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"Hydrochloric acid is a strong colourless acid formed when hydrogen
chloride gas dissociates in water, used in industrial and laboratory
processes. Hydrochloric acid is also produced in the stomach, where it
initialises the digestion of proteins.
Sodium thiosulphate is a white crystalline salt used as a fixer in
photography and as a bleach."
Encarta World English Dictionary
When hydrochloric acid reacts with sodium thiosulphate, sulphur is
created. Sulphur is a pale yellow colour; after the reaction the
sulphur stays in the same container as the other products, meaning
that the solution will be a pale yellow colour.
"The sulphur does not start to form immediately, but begins to appear
after a short time, turning the mixture cloudy. You cannot tell
precisely when the reaction has stopped." AQA GCSE Higher
Sulphur dioxide (SO2) is a colourless gas, belonging to the family of
gases called sulphur oxides (SO_). It reacts on the surface of a
variety of airborne solid molecules, is soluble in water and can be
oxidised within airborne water droplets.
Many factors can affect the rate of reaction; concentration,
temperature, ratio of the substances, amount, stirring, and catalysts
are these factors. Rate of reaction is the speed of reaction.
In essays written by young scientists, they have used concentration as
a variable in their experiments; some have used a light sensor,
connected to a data logger, to measure the amount of light passing
through the "cloudy" liquid.
1 moles = 6.02 ´1023
The particles in the hydrochloric acid and sodium thiosulphate need to
collide to react, this is called the collision theory; a minimum force
of collision is required for the reaction to take place; this is
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The rate of a reaction depends on two factors.
1) The number of collision per unit of time between the reacting
2) The fraction of these collisions that are successful in producing a
If two or more molecules collide but are not orientated correctly then
no reaction will take place.
If there are more collisions then the rate of reaction (r) will
increase. R a Concentration, R a 1/Time
The rate of a chemical reaction is proportional to the product of the
concentrations of the reactants. The rate law expression is: r [A]m[B]n
where A and B represent the molar concentrations of A and B. m and n
are the powers to which the concentrations must be raised. k is a
constant of proportionality known as the rate constant.
First of all I did a practice experiment to familiarise myself with
it, also to find out what and whatnot to do.
After roughly measuring out 75ml of sodium thiosulphate and 25ml of
hydrochloric acid, I put them together and measured how long it took
for a pencil cross to disappear. It measured 1 minute 2 seconds. It
went cloudy and ended up a pale yellow colour, the cloudiness didn't
form immediately; only after at least 10 seconds did it start to form.
After leaving the liquid for a few minutes, the pale yellow colour
separated, forming two layers; I suspect that the pale yellow at the
bottom layer is the sulphur.
I have decided to do the experiment with different concentrations of
the hydrochloric acid; it is the most simple but most effective. I
will only use 40ml of sodium thiosulphate and 15ml hydrochloric acid,
I used too much for the preliminary experiment; if I use less it will
bring me closer to the actual time taken for the reaction, if there is
more liquid, then there is a bigger chance that the amount of liquid
will cover the cross instead of the cloudiness.
I can't measure the amount of light passing through the liquid because
I don't have a light sensor and a data logger.
It's impossible to control stirring by hand, if you stir too fast the
rate of reaction will increase, although this is what we want there is
no way to tell how fast or slow you are stirring.
Using temperature would be quite effective as you can measure
temperature; the problem is you can't stop the temperature from
decreasing during the actual experiment. You can make the temperatures
of all the attempts to decrease, but when the temperature is lower it
will decrease less making it unfair.
Using ratio would mean using more liquid, but I could scale the
experiment down; e.g. instead of 100ml to 50ml I will do 30ml to 15ml,
and instead of 50ml to 40ml I will do 25ml to 20ml, this way it will
always be the same amount of liquid but would mean a big hassle.
To find how concentration affects the rate of reaction of sodium
thiosulphate and hydrochloric acid.
Because the reaction will make a "cloudy" solution we will put a cross
on the bottom of the container, and see how long it takes for the
reaction to make a "cloudy" solution strong enough to block out the
cross, as a means to measure the rate of reaction.
1. Draw a cross on the bottom of a conical flask; put sticky tape over
the cross so it doesn't rub off.
2. Measure 40ml of sodium thiosulphate and pour into the conical
3. Measure 15ml of hydrochloric acid; get a stopwatch. Pour the
hydrochloric acid into the conical flask; start the stopwatch when the
hydrochloric acid is poured into the sodium thiosulphate.
4. Stop the stopwatch when the cross becomes invisible.
5. Repeat the same experiment at least once more, and find the average
time for more accuracy.
6. Repeat all the above, but with different concentrations
When doing any experiment safety comes first. Wear goggles when the
experiment commences. Wash your hands if acid gets on them. Always
close the tap on the beau ret when pouring into it, spillage at the
bottom could get on you hands; if you spill anything wipe it clean
immediately. Always stand up when doing the experiment, especially
when pouring the liquids into the beau ret, if you are standing then
you can get out of the way if any liquid is spilt. Always carry
bottles at the main body, not the neck, there's a bigger chance of
dropping the bottle if held at the neck.
To make this experiment as fair as possible, I have to make sure that
only the variables are changed. I wouldn't expect the results to be
perfect because human errors are inevitable.
Keep the same
The starting and stopping of the stop watch.
The amount of sodium thiosulphate and hydrochloric acid.
The concentration of hydrochloric acid.
Measuring the amount of each substance.
The pencil cross.
The speed of the pouring.
The person who observes the experiment and controls the stopwatch.
Not using the same measuring equipment.
To ensure a fairer test I will, use the same measuring equipment, let
the same person measure out the substances each time, have one person
to time all the experiments.
There are other errors that could occur, when the hydrochloric acid is
poured, there will definitely be some of it left in the container. As
this will happen every time I pour, there will more or less be the
same amount each time. This wouldn't affect the results much.
Another inevitable problem would be the speed of the pouring; it will
be virtually impossible to ensure the same speed each time. If poured
to fast, an unfair amount of energy will be given to the particles,
and increasing the rate of reaction.
In my research, it mentioned that you couldn't tell precisely when the
reaction has stopped; you also cannot tell precisely when the cross
Not using the same cross can lead to different results; I'm going to
draw a cross on the bottom of the conical flask, sticky tape will be
put over the cross so it doesn't rub off.
When diluting the hydrochloric acid, leave to stand for a minute so
the particles are evenly spread, so when halving the liquid not too
much or less of the hydrochloric acid stays in the used half.
After cleaning out the beaker and/or conical flask, wipe it as dry as
possible, if mixed each substance will become diluted by a fraction
and have a bigger volume than the intended volume.
I predict that the higher the concentration of hydrochloric acid the
higher the rate of reaction. If there are more particles in the
hydrochloric acid, then there is a bigger chance for collision with
the sodium thiosulphate particles. If the hydrochloric acid is less
concentrated the rate of reaction will decrease because there are less
particles in the hydrochloric acid to collide. In my research I
included an extract from AQA GCSE Higher, it says that the sulphur
will not form immediately, so it's certain that it will take some
time; my preliminary experiment also backs this up.
I found that using the 2 molar acid increased the rate of reaction,
and took least time for the cross to disappear; the 0.5 molar acid
decreased the rate of reaction, and took longest for the cross to
My prediction was correct, the rate of reaction increased when the
concentration was increased. The rate of reaction and concentration
are inversely proportional to time, so it took less time when the
concentration was higher. The more particles, there are the more
likely it is to collide; the higher concentrations had more particles
so the rate of reaction increased.
The results are quite close to the trend line according to the graph,
this shows that the results are quite close to the correct and exact
I don't think there are any anomalous results, because none of the
points on the graph are far from the trend line; it's just a matter of
errors during the experiment.
The experiment wasn't perfect because of the points I listed in the
fair test section, however I did follow the fair test table. I didn't
get as many results as I would of liked in the time provided.
If I do this experiment again, I will allow myself more time for a 4th
attempt to obtain more accurate results, and use a wider range of
concentrations; and use another factor. When pouring the hydrochloric
acid I will start the stopwatch on the moment the hydrochloric acid
hits the sodium thiosulphate.
Not all of the hydrochloric acid was poured into the flask, resulting
in inaccurate results. When cleaning the beau rets, some water was
left in there so both substances were diluted by a minute fraction.
The time and equipment were insufficient to obtain much more accurate
results. The speed of pouring was varied, mainly due to human error
and because as time was limited the experiment was speeded up by
The experiment was carried out accurately, and kept it as fair as