Lesson Explainer: Rates of Reactions Science

In this explainer, we will learn how to describe the rate of a chemical reaction and explain the effect the type of reagent and the surface area have on it.

There are chemical reactions taking place all the time. Some chemical reactions can be seen happening; others are either too quick or too slow to be observed directly. Drop a piece of magnesium into a beaker of hydrochloric acid and a chemical reaction occurs. We can follow the chemical reaction and watch it occur over a time period of seconds and minutes.

However, leave a piece of iron outside and it will slowly rust. The chemical process of rusting takes place over weeks, months, and years, a much longer timescale. Other chemical reactions take place very quickly. Strike a match and the chemical reaction that produces a flame appears to occur almost instantly.

The table below shows examples of chemical reactions that take place over a variety of different time spans.

Chemical ReactionTimescale of Reaction
CombustionVery fast
The reaction of magnesium with waterFast
Rusting of ironSlow
Chemical weathering of rocksVery slow

Example 1: Choosing Which Chemical Reaction is Likely to Occur on a Certain Timescale

The chemical timeline below gives examples of different chemical reactions that take place over a range of timescales. Which of the following reactions is likely to occur at point X?

  1. Cooking food
  2. The reaction of iron and hydrochloric acid
  3. Striking a match
  4. The reaction of alkali metals with water
  5. The chemical weathering of rocks

Answer

From the diagram, we can see that point X corresponds to a reaction that occurs on a very slow timescale. While the diagram provides no details on exactly how slow, we can deduce that the reaction is slower than the rusting of iron.

There are many chemical reactions involved in cooking food; however, given how most food can be cooked in minutes or hours, cooking food is unlikely to be slower than the rusting of iron.

The next three options all describe fast chemical reactions. Iron reacts quickly with hydrochloric acid, the striking of a match initiates a very fast chemical reaction to produce a flame, and the dropping of alkali metals in water produces a vigorous and explosive reaction.

The chemical weathering of rocks is a slow process taking place over years, and therefore the correct answer is option E.

During a chemical reaction, reactants are converted to products. As the reaction takes place, the concentrations of reactants decrease. The formation of products results in the concentrations of products increasing.

Observing the change in concentrations of reactants and products is a way of following the process of a reaction. Let’s consider the following chemical reaction: A+BC

During the chemical reaction, the concentrations of A and B will decrease as they react with each other. However, the concentration of C will increase as it is being produced during the reaction.

The graph below shows how the concentrations of A and C change during the reaction. The horizontal axis shows the time, usually given in units of seconds. The vertical axis represents the concentrations of A and C, usually given in units of moles per litre.

The point at which the concentrations of the reactant and product do not change tells us that the reaction is complete. From the graph above, the concentration of the product C reaches its maximum value at 50 seconds. At this time, the concentration of reactant A is zero and therefore has been used up.

Example 2: Determining Graphically the Time Taken for a Reaction to Reach Completion

The graph below shows the concentration of oxygen gas produced during the following chemical reaction: 2HO2HO+O2222 How long does this reaction take to reach completion?

  1. 90 seconds
  2. 120 seconds
  3. 100 seconds
  4. 80 seconds
  5. 20 seconds

Answer

The graph shows how the concentration of oxygen gas changes during the decomposition of hydrogen peroxide (HO22). As a product, the concentration of oxygen gas initially starts at zero and starts to increase.

At the point when the reaction is complete, the concentration of oxygen will level off and not change. We can see from the graph that the maximum value of concentration that the oxygen gas reaches is 1.00 mole per litre.

The concentration of oxygen gas reaches a value of 1.00 mole per litre at 90 seconds. Therefore, the reaction takes 90 seconds to reach completion. This line of reasoning can be used to determine that option A is the correct answer for this question.

The speed at which a chemical reaction takes place is known as the rate of reaction. Usually, the rate of reaction describes how some variable changes over a certain rate of time. This could be a change in volume or mass per second or per minute.

However, a common measurement for the rate of reaction is the change in concentration over time.

Definition: Rate of Reaction

The rate of reaction measures how reactant or product concentrations change per unit of time.

When sodium hydroxide is added to blue copper sulfate, a chemical reaction occurs. During the reaction, the solution turns colorless as sodium sulfate is formed. In addition, a blue precipitate of copper hydroxide is also formed. This is shown in the image below.

By measuring the time taken for the solution to turn colorless, or the time taken for the copper hydroxide to appear, the rate of the reaction can be measured.

The rate of reaction can be affected by many factors. Changing these factors can either increase or decrease the rate of reaction. Some of the factors that affect the rate of reaction include

  • nature of reactants,
  • surface area,
  • concentration,
  • temperature,
  • catalysts.

Here, the focus will be on the nature of the reactants and the surface area.

It is important to note that changing these factors does not change the final concentration of products made. However, changing these factors will change the time taken for these final concentrations to be reached.

Example 3: Identifying Which Statement Best Defines the Rate of a Chemical Reaction

Which of the following statements best defines the rate of a chemical reaction?

  1. The measure of change in the concentration of reactants or products per unit of time
  2. The difference in mass between the reactants and the products
  3. The final concentration of the products following a chemical reaction
  4. The speed at which particles need to move in order to successfully collide
  5. The time at which the concentrations of products and reactants are equal

Answer

The rate of a reaction gives us information about the speed at which a chemical reaction takes place. There are several ways we can measure and describe a rate of reaction; however, most of them involve the amount something changes over some time period.

From the five statements given, we can see that statements A, D, and E include the words speed or time in their description. The correct statement is likely to be one of these three.

However, only statement A describes how something changes per unit of time. In this case, it is the change in the concentration of reactants or products per unit of time.

This is a good description of the rate of reaction, as the concentration of reactant will decrease as the reaction proceeds. Conversely, the concentration of the product will increase as it is being produced during the reaction.

Therefore, the statement that best describes the rate of a chemical reaction is A.

The nature of the reactants can affect the rate of reaction. The nature of reactants refers to whether the reactant is an ionic or covalent molecule.

Often, chemical reactions involving ionic reactants are much faster than covalently bonded reactants. Ionic reactants can decompose into ions, which can react quickly. Covalent molecules require breaking the old bonds and forming news bonds to undergo a chemical reaction, and this takes longer time than the reaction between ions.

As such, the reaction between ionic compounds such as sodium chloride and silver nitrate is much quicker than the reaction between oxygen gas and carbon monoxide: NaCl+AgNOAgCl+NaNOfast2CO+O2COslow3322()()

Another factor that can affect the rate of reaction is the surface area of the reactants.

Consider the reaction between two reactants A and B. In the images below, A is shown by the purple circles, and B is shown by the green circles.

During the reaction, the surface area of A will change, although its mass remains constant.

In the image below, A is a large, solid lump. Here, the surface area of A is low. Initially, the molecules of B can only react with the outer particles of A. Once the outer particles of A have reacted, the inner particles will be exposed.

However, if a lump of A with the same mass is cut into smaller pieces, then the surface area will have increased. The image below shows how more particles of A are now available to react with molecules of B. As a result, the rate of reaction will increase.

The surface area of A can be increased further by cutting it into even smaller pieces, as shown in the image below. Now, an even larger number of particles of A are available to react with B.

The rate of reaction will increase again and be the fastest of all three experiments.

Increasing the surface area therefore increases the rate of reaction.

Example 4: Determining the Correct Order of the Rate of Reaction for Different Reactions

The diagram below shows four different reactions of a sample of metal with an acid. From slowest to fastest, which of the following best predicts the order for the rate of reaction? Assume the total mass of the metal, the concentration of the acid, and the temperature are the same for all reactions.

  1. A, C, B, D
  2. D, B, C, A
  3. C, B, A, D
  4. B, D, C, A
  5. D, A, C, B

Answer

Each beaker shows a reaction between a metal and an acid. We are told that in all four reactions the total mass of metal used is the same. However, we can see that each beaker contains a different number of metal pieces. In other words, the surface area of the metal is different in each beaker.

The rate of reaction can be affected by surface area. Usually, the larger the surface area, the faster the rate of reaction.

A large surface area means there is more of the metal available to react. However, if the metal is a lump, the surface area is small and the atoms at the center cannot react until the outer atoms have.

By looking at each beaker, we can see that the metal in D has the largest surface area, followed by B, C, and finally A.

We would therefore expect the rate of reaction to be fastest for D, followed by B, C, and A. The question, however, asks us to order the rate of reactions from slowest to fastest. The correct answer is therefore option (A): A, C, B, D.

The effect of surface area on the rate of reaction can be demonstrated using the reaction of iron with hydrochloric acid. By reacting iron with hydrochloric acid and measuring the amount of gas produced within a certain amount of time, the relative rate of reaction can be determined.

An experiment can be set up as shown below. In the experiment, two conical flasks are filled with the same concentration of hydrochloric acid. An iron ribbon is put into one of the conical flasks and iron filings are put into the other. Both reactions occur at the same temperature. The gas syringe is used to measure how much hydrogen gas is produced as the iron filings or iron ribbon reacts with the hydrochloric acid.

The chemical equation for the reaction is Fe()+2HCl()FeCl()+H()saqaqg22

For each experiment, the volume of gas can be measured over 5-second intervals and the data can be plotted on a graph. The graph below shows the results of the two experiments.

From the graph, we can see that the final amount of gas produced in each reaction is the same. However, this amount of gas is produced faster when the conical flask contains iron filings. Therefore, the rate of reaction is fastest in the reaction with iron filings.

The results from the experiment are not surprising. The iron filings have a much greater surface area than the iron ribbon. Usually, the greater the surface area, the faster the rate of reaction.

Example 5: Identifying from a Graph in Which Reaction the Surface Area of a Solid Was Greatest

In an experiment, a student added a sample of a solid into a solution, resulting in a chemical reaction. The student repeated the experiment 5 times but changed the surface area of the solid. The graph showing how the concentration of one of the products changes over time for each experiment is shown below. In which experiment was the surface area of the solid the greatest?

Answer

The rate of a chemical reaction can be affected by changing the surface area of the reactant. However, the final concentration of the product does not change. We can see from the graph that in all five experiments, the final concentration of the product is 1.50 moles per litre.

Graphically, a faster rate of reaction will correspond to a steeper line. A steeper line means that the final concentration of the product is reached in a much shorter time. Therefore, we can see from the graph that line B corresponds to the fastest rate of reaction and line D corresponds to the slowest rate of reaction.

Next, we need to determine how the rate of reaction is affected by surface area. The greater the surface area, the more the molecules that are available to react and so the faster the rate of reaction.

However, if the surface area is small, then the molecules at the center cannot react until the outer molecules have reacted. As a result, the rate of reaction is slower.

We have now determined that the rate of reaction is fastest when the surface area is greatest. We have also determined from the graph that the fastest rate of reaction corresponds to line B.

Therefore, the surface area of the solid was greatest in experiment B.

Key Points

  • Chemical reactions can take place over a range of different timescales.
  • During a chemical reaction, the concentration of reactants decreases, while the concentration of products increases.
  • The speed at which a chemical reaction takes place is known as the rate of reaction.
  • The rate of a chemical reaction can be defined as the change in the concentrations of the reactants and products per unit of time.
  • The rate of reaction can be affected by the nature of reactants, surface area, temperature, concentrations, and catalysts.
  • Ionic compounds generally have a faster rate of reaction than covalently bonded molecules.
  • The greater the surface area, the faster the rate of reaction.

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