Factors affecting reaction speed

Reaction Speed Lab

13th March 2007

Title: Factors affecting reaction speed.

Research Question: To explore how temperature and concentration of reactants affect reaction speed of the reaction speed between sodium thiosulphate and hydrochloric acid?

Background Information: As the temperature of the reactants increases the reaction speed will increase. According to the collision theory, when two chemicals react, their molecules have to collide with each other with sufficient energy for the reaction to take place. The two molecules will only react if they have enough energy. By heating the mixture, you will raise the energy levels of the molecules involved in the reaction. According to the kinetic theory, increasing temperature means the molecules move faster. However, the main reason why it increases the rate of reaction is that more of the colliding particles will have the necessary activation energy resulting in more successful collisions

According to the collision theory, increasing the concentration of the reactants will increase the frequency of collisions between the two reactants. When collisions occur, they do not always result in a reaction, this is because the molecules in the reaction mixture have a range of energy levels. If the two colliding molecules have sufficient energy they will react.

The rate of reaction is limited by the concentration of the weaker solution, and increasing the concentration of the other makes no difference. You have to fix the concentration of one of the reactants to excess. Now an increase in the concentration of the other solution will produce an increase in the rate of the reaction.

Low concentration of reactants High concentration of reactants

Source of picture: http://www.wpbschoolhouse.btinternet.co.uk/page03/3_31rates.htm#3a

Source of notes: www.purchon.com/chemistry/rates.htm

Hypothesis: As the temperature increases the reaction speed will increase because the particles gain a lot of kinetic energy. This increases the chances of reaction when the molecules of reactants collide since they would be likely to achieve the activation energy required for the reaction to take place. As the concentration of the reactants increases the reaction speed will increase, since there are more molecules of the reactants in a particular volume, thus increasing the possibility of a collision and reaction.

Variables:

Independent variables:

Temperature
Concentration of reactants

Dependent variables:

Reaction speed

Controlled variables:

Pressure
Temperature (when investigating the effect of changing concentration of reactants)
Concentration of reactants(when investigating the effect of changing temperature)

Concentration of sodium thiosulphate will be decreased by adding a certain amount of water. The concentration of hydrochloric acid will remain unchanged.

Materials:

3 measuring cylinders
4 conical flasks
Paper marked with a “X”
Thermometer
Ice
Sodium thiosulphate
Hydrochloric acid
Water, both hot and cold
2 big beakers to act as water baths
Stopwatch

Method:

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Variables:

Independent variables:

Temperature
Concentration of reactants

Dependent variables:

Reaction speed

Controlled variables:

Pressure
Temperature (when investigating the effect of changing concentration of reactants)
Concentration of reactants(when investigating the effect of changing temperature)

Concentration of sodium thiosulphate will be decreased by adding a certain amount of water. The concentration of hydrochloric acid will remain unchanged.

Materials:

3 measuring cylinders
4 conical flasks
Paper marked with a “X”
Thermometer
Ice
Sodium thiosulphate
Hydrochloric acid
Water, both hot and cold
2 big beakers to act as water baths
Stopwatch

Method:

To investigate the effect of changing the concentration of reactants, measure 25cm3 of sodium thiosulphate and 10cm3 of hydrochloric acid using two different measuring cylinders for each.
Make a large “X” on a piece of white paper and place a conical flask on top of it.
Pour the sodium thiosulphate and hydrochloric acid into the conical flask.
Start the stopwatch as soon as the reaction begins.
Gently stir the mixture.
Stop the stopwatch as soon as the “X” disappears from view under the conical flask. Record the time.
Now add 5cm3 of water to 20cm3 of sodium thiosulphate and 10cm3 of hydrochloric acid in a conical flask, so as to leave the concentration of hydrochloric acid constant.
Repeat steps 4, 5 and 6.
Then do the experiment with 10cm3 of water and 15cm3 of sodium thiosulphate. Then do the experiment with 15cm3 of water and 10cm3 of sodium thiosullphate. Finally do the experiment with 20cm3 of water and 5cm3 of sodium thiosulphate.
To investigate the effect of changing the temperature, use 10cm3 of sodium thiosulphate and 10cm3 of water, so as to control the concentration of reactants to make sure it does not affect reaction speed.
As control, add the reactants at room temperature into a conical flask.
Start the stopwatch as soon as the reaction begins and gently stir. Stop it when the “X” disappears from view. Record this time.
Then using a water bath with some water and crushed ice, place the conical flask in it. Record the temperature inside the water bath.
Add the reactants and record the time it takes for the “X” to disappear from view.
Use the cold water bath to do it one more time at an even lower temperature by repeating the steps 13 and 14.
Use another water bath, but fill it with hot water.
Record the temperature inside the water bath.
Add 10cm3 of sodium thiosulphate and 10cm3 of hydrochloric acid.
Start the stopwatch as soon as the reaction begins and gently stir. Stop it when the “X” disappears from view. Record this time.
Use the hot water bath again at another high temperature.
Repeat steps 17, 18, and 19.

Data Collection:

There was an uncertainty of ±0.05cm3 whenever the volume of the reactants was measured. It was more when water and sodium thiosulphate were added together to reduce its concentration. The uncertainty for that is as follows:

Uncertainty = Uncertainty for sodium thiosulphate + Uncertainty for water

Uncertainty = ±0.05cm3 + ±0.05cm3 = ±0.10cm3

Other errors might have occurred when recording the time taken for the “X” to disappear from view. The stopwatch might not have been started instantaneously when the reaction began and it would also not have been stopped instantaneously when the “X” disappeared from view.

Data Processing and Presentation:

The following graph represents graphically how the time taken for the “X” to disappear from view changes with increasing concentration of reactants.

The above graph shows that the reaction speed increases with increasing concentration because the time taken for the “X” to disappear is decreasing. There is an inverse relationship between concentration of reactants and time taken. Thus, reaction speed is directly proportional to concentration of reactants.

The following graph will represent the direct relationship between reaction speed and concentration of reactants.

Thus it can be seen clearly as the volume of sodium thiosulphate increases and hence its concentration increases, the reaction speed also increases.

The following graph represents graphically how the time taken for the “X” to disappear from view, changes with increasing the temperature.

The above graph shows that the reaction speed increases with increasing temperature because the time taken for the “X” to disappear is decreasing. There is an inverse relationship between temperature and time taken. Thus, reaction speed is directly proportional to temperature.

The following graph represents the direct relationship between reaction speed and temperature.

Thus it can be seen clearly as the temperature increases, the reaction speed increases.

Conclusion:

The following experiment was carried out to explore how temperature and concentration of reactants affect reaction speed of the reaction speed between sodium thiosulphate and hydrochloric acid. It was hypothesized that the reaction speed would increase with an increase in the concentration of reactants and the temperature. The hypothesis was strongly supported by the results. As the concentration of sodium thiosulphate was increased the time taken for the “X” to disappear decreased. It was slowest at the minimum concentration when it took almost 118 seconds and it was quickest when the concentration was highest when it took almost 95 seconds. Therefore as we increase the concentration of the reactants, the possibility of a collision between molecules increases and a reaction takes place. It is also more probable that the molecules collide in the right order and orientation since they are so many per unit volume.

Also as the temperature increased the time taken for the “X” to disappear decreased. It was slowest at the lowest temperature of 9°C when it took 88.78 seconds and it was quickest at the highest temperature of 63°C when it took only 14.97 seconds. Therefore as the temperature increases, the molecules gain kinetic energy and move faster. This increases the probability of a collision between molecules since molecules are moving faster and also since the kinetic energy is high, they are likely to attain the necessary activation energy for the reaction to take place.

Evaluation:

The hypothesis was supported but there were a few experimental errors that occurred. The measurement of time was not completely accurate since the stopwatch was not started when the reaction actually began and also it was not stopped at the exact moment when the “X” disappeared from view. Another error was from the uncertainties in the equipment we used. There was an uncertainty of ±0.05cm3 when measuring the volume of hydrochloric acid and sodium thiosulphate. When mixing water and sodium thiosulphate this increased to ±0.10cm3. Also the measurement of temperature with the thermometer had an uncertainty of ±0.5°C. Since this was an experiment based on the judgement of the eye with regards to when the “X” actually disappeared, sometimes it may not correct because we may stop the time even though the “X” is still visible. There were changes in temperature while doing the experiment since this was not done in vacuum and outside conditions may have affected the results. Next time, to make this experiment better we can do it in vacuum and under a controlled environment where temperature and pressure are constant.

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