How important are chemical reactions?
Long ago, people discovered that:
- substances can react to make new substances
- these reactions can be controlled
The science of chemistry began when people began experimenting with chemical reactions systematically and organizing their results logically.
As chemistry developed, scientists learned that living things exist only as a result of chemical reactions – and this includes you and me.
Every living thing is a remarkably well-organized, self-managing chemistry set.
Our high-tech society could not exist without the ability of chemists to precisely control chemical reactions – the screen you’re viewing right now is one example of a device that would be impossible to make.
So, on a one-to-ten scale of scientific importance, chemical reactions rate ten.
People started reacting chemicals a long time ago
Our ancient ancestors learned:
- How to harness and control the oxygen-wood reaction to make fire.
- How to release metals from their ores to make efficient plows, axes, hammers and chisels.
- How to disinfect/fumigate rooms and buildings with sulfur dioxide, the gas released by burning sulfur.
- How to convert limestone into calcium oxide, the basis of cement and concrete. Large-scale construction projects such as ancient Rome’s Colosseum became possible.
Not all uses were peaceful
Our ancestors used new materials, such as iron and steel, to wage war. They discovered explosive chemical reactions and made gunpowder by mixing potassium nitrate, carbon, and sulfur.
We’re still making new discoveries
Although we know a lot more about chemistry than our ancestors did, we continue studying chemical reactions; this brings improvements in technology and our health.
The sheer excitement of making new discoveries also drives chemists to continue investigating reactions. Examples include:
- Lithium chemical reactions power our phones, tablet computers and cameras.
- Biochemical and electrochemical reactions enable our brains to store memories and to think.
- New chemical and biochemical reactions to produce new antibiotics, because bacteria are becoming increasingly resistant to the antibiotics we’re using today.
Reactants and Products
Chemical reactions begin with reactants and end with products. In chemical reactions:
Using the right-pointing arrow above implies that all reactants turn into products. For example, we can make water by reacting hydrogen with oxygen. The reactants are hydrogen and oxygen. The product is water in the gas phase because the reaction produces a lot of heat.
Often chemical reactions result in an equilibrium in which the reaction proceeds in both directions, leading to a mixture of reactants and products.
This is shown using a different arrow:
For example, the gas phase reaction used to make ammonia from hydrogen and nitrogen results in an equilibrium:
At equilibrium the reaction rates in the forward and reverse directions are equal.
How do we know there’s been a chemical reaction?
There has been a chemical reaction only when one or more new substances are made. Sometimes this is obvious, sometimes less so.
You will also often detect an energy change, because the reaction will heat up or cool down.
Food can react chemically with oxygen. Two ways of reacting food with oxygen are burning and respiration.
If the food is a carbohydrate, the word equation for both burning and respiration is:
The ash and flame make it obvious that new substances are being made and that energy has been released. Ash forms because food usually doesn’t burn cleanly. Although the carbon dioxide and water formed by the reaction are less obvious, the ash and flame make it easy to tell that a chemical reaction has taken place.
The evidence for a chemical reaction during respiration is more subtle than when we burn food. It’s not obvious that water and carbon dioxide are made by the reaction, although we can detect them. For example, we can use limewater to show that there is more carbon dioxide in the air we breathe out than in the air we breathe in. Breathing on a cold window produces condensation, indicating water is present in our breath. The energy released by respiration keeps all animals on our planet moving and keeps warm-blooded animals warm.
Signs of a chemical reaction
Look for one or more of these:
|Gas is released|
|A solid appears or disappears|
|There is a color change|
|The temperature changes|
|Sound is heard|
The signs above are all clues, but (except for flames) don’t prove there has been a reaction.
A chemical reaction has happened only if one or more new substances are made.
Here is an example of a chemical reaction. Identify the reactant(s) and the product(s).
You toast some bread and end up burning it. Has there been a chemical reaction? Why?Show Answer
A tree grows a little higher. Has there been a chemical reaction? Why?Show Answer
Name three visual clues that a chemical reaction might have happened.Show Answer
When water freezes, has there been a chemical reaction? Why?Show Answer