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Fast or Slow ... Chemistry Makes It Go!

By Lori R. Stepan

Everything around us is made of molecules, including the air we breathe, the objects we touch, and the bodies we use to run, learn, and laugh. Molecules are very small and can’t be seen with the naked eye, but they are very important!

Every molecule is made of two or more atoms, which are the building blocks of matter, like the individual bricks in a Lego sculpture. Molecules are in constant motion at all times. Whey they’re in a solid phase, they vibrate against each other. When they’re in a liquid phase, they slide past each other. And when they’re in a gas phase, they are very spread out. Chemistry is the study of these atoms and molecules, their properties, and their changes.

An illustration of a purple mole on a bicycle labeled "Catalyst"

Molecules can change into other molecules as they go through chemical reactions, which happen when molecules break the chemical bonds that hold them together or combine to make new bonds with other molecules, forming new molecules. The starting substances in a reaction are the reactants, and the substances produced at the end of a reaction are called products. Some chemical reactions are so slow that you can’t observe a change right away (such as the formation of rust on a piece of metal), and others are so fast that you might miss them if you blink (like when a firecracker pops).

What kind of things cause chemical reactions? What makes them go fast or slow? To answer these questions, let’s think about how chemical reactions are like a certain game you can play at a county fair or carnival.

Imagine you are going to a fair or carnival with a friend, smelling pretzels and cotton candy, hearing music, and watching all the happy people around you. Your friend suggests you try to win a prize at one of the booths by hitting a balloon with a dart, so you give it a go!

What will it take to break the balloon and win the prize? First, you must throw the dart with an excellent aim! Next, your dart must have a collision with the balloon. Is any collision good enough? No, the dart must have enough force to break the balloon, so you have to throw it hard enough to give it enough energy. What about the orientation (or direction) of the dart? If you throw it so that it hits the balloon sideways or on the wrong end, it’s unlikely to break the balloon. If you want to win, you have to do everything just right!

Molecules in Motion

A chemical reaction is in some ways similar to the balloon dart game. Molecules are constantly moving and colliding with each other, but they don’t always react. In order to have a successful reaction, molecules must have a collision with enough energy to react — this is what’s called the activation energy. If collision is not “active” enough, the molecules just bounce off each other like balls on a pool table, and go on their way. Finally, they have to be oriented correctly in order to break existing chemical bonds and make new ones. If the wrong ends of the molecules collide with each other, they might not react.

So, molecules are in constant motion, and the right kind of collisions can make new molecules. But what causes a reaction to go fast or slow? Anything that changes how effectively molecules collide will affect the speed of the reaction, which is also called the reaction rate. If there are more molecules present, or there’s a bigger surface area on which the reaction happens, there will be more successful collisions and the reaction will go faster.

Also, if the temperature is higher, more molecules will have enough energy to react, and the reaction will be faster. If the phase of matter (solid, liquid, or gas) of the reactants results in more collisions, the reaction will be faster. Gas molecules move fast, so gases usually react faster than liquids. Liquids move faster than solids, so liquids react faster than solids. If a substance called a catalyst is present, it can also help a reaction go faster.

In this edition of Celebrating Chemistry, you’ll find out lots more about the rate of chemical reactions, including how to slow down how quickly sliced apples turn brown, the very fast reactions that happen in cars, the enzymes in your body, what factors affect reaction rates, and more! Celebrate National Chemistry Week 2021 with the theme, “Fast or Slow … Chemistry Makes It Go!”

Lori R. Stepan, Ph.D. is an Associate Teaching Professor of Chemistry at Penn State University in State College, PA.