Chemical synthesis, Acids & Alkalis
Chemical synthesis means using simple substances to make new, useful chemical compounds. The chemical industry uses chemical synthesis to make chemicals for food aditives, fertilisers, dyes, paints, pigments and pharmaceuticals (medicines).
Litmus paper turns red in acids and blue in alkalis. Universal indicator is orange or red in acids and green to blue in alkalis. Pure acid compounds can be soilds liquids or gases. They can be dissolved in water to create dilute acids that can be tested using indicators.
Some examples of solid acids are citric acid and tartaric acid. Liquid acids include sulphuric acid, nitric acid and ethanoic acid. Hydrochloric acid is a gas.
Common alkalis include sodium hydroxide, potassium hydroxide and calcium hydroxide.
pH can be measured using either universal indicator or a pH meter.
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:
calcium + hydrochloric acid --> calcium chloride + hydrogen
Acids react with metal oxides and hydroxides to make a salt and water:
magnesium oxide + sulphuric acid --> magnesium sulphate + water
sodium hysroxide + nitric acid --> sodium nitrate + water
Acids react with metal carbonates to form a salt, water and carbon dioxide:
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. To work out the formula of a salt, the number of positive charges must equal the number of negative charges. For example, in potassium sulphate, two potassium ions (K+) are needed to balance the charge on the sulphate ion (SO4 2-), so the formula is K2SO4.
Relative atomic mass (for atoms) and relative formula mass (for compounds) can be used to work out the amounts of reactants and products in the reaction.
The balanced equation for a reaction can be used to work out the minimum quantity of reactants needed to make a particular amount of a product.
For example, what mass of magnesium chloride can be made by reacting 1 tonne of magnesium oxide with hydrochloric acid?
MgO + HCl --> MgCl2 + H2O
MgO=24+16=40 --> MgCl2=24+(2X35.5)=96
40 tonnes --> 96 tonnes
1 tonne --> 96/40=2.4 tonnes
When an acid reacts with an alkali it becomes neutral. This is called a neutralistaion reaction. 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 tritration.
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. The true value should fall within the range of readings. An estimate for the true value can be calculated by calculating the mean of the results. An outlier is a reading that is very different to the rest of the other values, Outliers should be left out when you calculate the mean because they are usually the result of errors in measurement.
The volumes obtained from a titration will always be in the same proportion if the same concentrations of the same solutions are used.
Explaining neutralisation & Energy changes
All acids contain hydrogen 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 and hydroxide ions join up to form water molecules:
H+ (aq) + OH- (aq) --> H2o (l)
The positive ion from the alkali and the negative ion from the acid make the salt.
Exothermic reactions give out heat energy and the surrounding temperature rises. Endothermic reactions take in heat energy, causing the surrounding temperature to fall. An energy level diagram summarises the energy changes in a reaction. (see diagram on next card)
Exothermic and endothermic energy level diagrams
Separating and purifying
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 too. Filtration can be used to separate a solid from a liquid or solution.
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 minimum amount of water necessary to dissolve the product).
- Filtering: filter off any solid impurities that do not dissolve. The solution that comes through the filter is the filtrate.
- Evaporating: the filtrate starts to crystallise as some of 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.
The percentage yield at the end of an experiment is worked out from the theoretical yield and the actual yield. The actual yield is the mass of the 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 equation for the reaction.
Percentage yield = (actual yield / theoretical yield) X 100%
For example, an experiment reacts 2.4g magnesium with hydrochloric acid. The actual yield of magnesium chloride is 5.7g. What is the percentage yield?
Step 1: work out the theoretical yield:
- Mg + 2HCl --> MgCl2 + H2
- Relative masses: 24 Mg --> 95 MgCl2
- Reacting masses: 2.4g --> 9.5g
- Theoretical yield of magnesium chloride = 9.5g
Step 2: work out the percentage yield:
- Percentage yield = (5.7 / 9.5) X 100% = 60%
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 as 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 the reaction occurs 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 economial as possible but they must 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
- 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 underneath the flask or beaker.
A colourimeter can be used to follow the rate of a colour change.
Changing rates of reactions
For reactions to occur, particles must collide. The more collisions, the faster the recation. A catalyst is a substance that speeds up a chemical reaction but is not used up.
Reactions are faster when:
- the temperature of the reactants increases
- the size of solid particles are smaller (increases surface area)
- the concentration of reactants in solution increases
- a catalyst is used
Rate of reaction increases when the frequency of collisions increases.