Chemical Reactions: Introduction
All chemical compounds consist of atoms of different elements in specific combinations and defined ratios. Two types of bonds form the attraction between the atoms in chemical compounds. The bonds found in inorganic compounds are termed ionic because positive and negative ions of atoms are attracted to one another to form this type of bond. A positive ion (cation) is formed when an electron(s) is lost resulting in a net positively charged atom. A negative ion or (anion) is formed when an electron(s) is gained resulting in a net negatively charged atom. The bonds found in organic compounds are termed covalent because no electrons are gained or lost but are shared by different joined atoms.
Look at the two different compounds below. Both are found in every kitchen – table sugar (black balls represent carbon atoms, red represent oxygen atoms, white represent hydrogen atoms) and table salt (green balls represent chloride ions, yellow represent sodium ions):
Table Sugar: sucrose
Covalent chemical bonds
Table Salt: Sodium Chloride
Ionic chemical bonds
The affinity of each particular element determines whether they bond with other elements through ionic or covalent bonds. Elements that have a high affinity for electrons gain electrons from elements that have a low affinity, resulting in the formation of an ionic bond between the resulting ions (as in sodium chloride). Elements with an intermediate electron affinity cannot gain or lose electrons, so they typically share electrons in covalent bonds (as in sucrose).
Chemical reactions are described by specific chemical equations that indicate which compounds are reactants and which are products. The reactants and products are designated by their formulas which are separated by a yield arrow. A chemical equation can be thought of as a sentence describing the reaction. The formulas of the reactants and products can be thought of as the words within the sentence.
When a chemical reaction occurs, bonds of reactant compounds are broken between different ions in the case of inorganic compounds or atoms in the case of organic compounds. When the bonds are reformed, the resulting compounds are the products. During the course of a chemical reaction, the reactants are typically consumed and no longer exist as reactant compounds. The product compounds are produced.
Despite the fact that reactant compounds no longer exist when a chemical reaction is completed, all chemical reactions are governed by the Law of Conservation of Matter (See Fun Facts in the right-hand column). The reactants may not exist in their original form but every atom and ion that was originally found in the reactant formulas can be found in the formulas of the products.
Look at the chemical reaction below between magnesium metal and hydrochloric acid. Notice how the formulas are written to account for the number of atoms of each element present both on the reactant and product side of the reaction. Sometimes you will see a number in front of a compound (example below: 2HCl). You simply multiply this number times the number of atoms in the compound that follow it (example below: 2HCl = 2 X H and 2 X Cl, or 2 hydrogen atoms and 2 chlorine atoms). The second way of showing the number of atoms in chemical reactions is by using subscripts. In the example reaction below, both chlorine and hydrogen are present on the product side as two atoms each (Cl2 and H2). Sometimes, although not in the example chemical reaction shown below, both forms of atom counting are used together. For example, 2MgCl2 would indicate 2 magnesium atoms and 4 (2 X 2) chlorine atoms. In any case, you will always find the exact same number of each atom on both sides of a chemical reaction formula. This follows the law of conservation of matter.
In a typical chemical reaction, energy is required to break the bonds of reactants since they possess a certain degree of stability. When the bonds of the products are formed, energy is released since typically the products of a reaction are of greater stability than the reactants. The release of energy to the surroundings reduces the total bond energy of the products making them more stable than the reactants. Whether a reaction requires energy or produces energy depends on the difference between the input energy required to break reactant bonds and the output energy released from the formation of the product bonds. If more energy is required to break bonds than is produced, then energy is required resulting in an endothermic reaction. If less energy is required to break bonds than is produced, then energy is released, resulting in an exothermic reaction. The suffix thermo means temperature. Thus, endothermic reactions take up/absorb thermo-energy and the reaction becomes cooler as it occurs. Conversely, exothermic reactions release thermo-energy and the reaction becomes warmer as it occurs.
The total energy change of a reaction, either the net input or net output, is usually measured as heat energy. This energy can be conveniently determined by measuring the temperature change of a reaction. The reactants are placed in a calorimeter, which is an insulated container that prevents a chemical reaction from gaining heat from or losing heat to its surroundings. Any change in the measured temperature is an accurate measure of the energy that is either required by the reaction (in an endothermic reaction) or gained by the reaction (in an exothermic reaction). We will make a simple calorimeter in Investigation 1 Lab and follow an exothermic chemical reaction by measuring its temperature.
The balanced chemical equation that represents each chemical reaction stipulates the relative numbers of reactants and products. Atoms and molecules react with each other in specified ratios of whole numbers. This is analogous to a cake recipe that stipulates specific amounts of ingredients. In each case, if one reactant is not present in the specified ratio with the other reactants, the amount of products will be less than anticipated. The reactant that is present in excess will be unreacted, and the reactant that is present in the lesser amount will be completely consumed.
The relative amounts of the reactants of a chemical reaction also determine the rate at which the reaction proceeds. The rate of any chemical reaction depends on the concentrations of all reactants. A high concentration of reactants results in a fast rate of reaction because, statistically, the molecules have a high chance of encountering each other and reacting. This corresponds to a decrease in the time it takes the reaction to be completed. A low concentration of reactants results in a slow rate of reaction because encounters between reactant molecules are less likely to occur. This corresponds to an increase in the time it takes the reaction to be completed.
The rate of a chemical reaction can be assessed by measuring how fast the reactants are consumed or how fast the products are produced. Increasing the concentration of one reactant will result in the reaction proceeding to completion at a faster rate. The reactants will be consumed more quickly and the products will be produced more quickly. Typically, some means is required to measure the decrease in reactant concentration or increase in product concentration. Often a spectrophotometer is used to measure the decrease in absorbance due to reactant consumption or the increase in absorbance due to product formation. We will perform such a reaction and use a spectrophotometer in Investigation 3 Lab.
- Fun Facts
- Learn the Lingo
- Get Focused
Careers in Chemistry
There are many career opportunities that are available to those that have been educated in chemistry. The following list is just a sample of the type of chemistry careers listed by the US Department of Education. There are many others.
- Atmospheric Chemistry and Climatology
- Analytical Chemistry
- Inorganic Chemistry
- Chemistry, General
- Organic Chemistry
- Physical and Theoretical Chemistry
- Polymer Chemistry
- Chemical Physics
- Geochemistry and Petrology
- Molecular Biochemistry
- Biochemistry/Biophysics and Molecular Biology
- Chemical Technology/Technician
The energy industry, in particular, will likely require much more research and development by chemists and engineers. While petroleum production is not very important, energy sources that do not rely on petroleum fossil fuel will be more and more important. Chemists will certainly play a major role in developing clean, renewable energy sources.
Conservation of Matter
The Law of Conservation of Matter or sometimes referred to as the Law of Conservation of Mass, was formulated by the brilliant eighteenth-century chemist Antoine Lavoisier.
The Law states that matter (or mass) cannot be created or destroyed. Matter, however, can change form. This is exactly what happens in chemical reactions. This is also the reason that, for every chemical reaction, we must account for each and every atom in both the reactants and products. This is one of the most important laws of chemistry and physics.
Unfortunately, during the French Revolution (1789-1799), Lavoisier was arrested for his previous affiliation with the Royal government and beheaded by guillotine in Paris (1794) at the age of 50.
LEARN THE LABLEARNER LINGO
The following list includes Key Terms that are introduced within the Backgrounds of the CELL. These terms should be used, as appropriate, by teachers and students during everyday classroom discourse.
- Chemical reaction: occurs when chemical reactants are converted into chemical products
- Reactant: a chemical compound that is consumed in a chemical reaction
- Product: a chemical compound that is produced in a chemical reaction
- Formula: a description of a chemical compound using the letter designations of the elements
- Chemical equation: Describes a chemical reaction by indicating the formulas of all reactants and products
- Yield: the amount of a product produced by a chemical reaction compared to the amount of reactant
- Calorimeter: an insulated container that prevents a chemical reaction from gaining heat from its surroundings or losing heat to its surroundings
- Chemical bonds: the forces between atoms that hold those atoms together to form compounds
- Atoms: the smallest particle of matter that still retains the properties of an element
- React: when reactants interact to form products – It always requires the breaking and reforming of chemical bonds
- Matter: anything with mass and volume
- Law of Conservation of Matter: matter cannot be created nor destroyed
- Consumed: when a reactant is converted to a product
- Unreacted: when a reactant is not converted to a product
- Rate: the quantity of reactants consumed or the quantity of products produced during a specific time period
- Wavelength: the distance between two adjacent crests or two troughs of a transverse wave of electromagnetic radiation
- Nanometer: wavelength is usually expressed in nanometers – A nanometer is a metric unit of length that equals 10-9 meters. 1,000,000,000 nanometers is equivalent to one meter.
- Spectrophotometer: an instrument that can quantitate the amount of light of a specific wavelength that is absorbed by a chemical compound
- Absorbance: the ability of a chemical compound to take in electromagnetic radiation of a specific wavelength – The energy of a specific wavelength, when absorbed, is dissipated in the electrons of the chemical compound.
- Catalyst: a chemical that accelerates a chemical reaction without being consumed in the reaction
The Focus Questions in each Investigation are designed to help teachers and students focus on the important concepts. By the end of the CELL, you should be able to answer the following questions:
- In a chemical reaction, what is the relationship between the amount of reactants and the amount of products?
- In a chemical reaction, do the amounts of the reactants affect the amounts of products produced?
- How does the Law of Conservation of Matter relate to chemical reactions?
- In a chemical reaction, what is the relationship among the reactants, the products, and the time it takes to complete the reaction?