Chemistry Year Two Practical Work

  • Created by: Estelles
  • Created on: 14-06-18 16:05

Test for NH4+ and NO3- ions

Test for NH3+ ions :

-add NaOH (aq) and gently heat 

-turn damp red litmus paper blue 

Test for NO3- ions: 

- add NaOH (aq) and heat gently 

- add Devardas Alloy

-  turn damp red litmus paper blue

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Measuring reaction rates using colourimetry

Colourimetry:

- a colourimeter measures the absobance of the solution, the more concentrated a colour, the higher the absorbance 

- works by filtering a source of white light, containing all frequecies of visable light, into monochromatic light (only has one frequency of colour)

-pick a suitable filter that produces a colour thats absorbed by the one you want to look at 

- set the colorimeter to zero with a blank sample of your solvent

- once data collected, calculate concentration using a calibration curve

--plot absorbance agaisnt concentration 

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Measuring reaction rates using gas volume, Loss of

Volume of gas :

-collect gas in a gas syringe 

-units are volume time so could be cm s 

Loss of mass 

- meausre mass at regular intervals with a balance 

-units will be mass time so coulb be g s 

PH 

-if an acid or a base can monitor ph 

-could use a ph meter or ph probe 

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Paper chromatography

1) draw a pencil line near the bottom of a piece of chromo paper  and then put a concentrated spot of the mixture 

2) place into beaker with solvent below the pencil line. use a watch glass to stop the solvent evaporating out 

3)when the solvents nearly at the top take out the paper and mark where the solvent has reached 

4) spray with ninhydrin to make the amino acid visible it will turn them purple 

5) work out RF values= distance travelled by spot / distance travlled by solvent 

It is used to analyse the amino acids given by hydrolyisis of a protien 

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Finding enthalpy of solution

1) dissolve the ionic substance in water

2) put reactants in a polysterine beaker with a lid, hole for thermometer 

3) record the temp change 

use Q= MC   T

-need to know the no. of moles, temp change and mass of water( or solution)

q= heat lost or gained 

m= mass of water 

c= specific heat capacity 

   t= temperature change 

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measuring the ph of strong acids and weak acids

  • Strong acids 

Strong acids ionise fully, they are monoprotic which means each mole of acid produces one mole of hydrogen ions. This means that the H+ conc is the same as the acid concentration. 

Ph = -log  [H+]

[H+] = 10 

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measuring the ph of weak acids

  • Weak acids 

Weak acids only dissociate only slightly so the H+ isnt the same as the acid concentraion, need to use Ka, acid dissociation constant

For a weak aq acid, HA, you get the following Eq 

assumptions - as a tiny amount of HA dissociates

  -  the dissociation of the acid is much greater than the dissociation of water so you can assume all the h+ ions in sol come from the acid 

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measuring PH of a strong base

Water acts as an acid and a base, it donates and accepts a proton. in water there will always be both hydroxonioum ions and hydroxide ions. water only dissociates a tiny amount so water conc is a constant value, ionice product of water Kw=                                                                                    in pure water u get one oh- for one h+ so h+=oh- so kw=[h+]2 

Use Kw to find ph, strong bases fully ionise in water, they donate one oh- ion per mole of acid, put all values into the kw eqaution to find the h+ conc then ph 

Find a weak acid ph, use ka and plug in info you know 

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Buffers

A buffer is a solution that resists changes in pH when small amounts of acid or base are added, or it's diluted. 

Acidic buffers are made froma weak acid and one of its salts, they have a pH of less than 7.            A good example is sodium ethanoate and and ethanoic acid                                                             The ethanoic acid is a weak acid so it only dissociates slighty:                                                            The salt fully dissocaites:                                                                                                                     In solution you have lots of undissocaited ethanoic acid and lots of ehanoate ions, when you alter the conc of H+ or OH- the eq moves to counteract the change, if you add a small amount of acid the h+ conc increases the extra h+ will combine with the ch3coo- to form ch3cooh, shifting the eq to the left and reducing the h+ conc close to original value. If a small amount of base is added the oh- conc increases, these react with the extra h+ from the water removing them from the sol. this cause smore ch3cooh to dissocaiate and form more h+ replacing them. 

Calculate pH of one by putting values into ka expression 

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Finding x in the formula of a hydrated solid

method: heat the hydrated substance to contant mass, rcord the mass before and after 

Calc: 

1) find the moles of water lost by finding the mass and dividing by mr 

2) find the moles of the anhydrous salt by fidning yhe mass and dividing by mr 

3) work out the ratio of moles in the form 1: n , anhydrous moles: water moles 

X must be a whole number, however you can round it  errors are expected in any experiment.

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Preparation of salts

Insoluble 

- use a precipitaion reaction, occur in aq solutions when cations and anions combine to form an insoluble ionic salt ( ppt) 

- need to pick two sols that contain the ions you need. acidand base reaction 

- filter the ppt from sol, wash it and dry pn filter paper. 

Soluble 

-need to pick the right acid plus a suitable metal or insoluble base

- add the solid metal, metal oxide or metal hydroxide to the acid, it will dissolve. you will know when reaction is finished as no more solid will dissolve. 

-filter out excess metal to get the salt solution 

-to make pure solid crystals, evap some of the water and then leave rest to evap slowly (cyrstilisation)

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salt reactions

acid + base = salt + water 

acid + metal carbonate = salt + hydrogen gas + co2 

acid + 

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Cation tests

Flame test 

-dip a nichrome wire loop in conc hcl, dip wire in to the compound, hold loop in clear blue part of flame 

Lithium - red, Sodium- yellow, potassium- liliac, calcium-brick red, barium- green, copper- blue green. 

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anion tests

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Showing trend of thermal stability of g2 carbonate

-use equal molar qauntities of the carbonates , record the mass of the carbonate before heating, heat, record mass after , expect gp2 to lose more mass going up the group, losing less mass going down the group, 

trend- themral stability increases going down g2 as the cations get larger down the gorup, the larger they are the less ability they have to polarise the carbonate anion electorn cloud. 

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How to make up a standard solution

1) work out how many moles of solute you have, the solute is the substance being dissolved in the solutuion 

2) work out how many grams of solute is needed

3) carefully weigh out the mass of solute you need, weigh beaker first record mass then add the mass accordingly

4) add a small amount of distilled water to the beaker and stir untill all the solute has dissolved 

5) tip the solution into a volumetric flask using a funnel

6) rinse beaker and stiring rod and tip into volumetirc fgalsk to ensure no sokute is stuck to them 

7) top flask up to correct volume with more distilled water, bottom of minescus and use a pipette 

8) invert to ensure all mixed                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          

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Finding enthalpy of combsution of a liquid fuel

1) you burn the liquid in a combustion chamber full of water with a thermometer. 

2) use q=mc  t

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Gas liquid chromotography

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