Describe a flame test to test for positive ions.
1) Clean a piece of nichrome wire with sandpaper, then water.
2) Dip wire into test substance.
3) Hold wire at the edge of a blue Bunsen flame.
What colour flame do each of these metal ions produce in a flame test:
Describe a sodium hydroxide test for positive ions.
1) Dissolve substance in water.
2) Add a sodium hydroxide solution until in excess.
In a sodium hydroxide test for positive ions, what ions produce no precipitates?
Sodium (Na+) or Potassium (K+)➡️Because these hydroxides are soluble.
In a sodium hydroxide test for positive ions, what ions produce white precipitates which do not redissolve?
Calcium (Ca2+) or Magnesium (Mg2+)➡️Because these hydroxides are both white and insoluble.
In a sodium hydroxide test for positive ions, what ions produce white precipitates which dissolve when excess sodium hydroxide is added?
Aluminium (Al3+)➡️Because the hydroxide is insoluble in water but soluble in sodium hydroxide solution.
In a sodium hydroxide test for positive ions, what ions produce a coloured precipitate and what colours are produced?
-Iron (II)(Fe2+)➡️Dirty green.
Describe a test for carbonates for negative ions.
1) Add dilute hydrochloric acid.
2) If it fizzes, test for CO2 with lime water (which goes cloudy).
This indicates a carbonate (CO32-).
Describe a test for sulphates for negative ions.
1) Add a little dilute hydrochloric acid followed by barium chloride solution.
2) A sulphate (SO42-) gives a white precipitate.
Describe a test for halides for negative ions.
1) Add a little nitric acid followed by a few drops of silver nitrate solution.
What coloured precipitates do non-metal ions produce in a test for halides?
-Bromide (Br-)➡️Cream coloured.
What are the advantages of using instrumental methods in chemical analysis?
✅You can use very small samples.
Balance this ionic equation:
Describe the method for titration.
1) Use a graduated pipette and safety filler to put 25cm3 of 0.1M sodium hydroxide solution into a 250cm3 conical flask.
2) Add 3-5 drops of indicator.
3) Place a white tile under the burette, then fill to the 0 mark with hydrochloric acid.
*Remember the bottom of meniscus must be in line with the 0 line*
4) Add the acid from the burette 1cm3 at a time until you approach the end point (given your indicator colour change). Swirl the conical flask constantly. Now add the acid one drop at a time until you have reached the end point, as seen by the colour change.
5) Repeat the experiment until 3 concordant results are obtained.
What is a titration used for?
A titration is used to measure accurately how much acid and alkali react together completely.
What is the difference between a pipette and burette?
-A pipette measures a fixed volume, has a single graduation and has no tap.
-A burette measures different volumes, has graduations and has a tap.
What are concentrations of solutions measured in?
g/dm3 or mol/dm3.
How can the number of moles of a solution be calculated?
n= no. of moles (mol).
v= volume (cm3).
c= concentration (moldm-3 or M).
We found that 25.2cm3 of 0.1moldm-3 hydrochloric acid was needed to neutralise a 25cm3 sample of sodium hydroxide.
Calculate the concentration of NaOH.
1) n=(25.2 x 0.1)/1000= 0.00252 moles of HCl.
2) HCl + NaOH➡️NaCl + H2O= 1 mole of HCl always reacts with exactly 1 mole of NaOH.
3) c= 1000n/v= (1000 x 0.00252)/25= 0.1008M.
How can the mass of a solute be calculated from the number of moles in it?
n= number of moles (mol).
m= mass (g).
Mr= relative formula mass.
What is the mass of sodium hydroxide in 100cm3 of a solution with a concentration of 0.2mol/dm3?
1 mol NaOH= 40g.
1) (100 x 0.2)/1000= 0.02mol.
2) 0.02 x 40= 8g.
How are qualitative methods for chemical analysis used?
Qualitative methods are used to find out if a substance is in a sample.
How are quantative methods for chemical analysis used?
Quantative methods can tell us how much of a substance is in a sample.
What is meant by equilibrium?
When the rates of the forward and reverse reactions of a reversible reaction are equal or when the amounts of reactants and products in a reversible reaction are constant.
How does changing the concentration of a reactant change the product?
Increasing the concentration of a reactant will cause more products to be formed as the system tries to achieve equilibrium.
If a product is removed, more reactants will react to try to achieve equilibrium and so more product is formed.
To make SO3, the reaction 2SO2 +O2⇌2SO3 is done in a reactor over a heated catalyst.
Why is the SO3 removed from the reactor as soon as it is made?
So that more of reactants react or so more SO3 is produced.
How does changing the pressure affect reversible reactions involving gases at equilibrium?
-Increasing the pressure favours the reaction with fewer molecules.
-Decreasing the pressure favours the reaction with more molecules.
How does changing the temperature in a reversible reaction affect the amount of products formed at equilibrium?
-Increasing the temperature favours the endothermic reaction.
-Decreasing the temperature favours the exothermic reaction.
What is the Haber process?
The Haber process is used to manufacture ammonia, which can be used to make fertilisers and other chemicals.
What is ammonia made from?
Nitrogen and hydrogen.