GCSE C2 OCR Gateway

An indicator is a dye that changes colour according to pH.

Universal indicator is a combination of dyes. It changes colour gradualy as the pH changes.

Its very useful for estimating the p of a solution.

Some indicators change colour quicker, E.G. Phenolphthalien changes colour suddenly from colourless to pink as the pH rises above 8

0-6.9=acdic

7=neutral

7.1-14=alkali

acids and base neutralise eachother.

an acid is a substance with a pH of less than 7.Acids form H+ ions in water.The pH of an acid is determined by the concentration of the H+ ions.

a base is a substance with a pH of greater than 7.An alkali is a base that is soluble in water.Alkalis form OH- ions in water.

The reaction between acids and bases is called neutralisation

acid+base->salt+water

It can also be seen in terms of H+ and OH-

 H+ + OH- (reversible sign) H(2)O

When the solution is neutralised, the products have a pH of 7

Some metal oxides and metal hydroxides dissolve in water. These soluble compounds are bases.

Insoluble bases also react with acids. These form salts and water.

acid + metal oxide -> salt + water

acid +metal hydroxide -> salt + water

Hydrochloric acid = (metal) chloride

Nitric Acid = (metal) nitrate

Phosphoric Acid = (metal) phosphtate

acids and carbonates produce carbon dioxide aswell as water.

Acid + carbonate -> salt + carbon dioxide + water

E.g. Hydrochloric acid + sodium carbonate -> sodium chloride + water +carbon dioxide.

Acids and ammonia produce ammonium salts.

Acid + Ammonia -> ammonium salts

E.g. Nitric Acid + Ammonia -> Ammonium nitrate

Thre main essential elements in fertiliser are Nitrogen, Phosporus and Potassium.

If the plants dont get enough of each of these elements, their growth and life processes are affected. These elements could be missing from the soil due to the previous crop.

fertlisers replace the missing elements and provide more. This increases crop yield, as the crops can grow bigger and faster.

For example, the nitrogen is absorbed through he roots and provide more proteins for the plant to grow.

the fertiliser must be soluble in water before it can be given to the crops.

Ammonia is a base, so it can be neutralised by an acid to produce ammonium salts. Its the key ingredient to many fertilisers.

1.Nitric acid + ammonia -> amonium nitrate

This is a good fertiliser, because both sources contain nitrogen.

2. Ammonia + sulfuric acid = ammonia sulphate

3. Ammonia + phosphoric acid = Ammonium Phosphate

4. potassium hydroxide + nitric acid = Potassium nitrate

The exponential rise in population means more food is required.

Fertilisers increase crop yeold, so the more fertiliser we make, the more crops we can grow.

But if we use to many fertilisers, they can pollute water supplies

This can lead to eutrophication.

1. When fertilisers are put on fields , some get washed off and find their way into streams and rivers.

2.The level of nitrates and phosphates in the water increases.

3.Algae living in the river use these nutrients to grow rapidly creating an algal bloom. this blocks off the light to plants below which stops them photosynthesising.

4.Aerobic bacteria feed on the dead plants and strat to multiply. As the bacteria multiply, they use up all of the oxygen in the water, this kills most of the remaining organisms in the water.

The mass of product that you end up with is called the yeild of a reaction.

You never get 100% yeild, as not all of the reactant is converted into the product.

The more reactant at the start = higher the yeild.

Percentage yeild doesnt depend on the amount of reactant started with, its a %

Percentage yeild is always between 0-100%

100% yeild means you get all of the expected yeild

0% yeild means no reactants were converted into the product.

The predicted yeild = amount of product you'd get if all of the reactant was converted into product

- Its a reversible reaction.

N(2) + 3H(2) (reversible symbol) 2NH(3)

1)The nitrogen comes from the air (78%)

2)The hydrogen comes from cracking oil fractions or natural gas

3)Because it's reversible, not all of the hydrogen and nitrogn will convert into ammonia but, the hydrogen and nitrogen are recycled and used again.

Industrial Conditions  

Pressure = 200 atmospheres

Temp. = 450 degrees celsius

Catalyst = Iron

Because the reaction is reversible, theres a compromise to be made.

High pressure favours the forwards reaction, so the 200 atmospheres increases the % yield of ammonia.

High temp. favours the backwards reaction. This decreases % yield.

But lower temps. decrease the rate of reaction, so manufacturers use high temps. anyway so that they can increase reaction rates.

450 degrees celsius is the optimum temp. - it gives a fast reaction rate and an acceptable % yield.

This is the compromise.

E.g. 20 secs for a 10% yield is better than 60 secs for a 20% yield

The iron catalyst used speeds up the reaction and dereases costs.

The iron makes the reaction go faster, which gets it to the equilibrium proportions faster. The catalyst doesn't affect the position of the equilibrium.

Without the catalyst, the temp. would have to be raised even further to get a quick enough reaction rate.

This rise in temp. would decrease % yield further.

there are five main factors which affect the cost of production;

1) Price of energy - Industry needs to keep energy bills as low as possible. If a reaction needs a high temp. then running costs will be higher.

2)Cost of raw materials - Recycling the materials reduces this, e.g. The Haber Process.

3)Labour Cost - Workers must be paid, labour intensive work can be expensive, automation reduces the need of people. Companies must weigh any savings they make on wages against the initial cost and running costs of machinery

4)Equipment - The cost of equipment depends on how specialised it is, e.g. Equipment used to withstand very high pressures costs more than one that has to withstand atmospheric pressure.

5)The faster the reaction, the less it costs to produce, catalysts can speed up reactions to save money on fuel etc.

Optimum conditions are those that cost the least per kg, even if this means getting less % yield or slower reaction rates.

Definition:

Optimum conditions are those that give the lowest production cost.

However, the rate of reaction and % yield must be high enought to make a sufficient amount of product each day.

Sometimes low percentage yields are okay, as long as the the starting materials can be recycled, e.g. The Haber Process.

In britain, salt is mined from underground deposits left millions of years ago when ancient seas evaporated. there are huge deposits of rock salt under Cheshire.

Rock salt is a mixture of salt and impurities

It can be mined by pumping hot water into the ground, the salt dissolves into the water and is forced to the surface by the pressure of the water. This is called solution mining. 

When the mining is finshed, the holes must be filled. The land could collapse and slide - this is called subsidence.

Rock salt can be used in its raw state for gritting roads, enhancing the flavours of foods, or making chemicals.

If the salt is to be used in chemicals, usually the first thing that must be done is to electrolyse the salt using the chlor-alkali process.

Electrolysis of rock salt gives hydrogen, chlorine and NaOH.

Concentrated brine (sodium chloride solution) is electrolysed industrially.

The electrodes are made of an inert material - so they dont react with the electrolyte or the products of the electrolysis.

There are three useful products;

1) Hydrogen gas is given off at the (-ve) cathode.

2) Chlorine gas is given off at the (+ve) anode.

3) Sodium hydroxide (NaOH) is formed from the ions left in solution.

The sodium chloride solution contains four different ions: Na+, OH-, Cl- and H+

At the cathode; two hydrogen ions accept one electron each to become one hydrogen molecule

cathode half equation; 2H+   +  2e-    ->   H(2)    REDUCTION REACTION

At the anode; two chloride ions lose one electron  each to become one chlorine molecule

anode half equation; 2cl-   -    2e-   ->   Cl(2)     OXIDATION REACTION

Oxidation = loss of electrons

Reduction = gain of electrons

The products of the chlor-alkali process is used for many things.

E.g. the hydrogen gas is used to make ammonia and margarine.

The chlorine is used to disinfect water and to make plastics (PVC), solvents or hydrochloric acid.

The sodium hydroxide is used to make soap or can be reacted with chlorine to make household bleach.

All of these uses of the products from electrolysis of brine makes the chlor-alkali industry essesntial to the economy, because lots of jobs are created and products are made.