23 Cie Organic Synthesis

CIE Chemistry A-Level Practicals for Papers 3 and 5 Graphs

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Drawing Graphs You will need a: ● Sharp HB pencil ● Pencil sharpener ● Clean eraser (mistakes need to be removed for CIE) ● Transparent ruler (can more easily read scale) ● Pen Plot points as a ​small cross​​, which falls exactly on grid lines. Use a sensible scale that is easy to use and takes up most of the grid.

Axes ● ●

The​ ​independent ​variable (one you are changing e.g. concentration) goes on the ​x-axis​​. The ​dependent​​ variable (what is being measured e.g. time) goes on the ​y-axis​​.

Best fit lines ●

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In paper 5, data will possibly have two separate ​problems intentionally in the question which will determine what type of line of best fit that would be illustrated (e.g. straight line or curve). Results given in the question will likely have some ​random error​​ ​included by examiners and thus plots will likely not fall on a straight line or smooth curve. Anomalous results means that the points don’t fit the trend so will need to be discounted. Later in the question you may be asked why that point is anomalous.

Straight line graphs ● ● ● ●

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If a point falls above the trend for all the rest of the points, double check that you have plotted the point correctly. Do not​​ ​include any anomalous points while drawing your line of best fit. Follow up questions may be : a smaller volume was recorded than you would expect , give reasons for this. There are many possible reasons such as you might have measured the volume incorrectly, recorded a smaller volume than you should have, product may have been lost from the apparatus before you collected it or you may have incorrectly weighed the reactant (use before and after weight method – see measuring mass). Answers to this type of question should have accurate explanations. "There is an experimental error.” will not be credited. Explain with ​detailed reasons​​ ​what may have gone wrong to cause the error in the method.

Decide whether the line will go through the ​origin ​(0, 0). If you had zero mass of reactant, you would get zero mass of products; hence the line must go through the origin. This gives you one point which is accurate with no error, which aids placement of a line of best fit.

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Draw your line of best fit with a ruler and pencil so that there’s an approximately even a number of points either side of the line of best fit. Discount the anomalous points.

Curve line graphs ● ● ● ●

Check for anomalies e.g if a point looks slightly lower than the trend don’t include it in curve of best fit. Decide whether your line will go through the origin e.g note that in a rates graph which is often a curve, that at time zero the concentration of reactant is exactly known at its highest. When drawing the curve free-hand, be careful in the way your wrist and arm moves. Sometimes it takes many attempts to draw the curve of best fit. Start off by drawing the line lightly, and then make it firmer when you are satisfied. You ​must​​ leave as you final answer one line with no shading.

Gradient of a straight line graph Gradient is measured by drawing a large triangle. Make the triangle large to make measurements more accurate. You should ​not​​ ​use points that you have plotted for the end points of the triangle unless they happen to fall exactly on the line of best fit. Where the line goes through the origin, you could use the origin as one of the ends of the triangle. The slope is given by the change in the y-axis value divided by the change in the x-axis value: Δy/Δx​​. You should record the coordinates of the points you have used to draw your triangle. They are used to check that you have calculated the slope accurately. You ​must​​ ​give the coordinates of each point in the standard format of ​(x, y)​​. In this case, you should quote the coordinates of the endpoints of the triangle. Gradient of a curve line graph The ​gradient is changing​​ ​all the time, and so you will be asked to find the gradient at some particular point. To do that, you draw a ​tangent​​ to the curve at the point, and use this to draw a triangle similar to in a straight line and remember to include the sign (+ for positive gradient, - for negative gradient).

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