Analysing Graphs
- The gradient of a graph can be found by:
- In the case of a straight line graph: using a triangle and the equation for a straight line
- In the case of a curve: drawing a tangent to the graph
- The triangle should be as large as possible to minimise precision errors
- The equation for a straight line is y = mx + c, where:
- y = dependent variable
- x = independent variable
- m = slope
- c = y-intercept
- The gradient or slope is therefore : m = ∆y/∆x
- This example from Kinetics illustrates the calculation of rates from a curve
The gradient can be found at different points on a curve. Here it has been multiplied by 60 to convert it from minutes-1 to seconds-1
- In the case of curves you will need a ruler to line up against the curve at the point you want to measure the gradient:
Lining up a ruler against the curve is essential to drawing a tangent accurately
Exam Tip
Be careful that you process the units correctly when finding the gradient. The gradient unit is the y-unit divided by the x-unit, so in the example above the gradient of the curve is measured in cm3 s-1
Sketched Graphs
- Sketched graphs are a way to represent qualitative trends where the variables shown are often proportional or inversely proportional
- Sketched graphs do not have scales or data points, but they must have labels as these examples from the Gas Laws show:
Sketched graphs show relationships between variables
Graphical Relationships
- In the first sketch graph above you can see that the relationship is a straight line going through the origin
- This means as you double one variable the other variable also doubles so we say the independent variable is directly proportional to the dependent variable
- The second sketched graph shows a shallow curve which is the characteristic shape when two variables have an inversely proportional relationship
- The third sketched graph shows a straight horizontal line, meaning as the independent variable (x-axis) increases the dependent variable does not change or is constant
Worked Example
Which graph shows the correct relationship between the number of moles of a gas, n, and the temperature, T, at constant pressure and volume?
Answer:
The correct option is D
- The Ideal Gas Equation is PV= nRT.
- If P, V and R are constant then PV/R = nT = a constant
- n must be inversely proportional to T, which gives graph D