Wednesday, December 2, 2009

FOSSIL FUEL -COAL / IGCSE CHEMISTRY / GCSE-CHEMISTRY

Just as coal has formed by the action of heat and pressureon the remains of trees and plants on land over millions of years,so oil and natural gas have formed by the action of heat and pressureon the remains of sea plants and

animals over millions of years.
The remains were buried in sediments which excluded the air (kept out oxygen) and stopped them decaying.
More sediment buried the remains deeper and deeper until pressure and heateventually turned them into coal, oil and natural gas.
They are called fossil fuels because they are buried underground(from Latin fossilis - dug up).
Fossil fuels are a finite resource and non-renewable.
The oil deposits are formed in porous rock sediments.
Porous rock has pores in it. Pores are small holes (see for example sandstone).The small holes allow the oil and natural gas to pass through the rockand rise until they are stopped by a layer of non-porous rock.
Non-porous rock (for example shale) has no holes,and acts as a barrier to prevent the oil and natural gas rising.The oil and natural gas become trapped underground.
The oil is called crude oil (or petroleum, from Latin - rock oil),and has natural gas in it or in a pocket above it trapped by non-porous rock.
Drilling through the rock allows the oil and gas to escape to the surface.
Natural gas is mostly methane (CH4).Crude oil is a mixture of substances (mostly hydrocarbons).

ACID RAIN -IGCSE/GCSE CHEMISTRY NOTES

2) Fossil fuels are burnt on a huge scale - Acid Rain.
Coal (and to a lesser degree Oil and Natural Gas) contain sulphur.When they are burnt the sulfur oxidises (reacts with oxygen)to form sulfur dioxide gas.
sulfur + oxygen sulphur dioxide.
S(s) + O2(g) SO2(g)
Sulfur dioxide gas is acidic, poisonous, and smells of bad eggs.It is removed from the burnt waste gases and used in the contact process.If it gets into the atmosphere it reacts with water and oxygen in the airto form a dilute solution of sulfuric acid.This sulfuric acid is the main pollutant in acid rain.Natural rain is slightly acidic due to dissolved carbon dioxide.Natural rain has a pH of 5·5, acid rain has a pH of
4.The second most important pollutant in acid rain is nitric acid - see next page.
Acid rain kills trees.It runs into rivers and gathers in lakes.Eventually, lakes become too acidic, and plants and fish begin to die.Acid rain reacts with limestone and damages limestone buildings.
Powdered limestone or slaked lime can be added to soils or lakesto make them less acidic. It would be better if we could avoid or reducepollutant gas emissions in the first place.

AMPHOTERIC WATER /IGCSE /GCSE CHEMISTRY

In a sample of water,a very small number of water molecules will form ions.
water hydrogen ion + hydroxide ion.
H2O(l) H+(aq) + OH-(aq)
This ionisation is reversible (shown by the arrow).
The hydrogen ion is acidic. The hydroxide ion is alkaline.Water forms equal amounts of both ions, and so water is neutral.
Compare this reaction with neutralisation

titraction IGCSE/GCSE/ GCE-CHEMISTRY NOTES

In the example below, an acid and an alkali react to make sodium chloride.
hydrochloric acid + sodium hydroxide sodium chloride + water.
HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)
The burette is filled with hydrochloric acid.A known quantity of alkali (say 50 cm3 sodium hydroxide)is released from a pipette into the conical flask.The tap on the burette is turned opento allow the acid to be added drop by drop into the alkali.The alkali contains an indicator (phenolphthalein)which is pink in an alkali and colourless in an acid.
When enough acid has been added to neutralise the alkalithe indicator changes from pink to colourless.
The titration can be repeated using the same amounts of acid and alkalibut without the indicator.Pure salt crystals which are free from indicatorcan then be crystallised from the neutral solution.
Alternatively a pH meter can be used to find the end point.At neutralisation the pH is 7.
Indicators used for weak Acids or Alkalis.
Indicator
Titration
Colour in Acid
Colour in Alkali
Methyl Orange
Strong Acid+Weak Alkali
Red
Yellow
Phenolphthalein


Strong Alkali+Weak Acid
Colourless
Pink
Either Indicator can be used for a Strong Alkali + Strong Acid.
Universal indicator is not usually used for a titrationbecause it changes gradually giving different colours for a different pH.Methyl orange or phenolphthalein are usedbecause they give a sudden change in colour at neutralisationwhich makes it easier to see the end point of the titration

Ph - acids and bases - igcse/gcse /chemistry notes

pH is a measure of how acidic or how alkaline a solution in water is.The pH scale goes from 1 to 14,with 1 being very strongly acidic,and 14 being very strongly alkaline.A pH of 7 is neutral.
You can measure the pH of a solution using universal indicator.Just as litmus paper will be red for an acid and blue for an alkali,so universal indicator is a mixture of indicatorswhich will give a different colour for a different pH.
Any acid will have a pH of less than 7.Any alkali will have a pH of more than 7.
A strong acid (HCl or H2SO4 or HNO3 )will have a pH of 1 (red).
A weak acid will have a pH of 3 to 4 (orange).Examples of weak acids are ethanoic acid (vinegar),citric acid (lemon juice) and rain water.
Rain water has a natural pH of 5·5 (see carbonic acid).
Water and salts are neutral, pH 7 (green).
A weak alkali (ammonia) will have a pH of 11 to 12 (blue).
A strong alkali (Ca(OH)2or NaOH) will have a pH of 14 (purple).

STRONG AND WEEK ACIDS / IGCSE/GCSE / GCE- CHEMISTRY

Acids and Alkalis
Strong and Weak Acids - Strength and Concentration.
Acids and alkalis can be described as strong or weak.This does not mean the same as concentrated or dilute.
The strength of an acid or alkali depends on how ionised it is in water.
A strong acid or alkali is completely (100%) ionised. For hydrochloric acid
hydrogen chloride (in water) hydrogen ion + chloride ionHCl(aq) H+(aq) + Cl-(aq)
All of the hydrogen chloride moleculesbecome hydrogen ions and chloride ions in water(see examples for other strong acids).
For sodium hydroxide
sodium hydroxide (in water) sodium ion + hydroxide ionNaOH(aq)
Na+(aq) + OH-(aq)
Sodium hydroxide exists as ions both in water and in the solid.(see examples for other strong alkalis).

A weak acid or alkali is only partly (less than 100%) ionised.
For ethanoic acid
ethanoic acid (in water) hydrogen ion + ethanoic ion

CH3CO2H(aq) H+(aq) + CH3CO2-(aq)
Some of the ethanoic acid molecules become ions in waterbut most of them stay as molecules.The reaction is reversible (shown by the arrow).
For ammonia
ammonia + water ammonium ion + hydroxide ion

NH3(g) + H2O(l) NH4+(aq) + OH-(aq)
Some of the ammonia molecules become ions in waterbut most of them stay as molecules.

IONIC EQUATION - NEUTALISATION / IGCSE /GCSE/GSE /O-LEVEL CHEMISTRY NOTES

If we take the reaction betweenhydrochloric acid and sodium hydroxide from the previous page,
HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)
We can write it in terms of ions,since both the acid and the alkali form ions in water.
H+(aq) + Cl-(aq) + Na+(aq) + OH-(aq) Na+(aq) + Cl-(aq) + H2O(l)
Chloride ions (Cl-(aq)), and sodium ions (Na+(aq))appear on both sides of the equation.They are spectator ions, they are not changed by the reaction,and so they may be left out of the equation.
This leaves the equation
hydrogen ion + hydroxide ion waterH+(aq) + OH-(aq) H2O(l)
Compare this reaction with the ionisation of water.
This is the reaction that always occurswhen an acid + alkali salt + water.The hydrogen ion of the acid + the hydroxide ion of the alkalicombine to form water,leaving the metal from the alkali and the non-metal from the acidto form a salt solution.
How much acid is needed to neutralise an alkali?

alkalies / IGCSE /GCSE CHEMISTRY NOTES

Alkali is pronounced like alcohol,with 'lie' at the end instead of 'hol'.
An alkali is any substance which produces OH- ions in water.OH- ions are called hydroxide ions
A substance which will neutralise an acid,but does not dissolve in water, is called a base.For example,copper(II) oxide, iron(II) oxide and zinc carbonate are bases,they do not dissolve in Any base which dissolves in water is called an alkali.
Acids and Alkalis
Arrhenius, Lowry and Brønsted.
Arrhenius defined an acid as a substance which produces hydrogen ionswritten H+(aq) in water
Lowry and Brønsted defined an acid as a proton donorand a base as a proton acceptor.
If you look at the reaction below
hydrochloric acid + water hydroxonium ion + chloride ionHCl(aq) + H2O(l) H3O+(aq) + Cl-(aq)
According to Arrhenius, hydrochloric acid is an acidbecause it produces hydrogen ions in water(hydrogen ions in water become hydroxonium ions).
According to Lowry and Brønstedhydrochloric acid is an acid because it is a proton donor.A proton is a hydrogen ion.A proton donor is a substance which gives a hydrogen ion away.If you look at the reaction abovehydrochloric acid gives a hydrogen ion to water.
A base is a proton acceptor.This means that a base will gain a hydrogen ion.Water is a base when it is put with hydrochloric acidbecause water will gain a hydrogen ion to become H3O+.
acid + base acid + base

HCl(aq) + H2O(l) H3O+(aq) + Cl-(aq)
On the right side of the arrow, H3O+ is an acidbecause it can give away a hydrogen ion to become H2O.Cl- is a base because it can gain a hydrogen ion to become HCl.
Is water always a base according to Lowry and Brønsted?

activation energy/ collision theroy /igcse/gcse chemistry notes

Collision theory

it says that a chemical reaction can only occurbetween particles when they collide (hit each other).Particles may be atoms, ions or molecules

activation energy

There is a minimum amount of energywhich colliding particles need in order to react with each other.If the colliding particles have less than this minimum energythen they just bounce off each other and no reaction occurs.This minimum energy is called the activation energy.
The faster the particles are going, the more energy they have.Fast moving particles are more likely to react when they collide.You can make particles move more quickly by heating them up

HOW FAR, HOW FAST? ENERGETICS/igcse/ gcse / chemistry notes

HOW FAR, HOW FAST? ENERGETICS
Energy changes in reactionsIn every chemical reaction bonds must be broken in the reactants and new bonds must be formed in the products.
For example in the Haber Process nitrogen reacts with hydrogen to produce ammonia, NH3.
Initially bonds must be broken between the nitrogen atoms in the nitrogen molecule and the hydrogen atoms in the hydrogen molecule.


As energy must be put in to break the bonds and energy is released when the new bonds form this means that most chemical reactions involve energy changes.
When we do reactions in the laboratory the apparatus is usually open to the atmosphere so, if the reaction produces a gas, it is allowed to escape into the air. This means that the reactions are occurring under conditions of constant pressure.
In this case, when a gas is produced in a reaction, it must push back the surrounding air to escape from the apparatus. This uses up some energy. The energy changes we measure under these conditions take account of all these factors and are know as enthalpy changes. They are given the symbol .
The energy changes are mainly in the form of heat though other forms of energy can be involved (eg. burning of magnesium releases a lot of light energy as well as heat).

When energy is lost (or given out) in a reaction it is called an exothermic reaction. As energy is given out in an exothermic reaction the mixture gets hotter and the temperature rises. This can be seen in the reaction between the magnesium and oxygen, as the temperature of the air around the burning magnesium gets very hot.
When energy is taken in by a reaction it is called an endothermic reaction. As energy is taken in the reaction mixture gets colder and its temperature falls.
Sign convention and energy diagramsEnthalpy changes are usually measured in kilojoules (or joules). The symbol for kilojoules is kj.
In enthalpy changes that are exothermic the energy has been lost in the reaction. It is therefore given a negative sign.
In changes that are endothermic the energy has gained by the reaction mixture so it is given a positive sign.


In energy diagrams the energy changes are shown as follows:
This is an example of an energy diagram showing an exothermic reaction.


Standard enthalpy changesSo that scientists can compare data from experiments a set of standard conditions have been defined. These are
Pressure of 1 atmosphere (or 101kPa or 101,000Pa using SI units)
Temperature of 25oC (or 298K)
All solutions have a concentration of 1M (1 mol.dm-3)Standard enthalpy changes are ones that carried out under standard conditions. The symbol used for a standard enthalpy change is---
A so-called thermochemical equation summarises all the information for a reaction.
It shows

1. The amounts in moles of the reactants and products.2. The quantities of energy.3. The conditions of temperature and pressure.

An example is
If half the amounts are used, then the energy change is halved as well.
It is important to include the state symbols (s) for solids, (1) for liquids, (g) for gases and (aq) for aqueous solutions in all equations connected with energy changes.

carbon chemistry -igcse/ gcse/ gce olecvel notes

CARBON CHEMISTRY
Carbon chemistry is also called organic chemistry. This is because most carbon compounds are obtained from living organisms.
You will spend a lot of time on this topic and this is because there are more compounds of carbon than all the other elements put together! This is because carbon is the only element than can form strong bonds to other carbon atoms. There is no limit to the numbers of carbons that can be joined together, and they can join together in an unlimited number of different patterns.
Before we look at organising and naming all these different compounds, let us first find out where we find these compounds.
1. Fossil Fuels.
Then test your understanding of this topic using:
You should now be an expert on the sources of carbon compounds! Crude oil is however a complex mixture of up to one hundred compounds and must be purified before we can use it. This is done using a process known as fractional distillation.
2. Fractional Distillation.
Then read through the notes on the purification of crude oil. You can then try the test at:

work sheet / chemical bonding / igcse/ gcse/ gce olevel

Checkpoint.
Q1. Look at the ten compounds below, and decide whether the bonding will be ionic, covalent or metallic. You may need a periodic table to help you decide which elements are metals and non-metals.
a. potassium chloride (KCl)
b. carbon dioxide (CO2)
c. hydrogen chloride (HCl)
d.carbon dioxide (CO2)
e. iron oxide (Fe2O3)
f. magnesium iodide (MgI2)
g. fluorine gas (F2)
h. brass (a mixture of zinc and copper)
i. ammonia (NH3)
j. calcium carbonate (CaCO3)
Q2. Which of the compounds above are molecules?

Kinetic Theory igcse/gcse/ olevel chemistry notes

Kinetic Theory
Three states of matter – solids, liquids and gases.
Matter made up of very small particles in constant motion.
In solids the particles are packed very close together. They vibrate about fixed positions and have strong forces of attraction between them.
Solids :
have a high density
can not be compressed
do not flow
have a fixed shape
have a fixed size

In liquids the particles are close together but not as close as they are in solids. They can move around in any direction and are not fixed in position. The forces of attraction between them are still quite strong but, again, not as strong as in solids.
Liquids :
have a medium density
can not be compressed
can flow
have the shape of their container
have a fixed size

In gases the particles are very far apart with large distances between them. They move around very quickly in all directions and the forces of attraction between them are very, very weak.
Gases :
have a very low density
can be compressed
can flow
have the shape of their container
have the size of their container

You can change the state of a substance by heating or cooling it.

When the change is from a solid to a liquid it is called melting.
When the change is from a liquid to a gas it is called evaporating.
When the change is from a liquid to a solid it is called freezing.
When the change is from a gas to a liquid it is called condensing.
When a solid is heated it changes to a liquid and then a gas. A graph of temperature against time for this process would look like this:

KEY
A:
Over portion A the particles in the solid are vibrating more and more as they gain the heat energy so the temperature rises.
B:
Over portion B, as the solid changes into a liquid, all the energy is being used to overcome the strong forces of attraction between the particles and separate them so the temperature does not rise
C:
Over portion C the particles move around faster as they gain energy so the temperature rises.
D:
Over portion D the heat energy is all being used to separate the particles and overcome the forces of attraction between them so again the temperature does not rise as the liquid changes to a gas
E:
Over part E the temp rises as the particles move around faster

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