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Chemistry Coursework Gcse Descalers

A kinetic study of the reaction between aqueous sodium thiosulphate and hydrochloric acid

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A kinetic study of the reaction between aqueous sodium thiosulphate and hydrochloric acid


Aim: To carry out a complete kinetic study of the reaction between
aqueous sodium thiosulphate and hydrochloric acid:

Equation: Na2S2O3(aq) + 2HCl(aq) 2NaCl(aq) + S(s) + SO2(g) + H2O(l)

PART A

To deduce the order of the reaction with respect to the concentrations
of sodium thiosulphate and hydrochloric acid the experiment will be
carried out at constant temperature and the time interval between the
addition of HCl and the obscuring of the ink cross on white paper by
the solid yellow sulphur precipitate will be measured for a constant
volume of solution that uses 3 varying concentrations of Na2S2O3(aq)
while maintaining the concentration of HCl(aq) and 3 varying
concentrations of HCl(aq) while maintaining the concentration of Na2S2O3(aq).

The rate of the reaction can be determined by the calculating the
amount of sulphur produced in the time recorded. This is given by the
equation:

Rate = Amount of sulphur

Time

The amount of sulphur needed to obscure the cross is assumed to be the
same in each reaction so therefore,

Rate = 1

Time

Then, the data will be placed in a table to determine the effect of
concentration on the rate of reaction and hence the order of both
reactants can be formed. Adding up both orders of the reactants gives
the overall order of the reaction.

PART B

To find out the effect of temperature on the rate of the reaction the
time interval between the addition of HCl and the obscuring of the ink
cross on white paper by the solid yellow sulphur precipitate at five
different temperatures must be recorded. A graph showing time taken
vs. temperature will produce a curve showing the effect of varying
temperature on the rate of reaction.

The Arrhenius equation ln k = ln A – (Ea / RT) can be shown
graphically by plotting a graph of ln (t) against ln (1/T). The
gradient of this graph = - (Ea / RT) which can be used to calculate
activation energy. The y-intercept of the line = ln A where A is the
Arrhenius constant for the reaction.

Hypothesis:

The rate of a chemical reaction can be obtained by finding out the
change in amount (or concentration) of a particular reactant or
product over the time taken for this change.

Many factors affect the rate of a reaction, one of which is
concentration. For any reaction to happen, the reactant particles must
first collide. This is true whether both particles are in solution, or
whether one is in solution and the other is a solid. If the
concentration is higher, there are more particles in the same volume

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Related Searches

Kinetic         Sodium Thiosulphate         Hydrochloric Acid         Effect Of Temperature         Varying Concentrations         Activation Energy         Graph        




so the chances of collision are greater therefore the rate of reaction
increases. However, this is not always the case because the rates of
most reactions can be related to the concentrations of individual
reactants by the rate law: Rate = k[X]n (where ‘n’ gives the order of
the reaction). This expression can only be determined experimentally
and cannot be predicted from the balanced equation or in any other
way. So if the concentration of one of the reactants is doubled and
the rate also doubles, the order of reaction (with respect to that
reactant) is 1, because it shows that rate is proportional to
concentration to the power 1. If the concentration of one of the
reactants in doubled and the rate is four times greater, the order is
2 (with respect to that reactant) because rate is proportional to
concentration to the power 2. If the order of either reactant is
calculated to be first order, then this can be verified by plotting
two separate graphs (one for the Na2S2O3(aq) and one for the HCl(aq to
show log (1/time) against log V (where V is the initial volume of
reactant). This is the equation for a straight line of the form y =
mx + c and the graph should give a straight line of slope ‘n’ if the
order is 1 with respect to that reactant.

As temperature is increased, the rate of reaction also increases. When
a substance is heated, the particles move faster and collide more
frequently speeding up the rate of reaction. However, the effect of
increasing collision frequency on the rate of the reaction is very
minor. A reaction will only occur if the colliding particles possess
more than a certain minimum amount of energy known as the activation
energy, which can alter chemical bonds by the rearrangement of atoms,
ions and electrons as the reaction proceeds. When temperature is
increased the proportion of reactant particles with the minimum
required amount of energy is also increased so the reaction can occur
faster.

GRAPH

Reactions with low activation energies are virtually instantaneous -
for example, the reaction between hydrogen ions from an acid and
hydroxide ions from an alkali in solution. The reaction between
thiosulphate ions and hydrogen ions occurs the instant HCl(aq) is
added to the Na2S2O3(aq) therefore the activation energy must be
fairly low.

The Arrhenius constant is also called the pre-exponential factor or
the steric factor. The constant is dependent on the frequency of
collisions and their orientation. It varies slightly with temperature,
although not much and is often taken as constant across small
temperature ranges so when I find the value at one temperature, I can
assume that is the same for the reaction at the other temperatures.

GRAPH

Apparatus:

PART A

- Safety glasses
- Three 50.00 cm³ burettes ± 0.02cm³
- 50.0ºC thermometer (± 0.05 ºC)
- Two 100 cm³ beakers ± 5 cm³
- Black pen
- Stirring rod
- Timer ± 1 s
- 1 M hydrochloric acid
- 0.5 M sodium thiosulphate
- Distilled water

Method:

PART A

1. Set up three 50 cm³ burettes in stands for hydrochloric acid,
sodium thiosulphate and distilled water.

2. Add 10 cm3 of the 1 M hydrochloric acid solution to a 100 cm3
beaker labelled X.

3. In a separate beaker (Y) place 30 cm3 of the 0.5 M sodium
thiosulphate solution and 10 cm3 of water.

4. Draw a black cross on a piece of paper and place beaker Y over it

5. Pour the HCl acid from beaker X into beaker Y and start a timer.

6. Stir the solution well with a glass rod and observe the
visibility of the black cross through the solution in beaker Y.

7. Stop the timer when the black cross on the paper just disappears.

8. Record the time and the temperature of the mixture in the results
table.

9. Make qualitative observations of the solid precipitate and opaque
solution now that the reaction has taken place.

GRAPH

Apparatus:

PART B

- Safety glasses
- Three 50.00 cm³ burettes ± 0.02cm³
- 100ºC temperature probe connected to TI – 83 calculator (± 0.005 ºC)
- Water baths at temperatures of 30ºC, 35ºC, 40ºC, 50ºC ± 1 ºC
- Two 100 cm³ beakers ± 5 cm³
- Black pen
- Stirring rod
- Timer ± 1 s
- 1 M hydrochloric acid
- 0.5 M sodium thiosulphate

Method:

PART B

1. Set up three 50 cm³ burettes in stands for hydrochloric acid,
sodium thiosulphate and distilled water.

2. Add 20 cm3 of the 1 M hydrochloric acid solution to a 100 cm3
beaker labelled X.

3. In a separate beaker (Y) place 20 cm3 of the 0.5 M sodium
thiosulphate solution and 10 cm3 of water. Record temperature of
mixture (for first run it should be at room temperature).

4. Draw a black cross on a piece of paper and place beaker Y over it

5. Pour the HCl acid from beaker X into beaker Y and start a timer.

6. Stir the solution well with a glass rod and observe the
visibility of the black cross through the solution in beaker Y.

7. Stop the timer when the black cross on the paper just disappears.

8. Record the time and the temperature of the mixture in the results
table.

9. Make qualitative observations of the solid precipitate and opaque
solution now that the reaction has taken place.

10. Repeat the procedure for the rest of temperature values )30ºC,
35ºC, 40ºC, 50ºC ± 1 ºC.)

PART A

Controlled variables

- Total volume of hydrochloric acid, sodium thiosulphate and distilled
water mixture.
- Size and shape of beaker Y
- Temperature of surroundings
- Paper with black cross on it
- Timer

Manipulated variable

- Concentration of hydrochloric acid and sodium thiosulphate in
solution.

Independent variable

- Time taken for sulphur precipitate to obscure black cross of paper.

- Order of each reactant, and therefore the overall order of the
reaction

PART B

Controlled variables

- Volume of hydrochloric acid, sodium thiosulphate and distilled water
mixture.

- Concentration of hydrochloric acid and sodium thiosulphate in
solution.

- Size and shape of beaker Y

- Paper with black cross on it

- Timer

- Thermometer

Manipulated variable

- Temperature of surroundings

Independent variable

- Time taken for sulphur precipitate to obscure black cross of paper.

- Rate of reaction.

- Activation energy of reaction

- Arrhenius constant for the reaction.

GRAPH



Is this a good enough hypothesis to get an A*?
My hypothesis is that dissolving Hydrochloric acid (HCl) in a small volume of water makes de-scaling quicker because there will be a higher concentration of acid, meaning that there will be more acid particles in a given volume, therefore there will be more available to react with Limescale (of which the main component is calcium chloride, CaCO3), meaning collisions between acid and Limescale particles will be more frequent and therefore there will be more successful collisions in a given time period, meaning a faster rate of reaction and therefore faster de-scaling than if the Hydrochloric acid was dissolved in a larger volume of water.

Also, any other help or tips to get top marks when planning the experiment or any other steps?
Thanks!

When writing a hypothesis for GCSE, I was taught that you needed to use the following format for top marks (and it did indeed bring me top marks in this section):
"If the...[dependent variable]...is related to the....[independent variable]...then this may result in....this is because...."
e.g. for your one, you should write "if the volume of HCl dissolved in water is related to the amount of descaling, then a small volume of water may result in descaling being quicker. This is because...[provide your reasoning/science/explanation]"

One key thing to note however, is because this is a HYPOTHESIS, you need to use words that denote uncertainty like "may" or might" or "should" etc throughout the entire hypothesis - do not write "will" (because that suggests you already know the answer before doing the experiment!).

Other tips:
-make sure your plan includes a diagram, method, risk assessment, identification of dependent, independent and controlled variables, results table
-make sure your plan also includes controlled variables and why/why not it may be possible to control them (I included some things like how temperature/pressure could affect it)
-for the risk assessment, it would be a very good idea to use the following headings: hazard (e.g. name of chemical), risk (e.g. toxic), how risk will be minimised(e.g.will be handled with care,not swallowed), what would happen if risk occurred(e.g.choking), action taken if risk occurred(first aid,ambulance called if serious)
-when drawing the results table, include units of measurement and headings
For the evaluation:
-make sure you label the axes, draw at least one graph for the evaluation bit- also do a few mathematical calculations
-make sure the research includes at least 3 (hopefully more sources)
-get in as much scientific info as you can for the evaluation (many students fail to do this and just describe their results) and make sure to make many links between your research and your experiment, identifying anomalies etc

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