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Experimental Error in Chemistry

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Experimental Error in Chemistry In an experiment, when you make a measurement of whatever kind, you cannot be sure just how close it is to the true value, that is, how accurate it is. There is an experimental uncertainty (often called 'experimental error'). Experimental uncertainty arises because of: * Limits in the how exact the measuring apparatus is. This is the precision of the apparatus. * Imperfections in experimental procedures. * Judgements made by the operator. When can my results be said to be precise? If you repeat a measurement several times and obtain values that are close together, your results are said to be precise. If the same person obtains these close values, then the experimental procedure is repeatable. If a number of different people carry out the same measuring procedure and the values are close the procedure is reproducible. What is a systematic error? A systematic error is one that is repeated in each measurement taken. If this is realised after the experimental work is done, it can be taken into account in any calculations. What are random errors? Even the most careful and experienced operator cannot avoid random errors. However, their effect can be reduced by carrying out a measurement many times (if the opportunity exists) and working out an average value. Let's look in more detail at 'built-in' uncertainty of some laboratory equipment... ...read more.


determines the number of significant figures. 6.3 x 5479 � 0.0053 = 6.5 x 106 The quantity with fewest significant figures determines the number of significant figures in the answer. 4.8729 becomes 4.873 (4 sf) 4.8729 becomes 4.87 (3 sf) 3.715 becomes 3.71 or 3.72 (3 sf) This is 'rounding off' a number to discard non-significant figures. Get Started with Reaction Kinetics These notes are intended to provide an introduction to Rate of Reaction. Work through these, and then refer to other available materials to build your understanding of the concepts. Answer the questions... 1. What is meant by the 'rate' of a chemical reaction? 2. How does the rate of a chemical reaction change as the reaction proceeds from start to finish? 3. How can we follow experimentally the changing rate of a chemical reaction as it takes place? A chemical reaction involves one or more reactants. We are interested in how the rate of reaction depends on each of these separately. Two experimental methods which can be employed to investigate reaction rate are: * Continuous (Progressive) method. * Initial Rates method. First of all, the Continuous method... Consider the decomposition of dinitrogen pentoxide dissolved in liquid tetrachloromethane at 30 �C. N2O5(sol) ? 2NO2(sol) + 1/2O2(g) The volume of O2(g) measured at a known temperature and pressure is related to the diminishing concentration of N2O5(sol) ...read more.


Now try these examples to get the ideas of the initial rates method... For the thermal decomposition of ethanal, CH3CHO CH3CHO(g) ? CH4(g) + CO(g) the following data at 800 K are given. Initial [CH3CHO(g)] (mol dm-3) Initial Rate of decomposition of CH3CHO (mol dm-3 s-1) 0.1000 9.0 x 10-7 0.2000 36.0 x 10-7 0.3000 81.0 x 10-7 0.4000 14.4 x 10-6 9. What is the order of reaction with respect to CH3CHO(g)? Write the rate equation for the reaction. 10. Calculate the rate constant for the reaction at 800 K. 11. Calculate the decomposition rate at 800 K at the instant when [CH3CHO(g)] is 0.250 mol dm-3. Again, when two reactants participate in a reaction, the rate equation may be derived by keeping the concentration of one reactant constant while varying the concentration of the other. The reaction NO(g) + 1/2Cl2(g) ? NOCl(g) has been studied at 50 �C. Initial [NO(g)] (mol dm-3) Initial [Cl2(g)] (mol dm-3) Initial rate of formation of NOCl(g) (mol dm-3 s-1) 0.250 0.250 1.43 x 10-6 0.250 0.500 2.86 x 10-6 0.500 0.500 11.4 x 10-6 12. Work out a rate equation for this reaction. 13. What is the overall order of the reaction? 14. Calculate the rate constant for the reation at 50 �C. 15. Calculate the rate of formation of NOCl when [NO] = [Cl2] = 0.110 mol dm-3. 16. At the instant when Cl2 is reacting at 2.21 x 10-7 mol dm-3 s-1, what is the rate at which NO is reacting and NOCl is forming? ...read more.

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