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IGCSE Chemistry, All atoms want to have a full shell of electrons. To do…

- - - - Alkanes are the simplest type of hydrocarbons. They have the general formula of CnH2n+2. Alkanes are saturated compounds, as they cannot form extra bonds.
      - The hydrocarbon properties change as the chain gets longer. The shorter the chain, the more runny it is. Shorter chains are also more volatile, and more flammable. They also have lower boiling points. They are often used as fuel.
        
        When they are used in fuel, they can combust to form some harmful chemicals.
      - The complete combustion of any hydrocarbon in oxygen always releases a lot of energy. The only waste product produced is carbon dioxide and water
        
        During combustion, both the carbon and hydrogen are oxidised. Hydrocarbons are used as fuels as they release a lot of energy.
  - - - At the top, liquefied petroleum gases collect as they have the lowest boiling point
      - Then petrol collects.
      - Then kerosene
      - Diesel
        
        These fractions have boiling points below room temperature, and have a low viscosity
      - Bitumen (tar)
        
        Has a very high boiling point, very viscous, and are difficult to ignite
  - - - Because they are unsaturated, they are more volatile, and are far more reactive, as compared to alkanes
      - When exposed to a large amount of oxygen, they combust completely to produce water and carbon dioxide.
        
        They wil
      - Straight chain alkanes have twice as many hydrogens as carbons in them.
    - - Alkenes react via addition reactions. All alkenes are in the same funcional group. Their carbon double bond will open up to accommodate 2 new atoms, which each bind to a carbon
        
        The addition of hydrogen is known as hydrogenation. 4 hydrogens are added.
        
        They will also react with halogens, such as bromine, iodine, or chlorine. 2 of them bind, for example, 2 bromine and ethene making dibromoethene
      - To test for Alkenes, mix Bromine water with the fluid. If there is an alkene present, .
  - - - Condensation polymers involves monomers that contain different functional groups
        
        They react together, and bonds form making polymer chains.
        
        For each bond that forms, a small molecule is lost (e.g water), resulting in condensation
  - - - The process of making alcohol is known as fermentation. It takes glucose, and converts them into Ethanol and CO2. This is also known as anaerobic respiration. This reaction can be catalysed by a naturally occurring fungus called yeast. The process is renewable, but very inefficient.
        
        Alcohols have the general formula is CnH2n+1IOH. Basically there is an extra hydrogen and carbon
      - We can also react Ethene (hydrocarbon) with steam to produce ethanol. This process is much more efficient, as it has no waste products and is very fast. It is non-renewable, and requires a lot of energy as you have to heat it. Phosphoric acid is used to catalyse this
        
        C6H12O6 -> 2C2H5OH + 2CO2
        
        A catalyst is an element that speeds up a reaction without changing the reactants or products.
        
        It works by providing an alternative path for energy, usually with lower reacting energy. This is the energy required for the particles to collide to react.
  - - - Carbon dioxide and water are produced, and the maximum amount of energy is given off. This results in a blue flame
  - - - They react like any other acids, with carbonates to produce a salt, water and carbon dioxide
        
        The salts end with 'anoate, for example ethanoic acid will result in methanoate
        
        These acids can dissolve in water. When they dissolve, they release Hydrogen gas. They do not ionise completely. They just form weaker solutions. This just means they have a higher PH.
    - - Esters are fiormed from an alcohol and a carboxylic acid. In these reactions, a concentrated acid is often used as a catalyst.
  - - - Addition reactions involve 1 product
      - Substitution reactions involve 2 reactants and 2 products
      - Combustion reactions involve the reaction of the product with oxygen.
- - - - Liquid
        
        Gas
        
        As a gas cools, it has less energy, so it doesn't have enough energy to break the bonds, so they reform. Eventually, enough bonds form so that the gas becomes a liquid. This is condensation.
        
        When a liquid is heated, it gains even more energy, which further weakens the bonds holding it together, until they eventually break, this is called boiling or evaporating
      - When a solid is heated, the particles move and collide more, which weakens the forces holding the solid together. At a specific temperature boiling Point, it becomes a liquid. This is called Melting
      - When a liquid cools, the particles have less energy, so more bonds form and it become a solid. This is Freezing.
- - - - To extract things from ores, we can use 2 methods based on their reactivity
        
        If they’re less reactive than carbon, we can extract them via displacement. This me that the carbon will react with the oxide, displacing the metal. This will work by heating the oxide with carbon in a fume cupboard.
        
        When it is more reactive than carbon, we need to use electrolysis.
        
        Electrolysis is the splitting up with electricity. In electrolysis, 2 nodes are inserted into the water, and each are charged.
        
        The positive ions in the electrolyte move towards the cathod, and gain electrons (reduction)
        
        The negative ions move towards the anode, gaining electrons. This is an oxidation
        
        During electrolysis of metals, the metal must either be aqueous or molten in order for the ions to move.
        
        In aqueous solutions, there is hydrogen formed at the cathode, as they are attracted to the negative charge of the cathode.
        
        At the anode, if OH- or Halide ions are present, they will form. If no halides are present, the OH- is discharged in the form of oxygen gas.
        
        For example, in an NaCl Solution, the sodium will form at the cathode, and chlorine (halide) will form at the anode.
        
        When electrons are transferred, this is known as a redox reaction. Oxidation is the gain of oxygen, or loss of electrons. Reduction is the gain of electrons.
        
        Redox can occur in displacement reactions, where one metal displaces another from a compound. Here, the more reactive ion will enter the compound, gaining electrons, while the less reactive ion loses them, as it is reduced
      - Metallic bonds are formed in lattices which consist of positively charged metal ions, suspended in a sea of delocalised electrons.
  - - - This can be measured with an indicator.
        
        Phenolphthalein turns from colourless in an acidic solution to pink in alkaline ones.
        
        Universal indicator is green in neutral, purple in alkaline, and red in acids.
        
        Methyl orange is red in acid, and yellow in alkaline
    - - An acid is a substance that forms aqueous solutions with ph <7. They form H+ Ions in water.
      - An alkali is a base that is soluble with a ph >7. They form OH- Ions in water
      - An acid and a base form Water and a salt.
    - - Strong acids fully ionise in water, releasing hydrogen ions. Weak acids do not fully ionise. Because stronger acids produce more hydrogen, the concentration is higher, meaning that their rate of reaction is higher.
      - aqueous metal oxides are aqueous, meaning that they can react with acids. This forms a salt, like copper chloride and water.
      - Acids and metal carbonates produce a salt, water, and CO2
      - You can make a soluble salt using an insoluble base.
        
        To do this, you would take an insoluble metal oxide, and an acid. For example, if you wanted copper chloride, you would take copper carbonate and hydrochloric acid.
        
        To do this, gently warm the acid, then add the insoluble base slowly, until it becomes saturated.
  - - - If the product of the reaction stores more energy than the original reactants, they will have taken energy in from the surroundings. This is called an endothermic reaction
        
        They are less common than exothermic reactions, but an example could be thermal decomposition, where energy is taken in to break apart a compound. They also have real life uses, such as in sports injury packs.
      - If the products have less energy than the original reactants, they will have released energy into the environment, This is called an exothermic reaction.
        
        The best example of this is combustion, or burning fuels. These reactions also have real life uses, such as hand warmers or self-heating cans.
        
        In these reactions, you can measure the amount of energy released by a chemical reaction (solution) by taking a temperature of the reactants, mixing them and measuring the temperature of the final product.
        
        The biggest problem of this is that temperature is lost. To Overcome this, make sure to insulate your can or vessel.
        
        You can represent this by using a reaction profile. This involves plotting your energy and progress onto a graph.
        
        Exothermic reactions will have their reactants with a decent amount of energy. The line will rise, showing the activation energy transferred, eventually plateauing and falling. This is the energy released
        
        In an endothermic reaction, the reactants will quickly gain energy, then slightly drop sitting at a higher level than they started at. This requires a lot of activation energy.
    - - Energy must always be supplied to break bonds. This means that breaking bonds is an endothermic reaction.
      - Energy is released when new bonds are formed, making bonding an exothermic reaction.
      - Every chemical bond has a specific bond energy associated with it. This varies between compounds, and these can be used to find the overall energy change in a reaction
        
        The overall change is the sum of the energy needed to break bonds, minus the energy released when new bonds are formed.
  - - - They can occur at many very different rates.
    - - The steeper the line on the graph, the faster the rate of the reaction. As it occurs, the graph shallows out, as reactants are used up.
      - To have a reaction, particles must collide with each other in order to generate energy.
        
        The collision frequency of reacting particles can affect the rate of reaction
        
        And the energy transferred during the collision. They have to transfer enough energy for the collision to be successful
      - To increase our rate of reaction, we can use a variety of methods.
        
        We can Increase the temperature. This means that our particles travel faster with more energy.
        
        You can also increase the surface area of your reactants. You can do this by blending or grinding down your reactants.
        
        You can stir the mixture, which means that there are more collisions
        
        You can introduce a catalyst, which increases speeds up the reaction without using it up. They do this by reducing the required activation energy.
    - - Rate of reaction = amount of reactant used OR product formed/time
      - You can record the visual change in a reaction, if the initial solution is transparent, and the product is a precipitate.
      - You can measure the change of mass, which comes from gas being given off. The quicker the reading on the balance drops, the faster the reaction
      - Finally, the volume of gas given off. The more gas given off during a time period, the faster the reaction
- - - - The closer your measured value is to the actual melting point, the purer your sample is. The melting point is changed by the presence of other compounds or elements.
  - - - Chromatography is used to separate substances in a mixture. It involves a stationary phase, usually a chromatography paper, and a mobile phase, usually a solvent like ethanol.
      - During a chromatography, substances constantly move between these 2 phases. The mobile phase moves through the stationary phase, and anything dissolved in this moves with it.
        
        The chemicals that spend more time in the mobile phase will move further through the stationary phase. This depends on the solubility of the substance, or how attractive the paper is.
        
        The result of chromatography is called a chromatogram.
        
        The RF value is the ratio between the distance travelled by the dissolved substance and the distance travelled by the solvent. The further through the stationary phase moves, the larger the RF value.
        
        We can calculate this using RF=Distance travelled by substance/distance travelled by solute
      - Chromatography is used to investigate if a certain substance is present in a mixture. To do this, we add a pure sample to the paper and observe if anything moves the same as it.
    - - To filter a substance, we can separate an insoluble substance from a liquid, using filter paper and a funnel
    - - Pour the solution into an evaporating dish, and as you heat it, the solvent will leave the mixture leaving you with pure dry crystals
    - - Heat the solution, and the part of the solution with the low boiling point will evaporate first, which goes through a condenser leaving you with 2 pure liquids,
  - - - Chloride forms a white precipitate of silver chloride
      - Bromides form a cream precipitate of silver bromide
      - Iodides form a yellow precipitate of silver iodide
  - - - Lithium burns red
      - Sodium burns yellow
      - Potassium burns lilac
      - Calcium burns orange-red
      - Copper burns green
      - To test for this, use a platinum wire loop and dip it into your sample, and place into a flame
    - - Calcium and Ca2+ forms a white precipitate
      - Copper II and Cu2+ forms a blue precipitate
      - Iron II and Fe2+ forms a green precipitate
      - Iron III and Fe 3+ forms a brown precipitate.
      - Aluminium and Al3+ forms a white precipitate, then redissolves into excess NaOH to form a colourless solution
      - Magnesium and Mg2+ forms a white precipitate
- - - - Protons have a relative mass of 1, and have a charge of +1. They decide the element that an atom forms. This is the atomic number
      - Neutron have a relative mass of 1 and have a charge of 0 The mass number of an element shows how many neutrons and protons are in an atom
      - Electrons have a relative mass of 0, and a charge of -1. The number of electrons is the same as the number of protons. This means that the atom has an overall charge of 0
  - - - Each element has it's own symbol on the periodic table. This symbol shows how many protons and the mass of the most common isotope for that element.
        
        Each element has isotopes. Each isotope has the same proton number, thus being the same element, but with a different number of neutrons. These all have a different mass, so to equally represent them, we use the average mass, known the relative atomic mass
      - An ion is an element with a different number of protons to electrons. This is usually because it has given it electron or taken one from/to another atom.
        
        This is represented by a charge, either becoming + when it loses an electron, as it has more protons, or becoming - when it gains one, as there is more negative than positive.
  - - - Ionic
        
        In ionic bonding, a non-metal and a metal bond together. The metal atom loses electrons to become + and the non-metal gains electrons to become -. This opposit charge makes it very strong.
        
        Ionic compounds all have similar properties.
        
        They all have a high melting point, due to the strong electrostatic forces
        
        They all cannot conduct electricity when solid, as nothing moves. While they are liquid, they can carry electricity, as the ions can move.
        
        Some can be dissolved in water, and can conduct electricity in this state (Aq)
        
        They form a giant ionic lattice. This is a closely packed regular lattice where there are strong electrostatic forces between all atoms.
      - Covalent
        
        Covalent bonds are bonds that happen between non-metals and metals. They share pairs of electrons, which orbit between both outer shells. These bonds are strong, as the negative electrons are attracted to the nucleus..
        
        Covalent bonds have very similar properties
        
        the smaller the structure, the lower the melting/boiling points. This is because of few weak inter-molecular bonds. However, larger structures have high melting points. This is because of lots of strong intra-molecular bonds to be broken.
        
        They tend to be hard to dissolve in water, due to full shells.
        
        They mostly don't conduct electricity, as the molecules don't move.
        
        Covalent compounds can form 2 different types of structure.
        
        Giant Covalent Structures are macromolecules. All of the atoms are bonded to each other by strong covalent bonds.
        
        They have very high melting and boiling points, due to the large number of strong bonds.
        
        They do not conduct electricity, as everything is fixed in place and nothing moves, even while molten.
        
        The main examples for these are Diamond, Graphite and Silica
        
        Diamond is very hard, as it consists of only carbon atoms, each which forms 4. covalent bonds. It is non-conductive because it carries no free electrons or ions
        
        Graphite contains sheets of hexagons. Each carbon atom forms 3 bonds. It also has one spare electron per atom, so it is conductive. It also has a high melting point, as it needs a lot of energy to break the bonds
        
        Graphene is one layer of graphite. It is very strong, due to all of the bonds. It also is conductive due to the loose electrons, and is very light. It is often used to reinforce composite materials
        
        Silica
        
        Silica consists of silicon and oxygen. It has a high melting point, and is very hard, due to the structure of it being similiar to diamond
        
        Fullerenes are molecules of carbon, shaped like hollow tubes or balls. They are mainly made up with carbon atoms in hexagons. They can be used to contain a molecule, allowing us to deliver drugs into the body. They also make great catalysts, due to their large surface area.
        
        Simple Molecular substances are made up of molecules containing a few atoms that are connected by covalent bonds. Here are a couple of examples.
        
        Chlorine
        
        Each chlorine atom only needs 1 electron, so 2 can form a ingle covalent bond.
        
        Hydrogen
        
        Hydrogen atom sonly need 1 more electron to finish their outer shell, so they often form shingle covalent bond with other hydrogen atoms, or other atoms
        
        Oxygen
        
        Each oxygen needs 2 more to complete it's shell, so in oxygen gas they form pair with double covalent bonds.
        
        Nitrogen
        
        Nitrogen needs 3 electrons, so 2 nitrogens share 3 pairs of electrons to fill their shell. This is a triple bond
        
        Methane
        
        Carbon has 4 outer electrons, so it can form 4 bonds with hydrogens to make methane
        
        Water
        
        Oxygen shares a pair of electrons with 2 hydrogen to complete it's shell.
        
        Hydrogen Chloride.
        
        Both only need 1 more, so they form a single bond.
        
        Substances that have covalent bonds usually have simple molecular structures. They often have similar properties.
        
        They have weak intermolecular bonds, so they boil and melt easily. These molecules are very easy to break apart.
        
        They area also often gases and liquids at room temperature.
        
        They do not conduct electricity, as there are no free electrons
      - Metallic
        
        Metallic bonds are between 2 metals. They consist of a lattice of positive ions, and have a sea of negative electrons between them, stopping them from repelling.
        
        Metallic compounds have a lot of similar traits.
        
        Metals consist of giant structures. This means that they have high melting points, due to lots of strong bonds. These take a lot of energy to break.
        
        They are good conductors of electricity and heat. This is due to the free-moving electrons that can carry energy and a charge,
        
        They are also malleable, as the electrons can move, so the layers can slide over each other
        
        Alloys are mixes of metals. They tend to be harder, as atoms of different size can distort the shape, meaning that the layers find it harder to move over each other.
- - - - One mole of any substance is simply a substance that contains an Avogadro's Constant number of particles (6.02 x 10^{23}), whether those be atyoms, molecules, ions or electrons.
        
        This number of particles is always equal to the atom's relative atomic mass. To find the number of moles in a compound, you divide the mass in G, over the Mr.
        
        Conservation of mass.
        
        During a chemical reaction, mass is always conserved. This means that no atoms are created or destroyed. Because of this, no mass is lost or gained. In the equation below, lithium reacts with fluorine to make lithium fluoride. There are 2 of each atom on each side, so mass is not gained or lost, and the equation is balanced.
        
        2Li +F2 -> 2LiF
        
        If mass seems to change, it usually is involved with a form of gas.
        
        If it increases, some reactants from the air has entered, for example oxygen.
        
        If it decreases, it is probably because some gas has escaped, and all the reactants are solid or liquid, or aqueous. If the vessel is not enclosed, the gas can escape, and you cannot account for it's mass.
        
        Despite tis, there can be limiting reactants, when one of the reactants is used up. To counter this, you usually use an excess of one of the reactants.
    - - If we know the masses of the reactants and products, we can work out the balanced symbol equation for the reaction.
      - Yield
        
        Percentage Yield compares the actual yield to the theoretical yield.
        
        The formula for this is mass of actual product (g)/ maximum theoretical mass (g). The result will always be between 100 and 0%. Industrial processes should always have as high a yield as possible, as to minimize waste.
        
        In reversible reactions, reactants can turn back so the yield will never be 100%.
        
        Also, reactants can react differently, perhaps with gases in the air.
        
        Finally, when you separate a solid from the reaction mixture, you will always lose some solid mass.