Additional Science-Chemistry

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  • Created by: Marvin
  • Created on: 20-11-10 23:16

Structures and Bonding

ATOMIC STRUCTURE
Nucleus contains (Nucleon) protons (+1) and neutrons (0)
Electrons (-1) orbit the nucleus
Proton no.=Electron no.=Atomic no.
Atomic Mass no.=Proton+Neutron
Arranged in order of atomic no.
CHEMICAL BONDING
Elements react to form compounds by gaining or losing electrons or by sharing e-
Sharing electrons - called covalent bonding (non-metal & non-metal react)
Transferring electrons - called ionic bonding (metal & non-metal react)
Elements in Group 1 react with the elements in Group 7 because Group 1 elements can lose an electron to grain a full outer shell. This electron can be given to an atom from Group 7, which then also gains a full outershell.
THE ARRANGEMENT OF ELECTRONS IN ATOMS

Electrons in atoms are arranged in energy levels or shells.
Due to electron negative charge it is attracted to the postively charge nucleus
Atoms with the same no. of electrons in outershell belong to the same group
No. of e- in outershell of atom shows the way that atoms behave in chemical reactions 


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Structure and Bonding

IONIC BONDING
Metals and Non-metals react.
Ionic bond is the name of the electrostatic force of attraction.
Ionic bonds between particles results in arrangement of ion that we call giant structure
Reactions can be presented by dot and cross diagrams.
Other elements that can form ionic compounds include those in group 2 & group 6
COVALENT BONDING
Nonmetal and Non-metal (sharing electrons)
Giant structures- Diamond- each carbon atom forms 4 covalent bonds
Silicon dioxide and graphite are other examples of Giant structures
BONDING IN METALS
Lattice of metal atoms arranged in regular layers.
Outer electrons form a 'sea' of free electrons surrounding positively charged ions.
sea of electrons are called delocalised electrons which help explain properties

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Structures and Properties

IONIC COMPOUNDS
It takes a lot of energy to break the bonds which hold a giant ionic lattice together. So ionic compounds have very high melting points - they are all solids at room temperature
Ionic compounds will conduct electricity when we melt them or dissolve them in water because their ions can then move freely.
SIMPLE MOLECULES
Substances made up of simple molecules have low melting points and boiling points
The forces between simple molecules are weak. These weak intermolecular forces explain their low melting points and boiling points.
Simple molecules have no overall charge, so they cannot carry electrical charge. Therefore substances cointaining simple molecules do not conduct electricity.

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Structures and Properties

GIANT COVALENT SUBSTANCES
Some covalently bonded substances contain giant structures.
These substances have high melting points and boiling points.
The giant structure of graphite contains layers of atoms that can slide over each other which make graphite slippery. The atoms in diamond have a different structure and cannot slide like this - so diamond is a very hard substance.
Graphite can conduct electricity because of the delocalised electrons along its layers.
Carbon molecules having different structures are called fullerenes
GIANT METALLIC STRUCTURES
We can bend and shape metals because the layers of atoms (or ions) in a metal can slide over each other.
Delocalised electrons in metals allow them to conduct electricity and heat well.
NANOSCIENCE AND NANOTECHNOLOGY
Nano means one thousand millionth - tiny things
Used for health etc.


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How much?

MASS NUMBERS
Relative mass of protons and neutrons is 1
The mass number of an atom tells you the total no. of protons and neutrons in the nucleus
Isotopes are atoms of the same element with different numbers of neutrons.
MASSES OF ATOMS AND MOLES
We compare the masses of atoms by measuring them relative to atoms of carbon-12
We work out the relative formula mass of a compound from the relative atomic masses of the elements in it.
One mole of any substance always contains the same no. of particles
PERCENTAGES AND FORMULAE
The relative atomic masses of the elements in a compound can be used to work out its percentage composition.
We can calculate empirical formulae given the masses or percentage composition of elements present.
EQUATIONS AND CALCULATIONS
Chemical reactions tell us the number of moles of substances in the chemical reaction
We can use chemiscal equations to calculate the masses of reactants and products in a chemical reaction from the masses of one moles at each of the substances involved in the reaction

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How much?

MAKING AS MUCH AS WE WANT
The yield of a chemical reaction describes how much product is made
The percentage yield of a chemical reaction tells us how much product is made compared with the maximum that could be made (100%)
Factors affecting the yield of a chemical reaction include product being left behind in the apparaturs and difficulty separating the products from the reaction mixture
It is important to maximise atom economy to conserve resources and reduce pollution
REVERSIBLE REACTIONS
In a reversible reaction the products of the reaction can react to make the original reactants
In a closed system the rate of the forward and backward (reverse) reactions are equal at equilibrium
Changing the reaction conditions can change the amounts of producs and reactants in a reaction mixture.
MAKING AMMONIA-THE HABER PROCESS
Ammonia is an important chemical for making other chemicals, including fertilisers
Ammonia is made from nitrogen and hydrogen in the Haber process
We carry out the Haber process under conditions which are chosen to give a reasonable yield of ammonia as quick as possible.
Any unused nitrogen and hydrogen are recycled in the Haber process

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Rates of Reaction

HOW FAST?
Knowing and controlling the rate of chemical reactions is important in living cells, in the laboratory and in industry.
We can measure the rate of a chemical reaction by following the rate at which reactants are used up. Alternatively, we can measure the rate at which products are made.
COLLISION THEORY
When reacting particles collide.
The minimum amound of energy that particles must have in order to react is called the activation energy
The rate of a chemical reaction increases if the surface area of any solid reactants is increased.

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Rates of Reaction

THE EFFECT OF TEMPERATURE
Reactions happen more quickly as temperature rises
A 10 degree celcius increase in temperature ar room temperature roughly doubles the rate of reaction
The rate of a chemical reaction increases with temperature because the particles collide more often and they have more energy.
THE EFFECT OF CONCENTRATION
Increasing the concentration of reactants increases the frequency of collisions between particles, increasing the rate of reaction
Increasing the pressure of reacting gases results in particles colliding more often increasing the rate of reaction
THE EFFECT OF CATALYSTS
A catalyst spreeds up the rate of a chemical reaction
A catalyst is not used up during a chemical reaction

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Energy and Reactions

EXOTHERMIC AND ENDOTHERMIC REACTIONS
Energy may be transferred of from the reacting substances in a chemical reaction
A reaction where energy is transferred from the reacting substances is called an exothermic reaction
A reaction where energy is transferred to the reacting substances is called an endothermic reaction
ENERGY AND REVERSIBLE REACTIONS
In reversible reactions, one reaction is exothermic and the other is endothermic
In any reversible reaction, the amount of energy released when the reaction goes in one direction is exactly equal to the energy absorbed when the reaction goes in the opposite direction.
We can change the amound of products formed at equilibrium by changing the temperature at which we carry out a reversible reaction

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Energy and Reactions

MORE ABOUT THE HABER PROCESS
The haber process uses a pressure of around 200 to 350 atmospheres to increase the amount of ammonia produced
Although higher pressure would produce more ammonia, they would make the chemical plant too expensive to build
A temperature of about 450 degrees celcius is used for the reaction. Although lower temperatures would increase the amount of ammonnia at equilibrium, the ammonia would be produced too slowly

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Electrolysis

ELECTROLYSIS - THE BASICS
Electrolysis involves splitting up a substance using electricity
Ionic substances can be electrolysed when they are molten or in solution
In electrolysis positive ions move to the negative electrode (cathode) and negative ions move to the positive electrode (anode)
CHANGES AT THE ELECTRODES
In electrolysis, the ions move towards the oppositely charged electrodes
At the electrodes, negative ions are oxidised whilst positive ions are reduced
Reactions where reduction and oxidation happen are called redox reactions
When electrolysis happens in water, the less reactive element is usually produced at an electrode.

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Electrolysis

ELECTROLYSING BRINE
When we electrolyse brine we get three products - chlorine gas, hydrogen gas and sodium hydroxide solution
Chlorine is used to kill microbes in drinking water and swimming pools, and to make hydrochloric acid, disinfectants, bleach and plastics
Hydrogen is used to make margarine and hydrochloric acid.
Sodium hydroxide is used to make bleach, paper and soap
PURIFYING COPPER
Copper is extracted from its ore contains impurities such as gold and silver
Copper is purifies by electrolysis to remove these impurities

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Acids, Alkalis and Salts

ACIDS AND ALKALIS
Acids are substances which produce H+ ions when we add them to water
Bases are substances that will neutralise acids
An alkali is a soluble base. Alkalis produce OH- ions when we add them to water
We use the pH scale to show how acidic or alkaline a solution is
MAKING SALTS FROM METALS OR BASES
When we react an acid with a base a neutralisation reaction occurs
The reaction between an acid anda  base produces a salt and water
Salts can also be made by reacting a metal with an acid. This reaction also produces hydrogen gas as well as a salt

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Acids, Alkalis and Salts

MAKING SALTS FROM SOLUTIONS
An indicator is needed when we produce a salt by reacting an alkali with an acid to make a soluble salt
Insoluble salts can be made by reacting two solutions to produce a precipitate
Precipitation is an important way of removing some substances from wastewater

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