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Rate of a Chemical Reaction: Modifying Factors

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  • 0:06 Reaction Rate
  • 2:23 Chemical Reactions: A…
  • 3:13 Temperature & Concentration
  • 5:36 Surface Area
  • 6:30 Catalysts
  • 7:26 Lesson Summary
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Lesson Transcript
Instructor: Elizabeth (Nikki) Wyman

Nikki has a master's degree in teaching chemistry and has taught high school chemistry, biology and astronomy.

Why are some reactions much faster than others? Is it possible to change how fast a reaction occurs? In this lesson, you will learn what the rate of a chemical reaction is. You will also discover how factors such as temperature, concentration, surface area, and catalysts impact reaction rates.

Reaction Rate

When you hear the word rate, what do you think of? An interest rate? A rate of speed? A growth rate? Or maybe a rate of pay? Most rates have something happening in a specific amount of time, like the percentage of interest you pay on a credit card every year, how far you drive in an hour, how fast you grow in a year, or how much you are paid every hour.

In chemistry, the rate of a reaction describes how fast a reaction proceeds over time. In other words, a rate of reaction measures how quickly reactants are changed into products. Reaction rate is the change in concentration of reactants over time or the change in concentration of products over time. Units for reaction rates are in terms of M/time. For faster reactions, seconds are used for units of time; for longer reactions, minutes are used.

Reaction rates may be expressed in terms of any chemical species involved in the reaction. Reaction rate can be written for the disappearance of a reactant or the appearance of a product.

Let's take an example of a pretty cool reaction: the reaction between hydrochloric acid and zinc metal. When combined, the acid eats away at the metal to produce hydrogen gas and some dissolved metal salts. The rate of this reaction could be measured in terms of the disappearance of the zinc or the rate of appearance of hydrogen gas.

If we were expressing reaction rate in terms of disappearance of zinc metal, we would write:

Reaction rate = -change[zinc]/time

The slope of the line would be negative, because the concentration of our reactant is constantly decreasing.

If we were expressing reaction rate in terms of appearance of hydrogen gas, we would write:

Reaction rate = change[hydrogen gas]/time

The slope of the line would be positive, because the concentration of our reactant is constantly increasing.

This reaction happens pretty quickly, but think...what are some ways that we could increase the rate of reaction? What are some ways that we could decrease the rate of reaction?

Before we talk about factors that influence reaction rate, let's look at a chemical reaction on the molecular level.

Chemical Reactions: A Molecule's View

Molecules are made up of atoms bonded together by the sharing of electrons. These bonds are relatively strong and require a certain amount of energy to break. The random bumping and colliding of molecules with each other generally do not contain enough energy to break these bonds and cause a chemical reaction. Additionally, molecules must collide with proper orientation.

According to collision theory, in order for a chemical reaction to happen there needs to be an effective collision between the reactants. To be effective, a collision must meet the following two requirements:

  • Molecules collide with enough energy to break bonds
  • Molecules collide with a favorable orientation

Any factor that affects the likelihood of an effective collision also affects the rate of reaction.

Temperature

You and your stomach might know from cooking experience that changing the temperature of reactants influences reaction rate. Generally, if we want to cook something, we heat it up. When we don't want our food to spoil, we cool it down. That's why we have refrigerators, freezers, microwaves, stoves and ovens in our kitchens!

Let's think about why temperature might affect reaction rate...what is temperature a measurement of?

Temperature is a measurement of the energy in motion within a system. The higher the temperature, the more energy the system has. Molecules with more energy are likely to have more effective collisions with each other, resulting in an increased rate of reaction. We could wait until our eggs cook themselves, but that's not likely to happen before they spoil. Instead, we add heat to increase the rate of reaction!

What would happen to the rate of reaction if we decreased the temperature?

Decreasing temperature decreases the energy of the molecules. The molecules collide less, resulting in fewer reactions. When we cool our food to preserve it, we are slowing down reaction rates. This keeps microbes like bacteria and mold from breaking down our food as fast, and it keeps fresh fruits and vegetables from starting to decompose.

In general, rate of reaction increases with increasing temperature.

Concentration

Imagine you're at the grocery store. The store is nearly empty and you have no problems cruising up and down the aisles. Suddenly, more and more people come into the grocery store. Pretty soon, you can hardly move your cart without colliding into another person!

The concentration of humans in the store has just increased, thereby increasing the likelihood of cart-human collisions.

By cramming more reactants into a space (or alternatively, decreasing the volume of the space) concentration is increased. Reactants collide with each other more, leading to an increased chance of an effective collision.

If the concentration of the reactants decreases, how will the rate of reaction change?

The lower the concentration of reactants, the lower the rate of reaction.

In general, the higher the concentration of reactants, the faster the rate of reaction.

For gaseous systems, the same effect can be achieved by changing the pressure of the system.

Surface Area

Let's do a little experiment. We are dissolving two pieces of zinc, each weighing 10 grams, in hydrochloric acid. One piece of zinc is in a perfectly round ball. The other piece of zinc is a thin sheet, only an atom thick. Which piece of zinc will disappear faster? Which will have the faster reaction rate?

The more surface area of a reactant that is exposed, the faster the reaction will happen. This is simply because there are more opportunities for effective collisions to occur. A slower reaction will have less surface area of a reactant exposed.

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