Rates of Reaction
Section: Chemistry | Syllabus: Cambridge Lower Secondary Checkpoint Science (0893)
What is Rate of Reaction?
- The rate of reaction measures how quickly reactants are used up or how quickly products are formed
- A fast reaction happens quickly (e.g. burning, explosions), while a slow reaction takes a long time (e.g. rusting, food decay)
- Rate of reaction: how quickly reactants are used up or how quickly products are formed during a chemical reaction
Examples of Different Reaction Rates
- Very fast: fireworks exploding, burning magnesium
- Moderate: cooking food, bleaching hair
- Very slow: iron rusting, concrete setting, rock weathering
Measuring the Rate of Reaction
- We can measure how fast a reaction occurs by monitoring changes over time
- There are several different methods depending on the type of reaction
Method 1: Measuring Gas Production
- Collect the gas produced in a gas syringe or upside-down measuring cylinder
- Measure the volume of gas produced at regular time intervals
- Example: reacting metal with acid produces hydrogen gas
- Rate of reaction = Volume of gas produced ÷ Time taken
Apparatus for Measuring Gas Production
- The reaction mixture is placed in a conical flask - its narrow neck and wide base reduce the risk of spillage and allow the mixture to be swirled
- The flask is sealed with a bung fitted with a delivery tube, which carries the gas produced out of the flask to where it is collected
- The gas is then collected and measured using one of two methods:
- a gas syringe (with a barrel and plunger) connected to the delivery tube - as gas enters, it pushes the plunger out and the volume is read directly from the scale on the barrel
- an inverted measuring cylinder filled with water and stood in a trough of water - the delivery tube passes under the water and into the cylinder; gas collects at the top and pushes water out, so the volume of gas can be read from the cylinder's scale
- The whole set-up must be gas-tight, with no gaps, or gas will escape before it is measured, giving an inaccurate reading
Method 2: Measuring Mass Loss
- Place the reaction container on a balance (scale)
- If a gas is produced and escapes, the mass will decrease
- Record the mass at regular time intervals
- Example: reacting marble chips (calcium carbonate) with acid releases carbon dioxide
- Rate of reaction = Mass lost ÷ Time taken
Method 3: Observing a Colour Change or Precipitate
- Some reactions produce a solid (precipitate) that makes the solution cloudy
- Measure the time taken for a mark underneath the container to disappear
- The shorter the time, the faster the reaction
Interpreting Rate of Reaction Graphs
- A steeper line = faster reaction
- A horizontal line = reaction has stopped (all reactants used up)
- The line eventually flattens when one reactant is completely used up
- Changing the temperature, concentration, or surface area changes how quickly the reaction happens, but (if the amount of reactants is unchanged) the total amount of product made does not change - a faster reaction reaches the same final (plateau) height on the graph sooner, it does not reach a higher one
Worked Example: Anomalous Results, Curves of Best Fit and Interpolation
Safia measures the total volume of hydrogen made by magnesium and hydrochloric acid every minute for 8 minutes, and plots her results on a graph. One result does not fit the pattern of the rest.
- Step 1: identify the anomalous result - look for the one point that is clearly off the general curve traced by the other points, and circle it
- Step 2: draw a smooth curve of best fit through the remaining (non-anomalous) points - do not simply join the points with straight lines, and ignore the anomalous point when drawing the curve
- Step 3: to predict a missing value, e.g. the volume of gas made at 3 minutes, read up from 3 minutes on the time axis to the curve, then across to the volume axis
- Reading a value from between two known points on a curve like this is called interpolation
Factors That Affect the Rate of Reaction
- Several factors can make a reaction go faster or slower
- Understanding these factors helps us control reaction rates in practical applications
Concentration
- Concentration: the amount of substance dissolved in a given volume of solution
- Higher concentration = faster reaction
- More particles in the same volume means more frequent collisions
- More collisions = faster reaction rate
- Example: concentrated acid reacts faster with magnesium than dilute acid
Common Mistakes
- Don't confuse concentration with amount - using MORE dilute acid won't speed up the reaction, you need a HIGHER concentration (more particles per cm³)
Temperature
- Higher temperature = faster reaction
- Particles move faster and have more energy
- This leads to more frequent collisions AND more energetic collisions
- More collisions with enough energy = faster reaction rate
- Example: food cooks faster at higher temperatures; food stays fresh longer in a cold refrigerator
- At higher temperatures, particles don't just collide more often - they also collide with more energy, meaning more collisions are successful
Surface Area
- Surface area: the total area of the surface of a solid that is exposed to react
- Larger surface area = faster reaction
- Smaller pieces (powder) have more surface area exposed than large chunks
- More surface area means more particles are available to collide and react
| Form | Surface Area | Rate of Reaction |
|---|---|---|
| Large lump | Small | Slow |
| Small chips | Medium | Medium |
| Powder | Large | Fast |
- Example: powdered chalk reacts faster with acid than a large chalk stick - this is why flour dust can cause explosions in mills
Catalysts
Interactive revision notes, videos and practice questions load below.