Pratical 2 - Concentration and Diffusion

What is Diffusion?

Movement of particles from a high concentration to a low concentration down a concentration gradient.

What is Concentration?

Refers to the number of particles in a particular concentration.

• Concentration (gdm-3) = Mass (g) / Volume (cm3)

• Concentration (moldm-3) = Moles (mol) / Volume (cm3)

What is the Concentration Gradient?

The greater the concentration gradient means there is a bigger difference in concentration between where the particles are and where they are moving to.

What would the rate of diffusion be like in a High/Low Concentration Gradient?

HIGH: If the concentration of molecules outside is very high relative to the inside concentration, the diffusion rate will also be higher.

LOW: If the difference in concentration is smaller, the rate of diffusion will be slower.

As the concentration gradient increases, the rate of diffusion also increases.

How did we work out the rate of diffusion in this pratical?

Sodium hydroxide (alkaline) agar filled with pink phenolphthalein indicator and a boiling tube of HCL.

We needed to know the shortest distance the HCl had to diffuse into the centre of the shape.

Rate = shortest distance/average time

What was the aim of this pratical?

To investigate the effect of concentration on the rate of diffusion.

What was the hypothesis of this practical -

As the concentration increases the rate of diffusion will increases.

This is because…

• The greater the concentration gradient means that there is bigger the difference in concentration, so the faster the particles will move.

• If the conc of molecules in the HCL solution is very high in comparison to the inside conc of the NaOH cyclinder, the quicker the particles in the HCl solution will diffuse into the NaOH due to the increased HCl particles.

• This will create a neutralisation reaction shown by the phenolpthalein indicator turning from pink in alkaline conditions to colourless in acid.

What are the Independent Variables?

Concentration of HCl - 0.5, 1, 1.5 moldm(-3)

What are the Dependent Variables?

Time for diffusion (seconds)

What are the Control Variables?

Volume of HCl - 10cm(3)

Surface area of agar

Concentration of Sodium Hydroxide (NaOH)

Indicator concentration (phenolphthalein)

Room temperature

What are the risks, precautions and emergency procedures of the hazard HCl?

Hazard = 1 Moldm(-3) HCl

Risk = Corrosive

Precaution = Wear gloves and goggles

Emergency Procedures = Rinse with water

What are the risks, precautions and emergency procedures of the hazard Phenolpthalein?

Hazard = Phenolpthalein

Risks = Laxative

Precautions = Do not swallow

Emergency Procedure = Seek first aid

What are the risks, precautions and emergency procedures of the hazard Sodium Hydroxide Cubes?

Hazard = Sodium Hydroxide Cubes

Risks = Corrosive

Precaution = Wear gloves/goggles

Emergency Procedure = Rinse with water

What are the risks, precautions and emergency procedures of the hazard Glassware?

Hazard = Glassware

Risks = Broken glass can cause slices/cuts

Precautions = Take care when handling; place away from the edge of the table

EP = dispose of broken glass by the teacher

What were the Equipment and its uses?

• 3x boiling tubes - to hold 0.5, 1, 1.5M hydrochloric acid.

• Boiling tube rack - to hold the three boiling tubes

• Measuring cylinder - to measure 10cm(3) of 0.5,1.0,1,5M hydrochloric acid

• Sharpie - to label each boiling tube with its amount of hydrochloric acid

• An agar cylinder containing sodium hydroxide and phenolphthalein, to show the reaction.

• Stop clock - to time the reaction from pink to colourless agar

What is the method?

1) Label 3 boiling tubes 0.5, 1, 1.5M, place them in the boiling tube rack.

2) Use a measuring cyclinder to measure 10cm3 of 0.5M hydrochloric acid.

3) Pour 10cm3 of 0.5M HCl into the correct boiling tube labelled 0.5M.

4) Place an agar cylinder containing sodium hydroxide and phenolphthalein into the boiling tube and start the stop clock.

5) Stop the stop clock when all the pink indicator colour disappears.

6) Record the time in seconds in your results table.

7) Repeat the experiment to get 3 repeat reults for 0.5M HCl.

8) Repeat steps 1-7 for each concentration of HCl 1 and 1.5M.

How do you calculate the rate of diffusion for each concentration?

shortest distance / average time

0.5/mean = …

What is title of the axis of the graph and what it should go up in?

y-axis (Vertical) = Rate of diffusion (sec-1)

• 0.0025 - 0.02.

x-axis (Horizontal) = Concentration of HCl (moldm-1)

• 0, 0.5, 1.0, 1.5.

Positive Correlation.

What was the conclusion?

As the conc of HCl increased, the rate of diffusion had increased, therefore demonstrating a positive correlation between the 2 variables.

For example, when the conc of HCl was 0.5cm-3, the rate of diffusion was 0.014 sec-1. Whereas, when the conc was 1.0cm-1, the rate of diffusion was 0.015 sec-1.

This is because due to the highest conc, the particles are going to move quicker to the area of low conc.

Furthermore, for 0.5cm-3 of HCl, it diffused the slowest of all, then 1cm-3 of HCl with the fastest being 1.5cm-3.

How would you improve this experiment?

Do more repeats for the same concentrations to give more reliable results.

Use a bigger range of concentrations, e.g. 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, to 2 to gather more data/results.

Use more precise measuring equipment such as a graduated pipette or micropipette to measure 10cm3 of HCl - Less % error and smaller scale.

How would you extend the experiment?

Use a different acid e.g. sulphuric acid/nitric acid.

Use a different shape/surface area/size cylinder e.g, 4cm diameter instead of 1cm.4

Use a different indicator e.g. methyl orange.

Use a different alkali e.g. ammonia.

Use a different concentration of alkali e.g. 0.5 moldm-3.

Use a different volume of alkali 50cm3.