Chemistry 4

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  • Created by: Ella Bond
  • Created on: 31-03-13 15:26

Tests For Positive Ions:

Some positive ions in groups 1 and 2 can be identified by a flame test. 

Lithium (Li+) = crimson

Sodium (Na+) = yellow

Potassium (K+) = lilac

Calcium (Ca2+) = red

Barium (Ba2+) = green

The hydroxides of most metals that have ions with 2+ and 3+ are insoluble in water. When sodium hydroxide is added to these solutions a precipitate of the metal hydroxide froms. Aluminium, calcium and magnesium form white precipitate. 

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Tests For Negative Ions:

There are three tests for negative ions:

Carbonate ions - add dilute hydrochloric acid to the substance to see if it fizzes. If it does and the gas produced turns limewater milky, it contains carbonate ions. 

Halide ions - add dilute nitric acid and then silver nitrate solution to the substance. Chloride ions give white precipitate, bromide ions give a cream precipitate and iodide ions give a yellow precipitate.

Sulfate ions - add dilute hydrochloric and then barium chloride solution. If a white precipitate forms, sulfate ions are present. 

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When solutions of an acid and alkali react to form a salt and water, a neutralisation reaction takes place. The volumes of solutions can be found exactly by using a titration. 

To do a titration, a pipette is ised to measure accurately the volume of alkali that is put into a conical flask. An indicator is added to the alkali. A burette is filled with acid which is then gradually added to the flask. When the indicator changes colour the end point has been reached. The volume of acid used is found from the difference between the initial and final burette readings. The titration should be repeated a few times to improve the repeatability of the results. 

Concentrations of solutions are measured in mol/dm3. If we know the number of moles of a substance dissolved in a given volume of solution we can calculate its concentration. If we know the volume of a solution and its concentration we can work out the number of moles of the substance dissolved in a given volume of solution:    Number of moles = mass in grams ~ relative formula mass 

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Chemical Equilibrium:

In a reversible reaction the products can react to reform the original reactants. In a closed system no reactants or products can escape. Equilibrium is achieved when the rates of the forward and reverse reactions are equal. Changing the reaction conditions can change the amounts of products and reactants in a reaction mixture at equilibrium. 

Qualitative methods are used to find if a substance is in a sample e.g ion testing. Quantitative methods tell us how much of a particular substance is in a sample e.g titrations.  

If you increase the pressure equilibrium will shift to the side with the fewest molecules. 

If you increase the concentration of one reactant euilibrium will shft in the opposite direction.

If you increase temperature, equilibrium will move in the endothermic direction.

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The Haber Process:

The haber process is used to manufacture ammonia which can be used to make fertilisers, dyes and household cleaners. In the haber process nitrogen from the air and hydrogen are purified and mixed together. The gases are passed over an iron catalyst at 450*C and a pressure of 200 atmospheres. These conditions are used to give a fast rate of reaction and produce a reasonable yield of ammonia. Ammonia is produced as a gas and as the temperature cools it turns into a liquid. 

The optimum pressure used in the haber process is 200 atmospheres because it gives a reasonable yield without costing too much so its a compromise.

The optimum temperature for the haber process is 450*C because it gives a reasonable yield of ammonia, lower temperatures would give a higher yield of ammonia however it would take too long to be produced so its a compromise.

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