The Chemical Industry
- The chemical industry synthesises chemicals on different scales according to their value.
- Bulk chemicals are made on a large scale, e.g. ammonia, sulfuric acid, sodium hydroxide, phosphoric acid.
- Fine chemicals are made on a small scale, e.g. drugs, food additives, fragrances.
- New chemical products or processes are the result of an extensive programme of research and development, for example, researching catalysts for new processes.
- Products have to be thoroughly tested to ensure that they are effective and safe to use.
- Governments have a duty to protect people and the environment from any dangers that could occur as a result of procedures involving chemicals.
- They impose strict regulations in order to control chemical processes, the storage of chemicals, the transportation of chemicals and the research and development of chemicals.
- In the UK, the Health and Safety Executive (HSE) is responsible for the regulation of risks to health and safety arising from the extraction, manufacture and use of chemicals. For example, all hazardous chemicals need to be labelled with standard hazard symbols.
- The production of useful chemicals involves several stages, including preparation of feedstock ,synthesis, separation of products, handling of by-products and waste and monitoring purity.
- Green chemistry is based on a number of principles, which if followed lead to more sustainable processes.
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- Atom Economy - The final product should aim to contain all the atoms used in the process, thereby reducing waste products and increased the yield.
- Atom economy = Mass of atoms in the useful product / Mass of atoms in the reactants X 100
- Use of Renewable Feedstocks - Whenever possible, a renewable raw material should be used. Crude oil (non-renewable) is currently the main source of chemical feedstocks. Several companies are developing new materials from plants, but plants take up a lot of land. Fertilisers can be used to increase productivity, but they use up a lot of energy during manufacture.
- Energy Inputs or Outputs - The energy needed to carry out a reaction should be minimised in order to reduce the environmental and economic impact. Where possible, the processes should be carried out at ambient temperature and pressure. Using catalysts makes reactions more efficient and can significantly reduce the amount of energy needed in the process.
- Health and Safety Risks - Substances used in a chemical process should be chosen to minimise the risk of chemical accidents, including explosions and fires. Methods need to be developed to detect harmful products before they are made.
- By-products or Waste - If waste is not made, then it will not have to be cleaned up. Where by-products are made, processes should be put in place to deal with them.
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Green Chemistry and Catalysts
- Environmental Impact - The environmental impact can be reduced by using alternatives to hazardous chemicals. Efficient chemical products could be designed that pose minimal harm to people or the environment. They should be able to be broken down into non-toxic substances that do not stay in the environment.
- Social and Economic Benefits - Social benefits include cleaner air quality and generally less creation of pollution. This will lead to cleaner buildings in towns and improved water quality in rivers and lakes. Economic benefits include reduced energy costs, as many industrial processes will be operated at lower temperatures and pressures.
- The chemical industry carries out research and development to ensure that its processes are sustainable. In recent years there has been a lot of research and development into catalysts. For example, an exothermic reaction can be started by using only a small amount of energy ad, as the catalyst remains unchanged, it can be used over and over again. This makes the process more sustainable. Some industrial processes use enzyme catalysts, which means that the temperature and pH of the reaction must be carefully monitored. The activation energy is the energy needed to break chemical bonds to start a reaction. Catalysts reduce the activation energy needed for a reaction by providing an alternative route for the reaction. This makes the reaction go faster.
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- Hydrocarbons are made up only of carbon and hydrogen atoms. The 'spine' of a hydrocarbon is made up of a chain of carbon atoms.
- There is a group of hydrocarbons called the alkanes. In an alkane the carbon atoms are joined together by single carbon-carbon bonds (C-C). So, all the carbon atoms are linked to four carbon or hydrogen atoms by single bonds. This means that all their bonds are single and the hydrocarbon is saturated.
- The general formula for alkanes is CnH2n+2
- Methane CH4, Ethane C2H6, Propane C3H8, Butane C4H10.
- Alkanes do not react with aqueous reagents because C-C and C-H bonds are difficult to break and are therefore unreactive. However, they do burn well in plenty of air to produce CO2&H2O.
- Some groups of hydrocarbons, e.g. the alkenes, are more reactive than the alkanes because they contain a reactive carbon-carbon double bond (C=C).
- Alkenes are unsaturated.
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- Alcohols form a homologous series with the functional group -OH. The presence of the -OH group gives alcohols their characteristic properties. The general formula for alcohols is CnH2n+1OH, when n is the number of carbon atoms.
- The two simplest alcohols are methanol (CH3OH) and ethanol (C2H5OH).
- Methanol is an important chemical feedstock. Methanol can be used in the manufacture of fuels, adhesives, foams, cosmetics and solvents.
- Ethanol can be used as a solvent, a fuel and a component in alcoholic drinks.
- Alcohols contain a hydrocarbon chain and an -OH group, so their physical properties can be compared with those of the alkanes and water.
- The hydrocarbon chain behaves like the alkane, e.g. it is less dense than water because the long hydrocarbon chains do not mix with water because the long hydrocarbon chains do not mix with water. This is seen with longer chain molecules.
- The -OH group behaves like water, which explains the higher than expected boiling point. The forces between the molecules are stronger than in the alkanes.
- Alcohols burn in air to produce CO2 and water due to the presence of the hydrocarbon chain.
- Sodium floats on water, melts, rushes around on the surface and rapidly reacts with water giving off hydrogen. // Sodium sinks in alcohol, does not melt and steadily reacts with alcohol giving off hydrogen. // There is no reaction between sodium and an alkane.
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- Raw materials:crude oil or natural gas and steam. Product:produces up to 96% pure ethanol on an industrial scale and is used as a feedstock, solvent or fuel.
- Preparation of Feedstock - Crude oil undergoes fractional distillation. The fractions containing the long-chained hydrocarbons are collected. The alkanes are then heated until they vaporise. The molecules are cracked by passing the vapour over a catalyst at high temperature (300 degrees C) and 60-70 atmospheres pressure.
- After purification by further fractional distillation, the ethene molecules produced in the cracking process are used for feedstock.
- The remaining 4% water is removed by zeolites, which absorb the water molecules to produce pure ethanol. This method replaced the old dehydration method, which used more energy and produced carcinogenic by-products.
- Synthesis of Ethanol - Ethene is continuously reacted with steam at a moderately high temperature and pressure by passing the gases over a catalyst. C2H4(g) + H2O(g) -// C2H5OH
- Recycling - Any unreacted products are recycled and fed through the system again. Ethene may also be obtained by cracking ethane obtained from natural gas.
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- Raw materials:natural sugars, yeast and water. Production: produces up to approximately 15% alcohol by volume.
- Water and yeast are mixed with sugars at just about room temperature. Enzymes (biological catalysts), found in the yeast, react with the sugars to form ethanol and carbon dioxide. The carbon dioxide is allowed to escape from the reaction vessel, but air is prevented from entering
- Water+Sugars+Yeast ----// Ethanol+Carbon dioxide. C6H12O6(aq) ---// 2C2H5OH(aq)+2CO2(g)
- Temperature and pH are important factors to consider when determining optimum conditions for the fermentation process. Enzymes use a 'lock and key' mechanism, which means that a specific reactant fits into a specific enzyme. If the temperature of the reaction rises too much, the enzyme is denatured (the shape is irreversibly changed) and the reactant can no longer fit into the enzyme. Distillation process is used to produce spirits such as whisky and brandy.
- If the pH of the mixture changes too much, the enzyme may also become denatured due to attractions of excess H+ or OH- ions.
- When ethanol solution is manufactured by fermentation, the concentration is limited. The main limiting factors are the amount of sugar in the mixture (reaction will stop after all used up) and the enzymes in the yeast (if conc of ethanol above approx 15%, they die of alcohol poisoning).
- When it's over, the concentration of the ethanol may be increased by distilling the mixture.
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- Raw materials:waste biomass and genetically engineered E. coli bacteria. Product: produces up to 95% pure alcohol.
- The biotechnology method uses genetically modified E. coli bacteria that have had new genes introduced. The new genes allow the bacteria to digest all the sugars in the biomass and convert them into ethanol.
- This means that a wider range of biomass, such as wood waste, corn stalks and rice hulls, can be converted to ethanol rather than remaining as waste. This method still needs a variety of organic substances for bacterial metabolism and growth.
- The optimum temperature for this reaction is 25-37 degrees C. The optimum pH level needs to remain fairly constant otherwise the enzyme will be denatured.
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- Carboxylic acids form a homologous series with the functional group -COOH. The presence of the -COOH group gives carboxylic acids their characteristic properties. The two simplest carboxylic acids are methanoic acid (HCO2H) and ethanoic acid (CH3CO2H).
- Vinegar is a dilute solution of ethanoic acid.
- Carboxylic acids are found in many substances and some have unpleasant smells and tastes. For example,they are responsible for the aroma of a sweaty shoe and the taste of rancid butter
- The general formula for carboxylic acids is CH2n+1COOH.
- Carboxylic acids are weak acids. In solution they have a pH from 3 to 6. Like all acids, they can react with metals, alkalis and carbonates to produce carboxylic acid salts, but they are less reactive than strong acids, e.g. hydrochloric acid. Reaction of a carboxylic acid with:
- a metal: Ethanoic acid + Sodium -----// Sodium ethanoate + Hydrogen.
- an alkali: Ethanoic acid + Sodium hydroxide ------// Sodium ethanoate + Water.
- a carbonate: Ethanoic acid + Sodium carbonate --// Sodium ethanoate + Water+Carbon dioxide
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Fats and Esters
- Fats and oils are naturally occurring esters. Living organisms make them to use as an energy store. Fats are the esters of glycerol (an alcohol with three -OH groups) and fatty acids (which are carboxylic acids with very long hydrocarbon chains). Animal fats, such as lard and fatty meat, are mostly saturated molecules. This means they have single carbon-carbon bonds and the molecules are unreactive.
- Vegetable oils, such as olive oil and sunflower oil, are mostly unsaturated molecules. This means that they contain some double carbon-carbon bonds (C=C). The presence of the C=C bonds means that the molecules are unreactive. E.g. vegetable oils are unsaturated fats.
- Carboxylic acids react with alcohols to form esters. The reaction is carried out in the presence of a catalyst. Esters have distinctive smells that are responsible for the smells and flavours of fruits. Due to their sweet smell, they are often used in the manufacture of perfymes, fragrances and food products. Esters are also found in products such as solvents in adhesives and plasticisers as they contain hydrocarbon chains.
- Ethanol and excess ethanoic acid are heated under reflux in presence of conc sulfuric acid.
- The ester is removed by distillation (ethyl ethanoate boils at 77 degrees C).
- The distillate is transferred to a separating funnel, where it is purified. A solution of sodium carbonate is added and the mixture is shaken up. This mixture will react with any remaining acid and extract it into the aqueous phase which is then run off leaving the ester in the funnel.
- The product is transferred to a conical flask and anhydrous calcium chloride is added to remove any remaining water molecules. The calcium chloride is removed later by filtration.
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Making and Breaking Bonds
- In a chemical reaction, new substances are produced. In order for this to happen, the bonds in the reactants must be broken and new bonds made to form the products.
- The activation energy is the energy needed to start a reaction, i.e. to break old bonds. This can be shown on an energy-level diagram.
- Breaking a chemical bond requires a lot of energy - this is an endothermic process. When a new chemical bond is formed, energy is given out - this is an exothermic process.
- If more energy is required to break old bonds than is released when the new bonds are formed, the reaction is endothermic.
- If more energy is released when the new bonds are formed than is needed to break the old bonds, the reaction is exothermic.
- The \------/ sign shows a reversible reaction.
- A + B \\---------// C + D
- This means that A and B can react together to produce C and D and C and D can react together to produce A and B.
- For example, solid ammonium chloride decomposes when heated to produce ammonia and hydrogen chloride gas, both of which are colourless. Hydrogen chloride gas and ammonia react to produce white clouds of ammonium chloride.
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- A reversible reaction will reach a state of equilibrium if it is in a closed system (a system where no reactants are added and no products are taken away).
- Att equilibrium the reaction appears to have stopped. However, neither the forward reaction (from left to right) nor the backward reaction (from right to left) are complete as both reactants and products are present at the same time. The concentration of the reactants and products does not change. The relative amounts of all the reacting substances at equilibrium depend on the conditions of the reaction.
- If the forward reaction (the reaction that produces the products C and D) is endothermic then: if the temperature is increased, the yield of products is increased and if the temperature is decreased, the yield of products is decreased.
- If the forward reaction is exothermic then: if the temperature is increased, the yield of products is decreased and if the temperature is decreased, the yield of products is increased. Although a reversible reaction might not go to complete, it could still be used efficiently in an industrial process. e.g. Haber process for ammonia.
- Once equilibrium is achieved, the concentration of the reactants and products does not change. The equilibrium can be approached from either direction.
- Chemical equilibriums are dynamic. Both the forward and backward reactions are still occurring, but at the same rate so, there is no overall change in concentration of substances.
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- Ammonia is a very important chemical as it is needed to make fertilisers (ammonium nitrate), nitric acid and explosives. Traditionally ammonia was made using sources of nitrate.
- The Haber process was a major scientific breakthrough as it allowed nitrogen from the air to be converted into ammonia on an industrial scale. This in turn has affected both society and the environment. E.g. ability to manufacture explosives prolonged the First World War.
- The raw materials for the manufacture of ammonia by the Haber process are nitrogen and hydrogen. Nitrogen is extracted from the air by fractional distillation of liquid air. Hydrogen is obtained by reacting natural gas (methane) with steam: CH4(g) + H2O(g)-----//3H2(g) + CO(g)
- The purified nitrogen and hydrogen are passed over an iron catalyst at a temperature of about 450 degrees C and a high pressure of about 200 atmospheres. The reaction is reversible and exothermic: Nitrogen + Hydrogen \\------// Ammonia. N2(g) + 3H2(g) \\----// 2NH3(g).
- Some of the hydrogen and nitrogen reacts to form ammonia. Some of the ammonia produced decomposes into nitrogen and hydrogen. However, the gases do not stay in the reaction vessel long enough to reach equilibrium. To increase the overall yield, the unreacted nitrogen and hydrogen are recycled.
- The conditions for the Haber process are low temperature and high pressure.
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- Low Temperature - The reaction in the forward direction is exothermic. If the temperature is lowered, the equilibrium will shift to the right to increase the temperature. The amount of ammonia formed will increase. Lowering the temperature slows down the reaction and a catalyst of iron is used to speed up the reaction.
- High Pressure - There are four molecules of gas on the left-hand side and two molecules of gas on the right-hand side. If the pressure is increased, the equilibrium will shift to the right to lower the pressure. The amount of ammonia formed will increase.
- Altering the temperature and pressure can have a big impact on the production of ammonia in the Haber process. The conditions have to be chosen very carefully to be economically viable and to make sure they can meet demand.
- The formation of ammonia is exothermic so a low temperature increases the yield, but the reaction is very slow. A high temperature makes the reaction faster but produces a lower yield. So a compromise is reached.
- The volume of ammonia produced is less than the total volume of the reactants (nitrogen and hydrogen) so a high pressure favours the production of ammonia, but this is very expensive. A low pressure is more affordable, but this produces a low yield. So, yet again, a compromise is reached.
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The Nitrogen Cycle
- The nitrogen cycle shows how nitrogen and its compounds are recycled in nature. Nitrogen is a vital element of all living things and is used to make proteins, which are used in plant and animal growth. All enzymes are proteins. Nitrogen gas in the air canot be used by plants and animals as it is inert (unreactive). Plants can only use it in the form of nitrates. The main processes in the nitrogen cycle are as follows:
- Nitrogen-fixing bacteria convert atmospheric nitrogen into nitrates in the soil.
- When plants are eaten the nitrogen becomes animal protein.
- Dead organisms and waste contain ammonium compounds.
- Decomposers convert urea, faeces and protein from dead organisms into ammonium compounds.
- Nitrifying bacteria convert ammonium compounds into nitrates in the soil.
- Denitrifying bacteria convert nitrates into atmospheric nitrogen and ammonium compounds.
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- Nitrogen fixation is the process by which nitrogen gas in the atmosphere is converted into more useful nitrogen compounds, such as ammonia, nitrates and nitrogen oxides. This can be done by nitrogen-fixing bacteria, which contain the enzyme nitrogenase. Nitrogenase is a biological catalyst that allows the process of fixing nitrogen to take place at room temperature and pressure.
- Some nitrogen-fixing bacteria live in the soil, while others are found in the root nodules of legumes such as peas, beans and clovers.
- Nitrogen can also be fixed during lightning storms. The enormous energy of lightning breaks the strong covalent bonds in the nitrogen molecule. This enables the nitrogen atoms to react with oxygen in the air, forming nitrogen oxides. Nitrates are formed as the nitrogen oxides dissolve in rainwater.
- Nitrogen can be fixed on an industrial scale through chemical reactions such as the Haber process, in which nitrogen from the air is combined directly with hydrogen to form ammonia. This method is not very sustainable as it is expensive to run as high temperatures and pressures are involved, requring specialised equipment, hydrogen comes from natural gas, which is non-renewable, it uses a lot of energy and the yield is low - only about 15%.
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Looking to the Future
- Scientists are working hard to develop new catalysts to try to improve the efficiency of the Haber process. They are particularly interested in producing a new catalyst that mimics the natural enzyme, nitrogenase. They know that it contains clusters of iron (Fe), molybdenum (Mo) and sulfur (S), and have been successful in making a molecule that shows some catalytic activity.
- If successful, this would be a major breakthrough as it would enable nitrogen to be fixed on an industrial scale at room temperature and pressure. It would provide a very sustainable source of ammonia and would solve most of the issues listed above.
- Over the years there have been some very specific developments in the industrial methods of nitrogen fixation, many of them involving more efficient catalysts. In the future scientists may need to look for a new source or feedstock for hydrogen if reserves of natural gas become depleted. Hydrogen obtained by the electrolysis of water could be an option, but this would depend on the availability of a large renewable energy source, e.g. solar energy or hydropower.
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- There are two types of analytical procedure: qualitative methods and quantitative methods.
- Qualitative analysis is any method used to identify the chemicals in a substance, e.g. using an indicator to find out if acids are present or using thin layer chromatography.
- Quantitative analysis is any method used to determine the amount of chemical in a substance, e.g. carrying out an acid-base titration to find out how much acid is present.
- Many of the analytical methods you have learned are based on samples in solutions.
- There are standard procedures for the collection, storage and preparation of samples for analysis.
- When collecting data, it is very important that the samples are representative of the bulk of the material under test.
- This is achieved by collecting multiple samples at random. After a sample has been collected, it should be stored in a sterile container to prevent change or deterioration.
- The container should be sealed, labelled and stored in a safe place.
- Using a system of common practices and procedures, such as ensuring that samples are not contaminated, can increase reliabilty since there is less room for human error. Different people can also repeat a test on the same sample and produce the same result.
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- Chromatography is a technique used to find out what unknown mixtures are made up of.
- Paper Chromatography - If the substance to be analysed is a solid, dissolve it in a suitable solvent (the solvent used will depend on the solubility of the substance).
- Place a spot of the resulting solution onto a sheet of chromatography paper on the pencil line and allow it to dry.
- Place the bottom edge of the paper into a suitable solvent.
- The solvent rises up the paper, dissolving the 'spot' and carrying it, in solution, up the paper.
- The different chemicals in the mixture become separated because their molecules have different sizes and properties. The molecules that bind strongly to the paper travel a shorter distance than the molecules that bind weakly to the paper.
- The chromatogram can then be compared to standard chromatograms (standard reference materials) of known substances to identify the different chemicals.
- The solvent that is used to move the solution is called the mobile phase.
- A range of aqueous and non-aqueous solvents may be used. Aqueous solvents are water based, whereas non-aqueous solvents are made from organic liquids such as alcohols.
- The medium that it moves through is called the stationary phase. In this case the paper is.
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- A chromatogram is formed when the chemicals come out of solution and bind to the paper, i.e. they move between the mobile phase and the stationary phase.
- For each component of the sample, a dynamic equilibrium is set up between the stationary and mobile phase.
- Different molecules in the sample mixture travel different distances according to how strongly they are attracted to the molecules in the stationary phase, in relation to their attraction to the solvent molecules.
- Therefore, the overall separation depends on the distribution of the compounds in the sample between the mobile and stationary phases.
- Thin Layer Chromatography - Thin layer chromatography (TLC) is similar to paper chromatography. However, the stationary phase is a thin layer of absorbent material supported on a flat, unreactive surface.
- There are several advantages of thin layer chromatography over paper chromatography which include: faster runs, more even movement of the mobile phase through the stationary phase and a choice of different absorbents for the stationary phase.
- As a result, thin layer chromatography usually produces better separations for a wider range of substances.
- Locating Agents - Some chromatograms have to be developed using locating agents to show the presence of colourless substances:
- Colourless spots can sometimes be viewed under ultraviolet (UV) light and then marked on the plate.
- The chromatogram can be viewed by being sprayed with a chemical that reacts with the spots to cause coloration.
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R, Value & Gas Chromatography
- In paper and thin layer chromatography, the movement of a substance relative to the movement of the solvent front is known as the R, value:
- R, value = Distance travelled by substance / Distance travelled by solvent.
- In gas chromatography (GC), the mobile phase is a carrier gas, usually an inert gas such as helium or nitrogen. The stationary phase is a microscopic layer of liquid on an unreactive solid support. The liquid and support are inside glass or metal tubing, called a column.
- A sample of the substance to be analysed is injected into one end of the heated column, where it vaporises. The carrier gas then carries it up the column, where separation takes place.
- GC has a greater separating power than TLC or paper chromatography and can separate complex mixtures. It can produce quantitative data from very small samples of liquids, gases and volatile solids.
- The size of each peak in the chromatogram produced by GC shows the relative amount of each chemical in the sample.
- GC can separate the components in a mixture because of their different solubilities in the stationary or mobile phases.
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GC and Quantitative Analysis
- Practical uses of gas chromatography include detecting banned substances in blood and urine and analysing the exact characteristics of oil or pesticide spills and matching them to samples from suspected sources, to identify where the pollution has come from.
- The time taken for each substance to pass through the chromatographic system depends on its solubility. This is called the retention time. In GC, the retention time is the time taken from the sample being injected into the system to when the substance is detected.
- Tables of relative retention times show the retention times of different chemicals relative to the retention time of a specific compound.
- Quantitative analysis determines the amount of a chemical in a sample. Stages are as follows:
- Choose a suitable analytical method.
- Take a sample, which represents the bulk material and make sure it is well mixed.
- Measure out a sample for analysis. Decide on the required level of accuracy before choosing the equipment.
- For solids, check the number of decimal places on the balance and dissolve the sample.
- For liquids, choose between burettes, pipettes and measuring cylinders.
- Measure a property of a solution. // Calculate a value. // Estimate the reliability of the results.
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- Many methods of quantitative analysis use solutions. The concentration of a solution is the quantity of solid dissolved in the liquid. The concentration of a solution is measured in g/dm3.The formula to calculate concentration is: concentration = mass / volume.
- The concentrations of standard solutions are known accurately. Therefore, these solutions can be used to measure the concentration of other solutions. A standard procedure is used to make up the solution.
- Acid-alkalis titration is an important method of quantitative analysis. Procedure is as follows:
- Fill a burette with the alkali and take an initial reading of the volume.
- Accurately weigh out a 4g sample of solid acid and dissolve it in 100cm cubed of distilled water
- Use a pipette to measure 25cm cubed of the aqueous acid and put it into a conical flask. Add a few drops of an indicator to the conical flask. Place the flask on a white tile.
- 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 a reaction, the indicator will start to turn the alkali colour. Keep swirling and adding the alkali until the indicator goes permanently pink on the addition of one drop of alkali, showing that the acid has been neutralised.
- Record the volume of the alkali added by subtracting the initial burette reading from the final burette reading.
- Repeat the whole procedure until you get two results that are the same or close or, repeat the procedure three times and take the average.
- As well as an indicator, a pH probe can be used to measure the pH change.. The end point can be determined from a pH/ volume graph.
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- The validity of an experiment can depend on the accuracy of the results. Accuracy describes how close a result is to the true value or 'actual' value.
- Inaccurate results can be the result of errors of measurements or mistakes. Mistakes are errors that are introduced when the person undertaking the experiment does something incorrectly, for example misreading a scale, forgetting to fill up a burette to the correct level and taking a thermometer out of the solution to read the scale.
- There are two general sources of measured uncertainty: systematic errors and random errors.
- Precision is a measure of the spread of the measured values. A big spread leads to a greater uncertainty.
- The degree of uncertainty is often assessed by working out the average results and stating the range.
- Systematic errors mean that repeat measurements are consistently too high or low. This could result from an incorrectly calibrated flask, e.g. a meniscus could affect the results. If the burette is used at a different temperature from the temperature it was calibrated at, then a systematic error might be introduced.
- Random errors mean that repeat measurements give different values, e.g. the meniscus may not be on the calibration line. Using the naked eye may introduce random errors.
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