Chemistry C2

  • Created by: BrownAnya
  • Created on: 23-11-16 20:43

Chemical bonding

Covalent bonding is sharing electrons

Ionic bonding is transferring electrons.

In Ionic bonding, atoms either lose or gain electrons to form changed compounds called ions. Elements lose electrons to form positive ions and gain electrons to form

Metals and non-metals form ionic compounds; the ions formed are held together by very strong forces of attraction between the oppositely charged ions. This electrostatic form of attraction is called ionic bonding. Ionic bonds result in a giant structure (or giant lattice). 

The ratio of the ions in the ionic compound depends on the charges of the ions within it. Ionic compounds are neutral- the charges of any ion within it must be balanced

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Covalent bonding and Metallic bonding

Atoms of non-metals need to gain electrons in order to get a full outermost energy field (outer shell) and be a stable electronic structure. 

This is done by sharing electrons- some atoms can form many covalent bonds. For example, carbon joins together in giant covalent structures, sometimes called macromolecules.

When metal atoms are packed closely together, the electrons in the outermost energy field become delocalised and can move freely between atoms. Delocalised electrons strongly attract the positive ions which hold the lattice together. 

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Giant ionic structures

Ionic compounds have giant structures in which many strong electrostatic forces hold the ions together. They are, therefore: Solids at room temperature, with high amounts of energy needed to break the strong ionic bonds they also have a high melting and boiling point.

When an ionic compound has been melted, the ions are free to move, which allows them to carry an electrical charge so they are able to conduct electricity as liquids. 

Some ionic compounds dissolve in water as the water molecules split up the lattice. The ions are free to move in the solution so they conduct electricity. 

Simple molecules have weak intermolecular forces which are easily overcome with low amounts of energy, so the molecules have a low melting and boiling point.

Simple molecules have no overall charge so therefore cannot carry electrical charge or conduct electricity

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Giant covalent structures/ Diamond

Giant covalent structures:

  • Also called macromolecules
  • Have no charge
  • Strong covalent bonds so high melting and boiling points
  • Don't conduct electricity except for graphite when molten.

Diamond:

  • Form of carbon with a regular 3D giant structure 
  • Each carbon atom is covalently bonded to 4 other carbon atoms, so no atom is able to move
  • So diamond is hard and transparent
  • The compound silica has a similar structure
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Giant covalent structure/Graphite

Graphite:

  • A form of carbon, atoms covalently bonded to 3 other atoms in flat 2D layers.
  • Giant covalent structure.
  • There are only weak intermolecular forces between the 2D layers, so they can slide over each other easily.
  • The covalent bonds have strong intermolecular forces so high melting and boiling points.
  • One electron in each carbon atom is delocalised so graphite can conduct electricity and heat.

 

 

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Fullerenes/Giant metallic structure

Fullerenes are large molecules formed from hexagonal rings of carbon atoms. 

The rings join together to form cage-like shapes with varying numbers of carbon atoms, some of which are nano-sized

Applications of fullerenes include drug delivery to the body, catalysts and reinforcing materials.

Giant metallic structure:

Metals are arranged in the giant metallic structure in layers that can slide over each other without breaking the structure.

Metals are good conductors of heat and electricity as delocalised electrons move throughout the lattice and can transfer energy quickly

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Polmers and nanoscience.

Properties of a polymer depend on the monomers used to make it. 

thermosoftening polymers:

  • Weak intermolecular forces, chains of polymers can slide over each other
  • Easy to melt, weak intermolecular forces need low energy to break bonds.
  • Can be re-shaped.

Thermosetting polymers:

  • Stronger intermolecular bonds- crosslinks
  • Hold chains together in a solid  structure so cannot be re-shaped.

Nanoscience:

  • 1-100 nanometers
  • Small size with large surface area- effective catalysts 
  • Effective sensors- can detect a single molecule
  • Nanomedicine- deliver drugs to body.
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Mass of atoms

Total number of protons and neutrons in an atom is the mass number

Atoms of the same element have the atomic number. Atoms of the same element with different numbers of neutrons are called isotopes.

Number of Neutrons = mass number - atomic number.

Relative atomic mass = mass number

Relative formula mass (Mr) is found by adding all the Relative atomic masses (Ar) of the atoms in the formula given.

Eg: (http://www.sigmaaldrich.com/content/dam/sigma-aldrich/structure7/109/mfcd00011106.eps/_jcr_content/renditions/mfcd00011106-medium.png)= Mg (24) + Cl2(71)= 95

(http://a.files.bbci.co.uk/bam/live/content/zkdkq6f/large)

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Percentage mass of an element in a compound

Percentage mass of element in a compound = Ar * no. of atoms /Mr (whole comp.)

Eg: Percentage mass of Na2CO3 

Ar Na = 23 Ar C = 12 Ar O = 16

Mr Na2CO3 = (2*23) +  12 + (3* 16) = 106

Percentage mass = ((2*23)/106)*100 = 43.6 %

Empirical formula:

  • List all elements in the compound
  • Write experimental mass/percentage
  • Divide each mass/percentage by the Ar for that element
  • Convert to simplest ratio possible
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Yield

Yield = (amount of product collected/total amount possible) * 100

Yield is not 100% as reactions may not go to completion, other reactions may happen and product may be lost when collected from apparatus.

Using reactions with high yields in the industry helps conserve resources and reduce waste.

Reversible reactions- the products can react to get the original reactants again. Two arrows used, each with half an arrowhead. 

Gas and Liquid chromatography. Chromatography separates the mixtures in order of mass number

Advantages: Quick, can measure very small volumes, higher accuracy.

Carrier gas moves vapour through the column. Compounds with stronger attractions have a longer retention time- move through the column slower.

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Rates of Reaction

Rate of reaction = Amount of reactant used/time taken 

Rate of reaction = Amount of product made/time taken.

Collision theory- reactions can only happen when particles collide with enough energy to change into new substances. The minimum energy required for this to happen is called activation energy.

Factors that increase the chance of collision or energy of the particles will increase the rate of reaction.

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ROR Temperature and Pressure

Higher temperature:

  • Particle have increased energy
  • Move faster
  • Collide more
  • More particles have equal to or higher than activation energy
  • So kore successful collisions.

Higher concentration or pressure:

  • Solution is more concentrated, more particles of reactant between water molecules
  • Likelihood of collisions higher. 
  • More frequent collisions under higher pressure.

 

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ROR Surface area and Catalysts

Larger surface area:

  • Breaking a solid up increases surface area
  • Increases area over which reaction can happen
  • Rate of reaction increases.

Catalysts:

  • Substances speed up a reaction without being changed or used up in the process
  • Large surface area increases the effectivity of the catalyst
  • Only work with one type of reaction.

 

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Energy/Enthalpy

Exothermic- transfers energy to surroundings and loses heat

Endothermic- Takes in energy from its surroundings and gains heat. 

(http://www.swotrevision.com/pages/alevel/chemistry/images/img_64.GIF)

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Acids and bases

Acid and a base makes a salt and water.

Acid and a metal makes a salt and hydrogen.

Acid and a metal oxide makes a salt and water

Acid and a metal hydroxide makes a salt and water

HCL- Hydrochloric acid

HNO3- Nitric acid

H2SO4- Sulfuric acid

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Electrolysis I

Ionic substances contain charged ions. When a current is passed through these ions during electrolysis, they are decomposed.  

Covalent structures cannot act as electrolytes as they contain neutral atoms. 

For electrolysis to happen, the ions must be free to move, so the substance is dissolved in water or melted. 

Anode- positively charged electrode

Cathode- negatively charged electrode 

Positively charged ions move to the cathode during electrolysis and are reduced- they receive electrons. 

Negatively charged ions move to the anode during electrolysis and are oxidised- they lose electrons. 

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Electrolysis II

Electrodes are inert so don't react with the substance produced however, when Aluminium is extracted using electrolysis, the carbon dioxide produced wears away the electrodes used in the process.  

Cryolite brings the melting temperature of aluminium down in order that electrolysis can take place- less energy needed for electrolysis and aluminium can actually be produced at the negative electrode.

Lead bromide half-equation:

Cathode: Pb2+ +2e - Pb

Anode: 2Br - Br2 + 2e

Hydrogen is produced if the other positive ions in the solution are those of a metal more reactive than hydrogen

Oxygen produced at the anode, however if of a halide ion is in the solution then the halogen will be produced.

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Electrolysis III

Electrolysis of brine. 

Brine is a solution of sodium chloride in water, containing Na+, Cl-,H+ and OH- ions. 

At the anode: 2Cl- ~ Cl2 + 2e-

At the cathode: 2H+ + 2e- ~ H2

Electroplating uses electricity to put a thin layer of metal onto a object. 

The negative electrode should be the object that will be plated

The positive electrode should be the metal that you want to coat the object with.

The electroylte should be a solution of the coating metal. 

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oregana345

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So helpful for my chemistry exam on Monday !!!!!!

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