Interpreting and sketching rate graphs for reactions
The rate and extent of chemical change • Rate of reaction
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Key concepts
What you'll likely be quizzed about
Axes and basic interpretation
Quantity of product formed or quantity of reactant used up appears on the vertical axis. Time appears on the horizontal axis. An upward-sloping curve shows product formation; a downward-sloping curve shows reactant consumption. Straight-line sections show constant rate. Curved sections show a changing rate. The steepness of the curve at any point indicates how fast the quantity changes at that time.
Common curve shapes and meanings
Rapid initial increase in a product graph appears as a steep slope near time zero and indicates a high initial rate. Gradual leveling off toward a horizontal line indicates rate decreasing as reactants get used up or as equilibrium is approached. A reactant graph that slopes steeply at first and flattens later indicates fast initial consumption followed by slowdown. A plateau means no further net change in the measured quantity.
Sketching graphs from experimental changes
Higher concentration, higher pressure (for gases), higher temperature, greater surface area, or presence of a catalyst cause a steeper initial slope. Cause → effect: change in experimental condition increases collision frequency or energy → increased rate → steeper initial gradient on the graph. Smaller surface area, lower concentration, lower temperature, or absence of a catalyst cause a shallower curve and earlier flattening. Cause → effect: fewer effective collisions → slower rate → lower gradient.
Reactant versus product plots
Plotting product amount against time produces curves that rise and often plateau when the reaction finishes or reaches equilibrium. Plotting reactant amount against time produces curves that fall and often plateau when reactants are exhausted or equilibrium is reached. Both plot types contain the same rate information but in opposite vertical directions. Comparison between experiments uses the steepness of the early part of the curve or the slope of tangents at matched times.
Tangents and instantaneous rate
A tangent is a straight line that just touches a curve at a single point and follows the curve's immediate direction at that point. Drawing a tangent at time t gives a line whose gradient equals the instantaneous rate at time t. Cause → effect: curved graph indicates changing rate → tangent isolates the local rate at the chosen time → slope of the tangent quantifies that local rate.
Calculating slope of a tangent
Slope of a tangent equals change in vertical axis divided by change in horizontal axis (rise/run). Units combine the quantity unit and time unit, for example grams per second or cm3 per minute. Practical method: draw a tangent, select two accurately read points on that tangent, calculate rise/run, and report the value with correct units. Larger numerical slope indicates faster reaction at that time.
Key notes
Important points to keep in mind