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111 Town Square Pl, Jersey City, NJ 07310, U.S.

Natalie Howari
What is the effect of the concentration of sodium thiosulphate (Na2S2O3) on the rate of reaction with Hydrochloric Acid (HCl)?
1.1 background information:
In class we have followed experiments that methods were produced by Ms. Holden. These experiments were to demonstrate the different types of rates of reaction. According to a rate of reaction could be defined as how fast or slow (speed) a reaction takes place. An example could be the oxidation of iron under the atmosphere which elucidating the impression of a slow reaction which can take many years, however the combustion of butane in a fire through lighters or other objects is a reaction that takes place in fractions of a second. In order for a reaction to take place particles must collide with sufficient energy; this is called activation energy. BBC.CO state four main factors in which effects the rate of reaction is:
Surface Area: Where the measure of how much exposed area of a solid object has to the reactant, expressed in square units. More surface area = Increase of rate of reaction
Temperature: Heating chemical substances could cause the molecules to separate and move around faster. An increase in the speed is created, in the substances or materials mix with surrounding molecules from other chemical substances, thus speeding up a reaction. Vice versa with low temperature.
Concentration: Solution that contains high concentration of reactants elucidates the impression of more collisions occurring this is best exemplified through an increased chance that the reactants would collide. Contrary of high concentrations if a solution has low concentration, the chances of the reactants colliding is less, this increases the rate of reaction.
Use of Catalyst: Whilst a catalyst is present then the reaction rate will increase due to the fact it replaces the amount of energy needed for the collision, allowing for more collisions to take place.
Lastly, before continuing through this experiment, we should be aware of the Collison theory. That the rate of a chemical reaction is proportional to the number of collisions between reactant molecules. The more often reactant molecules collide, the more often they react with one another, and the faster the reaction rate. (Marvin L. Goldberger & Kenneth M. Watson, 2004)
The investigation of the rates of reaction will be experimented by mixing together sodium thiosulfate (Na2S2O3) and hydrochloric acid to investigate the effect of sodium thiosulfate concentrations ( 50ml of Na2S2O3, 40ml of Na2S2O3 with 10 ml H20, 30ml of Na2S2O3 with 20ml h20, 20ml of Na2S2O3 with 30 ml of H20, 10 ml of Na2S2O3 with 40 ml of H20)on the rate of reactions of sodium thiosulfate with hydrochloric with dilution of the concentration with distilled water, by measuring precipitation (because Sulphur is produced) e.g. look for ‘X’ to disappear. What happens to the time it takes for sodium thiosulfate (Na2S2O3) to fully dissolve with the fixed concentration of hydrochloric acid (HCl ) when changing the increments of the concentrating of sodium thiosulfate (Na2S2O3) with distilled water (The diluting that will occur in the beaker after each attempt (5) per trail (3)) result a change in time taken for the sulfur precipitate to form, thus lead to change in time taken for the cross to disappear and the rate of reaction?
In this experiment, sodium thiosulphate (Na2S2O3) is the source of the thiosulphate ions, and the hydrochloric acid will be used as the source of hydrogen ions. By mixing the sodium thiosulphate and hydrochloric acid a chemical reaction shall occur which shall result in Sulphur being precipitated slowly. Sulfur dioxide is a gas at room temperature; however it is very soluble in water. Sulfur is water insoluble solid, this can partially be attributed to the solution becoming cloudy and then opaque in this reaction. Sulphur shall be formed and causes the solution to change from colorless to yellowish cloudy. The time taken for a certain quantity of Sulphur to form and cause the ‘x’ mark to disappear is used to determine the rate of reaction.

The balanced equation to represent this reaction is:
Na2S2O3(aq) + 2HCl(aq) ?S(s) + SO2(g) + 2NaCl(aq)

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The action that occurs:
Step 1: Na2S2O3 -> 2Na+ + S2O32-
Step 2: HCl -> H+ + Cl-
Step 3: S2O32- + H+ -> HS2O3-
Step 4: H-S-SO3- + nS2O32- -> H-S-(S)n-SO3- + nSO32-
Step 5: H-S-Sn-SO3- -> H+ + S–Sn-SO3-
Step 6: S–S7-SO3- -> S8 + SO32-
The speed the action occurs is what we are observing in this experiment.


As the concentration of Sodium Thiosulphate increases the length of time for the “X” to disappear should decreases (inverse). This is because by the increasing of the concentration of Sodium Thiosulphate it increases the rate of reaction between Hydrochloric acid and sodium Thiosulphate particles. Increasing the concentration of the sodium thiosulphate means that there will be more particles per dm3 of this substance. The more particles that there are in the same volume, the compact and closer the particles will be to each other. This can at least partially be attributed to that the particles collide more frequently with each other and the rate of the reaction increases.
So, as the concentration of sodium thiosulphate increased, the rate of reaction will increase, meaning less time taken for the “X?mark to disappear. Supposedly, the graph that will be plotted after the experiment based on the data that has been obtained from previous experiments will appear like the following:

Based on my intellectual assumptions, the order of reaction with respect to sodium thiosulphate is 1st order this is best exemplified through the graph of rate of reaction versus concentration of sodium thiosulphate which demonstrates the linear relationship. The order of reaction denotes 1st order because the concentration of one chemical changes when the amount of another chemical changes, hence a chemical reaction. Weather doubling a reactant and one observes the product concentration double, the order is said to be 1st order, with respect to the reactant this is because in a first order reaction, the rate is proportional to the concentration according to x

Independent Variable: The concentration for sodium thiosulphate (Na2S2O3) is the independent variable in this experiment. There will be a vary of the sodium thiosulphate (Na2S2O3) concentration by starting off with 50 ml of sodium thiosulphate and diluting it with H20 by 10 in each round, however keeping in mind that the volume of the of the concentration will always be 50 ml.
(starting with 50 ml of sodium thiosulphate, then in the next round will have 40 ml of sodium thiosulphate and 10ml of distilled water, then 30 ml of sodium thiosulphate with 20 ml of distilled water, then 20 ml of sodium thiosulphate with 30 ml of water, then 10 ml of sodium thiosulphate and 40 ml of water)
Dependent Variable: the time it takes for the sodium thiosulphate (Na2S2O3) to react with the Hydrochloric acid (HCL) in the varying the concentration level of sodium thiosulphate (Na2S2O3) with the specific dilution of water within each attempt; reaction time (in seconds) by observing when the drawn x under the conical flask disappears.
Controlled Variable:
Controlled Variable Method of Control Reason for Control
The amount and concentration of hydrochloric acid (HCL) that will be placed in the sodium thiosulphate (Na2S2O3) concentration.
The amount of the Hydrochloric Acid should be 10ml – This will be controlled by having a graduated cylinder that has a volume of 10ml or greater, in order to measure exactly 10 ml by using a pippete in order to get the exact amount of Hcl.

– dedicating one graduated cylinder for the hydrochloric acid (HCL) in order to prevent any mixture of other substance. Controlling the amount and concentration of the hydrochloric acid (HCL) is important because this chemical will be the reactant in this experiment, by using different amounts of hydrochloric acid (HCL) it will lead to varying the reactions times by shortening or lengthening of the time it takes for the hydrochloric acid (HCL) to react with the sodium thiosulphate (Na2S2O3) and provide inconsistent results.
The temperature for this experiment will be fixed to room temperature which is 27 degrees Celsius.
– This will be controlled by conducting the experiment in one location which is the school lab.

– By placing a thermometer in the substance to make sure that they are all equal in temperature, if the temperature was above room temperature or below a moderation of the temperature will occur by heating or cooling the substance. Controlling the temperature for this experiment is important to ensure the data is precise, if the temperature of the reaction were not constant, it will affect the rate of reaction by increasing it or deceasing (Background information).

The volume/ size/ shape of the conical flask This will be controlled by using a 100 ml conical flask with the same dimensions as the one that will be used in the first trial, in every reaction to ensure that the size of the conical flask is the same. If the volume/ size/ shape of the conical flask is not controlled, the result of the reaction (turning from clear to cloudy) would change this is best exemplified through thin conical flask will have the reaction on top of each other making the reaction appear faster however wide spread conical flask would appear slower, if the conical flask is small the volume will be small which compacts the reaction creating the appearance of a faster reaction (cloudiness reaction on top of each other making it hard to see the x) however if the conical flask is big the volume will be big which will not compacts the reaction creating the appearance of a slower reaction, so without controlled conical flask the experiment will provide inconsistent results.
Molarity of sodium thiosulphate (Na2S2O3) The molarity of the sodium thiosulphate will be controlled by using the same bottle that contains the sodium thiosulphate throughout the experiment if the molarity of the sodium thiosulphate is not controlled, the reaction between the sodium thiosulphate and Hydrochloric acid will vary since concentrations will be different, creating inadequate results to help prove or solve the experiment correctly.
The size and thickness of the x drawn on the This will be controlled by using the same black pen to draw the X on the paper (Same plack pen= same thickness and tone).
Same type of paper (Some paper have more absorpant factors than other papers )
The X shall be 2 inches wide and long If the size and thickness of the x are not controlled between each test, the time where the results reaction turns opaque might very since the deep color of the X might be light or dark effect the results provide inconsistent and unreliable results.

– A graduated cylinder (250 ml +- 2.00 ml): to place the Hydrochloric acid in proper measurements

– An iPhone stopwatch: to time how long it took for the reaction of the sodium thiosulphate and hydrochloric acid to turn cloudy giving a more precise reading than an analogue clock (mili seconds).

– Paper: to draw the ‘X” on, In order to see when the reaction has transformed into being opaque
– A pipette (3ml): to put small amounts of Hydrochloric acid into a graduated cylinder because it holds small amounts of acid giving actuate measuring.

– 2 Volumetric pipette (25 ml): to place the sodium thiosulphate and water in separate pipets to place in the conical , the pipette allows accuracy and preciseness and the room to be careful and precautions. There must be 2 pipettes because any leftover residue of one substance in one pipette can affect the other substance amount.

– A black marker: to draw the x with to see when the reaction becomes opaque
– Conical Flask: To put the solution of sodium thiosulphate, water and hydrochloric acid into inorder to react with each other.
– Sodium thiosulphate (0.2M): to react with the HCL; 150 ml needed + 20 ml for any mistakes (per trial)
– Hydrochloric acid(1M): to react with the sodium thiosulphate ; 25ml needed+ 10ml for any mistakes (Per trail)
– Thermometer: To control the temperature of the substances

– Distilled water: to duillet the water

– Goggles: to protect our eyes from any substances or object if it were to hit our eyes
– Lab coat: to protect our skin from any substances or object if it were to hit our or clothe
– Flume box: to collect any fumes the chemical reaction can create



Safety Concern: Reason for Concern/Risk: Safety Precaution:
Breaking the thermometer, whilst using it to control the temperature of the substances being used Most thermometers contain mercury in which it moves to tell the temperature, if the thermometer breaks, it can at least partially be attributed to the exposure of mercury. Mercury can evaporate in which increases the chance of a victim breathing the mercury increasing the potential chance of health issues such as migraines or harm to lungs, brain and so on forth. ( To be precautions when using the thermometer by:
– Not handling it with force which could lead to breaking the thermometer.
– exposing it to heat or excessive coldness in which could break the thermometer
-Being aware of placement of the thermometer; precautions of edges to prevent the thermometer from falling and so on forth

The use of Hydrochloric Acid in the experiment

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