Investigating a change in colour

Rate and extent of chemical change

What is a hypothesis?

A proposal that could explain a fact or observation. It must be testable.

Equation for disappearing cross reaction

Sodium thiosulfate solution + hydrochloric acid -

What does sulfur do in a solution?

Makes it go cloudy.

What is this cloudiness called?

Turbidity

How can this turbidity be used?

To see how long it takes the reaction to finish.

First step of practical

Use measuring cylinder to put 10cm³ of sodium thiosulfate in a conical flask.

Place conical flask on a printed black cross.

Second step of practical

Add 10 cm³ of hydrochloric acid into conical flask.

Third step of practical

Swirl solution and start a stopwatch. Look down through top of the flask.

After a certain amount of time, the solution will turn cloudy.

Fourth step of practical

Stop the clock when we can no longer see the cross.

Final step of practical

Repeat experiment using lower concentrations of sodium thiosulfate solution and calculate a mean time.

What do we do with anomalous results when calculating a mean?

Ignore them

What does it mean if a measurement is reproducible?

It can be repeated by another person or by using different equipment and still giving same result.

Weakness of this experiment

Different people have different eyesights so some people can see the cross for longer than others. However, because all students use same size printed cross, this problem may not be too great.

Equation for practical of measuring volume of hydrogen produced

Magnesium (or other) + Hydrochloric acid → MgCl + Hydrogen

First step of this second practical

Use a measuring cylinder to place 50cm³ of hydrochloric acid into a conical flask

Second step of second practical

Attach the conical flask to a bung and delivery tube. Place the delivery tube into a container filled with water.

Third step of second practical

Place an upturned measuring cylinder also filled with water over the delivery tube.

Fourth step of second practical

Add a 3cm strip of magnesium to the hydrochloric acid and start a stopwatch.

Fifth step of second practical

Reaction produces hydrogen gas which is trapped in the measuring cylinder

Sixth step of the practical

Every 10 seconds, measure the volume of hydrogen gas in the measuring cylinder.

How long should you continue the experiment?

Until no more hydrogen is produced.

What do you repeat with?

Different concentrations of hydrochloric acid.

What do both of these experiments show us?

That the greater the concentration of a chemical reaction, the faster the reaction takes place.