CR - Reaction Basics (Lesson)

Reaction Basics

A chemical change occurs when substances are transformed into new substances, typically with different compositions. Below are four observations that allow the inference that chemical change has happened.

Driving Forces of Reactions

The evidence of change is closely associated with the reason the reaction happens - the 'driving force' for the chemical change. There are several driving forces for reactions connected to the energy and organization of the materials and what drives a reaction will vary with the reactants and the conditions. Most involve the organization of the products as compared to the reactants. If a greater number of smaller molecules are created from fewer larger molecules, especially if a small gaseous molecule such as CO₂, N₂,or HCl is produced as a product, this increase in entropy drives the reaction. If stronger bonds are made then are broken in going from starting materials to products, this energy change or electron transfer drives the reaction and is related to the energy of the materials or enthalpy. There will be a more involved discussion on the driving forces for chemical reactions in the thermodynamics module towards the end of the course.

Chemical Equations

A chemical reaction is an event that happens at the atomic level and involves rearrangements of atoms, energy, and sometimes even electrons.

A chemical equation is a way of describing that event with chemical formulas and symbols to provide information about the event. An equation provides a before-and-after documentation of the chemical substances.

The image below is a depiction of the chemical reaction involving the combustion of the common fuel butane found in grill/fireplace lighters and some camping stoves. 

Writing and Balancing Equations

Chemical equations are shorthand ways of writing the quantitative relationships between compounds and elements in a chemical reaction. You can think of equations like recipes. The reactants are the ingredients and the products are what the recipe will make. Chemical equations must obey the Law of Conservation of Matter meaning the mass of chemicals at the beginning (reactants) must equal the mass of chemicals at the end (products). The reactants can be rearranged, but the same number of each chemical must be present before and after the reaction. The way that this is indicated in a chemical equation is by using coefficients, numbers placed in front of a formula, to indicate the numbers of molecules, atoms, or formula units of a substance. These coefficients must be written so that the equation is balanced, meaning that there are the same number of atoms of each type of element on each side of the reaction arrow. An important thing to remember is that when balancing equations, the formulas themselves can never be altered. The only means to change the number of each element on either side of an equation is by adding or changing coefficients.

View the following video to learn more about writing and balancing equations.

Particle Representation of Reactions

Particulate models can also be used to represent both chemical reactions and physical processes. The synthesis of ammonia can be shown at the particle level using such a diagram. What features do you notice about the diagram here showing the synthesis of ammonia from elements? Make yourself a list of features.

What was on your list? Did you notice the diatomic elements are shown with two touching atoms? How about the key, using a color code? Notice there was not a 'plus' sign on the container of the reactants? And since everything here is in gas form, the particles are spread out in the container drawn to hold them?

Consider this example:

Using three colors this time, but no container for the particles, again the reaction is shown at the particle level. Which features do you find most helpful in these two examples? Which will you be sure to include when asked to draw the particle level of a reaction yourself for this course?

Now, consider how particle models of reactions might be used in a multiple choice question.

Again, a single box shows the reaction container and a second box shows the same container after the reaction. Which shows water vapor forming from the elemental form of the reactants? All show the same representation of the water products. Since both hydrogen and oxygen are diatomic elements, the reactant container must also show them as molecules. This eliminates choices A and B from consideration, where H and O are shown as atoms.  How do we select between the remaining choices? This time, consider amounts. Count all the oxygen atoms from C reactants and compare to products. Do the same in choice D. Which most accurately accounts for all particles present at the start? Only choice C. Choice D does not show a balanced reaction as half of the oxygen atoms have disappeared between reactant and products. 

[CC BY 4.0] UNLESS OTHERWISE NOTED | IMAGES: LICENSED AND USED ACCORDING TO TERMS OF SUBSCRIPTION