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C3 Analysis and Synthesis
These tests give us results we can observe, and write about. (E.g. flame tests)
These tests give us numbers/amounts that we can use in calculations. (E.g. titrations)
Testing for Positive Ions
Reactions with Sodium Hydroxide
Testing for Negative Ions
Adding acid to a carbonate causes fizzing – producing carbon dioxide. This carbon dioxide can be bubbled through limewater to prove it has been made – a positive result would turn the limewater cloudy.
Adding dilute nitric acid, then silver nitrate to a solution containing halide ions would see a variation of off-white precipitates formed.
The image below shows the colours of precipitates formed.
From left to right: chloride, bromide, iodide.
Adding dilute hydrochloric acid (to remove carbonate ions), then barium chloride – if a white precipitate forms then sulfate ions are present.
A neutralisation reaction involves an exact amount of acid and a base reacting together, forming salt and water. You can use a titration experiment to neutralise either an acid, or a soluble base (an alkali), exactly. The point where the acid and alkali have reacted completely is called the end point. We decide if the reaction has reached the end point by using an indicator that changes colour on neutralisation.
A neutralisation reaction involves an exact amount of acid and a base reacting together, forming salt and water. If you only know the concentration of the acid (or base) you can use a titration experiment to work out the concentration of the other.
moles = concentration x volume
These calculations can appear a little tricky, so here's a video that explains how to do a titration and then goes through the calculation after (starts at 6min 10s).
Analysis in Forensic Science
Chemists solve crimes by analysing substances like drugs, paints, gunshot residues, toxic chemicals and DNA. To analyse DNA, chemists need to use a process called gel electrophoresis, which splits the DNA present into several bands to show what is present. Each person has their own unique bands (unless they have an identical twin).
Chemists also do analysis in pollution control, as well as for doctors. The results of the analysis often need to be matched against an existing database to identify unknown substances.
A closed system is a system where reactants and products are not added or removed. When a reversible reaction happens in a closed system, equilibrium is reached: this is where the rate of the forward reaction is the same as the backward reaction. The yield of a reaction depends on the balance of the forward and reverse reaction.
There are two main ways you need to know about to alter the equilbrium. These are defined under Le Chatelier's Principle:
- changing the concentration/pressure of a reaction
- changing the temperature of a reaction
Ammonia is an important chemical, as it is used to make fertilisers, explosives and cleaning products. It is produced in a reaction known as the Haber Process, and requires nitrogen and hydrogen gas. Ammonia is the only product made, and is a colourless gas, that forms a weak alkali when dissolved in water.
- hydrogen is obtained from natural gas reacting with steam, or from cracking oil
- nitrogen is obtained from the air
Le Chatelier's Principle explains how changing certain conditions can cause changes in where the equilibrium lies. These principles can be applied to the production of ammonia:
- increasing the pressure causes an increase in the yield of ammonia as there are only 2 moles of product, compared to 4 moles of reactant.
- decreasing the temperature would yield more ammonia as the forward reaction is exothermic, however reducing the temperature means less collisions and therefore less reactions - this lowers the rate of reaction. A compromise has to be made, faster rate of reaction, but slightly less yield.
- an iron catalyst is also used to speed up the reaction - it actually speeds up both the forward and reverse reaction, so does not affect the yield of ammonia, just how fast it is made.
The graph below shows how changing the pressure and temperature affects the yield of ammonia from the Haber process. As you can see, the conditions used in the Haber process don't yield the highest amount of ammonia (see diagram to left). Why aren't they maximising their yield?
Increasing the temperature and pressure are hugely expensive things to do in industry, and on top of that companies have other costs to consider too:
- energy needed (electricity)
- cost of starting materials (reactants)
- equipment costs
- labour costs (paying their workers)
Page last updated: 14/04/2017