Key Concepts and Summary
The rate of a chemical reaction is affected by several parameters. Reactions involving two phases proceed more rapidly when there is greater surface area contact. If temperature or reactant concentration is increased, the rate of a given reaction generally increases as well. A catalyst can increase the rate of a reaction by providing an alternative pathway with a lower activation energy.
Practice Problems: Kinetics – Factors Affecting Reaction Rates
Describe the effect of each of the following on the rate of the reaction of magnesium metal with a solution of hydrochloric acid: the molarity of the hydrochloric acid, the temperature of the solution, and the size of the pieces of magnesium.
Solution
Higher molarity increases the rate of the reaction. Higher temperature increases the rate of the reaction. Smaller pieces of magnesium metal will react more rapidly than larger pieces because more reactive surface exists.
Explain why an egg cooks more slowly in boiling water in Denver than in New York City. (Hint: Consider the effect of temperature on reaction rate and the effect of pressure on boiling point.)
Hint
Denver is a higher altitude than New York City (Denver famously being “Mile High”, and New York being at sea level). At higher altitude, atmospheric pressure will be lower.
Solution
At lower atmospheric pressure, boiling point will be lower (i.e. water will boil at 100°C in New York, but at about 94°C in Denver). This means the “reaction” of denaturing the egg proteins is happening at a lower temperature. From kinetic principles, we can say that the egg will take longer to fully react (i.e. to cook) since the reaction is happening at a lower temperature.
Chemical reactions occur when molecules collide with each other and undergo a chemical transformation. Before physically performing a reaction in a laboratory, scientists can use molecular modeling simulations to predict how the parameters discussed earlier will influence the rate of a reaction. Use the simulation embedded below to explore how temperature, concentration, and the nature of the reactants affect reaction rates.
This sim may take a minute to load. Click here to open the sim in a new tab. You can also download and run this simulation in Java: click here. The Java version is usually a bit faster and smoother to run.
In the PhET Reactions & Rates interactive embedded above, use the “Many Collisions” tab to observe how multiple atoms and molecules interact under varying conditions. Select a molecule to pump into the chamber. Set the initial temperature and select the current amounts of each reactant. Select “Show bonds” under Options. How is the rate of the reaction affected by concentration and temperature?
Solution:
You should find that as concentration (number of particles) increases, the reaction rate increases. As temperature increases, so does the reaction rate.
You may find it interesting to enable the “energy view” and explore how reaction rate is related to the interaction of temperature, activation energy, and overall mechanism (energy profile).
In the PhET Reactions & Rates interactive embedded above, use the Single Collision tab to represent how the collision between monatomic oxygen (O) and carbon monoxide (CO) results in the breaking of one bond and the formation of another. Pull back on the red plunger to release the atom and observe the results. Then, click on “Reload Launcher” and change to “Angled shot” to see the difference in behaviour.
(a) What happens when the angle of the collision is changed?
Solution
Depending on the angle selected, the atom may take a long time to collide with the molecule and, when a collision does occur, it may not result in the breaking of the bond and the forming of the other. Only collisions at a specific angle of “attack” will result in a reaction.
(b) Explain how this is relevant to rate of reaction.
Solution
Particles of reactant must come into contact with each other before they can react. The activation energy and Arrhenius factor A respectively each depend on the energy required for a “successful” collision, and how easy it is for the particles to collide at an appropriate angle to facilitate the reaction.
In the PhET Reactions & Rates interactive embedded above: Use the Many Collisions tab to set up a simulation with 15 molecules of A and 10 molecules of BC. Enabling the “Bar” chart to show the amount of each species in the mix may help.
(a) Leave the initial temperature at the default setting. Observe the reaction. Is the rate of reaction fast or slow?
Solution
(a) very slow.
(b) Click “Pause” and then “Reset All,” and then enter 15 molecules of A and 10 molecules of BC once again. This time, increase the initial temperature until, on the graph, the total average energy line is completely above the potential energy curve. Describe what happens to the reaction. Enabling the “Bar” chart to show the amount of each species in the mix may help.
Solution
As the temperature is increased, the reaction proceeds at a faster rate. The amount of reactants decreases, and the amount of products increases. After a while, there is a roughly equal amount of BC, AB, and C in the mixture and a slight excess of A.