Chemical reactions and how they break and form bonds between atoms. Balanced reactions, reversibility, and equlibrium.
Molecules—like the ones that make up your body—are just collections of atoms held together by chemical bonds. In many ways, they're a lot like Tinkertoy® building projects. In fact, if you take organic chemistry, you’ll most likely buy a model set that looks suspiciously similar to Tinkertoys®:
Ball-and-stick model of the atom proline made using a modeling kit.
Just as you can put Tinkertoy® wheels together in different ways using different stick connectors, you can also put atoms together in a different ways by forming different sets of chemical bonds. The process of reorganizing atoms by breaking one set of chemical bonds and forming a new set is known as a chemical reaction.
Chemical reactions occur when chemical bonds between atoms are formed or broken. The substances that go into a chemical reaction are called the reactants, and the substances produced at the end of the reaction are known as the products. An arrow is drawn between the reactants and products to indicate the direction of the chemical reaction, though a chemical reaction is not always a "one-way street," as we'll explore further in the next section.
For example, the reaction for breakdown of hydrogen peroxide () into water and oxygen can be written as:
In this example hydrogen peroxide is our reactant, and it gets broken down into water and oxygen, our products. The atoms that started out in hydrogen peroxide molecules are rearranged to form water molecules () and oxygen molecules ().
You may have noticed extra numbers in the chemical equation above: the s in front of hydrogen peroxide and water. These numbers are called coefficients, and they tell us how many of each molecule participate in the reaction. They must be included in order to make our equation balanced, meaning that the number of atoms of each element is the same on the two sides of the equation.
Equations must be balanced to reflect the law of conservation of matter, which states that no atoms are created or destroyed over the course of a normal chemical reaction. You can learn more about balancing reactions in the balancing chemical equations tutorial.
Reversibility and equilibrium
Some chemical reactions simply run in one direction until the reactants are used up. These reactions are said to be irreversible. Other reactions, however, are classified as reversible. Reversible reactions can go in both the forward and backward directions.
In a reversible reaction, reactants turn into products, but products also turn back into reactants. In fact, both the forward reaction and its opposite will take place at the same time. This back and forth continues until a certain relative balance between reactants and products is reached—a state called equilibrium. At equilibrium, the forward and backward reactions are still happening, but the relative concentrations of products and reactants no longer change.
Each reaction has its own characteristic equilibrium point, which we can describe with a number called the equilibrium constant. To learn where the equilibrium constant comes from and how to calculate it for a specific reaction, check out the equilibrium topic.
When a reaction is classified as reversible, it is usually written with paired forward and backward arrows to show it can go both ways. For example, in human blood, excess hydrogen ions () bind to bicarbonate ions (), forming carbonic acid ():
Since this is a reversible reaction, if carbonic acid were added to the system, some of it would be turned into bicarbonate and hydrogen ions to restore equilibrium. In fact, this buffer system plays a key role in keeping your blood pH stable and healthy.
Want to join the conversation?
- I understand that one of the ways to speed up a chemical reaction is to slice the solute into smaller pieces, this way it is more exposed to the solvent. However, what i fail to comprehend is how this method INCREASES the surface area of the solute. Wouldn't making the solute smaller DECREASE the surface area?(0 votes)
- It increases the surface area : volume ratio--there's the same amount of matter but more surface as more of it is exposed.
The apple example above is a good one--cutting something in half means new surface is exposed that was previously on the inside.
Surface area is the area of all the surfaces added together. A cube with length 1, width 1, and height 2 has 4 2x1 rectangles and 2 1x1 rectangles, which is 10 in total. Cut it in half and you have two cubes, each with 6 1x1 rectangles (half the height, same length and width). 2 x 6 = 12 so you now have a surface area of 12 instead of 10.(12 votes)
- Are all Hidrogen atoms equal to each other? Like totally equal no diference?(1 vote)
- Being as they are the same thing, if there is a difference, it would be so minuscule. But yes, There really isn't a genuine difference between multiple hydrogen atoms.(4 votes)
- Where are Hydrogen and Oxygen atoms found as single atoms?(3 votes)
- Oxygen and Hydrogen atoms are diatomic, meaning they can never be found alone in nature. If they fail to find another element to bond to, they are forced to bond with themselves.(7 votes)
- can radioactive elements set on fire?(4 votes)
- Sure, they pretty much act the same chemically as their stable isotopic counterparts.(5 votes)
- Why do atoms come together?(4 votes)
- Do you mean why several atoms come together in a molecule? They are more stable this way because when a bond is formed(by sharing electrons between atoms) energy is released.(4 votes)
- How do multiple chemical reactions occur simultaneously?(3 votes)
- Imagine a chemical reaction as people talking. As many people can hold conversations at the same time, chemical reactions can occur simultaneously.(1 vote)
- when water gets boiled by fire that is an example of a chemical reaction?(1 vote)
- No, it is a physical reaction because you still have water. However, the flame itself is a chemical reaction because the fuel no longer exists once burned.
A chemical reaction occurs when you no longer have the same substance or substances, but have some new substance or substances. Changing the state (such as boiling, freezing, melting, or grinding up) still leaves you with the same substance or substances, so such things are physical reactions.
While boiling or freezing water is just a physical reaction, cooking food is a chemical reaction. When you bake dough, you no longer have dough but bread -- cooling the bread back down does not turn it back into dough -- so that is a chemical reaction.(5 votes)
- why does there always have to be two sets of reactments to form a proper chemical reaction?(1 vote)
- Think about it like this, a woman or man cannot simply "create" a baby by themselves. You need both the sperm and the egg! If there was only one reactant.. it would just stay as it was... as soon as it was to change (chemically speaking) then it would have reacted with something.(3 votes)
- How can we tell which reactions are reversible or irreversible?(1 vote)
- It comes with experience, but even then it is not always easy or possible to decide just by looking. As you study more chemistry, you will come across equilibrium constants and thermodynamics, both of which help to determine reversibility.
One simple pointer is to see if a gas is liberated. For example, if oxygen is released (as shown in the H2O2 example above) the reaction will be irreversible if the gas escapes into the atmosphere.(4 votes)
- Are all reactions reversible?(1 vote)
- Hi! your answer is no.All reactions are not reversible.Mainly chemical reactions are not reversible. Examples include,germination of a seed,curdling of milk,etc. Hope this might help. Have a nice day.(4 votes)