0620 · 6.2Rate of reaction

Reaction rate and key factors

0:00Marble chips react with hydrochloric acid and release carbon dioxide. Using smaller pieces can make the bubbling much faster, even when the total mass of marble stays unchanged. The reaction produces the same substances, but they form at a different speed. Understanding that difference connects particle behaviour, practical measurements and reaction graphs. By the end of this video, you will understand how five factors affect reaction rate. You will also see how experiments measure that speed without watching individual particles.

0:27We will connect observations to collision theory, then interpret graphs and calculate rates. The aim is to explain the changes rather than memorise a list. A chemical reaction changes reactants into products. During that process, particles interact and their arrangement changes. In a solution, dissolved particles can move through the liquid. Gas particles move freely, while a solid usually reacts where its exposed surface meets another reactant. Rate of reaction describes how quickly a reactant is used up or a product is formed.

0:57A faster reaction produces a greater change during the same time interval. For example, collecting more gas in the first ten seconds indicates a faster average rate over that interval. Rate therefore connects a measurable change with elapsed time. We calculate an average rate by dividing the measured change by the time taken. Gas volume gives units such as cubic centimetres per second, while a change in mass is usually given in grams per second.

1:20Always identify the measured quantity before choosing the calculation and its units. Rate and final amount are different ideas. Two reactions can produce the same total volume of gas, but take different times to finish. One reaches the final amount quickly, while the other gets there more slowly. A change that speeds up a reaction does not automatically increase the amount of product. Increasing the concentration of a reacting solution usually increases the rate.

1:46Concentration tells us how much dissolved substance is present in a given volume. Increasing the pressure of reacting gases also usually makes their reaction faster. These changes affect how closely packed the reacting particles are. A larger exposed surface area makes a reacting solid react faster. Breaking marble into smaller pieces exposes more surface without increasing its total mass. Raising the temperature generally increases reaction rate, while cooling usually slows it.

2:12These observations apply when the other relevant conditions stay the same. A catalyst increases the rate of a reaction and is unchanged at the end. Adding a suitable catalyst makes the reaction faster than it would be without one. Removing it takes away that faster route, so the reaction proceeds more slowly. Enzymes are biological catalysts and follow the same basic definition.

Collision theory and successful collisions

2:33This next part is for the Extended paper. Collision theory explains the observations using the behaviour of reacting particles. For a reaction to occur, those particles must collide with sufficient energy. Collisions that lead to a reaction are called successful collisions. Not every collision produces a reaction because changing the arrangement of particles requires an initial energy input. Activation energy is the minimum energy that colliding particles must have for a reaction to occur.

3:00Think of it as an energy barrier that must be overcome. A collision below that barrier does not produce the required change. A useful way to organise collision theory is to consider two separate questions. First, consider how frequently the reactant particles collide. Then consider how many of those encounters have enough energy to succeed. Reaction rate depends on the number of successful collisions per unit time. Increasing concentration puts more reacting particles into each unit volume of solution.

3:28Those particles encounter one another more often, so collision frequency increases. At the same temperature, their average kinetic energy has not increased. The faster rate comes from more successful collisions per second because there are more encounters overall. Diluting a solution gives the opposite effect. There are fewer reacting particles per unit volume, making collisions less frequent. Consequently, fewer successful collisions occur each second, and the reaction slows. Saying that the particles move more slowly would be incorrect unless the temperature also decreases.

4:00Increasing gas pressure by compression places the same number of particles into a smaller volume. There are now more particles per unit volume, so collisions occur more frequently. More of these encounters are successful each second, increasing the rate. This explanation concerns reacting gases, not simply pressing on a solid. Lowering the pressure spreads the reacting gas particles through a larger volume. Their encounters become less frequent, so the rate decreases.

4:25Compression at constant temperature does not raise the average kinetic energy of the particles. The important change is the number of particles per unit volume. For a solid, much of the material is hidden inside, away from the other reactant. Splitting one piece into smaller pieces exposes surfaces that were previously internal. The total mass stays the same, but the total exposed area increases. More particles of the solid are now in contact with the acid.

4:50The larger surface area therefore allows more frequent collisions at the surface. More successful collisions occur per unit time, so the reaction is faster. Powder usually reacts faster than small chips, and large lumps react most slowly. This comparison assumes equal masses and otherwise identical conditions. Increasing temperature changes the kinetic energy of particles. Kinetic energy is the energy associated with movement, and hotter particles have greater average kinetic energy.

5:17They move faster, which increases collision frequency. More importantly, a greater proportion of collisions have energy equal to or greater than the activation energy. Both changes help increase the number of successful collisions per second. However, mentioning only faster movement misses an important part of the explanation. You should also explain that a greater proportion of collisions have sufficient energy. The activation energy itself has not decreased just because the reactants are hotter.

5:43A catalyst works differently from heating. It provides an alternative reaction pathway with a lower activation energy. At the same temperature, a greater proportion of collisions can now overcome the energy barrier. This increases the number of successful collisions per second without requiring greater average kinetic energy. The diagram shows both pathways starting and ending at the same energy levels. Only the height of the barrier is different. A catalyst is unchanged at the end, although it may participate in intermediate steps.

6:12It does not work by being permanently used up as an additional reactant. Enzymes also provide a pathway with lower activation energy. They catalyse particular reactions, so an enzyme must be suitable for the reacting substances. Excessive heating can alter an enzyme’s shape and prevent it from working properly. This is called denaturation, and it explains why enzyme-controlled reactions do not keep speeding up indefinitely as temperature rises.

Measuring rate in experiments

6:36The reaction between marble chips and hydrochloric acid is useful for investigating rate. Marble is calcium carbonate, and calcium carbonate plus hydrochloric acid gives calcium chloride plus water plus carbon dioxide. Because carbon dioxide is a gas, it can leave the reaction mixture. We can follow its formation through either mass loss or gas collection. For the mass loss method, place the reacting mixture in a flask on a balance.

7:00Fit a loose cotton wool plug into the neck. This reduces the escape of liquid droplets while allowing carbon dioxide to leave. A tightly fitted stopper would trap the gas and prevent the intended mass loss. Record the starting mass and begin timing as the reactants are mixed. Take further balance readings at regular intervals until the mass becomes constant. The measured mass falls because carbon dioxide leaves the apparatus.

7:22It does not fall because matter disappears during the chemical reaction. The mass lost between two readings represents the gas that escaped during that interval. A larger loss over the same duration indicates a faster average rate. The balance reads the flask and its contents together, so the empty flask need not be weighed separately. We use the difference between successive total readings. Suppose the total mass starts at one hundred and fifty-four point six grams.

7:46After twenty seconds, the reading is one hundred and fifty-four point two grams. Subtracting gives a loss of zero point four grams. Dividing by twenty seconds gives an average rate of zero point zero two grams per second. Alternatively, connect the reaction flask to a gas syringe using an airtight stopper and delivery tube. Carbon dioxide travels into the syringe and pushes its plunger outwards. Read the collected volume at regular time intervals.

8:11This method follows the formation of a product rather than the decrease in total measured mass. Set the gas syringe to its starting position before beginning, and check that the plunger moves freely. Start timing when the reactants come into contact. Gas produced before the apparatus is sealed may escape, especially during a fast reaction. That loss makes the collected volume smaller than it should be. To investigate surface area, compare equal masses of large and small marble chips.

8:37Keep the acid concentration, its volume and the temperature the same. Otherwise, a difference in rate could have more than one cause. The size of the solid pieces should be the intended changing variable. When investigating concentration, keep the solid’s mass and surface area constant, together with the temperature and volume of solution. For a temperature investigation, control the other conditions and maintain the chosen temperature. Repeat experiments to check whether the results are consistent.

9:03Wear eye protection because reacting acid can splash. These techniques can appear in the practical paper or the Alternative to Practical paper. You may need to describe apparatus, identify control variables or explain how readings are taken. A useful method states what you measure and when you measure it. Simply saying to watch the bubbles does not provide reliable quantitative data. This next part is for the Extended paper. Evaluating a method means judging how well it works for the particular reaction.

9:30You should connect each limitation to its effect on the measurements. Naming a problem without explaining its consequence gives a weaker evaluation. Mass loss is straightforward because the balance provides readings without a separate gas collector. However, it is unsuitable when the escaping gas has very little mass, such as hydrogen. Its loss may be too small for the balance to measure reliably. Droplets escaping from the flask would also cause an apparent gas loss that is too large.

9:56Evaporation can reduce the measured mass even when it is unrelated to the reaction’s gas production. This is especially relevant when the mixture is warm. The cotton wool plug reduces spray but does not prevent every source of mass loss. A useful evaluation distinguishes the desired measurement from other changes affecting the balance. A gas syringe can measure small gas volumes more precisely than inferring them from a very small mass loss.

10:18However, leaks cause gas to escape before collection, producing readings that are too low. A sticking plunger can delay movement and distort the measurements. The syringe must also have enough capacity for the expected volume. Gas volume depends on temperature and pressure, so comparison experiments should use consistent measurement conditions. Gas collection over water is another possibility, but it is unsuitable if the gas dissolves appreciably. Choosing a method therefore depends on the product’s properties and the available equipment.

10:46Neither technique is automatically best for every reaction.

Interpreting rate graphs

10:50A graph of gas volume against time shows how much gas has been collected at each moment. At first, its line is often steep because gas forms rapidly. As the reaction slows, the curve becomes shallower. Eventually, it levels off because no additional gas is being produced. For the marble and acid reaction, the curve commonly flattens as reactants are used up. The acid becomes less concentrated and the marble pieces become smaller.

11:15Successful collisions become less frequent, so the rate decreases. When a necessary reactant has been completely used up, this gas-producing reaction stops. The gradient, or steepness, indicates rate because it compares a volume change with a time interval. A steeper rising section means more gas forms during the same duration. Graph height gives the amount collected so far, not the rate at that moment. A high horizontal section therefore represents zero rate, despite showing a large total volume.

11:43Compare equal masses of small and large marble chips reacting with excess hydrochloric acid. Excess means enough acid is available to react with all the marble. The smaller chips give a steeper initial curve and reach the plateau sooner. Both experiments produce the same final volume because they contain the same amount of the reactant that is completely used up. That completely used reactant is called the limiting reactant. Equal amounts of it produce equal final amounts when the reaction and collection conditions are otherwise the same.

12:12Increasing concentration does not always leave the final volume unchanged. When the acid is limiting, the same volume at a higher concentration holds more acid. It then gives more gas as well as a faster rate. Consider these illustrative readings rather than results from a specific experiment. The collected volume reaches twenty cubic centimetres after ten seconds and thirty-two cubic centimetres after twenty seconds. During the next ten seconds, it rises to thirty-eight cubic centimetres.

12:39The successive increases are becoming smaller, showing that the reaction is slowing. To calculate the average rate between ten and twenty seconds, use the changes across that interval. Thirty-two minus twenty gives a volume change of twelve cubic centimetres. The time taken is twenty minus ten, which is ten seconds. Dividing twelve cubic centimetres by ten seconds gives one point two cubic centimetres per second. Dividing thirty-two by twenty would instead give the average rate from the start.

13:06That calculation would not answer a question about the later interval. Always match the two volume readings to the two stated times. The same reasoning applies when data are presented in a table rather than a graph. A total mass graph slopes downwards because gas escapes from the apparatus. A steeper downward section means faster mass loss and therefore a faster reaction. The rate of mass loss is normally reported as a positive amount per second.

13:30The graph’s downward direction describes the falling measurement, not a negative reaction speed.

Tangent rates and exam explanations

13:36This next part is for the Extended paper. A rate at one particular moment can be found from a tangent to a curved graph. A tangent is a straight line that matches the curve’s direction at that point. Its gradient estimates the rate at that instant rather than the average across a long interval. Draw the tangent carefully, then choose two well-separated points on that straight line. In this illustration, those points are zero seconds at five cubic centimetres and twenty seconds at thirty-five cubic centimetres.

14:03These points lie on the tangent, not on the reaction curve. Their purpose is to measure the tangent’s gradient accurately. The vertical change is thirty-five minus five, giving thirty cubic centimetres. Across the bottom, the time change is twenty seconds. Dividing thirty by twenty gives one point five cubic centimetres per second. That is the estimated rate at ten seconds, where the tangent touches the curve. Common mistakes include comparing graph heights instead of gradients and assuming every faster reaction must produce more product.

14:33Another error is saying that concentration or pressure increases particle kinetic energy. Those factors change the number of particles per unit volume when temperature is unchanged. Heating affects kinetic energy, while a catalyst lowers activation energy. An exam may ask you to describe the difference between curves, then explain it using collision theory. Describe means state the observable differences without replacing them with an explanation. Explain requires the particle-level reasons behind those observations.

15:01Identify the changing factor before deciding which part of collision theory applies. The curve for smaller chips has a steeper initial gradient. It reaches its final volume sooner. Both curves finish at the same volume because equal amounts of calcium carbonate react with excess acid. These sentences describe the graph and account for the equal endpoint. For the explanation, smaller chips have a larger total exposed surface area. Collisions between acid particles and the solid occur more frequently.

15:28There are therefore more successful collisions per unit time. Do not claim that crushing the marble raises kinetic energy or lowers activation energy, because neither is the relevant change. If asked to state the effect of a catalyst, say that it increases reaction rate and is unchanged at the end. For an Extended explanation, add that it provides a pathway with lower activation energy. A greater proportion of collisions then have sufficient energy to react.

15:53Keep these separate creditworthy points clear rather than replacing them with the vague phrase that it helps the reaction. The central idea is that reaction rate measures how quickly reactants disappear or products form. Concentration, gas pressure and exposed surface area affect how frequently reacting particles meet. Higher temperature increases kinetic energy and the proportion of collisions that overcome the barrier. Catalysts, including enzymes, increase rate by lowering activation energy.

16:20Experiments turn those invisible events into measurable changes in mass or gas volume. Graph gradients show rate, while final levels show the total measured change. Control variables carefully, consider limitations, and distinguish average rates from rates at a particular moment. These connections allow you to explain both the observations and the evidence behind them.