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Rates of Reaction

How can the Rate of the Reaction between
Sodium Thiosulfate and dilute Hydrochloric Acid be Measured?

HCl + sodium thiosulfate arrow sodium chloride + sulfur dioxide + sulfur + water.
2HCl(aq)  +  Na2S2O3(aq)  arrow        2NaCl(aq)    +      SO2(g)    +   S(s)  +  H2O(l)

The rate of this reaction can be measured by looking at the
rate at which the product solid sulfur (S(s)) is formed.
The solid sulfur makes the colourless solution go cloudy.

This reaction is usually carried out in a flask placed on a piece
of white paper. The white paper has a black cross on it.
At the beginning of the reaction, the cross can easily be seen
through the solution in the flask. As the solution in the flask
becomes more and more cloudy, the cross gets harder to see.

You can measure the time from the start of the reaction
until the cross can no longer be seen.
This is a way of measuring the rate of formation of sulfur.

The reaction between magnesium metal and a dilute acid
can be followed in a similar way noting the
time taken for the bubbles of hydrogen gas to stop forming
or for the magnesium metal to disappear.

See also how the rate of this reaction
is affected by increasing the concentration of the solution.

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