Practicals paper 2

Question 1

When do you use this method and describe what you observe and what it means :

• Measuring the time it takes for a reaction mixture to become opaque or change colour

Answer 1

• do this if the initial solution is transparent and the product is a precipitate which clouds the solution (become opaque)
• the faster the change the quicker the reaction
• you can test how long for the solution to lose it’s colour if the product is colourless

Question 2

What wrong with this method :

• Measuring the time it takes for a reaction mixture to become opaque or change colour

Answer 2

it isn’t very accurate as different people might not agree over the exact point when the mark disappears or the solution changes colour.

You can’t plot a rate of reaction graph from the results

Question 3

When do you use this method and describe what you observe and what it means :

• Change in Mass (usually when Gas is given off)

(can you plot a graph?)

Answer 3

• As gas is released the mass disappearing is measured on the balance
• The quicker the reading drops = faster reaction
• If you take measurements at regular intervals you cab plot the rate of reaction graph and find the rate quite easily

Question 4

Whats the pros and cons of using this method:

• Change in Mass (usually when Gas is given off)

Answer 4

most accurate method because the mass balance is very accurate

but it releases gas straight to the room

Question 5

disappearing cross rection

Answer 5

Sodium thiosulfate + hydrochloric acid = sulfur (solid), which makes the solution cloudy

1) Use a measuring cylinder to 10cm3 of sodium thiosulfate solution into a conical flask
2) Place the conical flask onto a printed black cross
3) next add 10cm3 of hydrochloric acid into the conical flask
4) swirl the solution and start a stopwatch
5) After a certain time, the solution will turn cloudy
6) stop the clock when we can no longer see the cross
7) carry out the experiment again using lower concentrations of sodium thiosulfate solution
8) repeat the whole experiment and calculate mean values for each concentration of sodium thiosulfate solution

Question 6

Gas test (magnesium and hydrochloric acid)

measuring cylinder

Answer 6

1) add 50 cm3 of dilute hydrochloric acid to a conical flask
2) attach the conical flask to a bung and delivery tube
3) place the delivery tube into a container filled with water
4) Then place an upturned measuring cylinder also filled with water over the delivery tube
5) Add a 3cm strip of magnesium to the hydrochloric acid and start a stopwatch
6) the reaction produces hydrogen gas which is trapped in the measuring cylinder
7) Every 10 seconds, measure the volume of hydrogen gas in the measuring cylinder
8) continue till no more hydrogen is given off
9) continue with higher concentrations of hydrochloric acid

Question 7

Gas test (magnesium and hydrochloric acid)

Gas syringe

Answer 7

1) add 50 cm3 of dilute hydrochloric acid to a conical flask
2) attach the conical flask to a bung and delivery tube
3) place the delivery tube on a gas syringe that is held up by a clamp stand
4) Add a 3cm strip of magnesium to the hydrochloric acid, quickly put the bung in so no gas escapes and start a stopwatch
5) Every 10 seconds, measure the volume of hydrogen gas in the syringe
6) continue till no more hydrogen is given off
7) ) continue with higher concentrations of hydrochloric acid