Unit 2 : Elements from the Sea

Dot and Cross diagrams for covalent bonding

The shells of each atom are connected to each other, with a dot of a lone pair connecting to a cross of a lone pair from the other atom.

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Mass of Substance Calculation

Mass of Substance (no. with grams) = No. of Moles (amount present/molar mass, total top no.) x molar mass (total top no.)

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Measuring concentration of a solution

1) Number of moles = Concentration (no. of moles in solution) x Volume (in cm3) / 1000. 2) Work out molar mass (total top no.) 3) Mass = no. of moles x Mr

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Calculate molarities from titrations

1) Balance equation, decide what you know. 2) Work out no. of moles of reactant u know everything about conc (M) x volume (cm3) / 1000 3) Concentration = moles of step 2 x 1000 / volume in question.

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Balancing the charge in ionic equations

1) How many of each atom on each side. 2) Even each side out. 3) Balance out charges, if electron already present, that's classes as 1 charge

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Percentage yield

1) No. of moles = mass / molar mass. 2) Ratio 3) Molar mass of product with reactant in. 4) Theoretical y = 1 x 3. 5) % yield = Actual y / Theoretical y x 100

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Dot and Cross diagram for Dative Covalent Bonding

All atoms are in the square with an arrow pointing to the donor atom, a + is outside the square.

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Using Balanced equations to work out gas volumes

1) Molar mass of reactant (total top no.) 2) No. of moles = amount of reactant/step 1. 3) Mole ratio 1m reactant:1 mole product... 4) No. of moles x 24

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Chemical properties dependent on electronic structure

S block elements lose electrons, forming +ve ions with inert gas configuration. P block elements gain 1, 2 or 3 electrons forming -ve ions with inert gas configuration. Group 0 shells filled so already inert. D block lose s and d electrons= +ve ions

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Titrations

1) Measure out alkali using pipette, put in flask with indicator. 2) Do rough titration to get end point - where alkali exactly neutralised and colour change. 3) accurate titration. Run acid to 2cm3 of end pont, then drop-drop.

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Dot and cross model limitations

Most bonds aren't purely ionic/covalent they're in between, so compounds have a mixture of ionic and covalent properties.

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Ionic Bonding

Ions are formed when electrons are transferred from one atom to another. Happens between a metal and a non-metal. Can't conduct electricity in solid as in fixed position by strong ionic bonds, dissolve in H20 as molecules pull ions away from lattice.

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Calculating volume from titration

1) Write balanced equation, find what you don't know. 2) Work out no. of moles of reactant u know everything about concentration (M) x Volume (cm3) / 1000. 3) Ratio 4) No. of moles x 1000 / conc.

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Electronic structure

S=2, P=6, D=10, F=14. 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d10... S block - Hydrogen, and first 2 groups. D block - middle bit. P block - group 5, 6 and 7

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Dot and Cross diagrams for ionic compounds

The atom that's lost an electron has a + outside the square, the atom that's gained the electron has a dot to represent the new electron, and a - outside the square.

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Covalent Bonding

Molecules are formed when 2+ atoms bond together. Two atoms share electrons so they both have full outer shells. Happens between non-metals. Weak bonds, no charge carriers free to move so don't conduct electricity, insoluble

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Data Covalent Bonding

One atom donates both electrons to a bond

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How to work out molecular formulae

1) Total top no. 2) Molecular mass (given in question) / relative mass. 3) Times this no. by those in empirical formula

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2 electron pairs on central atom, no lone pairs

Linear - 180 degrees

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3 electron pairs on central atom, no lone pairs

Trigonal planar - 120 degrees

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3 electron pairs on central atom, one lone pair

Bent - 120 degrees

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4 electron pairs on central atom, no lone pairs

Tetrahedral - 109.5 degrees

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4 electron pairs on central atom, one lone pair

Trigonal pyramidal - 107 degrees

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4 electron pairs on central atom, two lone pairs

Bent - 104.5 degrees

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5 electron pairs on central atom, no lone pairs

Trigonal Bipyramidal - 90 degrees and 120 degrees

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6 electron pairs on central atom, no lone pairs

Octahedral - 90 degrees

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Electronegativity

The ability to attract the bonding electrons in a covalent bond. Measured using the Pauling Scale. It increases across periods, and decreases down groups accept noble gases.

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Predicting bond polarity in a covalent bond

Bonding electrons pulled to more electronegative atom. Covalent bonds in diatomic gases=non-polar so electrons are equal. In polar bond, difference in electronegativty=dipole, difference in charge between 2 atoms by shift in electron density.

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Deciding whether a molecule is polar/non-polar from its shape and polarity of its bonds.

Simple molecule=polar bond gives permanent dipole-polar. Complicated molecule=several polar bonds, if in opposite directions, cancel out-non-polar. If polar bonds in same direction-polar. Lone pairs cancel out dipole created.

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Instantaneous dipole-induced dipole bonds (Van der Waals) - Weakest

Electrons in charge clouds move. At one point, electrons likely to be more to one side=temporary dipole. This can cause temporary dipole in opposite direction of neighbouring atom, so both attracted to eachother. Domino effect, Atoms always attracted

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Permanent dipole-permanent dipole bonds

Charges on polar molecules cause weak electrostatic forces of attraction. Electrostatically charged rod makes liquid move towards rod. Polar liquids have molecules with p.d, which turn around so oppositely charged end attracted to rod.

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Intermolecular forces

Alkanes have covalent bonds with van der waals forces between molecules. Longer the chain=stronger V.D.W as more surface area for electrons to interact. Longer=more energy to overcome forces.More branches=can't pack close, small molecularsurface area

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Giant Ionic Lattice

Giant=same basic unit repeated over and over. In NaCl, ions packed together, cube shaped as different ionic compounds have different shaped structures.

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How to work out the oxidation state of atoms

Uncombined elements=0. Elements bonded to identical atoms=0. Monatomic ion (Na+)=same as charge. Compounds=ion charge. Neutral compound=0. Combined oxygen=-2. Combined hydrogen=+1.

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Half equations

Oxidation-electrons on right of arrow. Reduction-electrons on left of arrow.

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How the periodic table is layed out

It lists elements in order of atomic number and groups elements together according to their common properties.

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Ionisation enthalpy

The energy needed to remove the first electron. State symbol is g. The lower the ionisation enthalpy, the easier it is to form an ion.

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NO3^-

Nitrate = Nitrate(V)

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SO4^2-

Sulfate = Sulfate(VI)

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CO3^2-

Carbonate

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OH-

Hydroxide

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NH4+

Ammonium

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HCO3-

Hydrogencarbonate

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Example equation of first ionisation enthalpy

O(g) ---> O+(g) + e-

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Example equation of second ionisation enthalpy

O(g) ---> O2+(g) + e-

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What effects the size of ionisation enthalpies?

Atomic radius=Further away the outer electrons-lower the enthalpy. Nuclear charge=more protons in nucleus, more attraction between electrons and nucleus-enthalpy higher. Electron shielding=More shells, more shielding-enthalpy lower.

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Appearance, physical state and volatility (l.b.t), electronegativity of halogens at room temperature

F2=pale yellow, gas. Cl2=green, gas. Br2=red-brown, liquid. I2=grey, solid. Volatility decreases down group as stronger van der waals. Electronegativity decreases down the group.

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Halogens solubility in water and organic solvents

Halogens in natural state=covalent diatomic molecules so low solubility in water, but dissolve easily in organic solvents. Cl2=(water)colourless, (hexane) colourless, Br2=yellow/orange, orange/red, I2=brown, pink/violet

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Atom economy

What proportion of starting materials end up in useful materials. %atom economy = mass of desired product/total mass of reactants x 100

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Batch processing

Small reactants used, taken out, machine cleaned, start again. For small quantities, can make diff products. Labour intensive, contamination if cleaning not thorough, dye manufacture, steel making, paracetamol + aspirin, ethanol by fermentation.

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Continuous processing

Continuous manufacture. Large quantities, less variation, low labour costs, more expensive to build and run (unless running at full capacity) industrial ethanol, haber process for ammonia, blast furnace.

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Finding a suitable site for a plant

Near source of raw material or near a port. Near your customers. Access to river/sea. Near a power station if needs energy. Near suitable workforce. Best compromise.

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Relative oxidising ability of halogens

Displacement reactions with halide ions. Halogen displaces a halide thats below it on the P.T. Mixing with an organic solvent, the halogen will dissolve and settle as a layer on top of aqueous solution.

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Equations for redox reactions with halogens, e.g cl2 and br2.

Cl2 + 2Br- --> 2Cl- + Br2

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Electrolysing an aqueous solution containing a halide

Halogen elements released at the + anode, they lose electrons to electrode and are oxidised. Hydrogen ons form hydrogen gas at - cathode.

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Reacton between silver ions (Ag+) and Halide ions (X-)

Add dilute nitric acid to remove interfering ions. Add silver nitrate solution (AgNO3 aq) Precipitate forms. Ag+(aq) + X-(aq) ----> AgX(s)

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Colour of precipitate

F- = No precipitate. Cl- = White. Br- = Cream. I- = Yellow

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Risks of storage and transport of halogens

F=most hazardous, produced where used if poss, small containers lined with nickle/copper-nickle alloys. Cl=Liquid under pressure in cylinders. Br=liquid at room temp, easy to store. I= solid at r.t.p least reactive.

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Uses of halogen compounds

Fl=making PTFE non-stick coating for pans, HCFCs, toothpaste. Cl=PVC, bleach. Br=medicines, flame retardents. I=medicine, human nutrients

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Making a chloroalkane from an alcohol

1) Reaction=Shake 2-methylpropan-2-ol with concentrated hydrochloric acid for 20 mins in seperating funnel. 2) Separation=Allow mixture to settle to 2 layers, run off aqueous layer.

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Stage 3=

Add sodium hydrogencarbonate solution. Run lower layer off. Add distilled H2O, run off lower layer. Add anhydrous sodium sulfate (drying agent) to remove H2O. Distil mixture to remove organic impurities. Collect fraction that boils 48-53 degrees.

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Get Revising

Unit 2 : Elements from the Sea

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