Chromatography and Rf Values (GCSE Chemistry)

Chromatography

  • Chromatography can split up mixtures. By using chromatography , we tin separate mixtures. This will help us to identify the substances within the mixture.
  • Chromatography has 2 phases. There are 2 phases in chromatography, called the mobile phase and the stationary phase. These phases allow the mixture to separate.
  • The two phases reach an equilibrium. As compounds move through the two phases of chromatography, an equilibrium is formed.
  • Molecules can motility in the mobile phase. The mobile stage can be a liquid or a gas , allowing molecules to move.
  • Molecules cannot move in the stationary phase. The stationary phase tin can be a solid or thick liquid , which doesn't permit the molecules to move. In this stage, molecules can split out of the mixture.
  • Distribution betwixt phases determines separation. Depending on the distribution of the compound between the mobile and stationary phases, molecules will carve up.

Rf Values

  • Substances kickoff at the origin. The starting point of the compound is called the origin . This is where we measure the altitude moved by the substance.
  • Substances move different distances. Each substance will move a dissimilar distance . We tin mensurate the distance travelled by substance past going from the centre of the spot to the origin.
  • Rf values are ratios. An Rf value is the ratio of the altitude moved by the compound as compared to the distance moved past the solvent. If the solvent only moves a short distance, so the Rf value will be small.
GCSE Chemistry - Chromatography and Rf Values
GCSE Chemistry – Chromatography and Rf Values

Differences Betwixt Rf Values

  • Rf values can differ. Rf values are not the same for every substance and every solvent. The Rf value will change depending on the solvent used.
  • Rf values identify compounds. Due to the differing Rf values, substances tin be identified by chromatography. This allows united states of america to see the substances contained within 1 compound past comparing them to reference values.
  • Substances create a series of spots. When chromatography is carried out on a chemical compound, the substances separate out to create a series of spots . Each spot will represent one specific substance.
  • Pure compounds produce a single spot. When nosotros deport out chromatography on a pure compound, one unmarried spot will be formed. This indicates that there are no other substances nowadays, reinforcing that the chemical compound is pure.

Newspaper Chromatography

  • Paper chromatography separates mixtures. Paper chromatography is a course of chromatography. By using chromatography, nosotros can split up mixtures and identify compounds.
  • Paper chromatography identifies pure substances. As previously mentioned, we can place pure substances from chromatograms, as they produce i single spot on the chromatogram.
  • Paper chromatography has two phases. Newspaper chromatography has a mobile phase and a stationary phase. The mobile phase is the solvent , whilst the stationary stage is the chromatography paper .
  • Paper chromatography gives Rf values. Like any other form of chromatography, we tin can calculate Rf values from newspaper chromatography using the equation we saw previously.
  • Rf values are affected past the solvent. In paper chromatography, the Rf values are affected by the solvent used. The substances will movement at unlike rates depending on how soluble they are in the solvent, and how attracted they are to the newspaper.
  • Paper chromatography can place colourless substances . By adding locating agents to chromatograms with colourless substances, coloured products or ones which glow under ultraviolet lite can exist formed. An example is iodine vapour. Information technology turns brown when reacted with fats and oils.

Using Paper Chromatography

  • Paper chromatography separates mixtures. As nosotros've seen, paper chromatography is a form of chromatography. Past using chromatography, we can separate mixtures and identify compounds.
  • Paper chromatography separates coloured substances. Paper chromatography can be used to split up the compounds within coloured substances such equally pen ink.

Method

  1. Gather equipment. For this experiment, nosotros will need a solvent, some substances (in this case, different coloured inks), filter paper, a container, a pencil, a capillary tube , and a ruler.
  2. Depict the origin. Using the ruler and a pencil, depict a direct line almost 2cm from the bottom of the filter paper. This is the start line , where the ink droplets to exist separated will be placed. A graphite pencil is used equally graphite is insoluble and will not spread in water.
  3. Add together ink spots. Using the inks and a capillary tube, place a single spot of each ink on the outset line. Using a capillary tube will make sure that the spots of ink are all a similar size and on the starting time line. Make sure the spots are far plenty apart so they will not spread into each other.
  4. Place the paper into the solvent. Put a pocket-size amount of solvent into the container, then place the tip of the filter paper with the ink spots into the solvent. Brand sure to place the newspaper upright into the container and ensure that the solvent is below the ink spots.
  5. Place a lid onto the container. Make sure to identify a lid onto the container so that the solvent does non evaporate .
    GCSE Chemistry - Chromatography and Rf Values
    GCSE Chemistry – Chromatography and Rf Values
  6. Wait for the solvent to travel up the paper. At present, nosotros must wait for the solvent to travel about of the way up the paper. The solvent should not completely achieve the top of the paper. When the paper is removed, the terminal line of the solvent is chosen the ' solvent front .'
  7. Allow chromatogram to dry. When the chromatogram is ready, remove the paper from the solvent and let the chromatogram to dry out .
  8. Calculate Rf values. Using the chromatogram, Rf values can be calculated with the equation distance moved past substance / altitude moved by solvent . Every bit seen earlier, the distance moved past substance is the distance from the get-go line to the centre of the spot.

Worked example: Look at the chromatogram in the diagram above.

Describe three conclusions about the iii substances analysed by the chromatogram.

Answer: Any three from:

Substance A has 4 dissimilar substances, as at that place are four spots.

Substance B has three different substances, equally in that location are iii spots.

Substance C has 3 different substances, as there are iii spots.

Substance A and B both have two of the same substances (blood-red and pink spots).

Substances A and C both have ii of the aforementioned substances (blue and yellowish spots).

Substances B and C both have ane of the aforementioned substances (green spot).

Worked instance: Look at the chromatogram given hither. Calculate the Rf value of the red spot.

GCSE Chemistry - Chromatography and Rf Values
GCSE Chemistry – Chromatography and Rf Values

Answer: Rf value = altitude travelled by the spot/ distance travelled by the solvent = ⅗ = 0.6