Calculations in Chemistry: A Practical Approach


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Some coursebook chapters involve the use of quantitative techniques and when you carry out these investigations you will need calculators and equipment for drawing graphs. All techniques listed in the practical guidance are covered in the workbook. There are two parts to this practical guide. The first part deals with subject matter and practical techniques described in the AS level syllabus.

A variety of investigations introduces you to a range of experiments which will provide you with practice in manipulating apparatus and taking measurements. The various investigations and accompanying questions will help you gain confidence in tackling laboratory work and develop a wide range of skills related to practical chemistry.

Apart from the necessary preparation for both practical papers, it is hoped that these investigations will help you understand the importance of laboratory work in development and assessment of theoretical chemistry. In each investigation involving practical work, you are expected to: 1 Wear eye protection such as safety goggles or safety spectacles note that goggles give more protection 2 Tie back long hair and make sure that items of clothing are tucked in.

It is also advisable for you to wear a laboratory coat to protect your clothing from chemical splashes. The substance is corrosive. It will damage your skin and tissues if it comes into contact with them. The substance is an irritant. If it comes into contact with your skin it can cause blisters and redness.

Keep the substance away from naked flames and when heating reaction mixtures use the hot water from a kettle rather than using Bunsen burners. The substance is an oxidising agent. It will liberate oxygen when heated or in the presence of a suitable catalyst. As a learner you should take responsibility for working safely and you must learn the meanings of the safety symbols shown in the table below. Table S1 shows the most common hazard symbols found in school science laboratories. All chemicals should be treated as hazardous. If they are spilt on the skin you must wash them off immediately using plenty of water.

You may not be aware of the dangers of particular chemicals and therefore using them without safety precautions can lead to unforeseen problems. Remember that you should also think about the hazards of all of the substances that are being produced in a chemical reaction, especially when a gas is given off. Chemical reactions which produce hazardous gases should be done in a fume cupboard or well-ventilated room. Practical work has its own set of skills. A number of these are related to working safely.

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Working safely is essential in getting the maximum advantage from your practical work. Skills chapter Chapter outline This chapter introduces the key practical methods, processes and procedures that you will use regularly throughout your course. This chapter covers the following:. Strategies for measuring heat changes a Temperature—time graphs b Calculating enthalpy heat changes Drawing graphs and charts Calculating errors in your experiments a Calculating systematic errors due to apparatus inaccuracy i Temperature readings ii Measuring cylinders iii Burette readings iv Top-pan balance readings Using significant figures.

Preparing a standard solution a Calculating the mass of solute required b Making cm3 of a standard solution Carrying out an acid—base titration a preparing the burette b pipetting a solution c carrying out the titration d processing your results Gas collection and measurement a Choosing your apparatus b General advice for measuring volumes of gases Qualitative analysis: testing for gases and ions a Tests for gases b Tests for ions. A standard solution is one that has a known concentration. With a standard solution, it is possible to investigate the concentration of other solutions of unknown concentration by titration see section 2.

A standard solution is made by dissolving an accurate mass of solute into a known volume of water. The first step is to calculate the mass of solute required to make up a standard solution.

Please note that not all substances make good standard solutions. This is due to the fact that some substances can be difficult to obtain in a completely pure form, are unstable in air or not readily soluble in water. To ensure there is no solute remaining in the weighing boat, wash the weighing boat twice using distilled water from a wash bottle pouring the washings into the beaker. Stir the mixture with the stirring rod until all the sodium carbonate has dissolved. Repeat this several times. You must also rinse the stirring rod.

HINT If you go over the mark and the level of the liquid is above the line then you must reweigh your solute and repeat the preparation of the solution. If you move the flask and still see swirling currents in the liquid you have not mixed enough — just turn upside down a few more times. Titrations are used to measure the volume of one solution that exactly reacts with another solution.

Titration is an analytical technique widely used in industry and is an essential chemical skill. The food industry, for example, uses titrations to determine the amount of salt or sugar in a product or the concentration of beneficial vitamins. Acid—base titrations involve neutralisation between an acid and a base when mixed in solution. An indicator is used to determine the end-point of the titration as it changes colour. This technique is also used in other areas of your syllabus, for example redox reactions.

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Additional advice When doing acid—base titrations, it is best if the acid is delivered from a burette. This is because alkalis and soluble carbonates can form solids in the taps of burettes and clog them up. It is often a good idea to place the burette and clamp stand on a chair or stool.

This will make it easier to fill the burette. Most pipette fillers have a way of pushing the liquid out of the pipette. Unfortunately, this method is often very difficult to use to the fine level necessary. Close the tap on the burette. Use the beaker to add a few cm3 of acid to the burette; you are only rinsing the burette at this stage.

Ensure the tap is closed, then fill the burette to above the zero mark. Remove the funnel from the burette. Remove the funnel from the top of the burette. HINT There is no need to adjust the volume of the solution to exactly 0. Quickly remove the pipette from the pipette filler and cover the open end with your index first finger as shown in Figure 0.

HINT There will be a very small amount of solution in the end of the pipette. Do not add this small drop of solution! The pipette is calibrated to deliver the exact volume with this drop remaining in the pipette. When the end of the pipette is over the conical flask release your index finger and let the solution run into the flask. You will need to repeat the titration at least three times, usually more.

The first titration is a rough titration which will help you to be more accurate when you repeat the process. You will need to prepare a results table in which to record your results. You will add acid from the burette 1. After each addition, swirl the flask and if the indicator does not change colour continue adding 1. When you are near the end-point, the colour of the indicator will take longer to return to its original colour.

As soon as the colour does not change back you know you have added exactly the right amount of acid. Note down the volume. What does this result tell you? If the indicator changed colour after You now know that you can safely run in If you have enough acid left in your burette to repeat the titration, go ahead; if not you will need to fill the burette up again taking care to record the starting level and then repeat. Then you will know that you have accurately estimated the volume of acid required to react with the Before completing calculations using your results you need to check and process your results to determine the average titre.

Figure 0. Note that no time was wasted in adjusting the volume to 0. In this titration, acid was added until a reading of You can investigate a chemical reaction by measuring the volume of gas given off evolved at certain time intervals when investigating reaction rates , or measuring the total volume of gas produced. There are different techniques to collect gas during an experiment and your choice of apparatus depends on the volume of gas produced and the apparatus that is available. The gas produced must be only slightly soluble or insoluble in water.

In a the gas produced is collected in a gas syringe. In b the gas is collected by the displacement of water. It is a suitable method for gases that are insoluble in water such as hydrogen. It is important to have some idea of the volume of gas that will be generated during your experiment so that you can choose an appropriate size of syringe or measuring cylinder. The volume of measuring cylinder chosen should be about 2—3 times the volume of gas.

Remember that the larger the volume of measuring cylinder that you use, the greater the error in measurement e. Trial runs are therefore essential to make sure that your approach will work. Example 1 When asked to investigate the effect of concentration of hydrochloric acid e. This will indicate which size of measuring cylinder is correct to use.

Knowing how to identify different ions and gases is a key skill for all chemists. In particular it is important to understand the chemical basis for each test. The practical exam will test your knowledge of common tests and their expected results. After collecting gas during an experiment, it will be necessary to complete a test to establish what gas you have. The tests for common gases and the method used in the testing are shown in Table 0. Method Using dropper, collect gas from above the surface of the reaction mixture; bubble gas through limewater solution.

Collect gas in upside down test tube above reaction mixture. Then insert lighted splint into test tube. Universal Indicator UI or red litmus paper. Universal Indicator UI or blue litmus paper. Chlorine gas bleaches moist UI or blue litmus paper. Acidified potassium manganate VII solution. SO2 decolourises acidified potassium manganate VII solution on filter paper.

Add 2—3 drops of potassium manganate VII solution to filter paper and hold near mouth of test tube. Must be carried out in a fume cupboard. Carbon dioxide CO2 Bubble gas through limewater calcium hydroxide solution. Turns cloudy in presence of CO2. When presented with a substance or solid to analyse it may be necessary to first prepare a solution of the compound. Here is some general advice regarding the making and testing of solutions. Example 1 When testing for halide ions you use silver nitrate solution. But before adding the silver nitrate solution, you add nitric acid. Example 2 When testing for sulfate ions you use barium chloride solution.

But before adding the barium chloride solution, you add a drop or two of hydrochloric acid, but sulfate ions SO42— can be distinguished from sulfite SO32— ions by first adding the barium chloride and then adding the hydrochloric acid. Strategies for measuring heat changes Introduction One of the main challenges when taking measurements of heat change is avoiding heat loss, mainly by conduction and convection.

There is also the issue that should the reaction take a long time to go to completion then the maximum temperature may not be reached. Nguyen, C. Jackels, M. Gu, I. Rossi, P. Fast, S. Clayton, V. Melissas, B. Garrett, A. Isaacson, D. Schatz, M. Truhlar, A. Isaacson, B. Garrett, Theory of Chemical Reaction Dynamics. Bauer, Ed.


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Truhlar, B. Garrett, S. Klippenstein, J. Truhlar, S. Rai, R. Steckler, G. Hancock, B. Garrett, M. Redmon, Comp. Kreevoy, D. Bernasconi Ed. Fonseca, J. Gomes, P. Grigolini, F. Marchesoni, Adv. Laidler, M. King, J. Truhlar, W. Hase, J. Hynes, J. Pechukas, Ann. Walker, J. Light, Ann. George, J. Ross, Ann. Light, Adv. Waage, B. Rabinovitch, Chem. Billing, K.

Bimolecular Collisions M. Ashford, J. Battott, Eds. Schatz, Ann. Truhlar, J.

Muckerman, Atom-Molecule Collision Theory. A Guide for the Experimentalist R. Bernstein, Ed. Widom, Adv. Klessinger, Theoretical Organic Chemistry C. Simons, J. Kocharovsky, V. Kocharovsky, S. Tasaki, Adv. Gaspard, I. Burghardt, Eds. Serrano-Andres, M. Merchan, I. Nebot-Gil, R. Lindh, B. Roos, J. Baer Ed. Ovchinnikov, M. Ovchinnokova, Adv. Quantum Chem. Cohen-Tannoudji, B. Diu, F. Nikitin, Adv. David C. In the chapter on reaction rates, it was pointed out that the perfect description of a reaction would be a statistical average of all possible paths rather than just the minimum energy path.

In order to examine these ideas computationally, the entire potential energy surface PES or an approximation to it must be computed. A PES is either a table of data or an analytic function, which gives the energy for any location of the nuclei comprising a chemical system. The SLE analysis with a short calculation time enables one to examine the various parameter sets for fitting the experimental date.

Our study demonstrates that simultaneous fitting of the time evolution of the MFE and of the magnetic field dependence of the MFE provides valuable information on the diffusion motions of the radical pairs in nano-structured materials such as micelles where the lifetimes of radical pairs are longer than hundreds of nano-seconds and the magnetic field dependence of the spin relaxations play a major role for the generation of the MFE. The article was received on 30 Jan , accepted on 02 Mar and first published on 03 Mar If you are not the author of this article and you wish to reproduce material from it in a third party non-RSC publication you must formally request permission using Copyright Clearance Center.

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Calculations in Chemistry: A Practical Approach Calculations in Chemistry: A Practical Approach
Calculations in Chemistry: A Practical Approach Calculations in Chemistry: A Practical Approach
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