Science Study Guide for the TEAS

Chemistry: The Interactions of Substances

Chemical Reactions

Chemical reactions can occur when particles collide with each other under the right conditions. The minimum amount of energy required to start a reaction is known as the activation energy, and it is the amount of energy required for a specific reaction to occur. The rate of a reaction can be increased by increasing the temperature, pressure, or concentration and by adding a catalyst. In industry, iron is added to catalyze the reaction between nitrogen and hydrogen, which makes ammonia. It is important to be able to describe chemical reactions.

Valence Electrons

Atoms form bonds with other atoms using the electrons in the outermost electron shell, known as the valence electrons. An atom is most stable when its valence shell is full.

Covalent and Ionic Bonds

Covalent bonds form between atoms that share electrons. Ionic bonds form between an atom that has pulled electrons away from another atom, causing one to have a negative charge and the other to have a positive charge, which results in an attraction between the atoms.

Chemical Reaction Equations

The substances present at the beginning of a chemical reaction are known as the reactants. After the chemical reaction, the products are formed. In a chemical equation, the arrow represents the direction of the reaction. The word “yields” is used to indicate the change from reactants to products.

Balancing Chemical Equations

The law of conservation of mass states that matter cannot be created or destroyed. In a chemical reaction, atoms change their bonds and create new substances, but the number of atoms of each type in the reactants needs to balance the number of atoms of each type in the products.

Moles

A mole is an SI unit of measurement in chemistry that indicates a certain number of atoms. One mole of a substance is equivalent to \(6.022 \times 10^{23}\), or \(602\) sextillion atoms. Different elements will have different molar masses because the atoms they are made up of vary in the number of subatomic particles.

Conditions and Chemical Reactions

Chemical reactions will only occur under certain conditions. The conditions cannot only determine whether or not a reaction occurs but can change the rate of the reaction. It’s important to understand how reactions are induced and controlled.

Affecting Factors

Chemical reactions are affected by the temperature and pressure of the surrounding environment, the concentration of the elements and molecules involved, and the presence of catalysts. Scientists can control the conditions of a reaction to achieve a desired result.

Pressure

Pressure can affect whether compounds are in a liquid or gaseous state, which can change the progress of the reaction. Increasing the pressure compresses molecules, especially gas molecules, so they are closer together and are more likely to react.

Concentrations

The higher the concentration of reactants, the faster the reaction will occur. The reacting molecules are more likely to collide at higher concentrations.

Temperature

Raising the temperature of a reaction usually increases the rate of the reaction, since the molecules will have more energy and will collide with each other more often with enough energy to react.

Catalysts

A catalyst speeds up a chemical reaction. Specific reactions have specific catalysts that can help increase their rates.

Chemical Equilibria

Some reactions are reversible, which means that they occur simultaneously in both directions. These reactions are indicated by the use of a double arrow in the equation, one that points toward the products and another that points toward the reactants. When the forward and reverse reactions take place at the same rate, chemical equilibrium has been reached.

Endothermic Reaction

Endothermic reactions take in energy as they progress. When an endothermic reaction occurs in a test tube, the tube will feel cold to the touch. Photosynthesis and the electrolysis of water into hydrogen and oxygen are examples of endothermic reactions.

Exothermic Reaction

Exothermic reactions give off energy as they progress. They require the input of activation energy to get started but create energy after that requirement has been met. Cellular respiration is an exothermic reaction, as is the combustion of hydrogen gas with oxygen to create water.

Catalysts

Catalysts are substances that speed up chemical reactions without being used up during the reaction. Biological catalysts found in the cells of living things are called enzymes. They provide a site for the reactants (called substrates), making it possible for them to react faster than they would without the catalyst.

Solutions

A solution is a homogeneous mixture of two substances. Most liquids on Earth are solutions, with water being the most common solvent. The cytoplasm inside of cells consists mainly of a water-based solution, as do all of the liquids important to the functions of the human body.

Polarity of Water

Water can dissolve many substances because it is polar. This means the molecule has two oppositely charged poles, or ends. The hydrogen side has a slight positive charge, and the oxygen side has a slight negative charge. This makes water molecules attract each other, but also aids it in pulling apart other atoms in the process of dissolving.

Solvents and Solutes

A solvent is a substance that can dissolve another substance. In a solution, it is the substance that there is most of. A solute is a substance that gets dissolved. It can be a solid, liquid, or gas.

Hydrophilic Substances

Hydrophilic means “water-loving.” These are substances that are attracted to water’s polar charges. Many hydrophilic substances can dissolve in water. Some, such as glass, do not, but have an attraction that makes them stick to water (adhesion).

Hydrophobic Substances

Hydrophobic means “water-fearing,” and these substances repel water. They do not dissolve. Hydrophobic substances, such as fats and oils, will separate over time when mixed with water.

Concentration and Dilution of Solutions

The concentration of a solution tell us the ratio of solute to solution. It is often expressed as a percentage. For example, 10 g of salt dissolved in enough water to make 100 g of solution would yield a 10% salt solution. The concentration can be increased by adding more salt or diluted by adding more water.

Units to Use

There are many ways to express the concentration of a solution. This helps scientists quantitatively compare various solutions.

Molarity—Molarity (\(\text{M}\)) is the number of moles of a solute dissolved per liter of solution.

\[\text{Molarity} = \frac{\text{moles of solute}}{\text{liters of solution}}\]

Mole Fraction—Mole fraction describes the number of moles of a solute per total number of moles of the solvent and solute. It does not have a unit, since you are dividing moles by moles and the units cancel out.

\[\text{Mole fraction} = \frac{\text{moles of solute}}{\text{moles of solute + moles of solvent}}\]

Molality—Molality (\(\text{m}\)) is the number of moles of solute dissolved per kilogram of solvent. It is also called the molal concentration.

\[\text{Molality} = \frac{\text{moles of solute}}{\text{kilograms of solvent}}\]

Mass Percentage—Mass percentage is similar to molarity in that it is a ratio of solute to solution. Instead of the moles of each being compared, it’s the mass of each being compared.

\[\text{Mass percentage} = \frac{\text{mass of solute}}{\text{mass of solution}}\]

Parts Per Thousand (ppt)—Parts per thousand measures how many units of solute there are per thousand units of solution. One way to calculate this is by using the mass of each, as shown in the equation below. Remember one gram is \(1\text{,}000\) milligrams.

\[\text{Parts per thousand} = \frac{\text{mg of solute}}{\text{g of solution}}\]

Parts Per Million (ppm)—This is similar to parts per thousand, but used for expressing smaller concentrations.

\[\text{Parts per million} = \frac{\text{mg of solute}}{\text{kg of solution}}\]

Parts Per Billion (ppb)—This calculation is used for extremely small concentrations of solutes. For this, the solute is measured in micrograms (\(\mu g\)).

\[\text{Parts per billion} = \frac{\mu g \text{ of solute}}{\text{kg of solution}}\]

Osmosis and Diffusion

Diffusion is the movement of particles from an area of high concentration to an area of low concentration. In other words, they spread out from where there are more of them to where there are fewer. Diffusion is an important process in the body, and it occurs without the use of energy. Osmosis is the diffusion of water. It occurs when water can cross a membrane that other substances cannot. The water will flow from the solution with more water (and fewer solutes) into the solution with less water (and more solutes) to balance the concentrations.

Acids and Bases

Acids and bases are compounds that produce different types of ions when dissolved in water, giving them unique properties. Acid/base chemistry is very important in understanding human physiology. For example, many enzymes that speed up reactions in the body can only operate within a narrow pH range. The pH of the blood must be kept between 7.35-7.45 or serious problems can occur.

Properties of Acids and Bases

Acids and bases have different chemical and physical properties. Solutions of acids and bases are electrolytes, which means they can conduct electricity. Acids and bases can react together to form water and a salt.

Acids

Acidic compounds create hydrogen ions in water. Most of their chemical formulas begin with H. They have a sour taste and can corrode certain metals.

Bases

Basic compounds create hydroxide ions when dissolved in water. The hydroxide ion’s formula is OH, and the chemical formulas of bases have OH at the end. They have a bitter taste and feel soapy or slippery to the touch.

The pH Scale

The pH scale is a way to express the strength of acids and bases. It ranges from 0 to 14, with 7 being neutral. Solutions lower than 7 are acidic, and the lower the number the more acidic the solution. Solutions higher than 7 are basic, and the higher the number the more basic the solution is.

Buffers

A buffer is a solution in which the pH remains relatively constant. It is able to resist drastic changes in pH when acids or bases are added to it in small amounts. A buffer solution contains a weak acid and one of its salts, or a weak base and one of its salts. Buffers in the blood are important to resist changes in blood pH.

Neutralization Reactions

A neutralization reaction occurs when an acid and base react in a solution. The product is water (made by combining the H from the acid and the OH from the base) and a salt. One example of a neutralization reaction is the use of antacids to control an upset stomach; the base in the medicine neutralizes the excess stomach acid.