8303242-72-Graphical-Tech - Chem Factsheet...

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Graphical Techniques Number 72 1 C hem F actsheet www.curriculum-press.co.uk The AS/A2 Specifications state: Students should be able to: plot two variables from experimental or other data; understand that y = mx + c represents a linear relationship; determine the slope and intercept of a linear graph; draw and use the slope of a tangent to a curve as a measure of rate of change (A2 only). This Factsheet will review the methods and knowledge needed for students to achieve these objectives. The scale should be linear - i.e. the scale goes up in equal units (like 20, 40, 60 etc). (The only exception to this is in logarithmic graphs , which you may encounter for ionisation energy etc, but will not be required to plot.) You do not have to: use the same scale on each axis (most of the time you won't be able to!) start the scale at 0, unless you want to show proportionality (i.e. that your line or curve passes through (0, 0)) or need to find the y-intercept (in which case, you only have to start the scale on the x-axis at 0) It is often advantageous not to start a scale at 0 - for example, if you were plotting the values 1050, 2040, 2890 and 3250, you would have no points between 0 and 1000, with the rest of your points comparatively cramped up. If you started the scale at 1000 instead, you could use a larger scale, hence spreading the points out more. Accurate plotting Most of the points to be considered here are obvious, but many students lose marks from sloppy work! use a pencil not a pen so you can erase mistakes use a sharp hard pencil make sure you put your dot (or the centre of your cross) exactly on the appropriate point take the time to read your scale carefully - work out how many units each small square is worth. you probably cannot plot much more accurately than half a small square - but nor should you plot less accurately than this! Correct drawing of line or curve There are three cases to consider Graphs where it is inappropriate to join the points up in any way . These include any where points half way between the ones plotted are meaningless - for example, there is no element half way between lithium and beryllium, and no ionisation energy half way between the 1 st and the 2 nd . (Warning - you will often see these graphs with lines joining the points in textbooks and on the internet! But that doesn't mean it is correct!) Graphs where you should draw a best straight line Obviously these need to look at least approximately like a straight line! You must also consider the underlying chemistry - you might expect a graph of rate of reaction against concentration to possibly be a straight line for a first order reaction, but you should be surprised to have a titration curve looking linear. Note that the "best" straight line is not necessarily the one going through the most points; you should aim to have roughly the same number of points above the line and below the line. If you have one obviously strange result, ignore it when drawing the line.
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This note was uploaded on 03/08/2011 for the course CHEM 101 taught by Professor Hard during the Spring '11 term at UT Arlington.

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8303242-72-Graphical-Tech - Chem Factsheet...

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