Additional Science Chemistry

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Atomic Structure.

ATOMS consist of ELECTRONS SURROUNDING A NUCLEUS that contains PROTONS and NEUTRONS.

NEUTRONS ARE NEUTRAL, BUT PROTONS AND ELECTRONS ARE ELECTRICALLY CHARGED.

The number of PROTONS in an atom is called its ATOMIC NUMBER.

ELECTRONS are arranged in energy levels or shells.

Different ENERGY LEVELS can hold a different number of ELECTRONS.

There is a link between the position of an element in the periodic table and its ELECTRONIC STRUCTURE.

NUMBER OF PROTONS IS ALWAYS EQUAL TO THE NUMBER OF ELECTRONS.

 

 

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The Structure of an atom diagram.

(http://www.bbc.co.uk/schools/gcsebitesize/science/images/19_1_atoms__isotopes.gif)

At the centre of an atom is a nucleus containing protons and neutrons. Electrons are arranged around the nucleus in energy levels or shells.

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Ionic Bonding.

IONS ARE ELECTRICALLY CHARGED PARTICLES FORMED WHEN ATOMS LOSE OR GAIN ELECTRONS. They have the SAME ELCETRONIC STRUCTURES as NOBLE GASES.

METAL ATOMS FORM POSITIVE IONS.

NON-METAL ATOMS FOR NEGATIVE IONS.

The STRONG ELECTROSTATIC FORCES OF ATTRACTION between OPPOSITELY CHARGED IONS are called IONIC BONDS.

IONIC COMPOUNDS HAVE HIGH MELTING AND BOILING POINTS.

AN IONIC BOND INVOLVES MOVING ELECTRONS BETWEEN ATOMS.

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Magnesium oxide, MgO dot and cross diagram.

Diagram of bonding in magnesium oxide. A magnesium ion (2,8)2+ gives two electrons to an oxide ion (2,8)2-. Both ions have full highest energy levels (http://www.bbc.co.uk/schools/gcsebitesize/science/images/diag_mag_oxide.gif)

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Covalent Bonding.

A COVELENT BOND is a STRONG BOND BETWEEN NON-METAL ATOMS. It consists of a SHARED PAIR OF ELECTRONS.  A covelent bond can be REPRESENTED BY A STRAIGHT LINE OR DOT-AND-CROSS DIAGRAM.

HYDROGEN AND CHLORINE CAN EACH FORM 1 COVALENT BOND.

OXYGEN 2 BONDS, NITROGEN 3 BONDS, WHILE CARBON CAN FORM FOUR BONDS.

Molecules can have a DOUBLE COVALENT BOND - meaning they have TWO SHARED PAIRS OF ELECTRONS.

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Models for covalent bonds.

Element - Methane

Formula = CH4H - C - H in a row, line from above the C to an H, line from below the C to an H  (http://www.bbc.co.uk/schools/gcsebitesize/science/images/methane_chem_struc.gif)five atoms joined (http://www.bbc.co.uk/schools/gcsebitesize/science/images/methane_model.gif)

 

 

Element - Water

Formula H2OH - O - H (http://www.bbc.co.uk/schools/gcsebitesize/science/images/water_chem_struc.gif)three atoms joined (http://www.bbc.co.uk/schools/gcsebitesize/science/images/water_model.gif)

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Different substances and their properties.

Ionic substances form giant ionic lattices containing oppositely charged ions. They have high melting and boiling points, and conduct electricity when melted or dissolved in water.

Simple molecular substances consist of molecules in which the atoms are joined by strong covalent bonds. Their molecules are held together by weak forces, so these substances have low melting and boiling points. They do not conduct electricity.

Graphite and diamond have different properties because they have different structures. Graphite conducts heat and electricity well because it also has free electrons.

Metals form giant structures containing free electrons, making them good conductors of heat and electricity.

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Chemical calculations.

The concentration of a solution is measured in MOLES PER CUBIC DECIMETRE.    (mol/dm3)

Percentage yield is the ratio of actual mass of products obtained compared to the maximum theoretical mass. Percentage yield = 100 x actual mass divided by theoretical mass (http://www.bbc.co.uk/schools/gcsebitesize/science/images/add_aqa_equa_percyield.gif)

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Rates of reaction.

The rate of a reaction can be measured by the RATE AT WHICH A REACTANT IS USED UP, OR THE RATE AT WHICH A PRODUCT IS FORMED.

Temperature, concentration, pressure of reacting gases, surface area of reacting solids, and the use of catalysts, are all FACTORS which affect the RATE OF A REACTION.

Chemical reactions can only happen if REACTANT PARTICLES COLLIDE WITH ENOUGH ENERGY. The more frequently particles collide, and the greater the proportion of collisions with enough energy, the greater the rate of reaction.

There are two ways to measure the rate of a reaction:

1. Measure the rate at which a REACTANT is USED UP.

2. Measure the rate at which a PRODUCT  is FORMED.

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Factors affecting the rate.

Rate of reaction = AMOUNT OF REACTANT USED OR AMOUNT OF PRODUCT FORMED / TIME TAKEN

The rate of reaction is equal to the amount of reactant used divided by the time taken. Or it can expressed as the amount of product formed divided by the time taken (http://www.bbc.co.uk/schools/gcsebitesize/science/images/add_aqa_equa_ratereac.gif)

How to INCREASE THE RATE OF A REACTION.

  • TEMPERATURE INCREASED.Graph showing rates of reaction under changing conditions. At a lower temperature, lower concentration, or with larger pieces, the rate of reaction is slower than at higher temperatures, higher concentrations, or with smaller pieces (http://www.bbc.co.uk/schools/gcsebitesize/science/images/gcsechem_19.gif)
  • CONCENTRATION of a DISSOLVED REACTANT is increased.
  • PRESSURE of a reacting GAS is INCREASED.
  • Solid reactants are broken into SMALLER PIECES.
  • A CATALYST is used.
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Collisions and reactions.

Why changing the conditions of a reaction changes its rate.

Changing concentration or pressure.

If the concentration of a dissolved reactant is increased, or the pressure of a reacting gas is increased:

  • There are more reactant particles in the same volume.
  • Greater chance of the particles colliding.
  • The rate of reaction increases.

Changing particle size.

If a solid reactant is broken into small pieces or ground into a powder:

  • Its SURFACE AREA is INCREASED.
  • RATE OF REACTION INCREASES.
  • More particles are exposed to the other reactant
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Collisons and reactions CONTINUED.

Using a catalyst

A catalyst increases the rate of reaction by LOWERING the ACTIVATION ENERGY NEEDED. With a catalyst, more collisions result in a reaction, so the rate of reaction increases. DIFFERENT REACTIONS need DIFFERENT CATALYSTS.

Catalysts reduce costs.

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Energy changes.

EXOTHERMIC reactions transfer energy to the surroundings.

ENDOTHERMIC reactions take in energy from the surroundings.

REVERSIBLE REACTIONS are where the PRODUCTS can react to REMAKE THE ORIGINAL REACTANTS. If the forward reaction is EXOTHERMIC, the REVERSE REACTION is ENDOTHERMIC.

Some examples of exothermic reactions are:

  • BURNING.
  • NEUTRALISATION reactions between acids and alkalis.
  • The reaction between water and calcium oxide.

Some examples of endothermic reactions are:

  • ELECTROLYSIS.
  • The reaction between ethanoic acid and sodium carbonate.
  • The THERMAL DECOMPOSITION of calcium carbonate in a blast furnace.
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The Haber process for making ammonia.

Hydrogen is extracted from the reaction between methane and steam. Nitrogen is extracted from the combustion of hydrogen in air. Hydrogen and nitrogen are combined at a pressure of 200 atmospheres and a temperature of 450°C, with iron as a catalyst, to produce ammonia (http://www.bbc.co.uk/schools/gcsebitesize/science/images/gcsechem_41.gif)

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Reversible reactions.

When REVERSIBLE REACTIONS REACH EQUILIBRIUM THE FORWARD AND REVERSE REACTIONS ARE STILL HAPPENING BUT AT THE SAME RATE, so the CONCENTRATIONS OF REACTANTS AND PRODUCTS DO NOT CHANGE.

THE BALANCE POINT CAN BE AFFECTED BY TEMPERATURE, AND ALSO BY PRESSURE FOR GASSES IN EQUILIBRIUM.

If the forward reaction is EXOTHERMIC and the temperature is increased, the yield of products is decreased. If the temperature is DECREASED, the yield of products is INCREASED.

If the forward reaction is ENDOTHERMIC and the TEMPERATURE is INCREASED, the yield of products is INCREASED. If the TEMPERATURE is DECREASED, the yield of products is DECREASED.

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Effect on ammonia when pressure changes.

Changing the pressure and temperature in an equilibrium reaction (http://www.bbc.co.uk/schools/gcsebitesize/science/images/gcsechem_21.gif)

For any given temperature the YIELD OF AMMONIA INCREASES AS THE PRESSURE INCREASES. For any given pressure, the YIELD GOES DOWN AS THE TEMPERATURE INCREASES. This is because the forward reaction is EXOTHERMIC.

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Electrolysis.

ELECTROLYSIS is the process by which IONIC SUBSTANCES ARE BROKEN DOWN INTO SIMPLER SUBSTANCES USING ELECTRICITY. During electrolysis, metals and gases may form at the electrodes.

IONIC SUBSTANCES FORM WHEN A METAL REACTS WITH A NON-METAL. THEY CONTAIN CHARGED PARTICLES CALLED IONS.

IONIC SUBSTANCES ARE BROKEN DOWN BY ELECTRICITY.

FOR ELECTROLYSIS TO WORK, THE IONS MUST BE FREE TO MOVE. IONS ARE FREE TO MOVE WHEN AN IONIC SUBSTANCE IS DISSOLVED IN WATER OR MOLTEN (MELTED).

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Electrolysis 2.

What happens in electrolysis. Positively charged ions move to the negatively charged electrode; negatively charged ions move to the positive electrode (http://www.bbc.co.uk/schools/gcsebitesize/science/images/gcsechem_74.gif)What happens during electrolysis:

  • Positively charged ions move to the negative electrode during electrolysis. They RECEIVE electrons and are REDUCED.
  • Negatively charged ions move to the positive electrode during electrolysis. They LOSE electrons and are OXIDISED.
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Electrolysis 3.

At the negative electrode, positively charged ions GAIN ELECTRONS. This is REDUCTION, and you say that the ions have been reduced.

At the positive electrode, negatively charged ions LOSE ELECTRONS. This is OXIDATION, and you say that the ions have been oxidised.

Copper is a good conductor of electricity, an is used to make electrical wiring and components. The extraction of ore is done by reduction with carbon. However, the copper produced is not pure enough for use as a conductor, so it is purified using electrolysis.

Brine is concentrated sodium chloride solution. If an electric current is passed through it, hrdrogen gas forms at the negative electrode and chlorine gas forms at the positive electrode. A solution of sodium hydroxide forms.

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Electrolysis 4.

electricity is passed through sodium chloride solution (brine). The reaction at the cathode (-) forms hydrogen gas and sodium hydroxide solution. At the anode (+) chlorine gas is formed.  (http://www.bbc.co.uk/schools/gcsebitesize/science/images/52_substances_from_salt.gif)

HYDROGEN, CHLORINE AND SODIUM HYDROXIDE - have important uses in the chemical industry:

HYDROGEN:

  • Making ammonia.
  • making margarine.

Chlorine:

  • Killing bacteria in swimming pools.
  • Making bleach.
  • Making disinfectants.

SODIUM HYDROXIDE:

  • Making soap.
  • Making paper.
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Electrolysis 5 - Higher.

Electrons are shown as e-. A half-eqaution is balanced by adding, or taking away, a number of electrons equal to the total number of charges on the ions in the equation.

At the NEGATIVE ELECTRODE positive ions GAIN ELECTRONS, so are REDUCED.

At the POSITIVE ELECTRODE negative ions or neutral atoms LOSE ELECTRONS, so are OXIDISED.

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Acids, bases and salts.

ACIDS have a pH of LESS THAN 7. The stronger the acid the lower the pH number.

BASES have a pH of MORE THAN 7. When bases are DISSOLVED IN WATER, THEY ARE KNOWN AS ALKALIS. The stronger the alkali, the higher the pH number.

SALTS are made when an ACID REACTS WITH A BASE, CARBONATE OR METAL. The name of the salt formed depends on the metal in the base and the acid used. For example, salts made using hydrochloric acid are called chlorides.

Neutral solutions have a pH of 7.

When acids react with bases, a salt and water are made. This reaction is called neutralisation. In general:

acid + metal oxidesalt + water.

acid + metal hydroxidesalt + water.

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Acids, alkalis and neutralisation - higher.

When ATOMS or groups of atoms LOSE OR GAIN ELECTRONS, charged particles called IONS are formed. Ions can be either positvely or negatively charged.

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Comments

Immi

it ws helpfull but I already knw all these

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