Chemistry Revision

Bonding

The three types of bonding are ionic bonding, covalent bonding and metallic bonding. Ionic bonding is between metals and non-metals. Electrons are transferred between atoms and create charged particles. Covalent bonding is between 2 or more non-metals. Electrons are shared between the atoms. Metallic bonding is between 2 or more metals. The electrons become delocalised and the metal atoms become positive.

Seperating Compounds/Mixtures

Distillation separates substances by evaporating the mixture and condensing it. Different substances within the mixture will evaporate and condense at different rates, and therefore be separated. Chromatography separates out liquids at different densities. If a less dense liquid is present, it will rise higher up the paper than the more dense liquid. Evaporation separates dissolved solids from the liquid they are dissolved in. The solution is heated, and the liquid evaporates, leaving the solid.

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Caption: : Distillation

Caption: : Chromotography

Titrations

If the sample is a solid, it is weighed using an accurate balance, and then dissolved to make up a known volume of solution (usually 100 cm^3). A pipette is used to measure accurately a volume of this solution - for example, 10cm3. A safety pipette filler is used to draw solution into the pipette. This is emptied into a conical flask. A few drops of an indicator may be added to the conical flask. This will show a change of colour when the titration is complete. A second chemical is placed in a burette. This other solution is of a chemical that will react with the synthesised chemical sample in the conical flask. Often the solution in the burette is an acid or alkali, and it must be of a precise, known concentration. The solution from the burette is run into the conical flask. The solution is added one drop at a time, with swirling to mix the solutions as the end-point is approached. Eventually, a colour change shows that the correct amount has been added to react completely with the synthesised chemical in the sample. The volume of solution added from the burette is noted. The titration results can then be used to calculate the amount of the synthesised chemical in the sample, and therefore find its purity.

Titration Calculations

27.5 cm3 of 0.2 mol/dm3 hydrochloric acid is needed to titrate 25.0 cm3 of sodium hydroxide solution. What is the concentration of the sodium hydroxide solution? Step 1: Convert all volumes to dm3 27.5 cm3 = 27.5 ÷ 1000 = 0.0275 dm3 25.0 cm3 = 25.0 ÷ 1000 = 0.025 dm3 Step 2: Calculate the number of moles of the substance where the volume and concentration are known number of moles = concentration × volume number of moles of hydrochloric acid = 0.2 × 0.0275 = 0.0055 mol (5.5 × 10–3mol) Step 3: Calculate the unknown concentration We can say that 0.0055 mol of acid will react with 0.0055 mol of alkali concentration of alkali = moles ÷ volume = 0.0055 ÷ 0.025 = 0.22 mol/dm3

Bond Energies

Bond Energy is the amount of energy needed to break a mole of a particular bond. To calculate energy change, add together the bond energies for all the bonds in the reactants – this is the ‘energy in’. Add together the bond energies for all the bonds in the products – this is the ‘energy out’. Calculate the energy change = energy in – energy out. H−H + Cl−Cl → 2 × (H−Cl) H−H 436 Cl−Cl 243 H−Cl 432 Energy in = 436 + 243 = 679 kJ/mole. Energy out = 2 × 432 = 864 kJ/mole. The energy change is negative, showing that energy is released to the surroundings in an exothermic reaction. 2 × (H−Br) → H−H + Br−Br H−Br 366 H−H 436 Br−Br 193 Energy in = 366*2 = 732 kj/mole. Energy out = 436+193 = 629 kJ/mole. The energy change is positive, showing that energy is gained from the surroundings in an endothermic reaction.

Endothermic and Exothermic

Endothermic reactions are reactions that take in energy while reacting. Some examples are Electrolysis, he reaction between ethanoic acid and sodium carbonate and the thermal decomposition of calcium carbonate in a blast furnace. Exothermic reactions are reactions that give out energy while reacting. Some examples are combustion, neutralisation reactions and oxidation reactions

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