do opaque mean?

Answer 13

not able to be seen through, not transparent.

Question 14

How do you measure the speed of a reaction that produced a gas?

Answer 14

by using a mass balance

Question 15

As the gas is released, the mass disappearing is measured on what?

Answer 15

on the balance

The quicker the reading on the balance frops, the faster the reaction.

Question 16

if you take measurements at regular intervals, by measuring the mass balance, how can you use this information?

Answer 16

you can plot a rate of reation graph and find the rate quite easily (see page 146 for more information).

This is the most accurate of the three methods described on this page because the mass balance is very accurate. But it has the disadvantage of releasing the gas straight into the room.

Question 17

How do you measure the volume of gas given off?

Answer 17

this involves the use of a gas syringe.

Question 18

What is meant by the more gas given off during a given time interval?

Answer 18

the faster the reaction is

Question 19

Gas syringes usually give volumes accurate using what units?

Answer 19

cm³, so they are quite accurate.

you can take measurements at regular intervals and plot the rate of reaction graph using this method too.

You need to be very careful though - if the reaction is too vigourous, you can easily blow the plunder out of the end of the syringe.

Question 20

What are the three measuring rates of reaction?

Answer 20

Precipitation and colour change

Change in mass (usually gas given off)

the volume of gas given off.

(make sure you’ve learnt the three different methods for your exam)

Question 21

The reaction between solid na2C03 and aqueous HCl releases CO2 (a gas)

a) Describe an experiment that would allow you to measure the rate of this reaction (3 marks)

b) suggest units that would be appropriate for expressing the rate of this reaction (1 mark)

Answer 21

a) Add Na2CO3 to a flask containing HCl (1mark) and take reakings of the mass of the flask at regular intervals (1 mark) using a mass balance (1 mark) Add Na2CO3 to a flask containing HCl (1 mark) and take regular readigs of the volume of gas released (1 mark) using a gas syringe 1 mark)

b) E.g. cm³/s (1 mark)