7.3.4Additional lessons
Graphing in Chemistry — IB Chemistry HL Revision Notes & Topical Questions
5 min read 6 parts Lesson 166 of 172
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 6Drawing a Good Graph
A graph reveals patterns that a table of numbers hides.
Axes
- Axes The independent variable goes on the x-axis and the dependent variable on the y-axis, each labelled with the quantity and its unit.
Scale
- Scale Choose a scale that uses more than half of each axis and makes each small square a simple value such as 1, 2 or 5.
Plotting
- Plotting Mark each point accurately with a small cross or dot, including any anomalous point.
Title
- Title A short title states what is plotted against what, which makes the graph understandable by itself.
Section 2 of 6Lines of Best Fit
Lines of Best Fit
Straight line
- Straight line Draw a single straight line with points scattered evenly above and below it,
- and not forced through the origin unless the theory requires it.
Curve
- Curve If the points follow a smooth curve, draw a curve rather than joining the dots with straight segments.
Outliers
- Outliers Points far from the trend should be identified and may be excluded from the line, but should be explained.
Error bars
- Error bars Bars show the uncertainty of each point and the line of best fit should pass through most of them.
Section 3 of 6Finding Information from a Graph
1
Choose two points far apart on the line of best fit, not necessarily data points, to calculate the gradient.
2
Gradient = change in y ÷ change in x, with units equal to the y unit divided by the x unit.
3
Read the y-intercept where the line crosses the y-axis, which may have chemical meaning.
4
For a curve, draw a tangent at the point of interest and find the gradient of the tangent.
5
State the result with the correct unit and the right number of significant figures.
Section 4 of 6Linearising and Interpreting
Linear form
- Linear form Plotting ln k against 1/T gives a straight line of gradient −Eₐ/R, from which activation energy is found.
Direct proportion
- Direct proportion A straight line through the origin shows that the two variables are directly proportional.
Rate graphs
- Rate graphs The gradient of a concentration–time graph is the rate of reaction at that moment.
R² value
- R² value This number shows how closely points fit a trendline, but it does not show that the relationship is correct.
Section 5 of 6Worked Example
Question
- In a gas-collection experiment the tangent to the volume–time curve at t = 0 passes through (0 s, 0 cm³) and (50 s, 20 cm³).
- Find the initial rate.
- Gradient = change in y ÷ change in x.
- Gradient = (20 − 0) cm³ ÷ (50 − 0) s.
- Gradient = 0.40 cm³ s⁻¹.
Answer Initial rate = 0.40 cm³ s⁻¹
Section 6 of 6Exam Checklist & Tips
7.3.4 – Key Points for the Exam
- Independent variable The variable that is changed deliberately and plotted on the x-axis.
- Line of best fit A line drawn to follow the trend of the plotted points.
- Gradient The slope of a line, found as change in y divided by change in x.
- Tangent A straight line that touches a curve at one point and has the same gradient as the curve there.
- Intercept The value at which a line crosses an axis.
Common Mistakes
- Joining data points with straight lines instead of drawing a best-fit line or curve.
- Taking the gradient from two data points close together, which magnifies error.
- Leaving out units on axes or on the gradient.
- Choosing an awkward scale, such as 3 units per square, which makes plotting inaccurate.
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