1. structures and bonding

2. structures and properties

3. how much?

4. rates of reactions

5. energy and reactions

6. electrolysis

7. acids, alkalis and salts

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  • Created by: Emma
  • Created on: 25-10-11 09:59

Structures and Bonding 1

  • atoms are made of protons, neutrons and electrons.
  • protons and electrons have equal and opposite electric charges.  Protons are positively charged, and electons are negatively charged.
  • atoms are arranged in the periodic table in order of their atomic number
  • neutrons have no electic charge.  They are neutral.
  • the electrons in an atom are arranged in energy levels or shells
  • atoms with the same number of electrons in their outer shell belong in the same group of the periodic table.
  • the number of electrons in the outer shell of an atom determines the way that the atom behaves in chemical reactions.
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Structures and Bonding 2

  • elements react to form compounds by gaining or losing electrons or by sharing electrons.
  • the electrons in Group 1 react with the elements in Group 7 because Group 1 elements can lose an electron to gain a full outer shell.  This electrons can be given to an atom grom Group 7, which then also gains a full outer shell.
  • ionic compounds are held together by strong forces between the oppositely charged ions.
  • other elements that can form ionic compounds include those in Group 2 and 6.
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Structures and Bonding 3

  • covalent bonds are formed when atoms share electrons.
  • many substances containing covalent bonds consist of molecules, but some have giant covalent structures.
  • the atoms (or ions) in metals are arranged in regular layers.
  • the positive ions in metals are held together by electrons from the outer shell of each metal atom.  These delocalised electrons are free to move throughout the metal lattice.
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Structures and Properties 1

  • it takes a lot of eneergy 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.
  • substances made up of simple molecules have low melting points and boiling points.
  • the forces between simple molecules are small.  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 containing simple molecules do not conduct electricity.
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Structures and Properties 2

  • some covalently bonded substances contain giant structures.
  • these substances have high melting points and boiling points.
  • the giant structure of graphite contains layers of aoms 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.
  • 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 heat and electricity well.
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How much? 1

  • the relative mass of protons and neutrons is 1.
  • the mass number of an atom tells you the total number of protons and neutrons in its nucleus.
  • isotopes are atoms of the same element with different numbers of neutrons.
  • we compare the masses of atoms by measuring them relative to atoms of carbon-12.
  • we work out the relataive formula mass of a compound from the relative masses of the elements in it.
  • one mole of any substance always contains the same number of particles.
  • the relative atomic masses of the elements in a compound can be used to work out its percentage composition.
  • we can calculate emphirical formulae given the masses or percentage composition of elements present.
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How much? 2

  • chemical equations tell us the number of moles of substances in the chemical reaction.
  • we can use chemical equations to calculate the masses of reactants and products in a chemical reaction from the masses of one mole of each of the substances involved in the reaction.
  • the yield of a chemical reaction describes how much productis made.
  • the percentage yield of a chemical reaction tells us how much product is made compared with the maximum amount that would be made (100%).
  • factors affecting the yield of a chemical reaction include product being left behind in the apparatus and difficulty separating the products from the reaction mixture.
  • it is important to maximise atom economy to conserve resources and reduce pollution.
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How Much? 3

  • In a reverse reaction the products of the reaction can react to make the original reactants.
  • in a closed system the rate of the forward and backwards (reverse) reactions are equal at equilibrium.
  • changing the reaction conditions can change the amounts of products and reactants in a reaction mixture.
  • ammonia is an important chemical for making ither 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 seasonable yield of ammlonia as quickly as possible.
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Rates of Reactions 1

  • knowing and controlling the rate of chemical reactions is important in living cels, 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.
  • the minimum amount 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 reactant is increased.
  • reactions happen more quickly as the temperature increases.
  • a 10 degrees C increase in temperature at room temperature roughly doubles the rate of a reaction.
  • the rate of a chemical reaction increases with temperature because the particles collide more often and they have more energy.
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Rates of Reactions 2

  • 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 collide more often, increasing the rate of reaction.
  • a catalyst speeds up the rate of a chemical reaction.
  • a catalyst is not used up during a chemical reaction.
  • catalysts are generally quite expensive because they are made of precious metals.
  • catalysts lower the activation energy needed for a chemical reaction to occur.
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Energy and Reactions 1

  • energy may be transferred to or from the reacting substances in a chemical reaction.
  • a reaction where energy is transferred from the reacting substances into its surroundings is called an exothermic reaction.
  • a reaction where energy is transferred to the reacting substances from its surroundings is called an endothermic reactions.
  • in reverse 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 amount of products formed at equilibrium by changing the temperature at which we carry out a reverse reaction.
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Energy and Reactions 2

  • the Haber process uses a pressure of around 200 to 350 atmospheres to increase the amount of ammonia produced.
  • although higher pressures would produce more ammonia, they would make the chemical plant too expensive to build.
  • a temperature of about 450 degrees C is used for the reaction.  Although lower temperatures would increase the amount of ammonia at equilibrium, the ammonia would be produced too slowly.
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Electrolysis 1

  • 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).
  • in electrolysis, the ions move towards the oppositely charged electrodes.
  • at the electrodes, negative ions are oxidised while 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 2

  • 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 mke bleach, paper and soap.
  • copper extracted from its ore contains impurities such as gold and silver.
  • copper is purified by electrolysis to remove these impurities.
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Acids, Alkalis and Salts 1

  • acids are substances which produce H+ ions when we add them to water.
  • bases are substances that will neutralise acids.
  • an alkali is a souble 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.
  • when we react an acid with a base a neutralisation reaction occurs.
  • the reaction between an acid and a base produces a salt and water.
  • salts can also be made by reacting a metal with an acid.  This reaction prduces hydrogen gas as well as a salt.
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Acids, Alkalis and Salts 2

  • 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.

Mole: The relative formula mass of a substance in grams.

Precipitate: A solid material produced from a solution.

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