C6- Chemical synthesis

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Making chemicals

  • Hazard symbols are used to show that chemicals are hazardous.
  • Chemists and engineers must assess the risks before using chemicals to make a new product.
  • Chemical synthesis means using simple substances to make new, useful chemical compounds.
  • The chemical industry uses chemical synthesis to make chemicals for food additives, fertilisers, dyes, paints, pigments and pharmaceuticals (medicines).
  • Food additives- Aspartame- Sweetening food
  • Fertilisers- Ammonium nitrate- Improving crop yields
  • Dyestuffs- Procion- Colouring clothes
  • Paints- Emulsion paint- Decorating homes
  • Pharmaceuticals- Aspirin- Relieving pain
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Acids and alkalis

  • Indicators turn different colours in acids and alkalis. Litimus turns red in acids and blue in alkalis. Universal indicator is orange or red in acids and green to blue in acids.
  • Pure acid compounds can be solids,liquids or gases. These compounds dissolve in water to form dilute acids that can be tested using indicators.
  • Sodium hydroxide, potassium hydroxide and calcium hydroxide are common alkalis.
  • The PH scale is a measure of how strong an acid or an alkali is.
  • The PH can be measured using universal indicator or a PH meter. The colour of the universal indicator can be compared to a colour chart to find the PH number of a sample.
  • Neutral solutions have a PH of 7. PH numbers for acids are below 7 and for acids are above 7.
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The PH scale

(http://ts4.mm.bing.net/th?id=HN.608007459248145202&w=221&h=107&c=7&rs=1&pid=1.7)

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Reactions of acids

  • Acids react with many metals and metal compounds to make a salt.
  • Acids react with many metals to form a salt and hydrogen gas, e.g: Calcium+ hydrochloric acid= calcium chloride+ hydrogen
  • Acids react with metal oxides and hydroxides to form a salt and water e.g: Magnesium oxide+ sulfuric acid= Magnesium sulfate+ water.
  • Acids react with metal carbonates to form a salt, water and carbon dioxide gas e.g: Calcium carbonate+ hydrochloric acid= calcium chloride+ water+ carbon dioxide.
  • Salts are ionic compounds and contain a positively charged metal ion and a negative ion from the acid.
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Continued

  • The reactions of acids can be shown using symbol equations with state symbols.
  • The state symbols can be either solid (s), liquid (l), gas (g) or a solution in water (aq).

To work out the formula of a salt, the number of positive charges must equal the number of negative chrages. For example in potassium sulfate, two potassium ions (K+) are needed to balance the charge on the sulfate ion (SO4 2-) so the formula is K2SO4.

A balanced equation has the same number of each type of atom on each side of the equation.

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Reacting amounts

  • The formula of a compound is the simplest ratio of the numbers of atoms that are in the compound.
  • The relative atomic masses of atoms can be found on the periodic table.
  • The relative formula mass of a compound is the sum of all of the relative atomic masses of all the atoms in the formula.
  • A balanced equation shows the atoms and molecules in a reaction. The number of atoms of each type of element is the same on both sides of the equation.
  • Relative atomic mass and relative formula mass can be used to work out the amount of reactants and products in the reaction.
  • The balanced equation for a reaction can be used to calculate the minimum quantity of reactants needed to make a particular amount of a product.
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Titrations

  • When an acid reacts with an alkali it becomes neutral, this is neutralisation.
  • A titration is used to measure the volume of acid and alkali that exactly react together.
  • An indicator is added so that you can see when neutralisation happens. The indicator suddenly changes colour at the end point of the titraion.
  • A titration is repeated to check that the results are close together. Variations between readings are small differences that happen due to small experimental errors.

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Continued

  • The range of readings is the spread of readings from the highest to the lowest
  • The true value should fall within the range of the readings.
  • An estimate of the true value can be worked out by calculating the mean of the results.
  • An outlier is a reading that is very different to most of the others. Outliers should be left out when calculating the mean ,because they are usually a result of errors in the measurement.
  • The volumes from a titration will allways be in the same proportion if the same concentrations of the same solutions are used.
  • (http://ts1.mm.bing.net/th?&id=HN.607993663824724674&w=300&h=300&c=0&pid=1.9&rs=0&p=0)
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Neutralisation

  • When an alkali reacts with an acid a salt and water are allways made.
  • All acids contain hydrogen ions (H+) when they are dissolved in water. The PH of an acid is related to the concentration of H+ ions in the acid solution.
  • All alkalis contain hydroxide ions (OH-), when they are dissolved in water.
  • In neutralisation reactions the hydrogen ions and hydroxide ions join up to form water molecules- H+(aq) +OH-(aq)= H20(l)
  • This is the ionic equation for all neutralisation rections. The negative ion from the acid and the positive ion from the alkali are left inthe solution to form the salt:
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Neutralisation continued

  • The positive ion from the alkali and the negative ion from the acid make the salt.
  • The formula of the salt can be worked out by looking at the chrages of the two ions,
  • For example: Sodium hydroxide+ Hydrochloric acid= Sodium chloride + water.
  • Salt: Sodium chloride.
  • Positive ion: Na+.
  • Negative ion: Cl-.
  • So the formula is NaCl.

Remember- Hydrochloric acid always forms chloride salts, sulfuric acid forms sulfate salts, and nitric acid forms nitrate salts.

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Energy changes

  • Exothermic reactions give out heat energy. The temperature of the surroundings rises.
  • Endothermic reactions take in heat energy. The temperature of the surroundings falls.
  • An energy level diagram summarises the energy changes in a reaction.
  • Energy changes for large scale-scale reactions in industry need to be carefully controlled, because very extreme temperature changes could cause overheating, explosions or fire.
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The importance of purity

A pure substance has nothing else mixed with it.

In industry pure substances need to be separated from impurities, such as left over reactants or other products, before they are used . Some impurities may be harmful.

Filtration can be used to seperate a solid from a liquid or from a solution.

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Crystallisation

Crystallisation is used to purify impure solid crystals. The process has several steps:

  • Dissolving- dissolve the product in a small amount of hot water (use only the minimum amount of water necessary to dissolve the product).
  • Filtering- filter off any off any solid impurities that do not dissolve. The solution tht comes through the filter is the filtrate.
  • Evaporating- The filtrate starts to crystallise as someof the water evaporates off. Cool the filtrate while the product continues to crystallise.
  • Filtering- Filter off the crystals, leaving any soluble impurities in the solution.
  • Drying- Dry the crystals in a dessicator or oven.
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Percentage yield

  • The percentage yield at the end of an experiment is worked out from the actual yield and the theoretical yield.
  • The actual yield is the mass of product measured at the end of the experiment.
  • The theoretical yield is the predicted yield. It is calculated from the amount of reactants used and the equationfor the reaction.
  • Percentage yield= actual yield/ theoretical yield x 100%
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Measuring rates of reaction

  • The rate of reaction is the amount of a product produced or the amount of reactant used up in a certain time. It is usually measured in the amount per second.
  • Chemical engineers look for ways to control reactions. By speeding the reaction up they can make them more economical. They also need to ensure that reactions occur at a safe rate.
  • In industry, chemical engineers aim to produce the most amount in the minimal time.  They change conditions to make reactions faster to make the process as economical as possible, but they also consider the cost of the energy and safety.
  • If the reaction makes a gas the rate can be followed by- measuring the volume of gas made at set times, e.g. every 30 seconds- measuring the decrease in mass of the flask as the gas leaves the reaction.
  • If the reaction makes a solid, the rate can be followed by measuring the time taken until you cannot see a cross under the flask or beaker.
  • A colorimeter can me used to measure the rate of a colour change.
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Rate of reaction graphs

  • Rate of reaction graphs show the change in the amount of of reactant or product against time.
  • The gradient of the curve at any point gives the rate of reaction.
  • The steeper the gradient, the greater the rate of reaction.
  • The reaction stops when the line becomes horizontal.
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Changing rates of reactions

  • For reactions to happen, particles must collide. The more collisions the faster the reaction.
  • A catalyst is a substance that speeds up a chemical reaction, but isn't used up.
  • Reactions are faster when:
  • The temperature of the reactants increases
  • The size of solid particles are smaller (this increases the surface area)
  • The concentration of reactants in the solution increases (concentration is measured in grams per dm3 of the solution.
  • To investigate the effect that one of these factors has on the rate of the reaction, it is important that all other factors are kept constant.
  • The rate of reaction increases when the freuency of collisions increases.
  • Rate of reaction increases with a greater surface area of solids and with a higher concentration of of solutions because the frequency of collisions increases.
  • Chemical engineers control rates by controlling th factors that affect the rate; e.g. temperature, concentration, particle size or using a catalyst.
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