Chemistry unit 2

C2
1. Structure and bonding
  1. Chemical bonding
    1. Elements react to form compounds by gaining or losing electrons, or by sharing electrons
    2. Atoms of metals in Group 1 combine with atoms of non-metals in Group 7 by transferring electrons to form ions that have the electronic structures of noble gases.
  2. Ionic bonding
    1. Ionic compounds are held together by strong forces between the oppositely charged ions. this is called ionic bonding
    2. The ions form a giant structure or lattice. The strong forces of attraction act throughout the lattice.
    3. We can represent atoms and ions using dot and cross diagrams
    4. The ratio of ions in the structure of an ionic compound depends on the charges on the ions. for example, calcium ions are Ca^2+ and chloride ions are Cl^-, so calcium chloride contains twice as many chloride ions as calcium ions and its formula is CaCl2.
    5. Between metal and non-metal.
  3. Formulae of ionic compounds
    1. The charges on the ions on an ionic compound always cancel each other out.
    2. The formula of an ionic compound shows the ratio of ions present in the compound.
    3. Sodium chloride -> one sodium (Na^+) ion for every chlorine (Cl^-) ion -> NaCl
    4. Sometimes we need brackets to show the ratio of ions in a compound, e.g. magnesium hydroxide, Mg(OH)2
    5. Do not forget that ionic compounds are giant structures. The formula of the ionic compound is the simplest ratio of the ions on the compound and does not represent the molecule
  4. Covalent bonding
    1. Non-metals only.
    2. A covalent bond is formed when two atoms share a pair of electrons.
    3. The number of covalent bonds an atoms forms depends on the number of electrons it needs to achieve a stable electronic structure.
    4. Many substances containing covalent bonds consist of simple molecules, but some have giant covalent structures.
    5. Covalent bonds join together to form molecules. You should only use the word 'molecule' when describing substances that are covalently bonded.
  5. Metals
    1. The atoms in metals are closely packed together and arranged in regular layer.
    2. The electrons in the highest energy level (outer shell) are delocalised. The strong elastic forces between these electrons and the positively charged metal ions hold the metal together.
2. How Much?
  1. The mass of atoms
    1. The relative atomic mass of protons and neutrons is 1, and electrons, very small (1/2000)
    2. The atomic number of an atom is its number of protons (which equals its number of electrons)
    3. The mass number of an atom is the total number of protons and neutrons in its nucleus
    4. Isotopes are atoms of the same element with different numbers of neutrons.
  2. Masses of atoms and moles
    1. One mole of any substance is its relative atomic mass in grams.
    2. we work out the relative formula mass of a compound by adding up the relative atomic masses of the elements in it.
    3. We use relative atomic masses to compare the masses of atoms.
    4. The relative atomic mass of an element is an average value for the isotopes of an element.
      1. Ar
        The mass number of the element.
    5. Relative formula mass (Mr).
  3. Percentages and formulae
    1. WORKED EXAMPLE: What is the empirical formula of the hydrocarbon that contains 80% carbon?
    2. The relative atomic masses of the elements in a compound and it formula can be used to work out its percentage composition.
    3. We can calculate empirical formulae given the masses or percentage composition of elements present.
  4. Equations and calculations
    1. Balanced symbol equations tell us the number of moles of substances involved in a chemical reaction.
    2. We can use balanced symbol equations to calculate the masses of reactants and products in a chemical reaction.
  5. The yield of a chemical reaction
    1. The yield of a chemical reaction describes how much product is made.
    2. percentage yield= (amount of product collected/maximum amount of product possible) x100%
    3. The percentage yield of a chemical reaction tells us how much product is made compared with the maximum amount that could be made.
    4. It is important to maximise yield and minimise energy wasted to conserve the Earth's limited resources and reduce pollution.
    5. The products often don't go to completion, other reactions may happen and some product may be lost when it is separated or collected from the apparatus.
  6. Reversible reactions
    1. In a reversible reaction the products of the reaction can react to make the original reactants.
  7. Instrumental analysis
    1. Compounds in a mixture can be separated using a gas specrometry
      1. Once separated, compounds can be identified using a mass sectrometer
    2. Modern instrumental techniques provide fast, accurate and sensitive ways of analysing chemical substances.
    3. The mass spectrometer can be used to find the relative molecular mass of a compound from its molecular ion peak
  8. Analysing substances
    1. Chemical analysis is used to identify food additives
    2. Paper chromatography can detect and identify artificial colours.
3. Rates and Energy
  1. How fast?
    1. The rate of reaction found by measuring reactants used up over time or products made over time.
    2. The gradient or slope of the line on a graph of amount of reactant or product against time tells us the rate of reaction at that time. The steeper the gradient, the faster the reaction.
  2. Collision theory and surface area
    1. Particles must collide with a certain amount of energy before they can react.
    2. The minimum amount of energy that particles must have in order to react is called the activation energy.
    3. The rate of a chemical reaction increases if the surface area of any solid reactants is increased. This increases the frequency of collisions between reacting particles
  3. The effect of temperature
    1. Reactions happen more quickly as the temperature increases.
    2. Increasing the temperature increases the rate of reaction because particles collide more frequently and more energetically.
    3. At a higher temperature more of the collisions result in a reaction because a higher proportion of particles have energy greater than the activation energy.
  4. The effect of concentration or pressure
    1. Increasing the concentration of reactants in solutions increases the frequency of collisions between particles, and so increases the rate of reaction.
    2. Increasing the pressure of reacting gases also increases the frequency of collisions and so increases the rate of reaction.
  5. The effect of catalysts
    1. A catalyst speeds up the rate of a chemical reaction
    2. A catalyst is not used up during a chemical reaction.
    3. Different catalysts are needed for different reactions.
  6. Catalysts in action
    1. Catalysts are used in industry to increase the rate of reactions and reduce energy costs.
    2. Traditional catalysts are often transitional metals or their compounds.
    3. Enzymes are catalysts.
    4. Modern catalysts are being developed in industry which result in less waste and are safer for the environment.
  7. Exothermic and endothermic reactions
    1. Energy may be transferred to or from the reacting substances in a reaction.
    2. A reaction in which energy is transferred from the reacting substances to their surroundings is called an exothermic reaction.
    3. A reaction in which energy is transferred to the reacting substances from their surroundings is called an endothermic reaction.
  8. Energy and reversible reactions
    1. In reversible reactions, the reaction in one direction is exothermic and in the other direction it is endothermic.
    2. 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.
  9. Using energy transfers from reactions.
    1. Exothermic changes can be used in hand warmers and self-heating cans
    2. Endothermic changes can be used in instant cold packs for sports injuries.
4. Structure and Properties
  1. Giant ionic structures
    1. Ionic compounds have high melting points and they are all solids at room temperature
    2. Ionic compounds will conduct electricity when we melt them or dissolve them in water. Their ions can then move freely and can carry charge through the liquid.
  2. Simple molecules
    1. Substances made up of simple molecules have low melting and boiling points.
    2. Simple molecules have no overall charge, so they cannot carry electrical charge and do not conduct electricity.
    3. The weak intermolecular forces between simple forces between simple molecules are why substance made of simple molecules have low meltin points and boiling points
  3. Giant covalent structures
    1. Covalently bonded substance with giant structures have very high melting points.
    2. Diamond is a form of carbon whose atoms each form four covalent bond.
    3. Graphite is another form of carbon where the carbon atoms form layers that can slide over each other.
      1. It can conduct electricity as it has delocalised electrons.
      2. Weak intermolecular forces.
    4. Carbon also exists as fullerences.
      1. Fullerences are large molecules formed from hexagonal ringsof carbon atoms. The rings join together to form cage-like shapes with different numbers of carbon atoms, some of which are nano-sized.
  4. Giant metallic structures.
    1. Alloys are mixtures of metals and are harder than pure metals because the layers in the structure are distorted.
    2. When we bend and shape metals the layers of atoms in the giant metallic structure slide over each other.
    3. If a shape-memory alloy is deformed, it can return to its original shape on heating.
    4. Delocalised electrons in metals enable metals to conduct heat and electricity well.
  5. The properties of polymers
    1. The properties of polymers depend on the monomers used to make them.
    2. Changing reaction conditions can also change the properties of the polymer that is produced.
    3. Thermosoftening polymers soften or melt easily when heated.
      1. Forces between polymer chains are weak and, when heated the intermolecular forces are broken.
    4. Thermosetting polymers do not soften or melt when heated.
      1. They have strong covalent bonds which form cross-links between their polymer chains.
  6. Nanoscience
    1. Nanoscience is the study of small particles that are between 1 and 100 nanometres in size.
    2. Nanoparticles behave differently from the bulk materials they are made from.
    3. Nanotechnology uses nanoparticles as highly selective sensors, very efficient catalysts, new coatings, new cosmetics such as sun screens and deodrants, and to give construction materials special properties.
    4. If nanoparticles are used more and more there will be greater risk of them finding their way into the air and our bodies. This could have unpredictable consequences on our health and the environment. More research needs to be done to find out their effects.
5. Salts and Electrolysis
  1. Acids and alkalis
    1. When acids are added to water they produce hydrogen ions, H^+ (aq), in the solution.
    2. Bases are substances that will neutralise acids.
    3. Alkalis dissolve in water to give hydroxide ions, OH^- (aq), in the solutions
    4. The pH scale shows how acidic or alkaline a solution is.
  2. Making salts from metals or bases
    1. Acid + metal -> a salt + hydrogen
    2. Acid + base -> a salt + water
    3. When an acid reacts with a base a neutralisation reaction takes place and produces a salt and water.
    4. Some salts can be made by the reaction of a metal with an acid. this reaction produces hydrogen gas as well as a salt.
    5. Salts can be crystallised from solutions by evaporating off water.
  3. Making salts from solutions
    1. When a soluble salt is made from and alkali and an acid, an indicator can be used to show when the reaction is complete, this is because it will be neutral.
    2. Insoluble salts can be made by reacting two solutions to produce a precipitate.
    3. Precipitation is an important way of removing some metal ions from industrial waste water.
    4. Acid + alkali -> salt + water
    5. OH^- (aq) + H^+ (aq) -> H2O(l)
  4. Electrolysis
    1. Electrolysis splits up substances using electricity.
    2. Ions compounds can only be electrolysed when they are molten or in a solution because then their ions are free to move to the electrodes.
    3. In electrolysis, positive ions move to the negative electrode (Anode) and negative ions move to the positive electrode (Cathode).
  5. Changes at the electrodes
    1. Negative ions lose electrons and so are oxidised at the positive electrode.
    2. Positive ions gain electrons and so are reduced at the negative electrode.
    3. When aqueous solutions are electrolysed, oxygen gas is produced at the positive electrode unless the solution contains halide ions.
    4. When aqueous solutions are electrolysed, hydrogen gas is produced at the solution contains ions of a metal less reactive than hydrogen.
    5. OILRIG--- Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons).
  6. The extraction of aluminium
    1. Aluminium oxide is electrolysed to manufacture aluminium.
    2. The aluminium oxide is mixed with molten cryolite to lower its melting point.
    3. Aluminium forms at the negative electrode and oxygen at the positive electrode.
    4. The positive carbon electrodes are replaced regularly as they gradually burn away.
  7. Electrolysis of brine
    1. When we electrolyse brine we get three products - chlorine gas, hydrogen gas and sodium hydroxide solution.
    2. The products are important reactants used in industry.
  8. Electroplating
    1. We can electroplate objects to improve their appearance, protect their surface, and to use smaller amounts of precious metals.
    2. The object to be electroplated is made the negative electrode in an electrolysis cell. The plating metal is made the positive electrode. The electrolyte contains ions of the plating metal.
Small high digits are represented by ^, Small after digits are left but large numbers NEVER occur after an atom but are always big before :)

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