C6: Chemical Synthesis

Chemicals are all around us and we depend on them daily. Chemical synthesis is the process by which raw materials are made into useful products such as food additives, fertilisers, dyestuffs, pigments, pharmaceuticals, cosmetics and paints.

The chemical industry makes bulk chemicals, such as sulfuric acid and ammonia, on a very large scale (millions of tonnes per year). Fine chemicals, such as drugs and pesticides, are made on a much smaller scale.

The range of chemicals made in industry and laboratories in the UK are as follows: Pharmaceuticals 31.5%, paint, varnishes and printing inks 8%, agrochemicals 3%, industrial glass 5%, dyes and pigments 3%, basic inorganics 2.5%, basic organics 12%, fertilisers 1%, plastic and synthetic rubber 7.5%, synthetic fibres 2%, other specialities 13% and soaps, toiletries and cleaning preparations 11.5%.

Many of the raw materials are hazardous, therefore it is important to recognise the standard hazard symbols and understand the necessary precautions that need to be taken.

The pH scale is a measure of the acidity or alkalinity of an aqueous solution, across a 14-point scale. Acids are substances that have a pH less than 7. Bases are the oxides and hydroxides of metals. Those which are soluble are called alkalis and they have a pH greater than 7.

Indicators such as litmus paper, universal indicator and pH meters are used to detect whether a substance is acidic or alkaline. The pH of a substance is measured using a full range indicator, such as universal indicator solution or a pH meter.

Acidic compounds produce aqueous hydrogen ions when they dissolve in water. Some common acids are; citric acid - solid, tartaric acid - solid, nitric acid - liquid, sulfuric acid - liquid, ethanoic acid - liquid, hydrogen chloride (hydrochloric acid) - gas.

Alkalis produce aqueous hydroxide ions when they dissolve in water. Some common alkalis are; sodium hydroxide, potassium hydroxide, magnesium hydroxide and calcium hydroxide.

When an acid and a base are mixed together in the correct amounts, they 'cancel' each other out. This reaction is called neutralisation because the solution that remains has a neutral pH of 7.

Acid + Base ----------// Neutral salt solution + Water

A balanced equation for a chemical reaction shows the relative numbers of atoms and molecules of reactants and products taking part in the reaction.

During neutralisation, the hydrogen ions from the acid react with the hydroxide ions from the alkali to make water. The simplest way of writing a neutralisation equation is:

H+ (aq) +OH- (aq) ------------// H20 (l)

The salt produced during neutralisation depends on the metal in the base and the acid used. Hydrochloric acid produces chloride salts; sulfuric acid produces sulfate salts; and nitric acid produces nitrate salts.

• Sodium chloride NaCl
• Potassium chloride KCl
• Sodium carbonate Na2CO3
• Sodium nitrate NaNO3
• Sodium sulfate Na2SO4
• Magnesium sulfate MgSO4
• Magnesium carbonate MgCO3
• Magnesium oxide MgO
• Magnesium chloride MgCl2
• Calcium carbonate CaCO3
• Calcium chloride CaCl2

• Chlorine Cl2
• Hydrogen H2
• Nitrogen N2
• Oxygen 02

When chemical reactions occur, energy is transferred to or from the surroundings. Therefore, many chemical reactions are accompanied by a temperature change.

Exothermic reactions are accompanied by a temperature rise. They transfer heat energy to the surroundings, i.e. they give out heat. The combustion of carbon is an example of an exothermic reaction: carbon + oxygen-----// carbon dioxide    C +O2 ---------// CO2 

It is not only reactions between fuels and oxygen that are exothermic. Neutralising alkalis with acids and many oxidation reactions also give out heat.

The energy change in an exothermic reaction can be shown using an energy-level diagram. Energy is lost to the surroundings during the reaction, so the products have less energy than the reactants.

Endothermic reactions are accompanied by a fall in temperature. Heat is transferred from the surroundings, i.e. they take in heat. The reaction between citric acid and sodium hydrogencarbonate is an example of an endothermic chemical reaction. Dissolving ammonium nitrate crystals in water is an example of a physical change that is exothermic. 

Ammonium nitrate + Water ----// Ammonium nitrate solution.

NH4NO3 (s) + H2O (l) ------// NH4NO3 (aq)

Thermal decomposition is also an example of an endothermic reaction.

The energy change in an endothermic reaction can be shown using an energy-level diagram. Energy is taken in during the reaction, so the products have more energy than the reactants. 

Since energy changes play a major role in chemical reactions, it is very important that these changes are managed during chemical synthesis. For example, there could be a nasty accident if too much energy was given out during a reaction.

Whenever chemical synthesis takes place, the starting materials (reactants) react to produce new substances (products). The greater the amount of reactants used, the greater the amount of product formed. The percentage yield can be calculated by comparing the actual amount of product made (actual yield) with the amount of product you would expect to get if the reaction goes to completion (theoretical yield). Percentage yield = Actual yield/Theoretical yield X 100

Stages of Chemical Synthesis of an Inorganic Compound:

• Establish the reaction or series of reactions that are needed in order to produce the product.
• Carry out a risk assessment.
• Calculate the quantities of reactants to use.
• Carry out the reaction under suitable conditions, e.g. temperature, concentration and presence of a catalyst.
• Separate the product from the reaction mixture.
• Purify the product to ensure it is not contaminated by other products or reactants (e.g. by evaporation, crystallisation or drying in an oven or desiccator).
• Measure the yield,
• Check the purity (e.g. by titration) to make sure you have made the correct product.

Steps to make sodium chloride:

• NaOH (aq) + HCl (aq) -------// NaCl (aq) +H2O (l)
• Wear safety glasses and use dilute hydrochloric acid.
• Calculate the quantities of reactants (shown on next notecard).
• Measure out 25cm3 of hydrochloric acid in a measuring cylinder and pour it into a beaker.
• Add a few drops of indicator, such as phenolphthalein, which will stay colourless unless alkali is present. Alternatively, use a pH meter to follow the reaction.
• Slowly add sodium hydroxide until the indicator changes or the pH meter reads 7. Pink is the alkali colour for phenolphthalein.
• Mix a spatula of activated charcoal (charcoal treated with oxygen) into the solution. The indicator molecules will be absorbed onto the surface of the charcoal. Filter to remove both the activated charcoal and the indicator.
• Remove the water by gently heating the solution to evaporate and form crystals after cooled.
• Wash the crystals to remove any excess NaOH and dry in a desiccator or oven.
• Weigh the mass and calculate the percentage yield.
• Check the purity by titration.

In order to work out how much of each reactant is required to make a known amount of product, you must understand:

• that a balanced equation shows the relative number of atoms or molecules of reactants and products taking part in the reaction.
• that the relative atomic mass of an element shows the mass of its atoms relative to the mass of other atoms.
• that relative atomic masses of elements can be found from the periodic table.
• how to calculate the relative atomic mass
• how to substitute the relative formula masses and data into a given mathematical formula to calculate the reacting masses and/or products from a chemical reaction.
• how to work out the ratio of the mass of reactants to the mass of products.
• how to apply the ratio to the question.

Titration can be used to calculate the concentration of an acid, such as citric acid, by finding out how much alkali is needed to neutralise it.

1. Fill a burette with the alkali sodium hydroxide (the conentration of the alkali must be known) and take an initial reading of the volume.

2. Accurately weigh out a 4g sample of solid citric acid and dissolve it in 100cm3 of distilled water.

3. Use a pipette to measure 25cm3 of the aqueous citric acid and put it into a conical flask. Add a few drops of the indicator, phenolphthalein, to the conical flask. The indicator will stay colourless. Place the flask on a white tile under the burette.

4. Add the alkali from the burette to the acid in the flask drop by drop.Swirl the flask to ensure it mixes well. Near the end of the reaction, the indicator will start to turn pink.Keep swirling and adding the alkali until the indicator is completely pink, showing that the citric acid has been neutralised. Record the final burette reading. Work out the volume of alkali added from: volume=final reading - initial reading.

Repeat the whole procedure until you get two results that are within +/- 0.05cm3

The rate of a chemical reaction is the amount of reaction that takes place in a given unit of time. Chemical reactions only occur when the reacting particles collide with each other with sufficient energy to react. Chemical reactions can proceed at different speeds, e.g. rusting is a slow reaction, whereas burning is a fast reaction.

The rate of a chemical reaction can be found in different ways:

• Weighing the reaction mixture - If one of the products is a gas, you could weigh the reaction mixture at timed intervals. The mass of the mixture will decrease.
• Measuring the volume of gas produced - You could use a gas syringe to measure the total volume of gas produced at timed intervals.
• Observing the formation of a precipitate - This can be done by either watching a cross (on a tile underneath the conical flask to see when the cross disappears) in order to measure the formation of a precipitate or by monitoring a colour change using a light sensor which will lead to a more reliable and accurate results as there is a definite end point. There are also more data points collected, especially if it is interfaced with a computer.
• Observing the loss of colour or loss of a precipitate - This is essentially the opposite of the previous one.

Graphs can be plotted to show the progress of a chemical reaction - there are three things to remember:

1. The steeper the line, the faster the reaction.

2. When one of the reactants is used up, the reaction stops (line becomes flat).

3. The same amount of product is formed from the same amount of reactants , irrespective of rate.

There are four important factors that affect the rate of reaction - temperature, concentration of dissolved reactants, surface area and the use of a catalyst.

1. Temperature of the Reactants - In a cold reaction mixture, the particles move quite slowly. They will collide less often, with less energy, so fewer collisions will be successful. In a hot reaction mixture, the particles move more quickly. They will collide more often, with greater energy, so many more collisions will be successful.

2. Concentration of the Dissolved Reactants - In a low concentration reaction, the particles are spread out. The particles will collide with each other less often, resulting in fewer successful collisions. In a high concentration reaction, the particles will collide with each other more often, resulting in many more successful collisions.

3. Surface Area of Solid Reactants - Large particles (e.g. solid lumps) have a small surface area in relation to their volume, meaning fewer particles are exposed and available for collisions. This means that particles will collide with each other less often, resulting in fewer successful collisions and therefore a slow rate of reaction. Small particles have a large surface area in relation to their volume so more particles are exposed and available for collisions. This means that particles will collide with each other more often, resulting in more successful collisions and a faster rate of reaction.

4. Using a catalyst - A catalyst is a substance that increases the rate of a chemical reaction, without being changed during the process. Consider the decomposition of hydrogen peroxide: Hydrogen peroxide --------// Water + Oxygen.........2H2O2 (aq) --------// 2H2O (l) + O2 (g)

We can measure the rate of this reaction by measuring the amount of oxygen given off at one-minute intervals. This reaction happens very slowly unless we add a catalyst of manganese(IV) oxide. With a catalyst, plenty of fizzing can be seen as the oxygen is given off.

The same amount of gas is given off,but it takes a far shorter time when a catalyst is present.

Catalysts work by lowering the amount of energy needed for a successful collision. They are specific to a particular reaction amd are not used up during the reaction. Consequently, only small amounts are needed.

Chemical reactions only occur when particles collide with each other with sufficient energy. Increasing temperature causes an increase in the kinetic energy of the particles, i.e. they move a lot faster. This results in more energetic collisions happening more frequently. The minimum energy required for a reaction will, therefore, be achieved more often, resulting in a greater rate of reaction. 

An increase in concentration or surface area results in more frequent collisions and, therefore, more collisions that are sufficiently energetic for a reaction to occur.

When carrying out a chemical synthesis on an industrial scale, there are also economic, safety and environmental factors to consider:

• The rate of manufacture must be high enough to produce a sufficient daily yield of product.
• Percentage yield must be high enough to produce a sufficient daily yield of product.
• A low percentage yield is acceptable, providing the reaction can be repeated many times with recycled starting materials.
• Optimum conditions should be used that give the lowest cost rather than the fastest reaction or highest percentage yield.
• Care must be taken when using any reactants or products that could harm the environment if there was a leak.
• Care must be taken to avoid putting any harmful by-products into the environment.
• A complete risk assessment must be carried out, and the necessary precautions taken.