Chemistry GCSE C3

Key points and memory tests for C3.

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C3 - History of the periodic table

Early 1800s they ordered it on atomic mass.
2 obvious ways to categorise elements:
- Their physical and chemical properties
- Their relative atomic mass

They had no idea about stmoic structure or of protons or electrons, so there was no atomic for them.

Back then, you could only measure relative atomic mass, and so the known elements were arranged in order of atmoic mass.

When this was done, a periodic pattern was noticed in the properties of the elements. 

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C3 - History of the periodic table

Newlands' law of octaves.
He noticed taht every 8th element had similar properties, and so he listed some of the known elements in rows of seven.

the patterns broke down on the third row, with transition metals like titanium (Ti) and iron (Fe).
It was because he left no gaps that his work was ignored.

1.His groups contained elements that didn't have similar properties. 
2.He mixed up metals and non-metals.
3.He didn;t leave any gaps for elements that hadn't been discovered yet.

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C3 - History of the periodic table

In 1869, Dmitri Meneleev left gaps and predicted new elements.

He put the elements in order of atomic mass, but mendeleev found he had to leave gaps in order to keep elements with similar properties in the same vertical groups - and he left very big gaps in the first 2 rows before the transition metals come in on the 3rd row.

The gaps were the reallyy clever bit because they predicted the properties of so far undiscovered elements.  

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C3 - History of the periodic table

Not all the scientists thought the periodic table was important because at the time there wasn't that much evidence to back up any of the predictions. 

After mendeleev released his work, newly discovered elements fitted into the gaps he left, this was convincing evidence in more favour of the periodic table.

Once there was more evidence, more scientists realised that the periodic table was a good and useful tool.

In the late 19th Century, scientists discovered "protons", neutrons and electrons. the periodic table matches up very well to what's been discovered about the atomic structures of the atom.

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C3 - The modern periodic table

When electrons, neutrons and "protons" were discovered the periodic table was arranged in order of atomic ("proton") number. All the elements were out into groups.

1.You can use the periodic table to work out the detailed arrangement of electrons in an atom of any element. Once you know the electron arrangement, you can predict the element's chemical properties.
2.Electrons  in an atom are set out5 in shells which correspond with energy levels.
3.The maximum number of electrons that can occupy each enrergy level is gievn by the formula 2 x nxn where the n is the energy level.
4.Apart from the transition metals, in the same group have the same number of outer electrons.
5.The +ve charge of the nucleus attracts electrons and holds them in place.
6.The attraction of the nucleus is even less when there are a lot of inner electrons.
7.Increased distance and increased shielding means electrons in a higher energy level is more easily lost.
8.Increased distance and shielding also means higher energy level is less likely to gain an electron. 

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C3 - Group 1: Alkali metals

Trends as you go down the group:

1.Bigger atoms - because there's one extra full shell of electrons for each row you go down.
2.More reactive - because the outer electron is more easily lost, because it's further from the nucleus. 
3.Higher density -  because the atoms have more mass. The 3 at the top are less dense than water.
4.Lower melting point.
5.Lower boiling point.

The alkali metals all have 1 outer electron.
The alkali metals all form 1+ ions.
They always form ionic compounds.
Their reaction with water produces hydrogen gas,
They form hydroxide in solution E.G. aqueous OH- ions.

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C3 - Group 7: The Halogens

Trends as you go down the group:

1. Less reactive
2.Higher melting point
3.HIgher boiling point

They are all non-metals with coloured vapours,
They all form molecules which are pairs of atoms E.G. F2, Cl2, Br2, I2.
They form both ionic and covalent bonds
- They form 1- ions when they bond with metals
- The form covalent bonds with non-metals.
The halogens react with metals to form salts.
More reactive halogens will displace less reactive ones.

Cl2 + 2KI --> I2 + 2KCl 

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C3 - Transition Metals

Properties of transition metals:
1.They're good conductors of heat and electricity.
2.They're very strong, dense and shiny.
3.Less reactive the group 1 metals - they don't react with water or oxygen.
4. They're also much denser stronger and harder than group 1 metals , and have much higher melting points.

They often have more than one Ion.
The compounds are very colourful:
- Potassium chromate (VI) is yellow.
- Potassium manganate (VII) is purple.
- Copper Sulphate (II) is blue
Transition metals and their compounds are what makes them good catalysts.
Their properties are due to the way their electron shells fill. 

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C3 - Acids and Alkalis

Arrhenius said acids release hydrogens in water.
HCl + H2O --> H+ + Cl-
H2SO4 + H2O --> 2H+ + SO42- 

He also said that alkalis form OH- ions in water.
NH3 + H2O --> NH4+ + OH-

Lowry and Bronsted said:.
Acids release H+ ions - i.e they're "proton" donors.
Bases accept H+ ions - i.e they're "proton" acceptors.

"Protons" are hydrated in water:
In acidic solutions: The acid molecules dissociate, releasing lots of H+ ions.
These H+ ions become hydrated. The protons are now called hydrated protons and can be represented by H+. It is these hydrated protons that make acids acidic.In basic solutions: Water molecules can dissociate into H+ ions and OH- ions. Some base molecules like ammonia can take hydrogen ions from water, causing more molecules to dissociate and leaving excess of OH- ions behind. 

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C3 - Acids, Alkalis and Titrations.

Acids can be weak or strong.
Strong acids: Ionise almost completely in water, this means almost every hydrogen atom is released to become a hydrated proton. (Sulphuric, hydrochloric and nitric)
Weak acids: Ionise only slightly - only some of the hydrogen atoms in the compound are released -  so only small number of H+ ions are formed. (Citric, ethanoic and carbonic)

The pH of a acid or alkali is a measure of the concentration of H+ions in a solution.

Titrations allow you to find out exactly how much acid is needed is needed to neutralise a quantity of alkali.
You put some alkali in a flask, along with some indicator.
Phenolphthalein is used for a weak acid and a strong alkali.
Methyl Orange is used for a strong acid  and a weak alkali.
Add the acid a bit at a time, using a burette - giving the flask a swirl as you do so.
The indicator will change colour when all the alkali has been neutralised. 

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C3 - Titration Calculations

Number of moles = concentration(mol/dm3) x volume(dm3)

Example 1:
25cm3 of sodium hydroxide = concentration is 0.1 mol/dm3
It takes 30cm3 of sulphuric acid to neutralise the sodium hydroxide.

Step 1:  work out how many moles of the 'known' substance you have:
0.1 mol/dm x (23/1000) dm3 = 0.0025 moles of NaOH

Step 2: write down a balanced equation of the reaction:
2NaOH + H2SO4 ---> Na2SO4 + 2H2O
and work out how many moles of the 'unkown' substance you have.
Using the equation you can see that for every 2 moles of sodium hydroxide you had there was one mole of sulphruic acid.
0.0025 / 2 = 0.00125 moles of sulphuric acid. 

Step 3: work out the concentration of the 'unkown' substance.
Concentration = Number of moles / volume
0.00125 mol / (30/1000) dm3 = 0.0417 mol/dm3 

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C3 - Titration Calculations

Mass (g) = number of moles (mol) x relative mass formula (Mr)

Example 2:
Find out the acid concentration in grams per cubic decimetre.

Step 1: work out the relative mass formula mass for the acid:
H2SO4 = (1x2) + 32 + (16x4) = 98

Step 2: convert the concentration in moles into concentration in grams.
0.0416666... x 98 = 4.08333...g

So the concentration in g/dm3 = 4.08g/dm3

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C3 - Water

The water cycle means water is endlessly recycled.

1. The sun causes evaporation of water from the sea. The water vapour is carried upwards as the warm air rises.
2. As the water vapour rises it cools. This fall in temperature means the water condenses to from clouds.
3. When the condensed droplets get too big they fall as rain.
4. When the water runs back into the sea. On its journey back to the sea the water will have absorbed different minerals.
5. The water cycle starts all over again.

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C3 - Solubility

Water is a solvent: It dissolves any other chemicals.

The solubility if a substance in a given solvent us the number of grams of the solute (usually a solid) that dissolve in 100g of the solvent (the liquid) at a particular temperature.

The solubility of (solid) solutes usually increases with temperature.

A saturated solution is one that cannot hold any more solid at that temperature - and you have to be able to see solid on the bottom to be certain that it's saturated.

Solubility Curves:
A solubility curve plots the mass of the solute dissolved in a saturated solution at various temperatures
The solubility of most solids increases as the temperature increases.
That means that cooling a saturated solution will usually cause some solid to crystallise out. 
The mass of the crystals formed can be calculated from a solubility curve. 

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C3 - Solubility

All gases are soluble.

Chlorine water is a solution of chlorine gas in water.  It is commonly used in swimming pools.

The amount of gas that dissolves depends on the pressure of the gas above it - the higher the pressure, the more gas that dissolves. 

Gases become less soluble as the temperature of the solvent increases, which is opposite to solids.

Aquatic life needs dissolved oxygen, but the oxygen levels in rivers can be lowered by pollution and a rise in temperature(caused by warm water from industries etc) causing problems. 

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C3 - Hard water

Hard water is caused by Ca2+ and Mg2+ ions.

Hard water makes scum.
Hard water doesn't lather soap easily. 
Hard water also causes scale which is like a thermal insulator, found in kettles.

Hard water isn't all bad it is good for bones and teeth.
And the scale is a protective coating. It stops poisonous metal ions getting into drinking water and protects pipes from rusting.

By removing the dissolved Ca2+ and Mg2+ ions the water becomes softer.
By adding sodium carbonate: the carbonate joins onto the calcium or magnesium ions and makes an insoluble precipitate.
By ion exchange column: this is when the water is fed through an ion exchange column and sodium or hydrogen ions exchange with calcium or magnesiumions.

Scale is mainly calcium carbonate and can be dissolved by acid. 

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C3 - Water quality

Drinking water needs to be a good quality and is treated.

1.The water passes through a mesh screen to remove large objects. 
2.It's treated with ozone or chlorine to kill microorganisms.
3.Chemicals are added to make solids and microorganisms stick together and fall to the bottom. Sometimes iron is added to remove dissolved phosphates. Bacteria are used to remove nitrates.
4.The water is filtered through gravel beds to remove all the solids. Nasty tastes and odors can also be removed by passing the water through 'activated carbon' filters or with 'carbon slurry'.
5.The pH is also corrected if the water is too acidic or too alkaline.
6.Water is chlorinated to kill off any harmful microorganisms left. 

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C3 - Energy

An exothermic reaction is one which gives out energy to the surroundings, usually in the form if heat and shown as a rise in temperature.

An endothermic reaction is one which takes in heat from the surroundings, usually in the from of heat and shown as a drop in temperature.

Energy must be supplied to break a bond....but energy is released when a bond is formed. 

Bond breaking is an endothermic reaction.

Bond making is an exothermic reaction. 

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C3 - Energy and fuels

Fuel energy is calculated using calorimetry.

1.Put 50g of water in the copper can and record its temperature.
2.Weigh the spirit burner and lid.
3. Put the spirit in the burner underneath the can and light the wick. Heat the water stirring constantly, until the temp. reaches about 50oC
4.Put the flame out using the burner lid and measure the final temp. of the water. 
5.Weigh the spirit burner and lid again.

Fuels provide energy.
Burning fuels has various effects on the environment, global warming and potential climate change.
It'll be expensive to try and fix what we've already caused, but we should try and start using less non-renewable resources and use more renewable resources. 

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C3 - Bond Energies

Energy level diagrams show if a reaction is endothermic or exothermic.

Exothermic reactions energy change is always -ve:
- the products are at a lower energy than the reactants.
- the height represents the energy given out in the reaction. (per mole)

Endothermic reactions energy change is always +ve:
- the products are at a higher energy than the reactants. 
- the height difference represents the energy taken in during the reaction 

The activation energy is the minimum amount of energy needed for the reaction to happen and cane be lowered by a catalyst.

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C3 - Bond Energy Calculations

Example: the formation of HCl 

Using known bond energies you can calculate the energy change for this reaction:
H2 + Cl2 --> 2HCl  
The bond energies you need are:
H-H + 436 kj/mol; Cl-Cl + 242 kj/mol; H-Cl + 431 kj/mol

Breaking one mole of H-H and one Cl-Cl bonds requires 436 + 242 = +678 kj/mol
Forming 2 moles of H-Cl bond releases 2x431 kj/mol
Overall more energy is released than is used to form the products:
862-678 =184 kj/mol
To show the energy change we need  to put a: energy change = -184 kj/mol 

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C3 - Energy & Food

Food energy is measured in calories and kilocalories.
1 calorie = amount of energy needed to raise the temp of 1g of water by 1oC.
1 calorie = 4.2 joules 

1 Calorie = amount of energy needed to raise the temp of 1kg of water by 1oC.
1 Calorie = 4200 joules

Taking in more fuel than you use means the excess is stored.

Glucose + Oxygen --> Carbon Dioxide + Water + ENERGY.

Fuel will be stored in the body as fat and can cause some serious health problems if a large amount is stored.
Fuel will not be stored if you do not overeat, and use up the calories/fuel of which you consume. 

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C3 - Test for Cations

Flame tests identify metal ions:
Lithium, Li+ burns with a crimson-red flame.
Sodium, Na+ burns with a yellow-orange flame.
Potassium, K+ burns with a lilac flame.
Calcium, Ca+ burns with a brick-red flame.
Barium, Ba+ burns with a green flame.

Some metals form coloured precipitate with sodium hydroxide, NaOH.
Calcium, Ca2+...white...Ca2+ + 2OH- --> Ca(OH)2
Copper (II), + 2OH- --> Cu(OH)2
Iron (II), Fe2+...sludgy green...Fe2+ + 2OH- --> Fe(OH)2
Iron (III), Fe3+...reddish brown...Fe3+ + 3OH- --> F3(OH)3
Aluminium, Al3+...white at first, redissolves into excess NaOH and goes colourless...Al3+ + 3OH- --> Al(OH)3 then Al(OH)3 + OH- --> Al(OH)4-
Magnesium< Mg2+...white...Mg2+ + 2OH- --> Mg(OH)2 


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