IGCSE Chemistry AIS :3 Sponsored by CSGO Lotto

C1. The particulate nature of matter

atom - the smallest part of an element that can take part in a chemical change

particle - a single piece of solid material

molecule - a particle made up of two or more atoms held together by covalent bonds

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C2. Experimental techniques

Chromatography: a method for separating and purifying coloured compounds using filter paper and a solvent

A chromatogram can be used to see which dyes are in different dyes, such as the one below.

Filtration: separating an undissolved solid from a solution by passing it through filter paper in a filter funnel.

  • solute - a substance that dissolves in a solvent
  • solvent - a substance that dissolves another substance

Crystallisation: a method used to obtain a crystalline solid from a solution.

  • Step 1: The solution is gently heated in an evaporating basin to concentrate it. The solvent (usually water) will evaporate until the crystallisation point is reached.
  • Step 2: The concentrated solution is left to cool, forming crystals at the bottom of the evaporating basin.
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C2. Experimental techniques pt. 2

Simple distillation: a method of distillation to obtain a solvent from a solution, and is often used to separate water from a dissolved salt

                                              

Fractional distillation: a method of distillation used to separate a mixture of liquids with different boiling points. The main different is this uses a fractionating column.

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C2. Experimental techniques pt. 3

Purity: Purity is important for things such as food additives, medical drugs, and baby food

Most pure substances have distinct melting and boiling points. If a substance is impure, the impurities can affect the melting and boiling points.

  • The melting and boiling points not having a narrow range, impurities will widen the range of temperatures over which melting and boiling occur.
  • The boiling point is increased by impurities, while the melting point is decreased

Apparatus for measuring: (volumetric pipette is most accurate, burette is second, measuring cylinders for large amounts where accuracy is not too important)

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C3. Atoms, elements and compounds, 3.1 and 3.2

Element - A substance containing only one type of atom

Compound - A substance made up of two or more different types of atom joined together by chemical bonds. There are two different compound types:

  • molecular compounds, where the atoms are bonded together (eg. water)
  • ionic compounds, where many charged atoms (ions) are joined together (eg. sodium chloride, or "salt")

Mixture - contains two or more elements or compounds that are not chemically bonded together. Mixtures can be separated by physical means.

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C3. Atoms, elements and compounds, 3.3

The atom is made up of a nucleus (that being made up of protons and neutrons) and shells of electrons. The outer shell of the atom is called the valence shell, and the electrons in the shell are known as valence electrons.

Electron shells can hold up to 8 electrons, however, the first shell can only hold 2.

Protons and Neutrons both have a relative mass of 1 and adding the masses of the two together make up the nucleon (or mass) number. Electrons have a relative mass of 0.00054, and for most occasions, the weight of the electrons will not be needed as the mass is so small.

The atomic (or proton) number is the number of protons in the atom. The same number of electrons are in the atom as protons.

Protons have a charge of 1, Electrons have a charge of -1, and Neutrons have no charge.

Isotopes: Atoms of the same element with a different number of neutrons.

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C3. Atoms, elements and compounds, 3.4

Ionic bonding of elements results in structures known as crystal lattice structures. Crystal lattice structures have incredibly strong intermolecular bonds, thus they have high melting and boiling points. 

An example of this can be found in diamonds and graphite. Diamonds have a crystal lattice structure and are incredibly tough, while graphite has layers and can be easily scratched due to it having weak bonding between layers. Both are made of carbon, but diamonds have a crystal lattice structure.

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C3. Atoms, elements and compounds, 3.5

Molecules and covalent bonds

Covalent bonding happens when 2 or more non-metals share electrons, meaning that both of them will use the same electron but it will be separate numbers to their valence shells. (eg. in water, hydrogen shares one of oxygen's electrons. this doesn't decrease the number of electrons in oxygen)

Compared to ionic bonding, covalent bonds have low melting and boiling points due to the forces between separate molecules being weak.

  • Ionic bonds are soluble in water but insoluble in organic solvents.
  • Ionic compounds conduct electricity only when molten or dissolved in water, due to the ions having the ability to move freely. Solid ionic compounds are too packed together.
  • Covalent compounds do not conduct electricity because they don't have any ions, however some compounds can react with water to form ions.
  • Covalent bonds are insoluble in water, with exceptions to sugar and some amino acids.
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C3. Atoms, elements and compounds, 3.6

As mentioned earlier, diamonds and graphite are both made of carbon but have different structures. The layers structure of graphite allow it to conduct electricity due to delocalised electrons (meaning not belonging to any particular atom) being able to move across the layers when a voltage is applied.

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C4. Stoichiometry

stoichiometry: the ratios of the reactants and products shown in a balanced chemical equation

Avogadro's number: 6.02 x 10^23

The amount of substance with the Avogadro number of particles is called the mole. This is a mole of the element's molecules, NOT ATOMS.

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

Arepresents relative atomic mass. Relative atomic mass is the mass (in grams) of other atoms found when compared to carbon-12 where carbon-12 has exactly 12 units.

Mrepresents relative molecular mass. Relative molecular mass is the sum of the relative atomic masses of all the atoms shown in the formula of a molecule.

eg. Aof hydrogen is 1, Aof oxygen is 6

      H x 2 + O x 1 = (1 x 2) + (16 x 1) = Mr

      Mof H2O is 8

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C4. Stoichiometry, Simple Chemical Calculations

Simple equations are a matter of simple proportion. Solving this requires a given amount of reactant and how much product is made.

eg. A student obtains 48g of magnesium sulfate from 9.6g of magnesium. What mass of magnesium sulfate can the student get from 1.2g of magnesium?

                9.6g of magnesium gives 48g of magnesium sulfate

therefore, 1.2g of magnesium gives (1.2/9.6) x 48 = 6g of magnesium sulfate

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C4. Stoichiometry, Chemical Calculations

Finding the number of moles of atoms, molecules or ions are found the same way:

no. of moles = mass of substance taken (g) / mass of one molecule of the substance (Ar/Mr)

eg. What is the mass of sodium hydroxide in 0.5 moles of sodium hydroxide? Mr of NaOH = 40

mass = (number of moles) 0.5 moles x (mass of 1 mole/formula mass) 40 = 20g of NaOH

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C4. Stoichiometry, 4.1

The mole concept

1 mol occupies 24dm^3

When working out gas volumes, you must first find the number of moles and then multiply this by 24. The answer is then in dm^3.

the volume of gas (in dm^3) = number of moles in gas x 24

1 cm^3 = .0001 dm^3

1 dm^3 = 1000 cm^3

eg. Calculate the mass of carbon dioxide present in 60cm^3 of carbon dioxide. Mr [CO2] = 44

moles of CO2 = volume of gas / 24 = 0.06 / 24

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C5. Electricity and chemistry

electrolysis: the breakdown of an ionic compound using electricity

electrolyte: a substance that conducts electricity when molten or dissolved in water

electrode: a rod of metal or graphite that leads an electric current into or out of an electrolyte

anode: the positive electrode

cathode: the negative electrode

Ionic compounds can be electrolysed when molten or dissolved in water.

When molten ionic compounds are electrolysed, a metal is formed at the cathode and a non-metal is formed at the anode.

When dissolved in water and metal ions are electrolysed, hydrogen is formed at the cathode. This can also happen for ionic compounds, however this depends on the discharge series.

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C5. Electricity and chemistry pt. 2

Discharge series:

for positive ions:      (least likely discharged) Na, Mg, Al, H, Cu (most likely discharged)

for negative ions: (least likely discharged) SO4, NO3, OH*, Cl, Br, I (most likely discharged)

*Take not the product would be oxygen, but you must know there are hydroxides present

Electrolysis examples:

molten lead(II) bromide --> produces bromine gas at anode and molten lead at cathode

(aq) copper chloride --> produces chlorine gas at anode and copper at cathode (shiny cathode)

(aq) copper sulfate:

inert electrodes --> oxygen at anode and copper at cathode, solution gradually loses blue colour

copper electrodes --> copper from anode and copper at cathode, solution remains blue

dilute sulfuric acid --> oxygen at anode and hydrogen at cathode (dilute=oxygen and hydrogen)

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