6.2Chemical Reactions
Rate of reaction: reading graphs — Combined Science 0653 - Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 8 of 10 Unit quiz
Lesson summary
Rate of reaction: reading graphs — COMBINED SCIENCE (CHEMISTRY) notes: Interpret rate-of-reaction graphs and calculate average and instantaneous rates from gradients.Reaction rate = how quickly a reactant is used up or a product is formed.Plot the measured quantity (e.g.
- Product formed
- Reactant remaining
- limiting reactant
- Average rate
- Rate at a particular time
- Plot the measured quantity (e.g. gas volume or mass) on the vertical axis and time on the horizontal axis
Part 1 of 3Reading the shape of a rate graph
Reaction rate = how quickly a reactant is used up or a product is formed.
Plot the measured quantity (e.g. gas volume or mass) on the vertical axis and time on the horizontal axis.
Product formed
Product formed graph: rises steeply, becomes less steep, then levels off.
- Steepest at the startmost reactant particles available to collide.
- Less steep laterreactant concentration falls as it is used up.
- Horizontalrate is zero, at least one reactant has run out, no more product forms.
Reactant remaining
Reactant remaining graph: falls instead; steepest downward section = fastest rate; horizontal = rate zero.
- Quote the size of the gradient as a positive rate even though the line slopes down.
Steeper line = faster reaction.
Limiting reactant
The final amount of product is set by the limiting reactant: more limiting reactant gives more product; more of the reactant in excess does not change it.
Several runs can be drawn on one grid to compare the effect of a variable.
Part 2 of 3Calculating rate from a graph
Calculating rate from a graph
Gradient
Rate = change in measured quantity ÷ change in time = gradient of the line.
Unit = vertical-axis unit ÷ time unit: cm3/s for gas volume in cm3 against seconds; g/s for mass in grams against seconds.
Average rate
Average rate over an interval: gradient of the straight line joining two points on the curve.
Rate at a particular time
(instantaneous): gradient of a tangent to the curve at that time.
1
Draw a tangent that just touches the curve at the chosen time.
2
Choose two well-separated points on the tangent and draw a large right-angled triangle.
3
Read the change in y and the change in x from the tangent's coordinates (not from other points on the curve).
4
Rate = change in y ÷ change in x, with the unit.
| Average rate | Rate at one time | |
|---|---|---|
| Method | Straight line between two points | Tangent at that point |
| Use when | Rate over an interval is asked | Rate 'at 20 s' is asked |
Part 3 of 3More from the specification
More from the specification
Tangent gradient in aqueous solution
- Tangent gradient in aqueous solution drawing tangents to concentration curves in solution provides an analytical means to measure initial reaction rates.
Interpreting Reaction Curves on Rate Graphs
- On a graph of gas volume produced against time, the initial rate is steepest where reactant concentrations are highest;
- as reactants in the reaction flask are depleted, collision frequency drops and the curve flattens.
Effect of Increasing Temperature on Collision Energy
- Increasing temperature gives reactant particles greater average kinetic energy, dramatically increasing the fraction of particles exceeding activation energy and raising the frequency of successful collisions.
Role of Solid Surface Area in Reaction Rates
- Increasing the exposed surface area of a solid reactant allows more particles to make simultaneous contact,
- which increases the rate of collisions without changing the total volume of gas produced.
Catalysts and Activation Energy in Chemical Flasks
- Adding a suitable catalyst to a reaction flask provides an alternative reaction pathway requiring lower activation energy,
- increasing the frequency of successful collisions without the catalyst being consumed.
Collision frequency and temperature
- Collision frequency and temperature increasing reaction temperature increases kinetic energy,
- causing more frequent and successful particle collisions.
Solid surface area
- Solid surface area crushing reactant lumps into powder increases exposed surface area, increasing the rate of collisions.
Catalyst addition
- Catalyst addition adding a suitable catalyst to the reaction flask lowers activation energy, speeding up gas evolution.
Theory
- A tested hypothesis that is widely accepted.
Hydrogen
- A gas that gives a squeaky pop with a lighted splint.
Successful collision
- A collision with the correct orientation and enough energy to react.
Activation energy
- The minimum energy colliding particles need to react.
Collision theory
- Particles react only if they collide with at least the activation energy and the correct orientation.
Thermal energy
- Total kinetic energy of the particles in a substance.
Temperature
- A measure of the average kinetic energy of the particles.
Catalyst
- A substance that speeds up a reaction and is not used up.
Pressure
- Force of gas particles colliding with a surface.
Key terms
| Term | Meaning |
|---|---|
| Rate of reaction | How quickly a reactant is used up or a product formed; change in quantity divided by change in time. |
| Gradient | Change in y divided by change in x; equals the rate on a quantity–time graph. |
| Tangent | Straight line touching a curve at one point; its gradient is the rate at that point. |
| Average rate | Rate over an interval, from the line joining two points. |
| Limiting reactant | Reactant that is used up first and so determines how much product forms. |
Avoid these mistakes
1
Using the average rate from 0 s to 20 s when asked for the rate at 20 s – draw a tangent at 20 s.
2
Taking coordinates from points on the curve instead of from the tangent when calculating the gradient.
3
Leaving out the unit – it must be (y-axis unit)/(time unit), e.g. cm3/s.
4
Giving a negative rate for a falling reactant graph – quote the magnitude of the gradient.
5
Thinking more excess reactant gives more product – only the limiting reactant sets the amount.
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