GCSE Chemistry Unit 3 Notes

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C31: Acids and Bases
Strength of Acids and Alkalis
The strength of an acid depends on the extent to which it ionises in water. A strong acid or alkali is
one which is 100% ionised in water. Hydrochloric acid, sulphuric acid and nitric acid are all strong
acids. Sodium hydroxide and potassium hydroxide are both strong alkalis. A weak acid or alkali is only
partly ionised in water. Ethanoic acid, citric acid and carbonic acid are all weak acids and ammonia
solution is a weak alkali.
We can detect strong and weak acids using their pH. This scale is a measure of the concentration of
hydrogen ions in a solution.
A strong acid, e.g. hydrochloric, will be
completely ionised, so the concentration of
hydrogen ions is 1 mol/dm³. However, a
weak acid, such as citric acid is only partly
ionised, so the concentration of hydrogen
ions will be much lower than 1 mol/dm³
Adding an acidic solution to an alkaline solution will produce a neutralisation reaction. They react
together and neutralise each other, producing a salt in the process. When a neutralisation reaction
takes place, the quantities of each solution used must be correct, because if a very strong acid and a
very strong alkali were mixed, if there was more acid solution, the whole alkali solution would be
neutralised, but not all of the acid solution would be ­ so the mixture would become slightly acidic
overall. We can measure precise volumes of acids and alkalis needed to react with each other using
In the neutralisation reaction, the point at which the acid and the alkali have completely reacted is
called the end point. We can show the end point using a chemical indicator. Indicators change
colour over different pH ranges. We have to choose suitable indicators when carrying out titrations
with different combinations of acids and alkalis:
strong acid + strong alkali ­ use any indicator
weak acid + strong alkali ­ use phenolphthalein
strong acid + weak alkali ­ use methyl orange
These are the steps to carry out a titration to calculate how much acid is needing to react with an
alkaline solution:
1. Measure an alreadyknown volume of the alkali solution into a conical flask using a pipette
2. Add an indicator solution to the alkali in the flask
3. Now put the acidic solution into a burette. This long tube has measurements down the side,
and a tap on one end and can accurately measure the amount entering the flask. So record the
reading on the burette (i.e. starting volume)
4. Open the tap to release the acid solution. The solution from the burette is released one drop at
a time, alongside swirling of the flask to make sure the solutions are mixed
5. Keep repeating Step 4 until the indicator changes colour to let you know the acid and the alkali
have completely mixed

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Record the amount of acid you entered by reading the measurement on the burette
Be sure to repeat the entire process two or three times at least to ensure accuracy.…read more

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­ but we can use it to compare energy changes from different fuels.…read more

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There is one vital piece of information
we need to know to do this:
4.2 joules of energy raises 1g of water by 1°C
Hence the units involved in this energy change will be kJ/g/°C (kilojoules per gram per degree). A
simple calorimeter is used to measure energy change in a reaction A + B C. So let's calculate an
example of such a reaction:
Question : 60cm³ of a solution containing 0.1 moles of A is mixed with 40cm³ of a solution containing
0.…read more

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These two energy
level diagrams show HH bonds being made and broken. The left diagram shows an already bonded
HH bond being broken. This has a bond energy of +436kJ/mol, so we write H = +436kJ/mol on the
diagram next to the change in height arrow. The right side is a diagram representing two separate
hydrogen atoms bonding. Obviously, this is bond making ­ which releases energy ­ so the energy
change is 436kJ/mol, written the same way as before, except with a minus sign.…read more

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The solubility curves for potassium nitrate, sodium nitrate and sodium
chloride are shown here. As you can see, the solubility of each one
increases with temperature as the rule states ­ but the rate of increase
differs between solutes. As you can see, sodium chloride barely
increases in solubility between 0°C and 100°C, whereas potassium nitrate
increases eightfold in the same period.
The thing that all of these solutes have in common is that they are all solid
solutes.…read more

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It is the dissolved substances which react with hard water to form scum.
In terms of economic factors, hard water is more expensive because more soap is required for the
same wash. The soap reacts with the magnesium and calcium ions in the water, forming salts called
stearates (the chemical name for scum). Only after all of the calcium and magnesium have reacted
can the soap begin to form a lather ­ and this is why so much more soap is needed per wash.…read more

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The problem with Newlands' table was that he was too determined to get it done and working that he
made some mistakes. What he didn't know was that there were still many elements to be found, so he
filled in octaves regardless of their properties, and some of them ended out not being similar at all. He
did this to make everything fit in, so as a result, his ideas were not accepted.…read more

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Group 1 ­ Alkali Metals
We call the first group (Group 1) the alkali metals. These are lithium, sodium,
potassium, rubidium, caesium and francium. The first three of those elements being
reactive, you may experiment with their reactions in class, but the last three of those
elements are extremely reactive, you won't get to play about with them in class.
Francium is even radioactive.…read more

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They are relatively typical nonmetals, in such that they are poor conductors of
heat and electricity, and that they have low melting and boiling points. At room temperature, fluorine is
a poisonous, yellow gas, whilst chlorine is a green poisonous gas. All of them are reactive.
All of the halogens exist in molecules, pairs of atoms. These molecules are made from a covalent
bond (see Atoms and Bonding, C2), and so we call the type of structure a diatomic molecule.…read more


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