Chemistry Study Guide for the HESI Exam

Compounds

Compounds are substances formed when two or more elements form a chemical bond. These elements can be the same or different, and they combine in fixed ratios. For example, water is a compound made of two hydrogen atoms and one oxygen atom, represented as \(\text{H}_2\text{O}\).

In chemical formulas, subscripts are the small numbers written to the lower right of an element symbol. They show the number of atoms of that element present in one molecule or unit of the compound. Subscripts are essential because they reflect the fixed ratio in which elements combine. If a compound includes parentheses, the subscript outside the parenthetical multiplies everything inside.

For example, in \(\text{Ca(OH)}_2\), the subscript \(2\) applies to both oxygen and hydrogen inside the parentheses. So, there are two oxygen atoms and two hydrogen atoms bonded to one calcium atom.

Chemical Equations

Chemical equations represent the process of a chemical reaction, in which reactants are transformed into products. These equations show what is happening during a reaction, allowing us to track how atoms and molecules change. There are three important parts of a chemical equation:

• Reactants are the substances that start a chemical reaction. They are placed to the left of the arrow.

• Products are the substances formed as a result of the reaction. They are placed to the right of the arrow

• The arrow (\(\rightarrow\)) indicates the direction of the reaction, showing the conversion of reactants to products.

For example, the reaction in which two molecules of hydrogen (\(\text{H}_2\)) and one molecule of oxygen (\(\text{O}_2\)) react to form two molecules of water (\(\text{H}_2 \text{O}\)) is represented by the following chemical equation:

\[2 \text{H}_2 + \text{O}_2 \rightarrow 2 \text{H}_2 \text{O}\]

Note: In nature, hydrogen and oxygen, as well as other elements like the halogens, exist as diatomic molecules (molecules made from two atoms) for stability reasons. That is why in the reaction to form water we use \(\text{H}_2\) and \(\text{O}_2\) instead of single atoms of \(\text{H}\) and \(\text{O}\).

Balancing a Chemical Equation

Balanced equations will have the same number of each type of atom on both sides. To balance equations, you have to change the number of molecules of one or more compounds on either side of the equation. So, for instance, assume you need to balance this equation:

\[\text{C}_2 \text{H}_6+\text{O}_2 \rightarrow \text{CO}_2+2\text{H}_2\text{O}\]

What will you do? The first step is to work out the number of atoms on each side:

\(\text{C} = 2\)		\(\text{C} = 1\)
\(\text{H} = 6\)		\(\text{H} = 4\)
\(\text{O} = 2\)		\(\text{O} = 3\)

The right side needs one more atom of \(\text{C}\) and two more atoms of \(\text{H}\), so we can add one more \(\text{CO}_2\) molecule and one more \(\text{H}_2\text{O}\) molecule and the equation becomes:

\[\text{C}_2\text{H}_6+\text{O}_2 \rightarrow 2\text{CO}_2+3\text{H}_2\text{O}\]

\(\text{C} = 2\)		\(\text{C} = 2\)
\(\text{H} = 6\)		\(\text{H} = 6\)
\(\text{O} = 2\)		\(\text{O} = 7\)

Because we have also added more atoms of \(\text{O}\) to the right, we now need seven atoms of \(\text{O}\) on the left. So, we’ll multiply the \(\text{O}_2\) by \(\frac{7}{2}\) on the left and the equation is balanced:

\[\text{C}_2\text{H}_6+\frac{7}{2}\text{O}_2 \rightarrow 2\text{CO}_2+3\text{H}_2\text{O}\]

\(\text{C} = 2\)		\(\text{C} = 2\)
\(\text{H} = 6\)		\(\text{H} = 6\)
\(\text{O} = 7\)		\(\text{O} = 7\)

However, chemical equations typically do not include fractional coefficients, so let’s multiply the entire equation by \(2\) to eliminate \(\frac{7}{2}\):

\[2 \text{C}_2\text{H}_6+7\text{O}_2 \rightarrow 4\text{CO}_2+6\text{H}_2\text{O}\]

Equilibrium

In a reversible reaction, the products can react to form the original reactants, creating a dynamic equilibrium. This means that the forward and reverse reactions occur at the same rate, and the concentrations of reactants and products remain constant over time.

Consider this reaction:

\[\text{N}_2 + 3 \text{H}_2 \rightleftharpoons 2 \text{NH}_3\]

At equilibrium, the rates of the forward and reverse reactions are equal, so the amounts of nitrogen (\(\text{N}_2\)), hydrogen (\(\text{H}_2\)), and ammonia (\(\text{NH}_3\)) stay constant.

Manipulating Reaction Rate

Particles in liquids and gases are constantly moving and colliding with each other. Under the right conditions, these particles can react. To start a reaction, the particles need to have a minimum amount of kinetic energy, known as the activation energy. This is the amount of energy required to break the bonds within each particle. These are ways to increase the reaction rate.

Temperature

The rate of a reaction increases as temperature increases because the particles will have more kinetic energy. This means that they will be moving faster and colliding more often and be more likely to have enough energy to break the activation barrier.

Surface Area

Increasing the surface area of a reactant increases the reaction rate because more particles are exposed and available for collision, speeding up the reaction. For example, powdered reactants react faster than large chunks of the same material.

Catalysts

Catalysts can also be added to increase the reaction rate. These work by lowering the activation energy. In the following example, platinum is used as a catalyst in the reaction, which makes nitric acid (\(\text{HNO}_3\)) from ammonia (\(\text{NH}_3\)). Notice that platinum does not appear in the equation. This is because a catalyst is neither consumed nor changed during a chemical reaction.

\[4 \text{NH}_3+5 \text{O}_2 \rightarrow 4 \text{HNO}_3+6 \text{H}_2 \text{O}\]

Concentration

Increasing the concentration of a reactant will also increase the rate of reaction because this will increase the number of particle collisions.

Solutions

A solution is a homogeneous mixture made of two or more substances. The substance in the smaller amount is called the solute, while the substance in the larger amount is the solvent. In a solution, the solute is dissolved evenly throughout the solvent, resulting in a uniform composition. For example, when salt (solute) dissolves in water (solvent), it forms a saltwater solution. The salt dissociates into ions and is evenly distributed in the water.

Solution Types

There are different types of solutions based on the nature of the solute and solvent involved.

Alloy

An alloy is a solid solution in which two or more solids are combined to form a new material with different properties. For example, bronze is an alloy of copper and tin. Depending on the relative sizes of the metals, alloys can be substitutional (when the sizes are similar), or interstitial (when one element is much smaller than the other).

Amalgam

An amalgam is an alloy in which one of the metals is mercury. Amalgams are typically used in dental fillings. Like alloys, amalgams are homogeneous mixtures, but they involve mercury as a key component.

Emulsion

Emulsions are mixtures in which two liquids that typically do not mix (such as oil and water) are dispersed together, forming a heterogeneous mixture. In an emulsion, one liquid is dispersed as tiny droplets within the other, like tiny droplets of fat dispersed in water to form milk or tiny droplets of water dispersed in oil to form butter. Emulsions are not true solutions as they are heterogeneous mixtures.

Compound

A compound is a substance formed by the chemical bonding of two or more different elements. These compounds can dissolve in solvents to form a solution, where the solute particles keep their identity and do not react with the solvent. Pure compounds, however, are not solutions.

Solution Concentration

The concentration of a solution conveys the amount of solute dissolved in a given amount of solvent. This measurement shows how strong or diluted a solution is. There are several ways to express concentration, depending on the context and the nature of the solution.

Percent Concentration

Percent concentration expresses the amount of solute in a solution as a percentage of the total solution. For example, in a \(10\%\) salt solution, there are \(10\) grams of salt in every \(100\) grams of solution. Letting \(PC\) be percent concentration, \(m\) be the mass of the solute, and \(T\) be the total mass of the solution, we have:

\[PC = \left( \frac{m}{T} \right) \times 100\%\]

Molar Concentration

Molar concentration (molarity) is the number of moles (mol) of solute dissolved in one liter of solution. It is commonly used for liquid solutions in chemistry and expressed with the symbol \({M}\). One mole equals approximately \(6.02 \times 10^{23}\) molecules or atoms of a substance.

This is also known as Avogadro’s number. The amount of a substance that contains this many molecules or atoms equals its atomic mass expressed in grams. For example, oxygen has an atomic mass of \(16.00\). So, \(16\) grams of oxygen contains Avogadro’s number of atoms and equals one mole of oxygen. For a molecule, add up the atomic masses of all of the atoms in that molecule. Sodium chloride (\(\text{NaCl}\)) has one sodium atom, mass \(22.99\), and one chlorine atom, mass \(35.45\), so one mole of that molecule is \(22.99+35.45 = 58.44\) grams.

Letting \(m\) be the number of moles of the solute and \(V\) be the volume of the solution (in liters), we have:

\[M = \frac{m}{V}\]

When a concentrated solution is diluted into a less concentrated solution, the concentration of the resulting solution can be found by applying the following formula to solve the dilution problem:

\[{M}_1{V}_1 = {M}_2{V}_2\]

where \({M}_1\) and \({M}_2\) are the concentrations of the initial and final solution, respectively, and \({V}_1\) and \({V}_2\) are the volumes of the initial and final solutions, respectively.

Chemical Reactions

A chemical reaction is a process in which one or more substances are converted into new substances with different properties. This involves the breaking and forming of chemical bonds, resulting in the rearrangement of atoms and molecules. The product of a chemical reaction is a molecule that has different properties from the reactants.

Here are some of the common symbols used to draw chemical reactions:

• \(\rightarrow\) (arrow)—indicates the direction of the reaction, from reactants (left) to products (right)
• \(\ce{<=>}\) (double arrow)—shows a reversible reaction, meaning it can proceed in both directions
• \(+\) (plus sign)—separates different reactants or products
• \((s)\), \((l)\), \((g)\), and \((aq)\) (solid, liquid, gas, and aqueous)—show the physical state of each substance
• \(\Delta\) (delta)—placed above the arrow to indicate that heat is required for the reaction

Reaction Types

Chemical reactions can be classified into several types based on how the reactants and products interact.

Synthesis

A synthesis reaction occurs when two or more simpler substances combine to form a single more complex product. This is the general pattern for such a reaction:

\[A + B \rightarrow AB\]

Decomposition

A decomposition reaction occurs when a single, more complex compound breaks down into two or more simpler substances in a reaction that often requires energy in the form of heat or light. This is the general pattern for decomposition reactions:

\[AB \rightarrow A + B\]

Combustion

A combustion reaction involves the combination of a substance with oxygen, releasing energy in the form of heat and light. If the fuel is a hydrocarbon, combustion produces water and carbon dioxide. For example, the combustion of methane is described by this reaction:

\[\text{CH}_4 + 2 \text{O}_2 \rightarrow \text{CO}_2 + 2 \text{H}_2 \text{O}\]

Single Replacement

A single replacement reaction occurs when an element replaces another in a compound. This is the general pattern for a single replacement reaction:

\[A + BC \rightarrow AC + B\]

Double Replacement

A double replacement reaction occurs when two compounds react and the cations and anions exchange partners to form new compounds. This is the general pattern for double replacement reactions:

\[AB + CD \rightarrow AD + CB\]

Chemical Bonding

Chemical bonding is the interaction between atoms that allows them to form molecules and compounds. Valence electrons (the outermost electrons) are responsible for the formation of chemical bonds between atoms.

Ionic Bond

An ionic bond is formed when one atom transfers electrons to another, resulting in the formation of positively (cations) and negatively (anions) charged ions that are attracted to each other. Ions are atoms or molecules that have gained or lost one or more electrons, resulting in a charged particle. Electrostatic forces hold these ions together. Ionic bonds usually occur between metals and nonmetals.

Covalent Bond

A covalent bond is formed when two atoms share electrons rather than transferring them to achieve a full outer electron shell. This bond usually occurs between nonmetals.

Bond Polarity

In a covalent bond, bond polarity arises due to the unequal sharing of electrons between atoms. This can result in a polar bond (when one end is negative and the other is positive), or a nonpolar bond (where the electrons are shared equally). The polarity of the bonds influences the strength of the intermolecular forces that occur between molecules.

Bonds can be classified according to the electronegativity difference of the atoms forming the bond, as shown in this chart:

Dispersion Forces

Dispersion forces (also called London dispersion forces) are weak intermolecular forces that arise due to temporary dipoles (a pair of equal and opposite electric charges separated by a distance). A molecule that has partial positive and partial negative regions has a dipole. In molecules that have no difference in electronegativity between their atoms, like \(\text{H}_2\), the dipoles occur randomly, creating London dispersion forces. These forces are present in all molecules but are significant forces in nonpolar molecules.

Dipole-Dipole Interaction

Dipole-dipole interactions occur between molecules that have a permanent dipole moment. These interactions occur when the positive end of a polar molecule is attracted to the negative end of another. Dipole-dipole interactions have intermediate strength between dispersion forces and hydrogen bonds.

Hydrogen Bond

Hydrogen bonds are a special type of dipole-dipole interaction that occurs when hydrogen is bonded to a highly electronegative atom (usually oxygen, nitrogen, or fluorine). These bonds are the strongest type of intermolecular force and are crucial for many biological processes.