Forms of Matter
A gas is one of the four fundamental states of matter. A pure gas is made up of either individual atoms or molecules made from one type of atom (such as Oxygen), or a compound molecule made up of variety of elements (such as Carbon Dioxide). Air is a mixture of gases; in this case, several pure gases occupy the same space. The particles in gases are widely separated from each other making colorless gases invisible.
There are 5 elemental gases that are stable at standard temperature and pressure, these include Hydrogen (H2), Oxygen (O2), Nitrogen (N2), Flourine (F2), and Chlorine (Cl2).
The term gas was first coined by the Flemish Chemist Jan Baptist van Helmont and there are several theories about the origin of the word itself including references such as ghost (gahst), froth (Gäscht) or chaos (Greek).
Gases can be described using four physical properties: Pressure, Volume, Temperature and Number of particles. These properties were extensively studied by Chemists such as Jacques Charles, Robert Boyle, John Dalton, Amedeo Avogadro and Joseph Gay-Lussac resulting in the gas laws that will be covered in this chapter.
As the pressure on a gas increases, the volume of the gas decreases proportionally, provided that the temperature and chemical amount of the gas remains constant.
The volume of a gas in inversely proportional to the pressure. Another way to think about it is - when you multiply the pressure of a gas and the volume, you will get a constant value, as long as the temperature and the amount of gas remain constant as well.
Using Boyle's law, we are able to predict the change in one property, if the other property also changes. We can express the initial Pressure and Volume and P1 and V1 and the new Pressure and Volume as P2 and V2. Because the product of P and V is always a constant, we can say -
A graphical illustration of the relationship between pressure and volume - Boyle's law. (Source: Wikipedia-CC BY-SA 3.0)
The animation illustrates the relationship between pressure and volume; as the pressure increases, the volume decreases proportinately - Boyle's law. (Source: Wikipedia-CC BY-SA 3.0)
For an ideal gas at constant pressure, the volume is directly proportional to its temperature.
An illustration of the relationship between temperature and volume of a gas. As the temperature increases, the volume of the gas also increases. (Source: Wikipedia-CC BY-SA 3.0)
A combination of both Boyle's and Charles' laws results in the following equation:
This is referred to as the Combined Gas Law.
Is also called the Law of Combining Volumes. It states that, when measured at the same temperature and pressure, the volumes of gaseous reactants and products of chemical reactions are always in simple ratios of whole numbers.
1 volume unit of Hydrogen + 1 volume unit of Chlorine = 2 volume units of Hydrogen Chloride.
Equal volumes of gases at the same temperature and pressure contain equal number of molecules. In other words, the volume occupied by an ideal gas is proportional to the number of moles (or molecules) present in the container.
where n is the number of moles of gas.
The Avogadro's constant represents the number of (particles - atoms) found in 12 grams of elemental carbon-12 (6.022×1023 / mol). This specific number of gas particles, at standard temperature and pressure (STP) occupies 22.40 liters and at Standard Atmospheric Temperature and Pressure (SATP) occupies 24.8 liters. This is referred to as the molar volume.
Pressure can be expressed using several units. The Pascal is the internationally accepted Unit. 1 Pascal (1Pa) is 1 N/m2. That is the amount of pressure expressed by a mass of 1 Newton / sq meter.
An 'Atmosphere' (1 atm) is equal to 101.325 kPa.
Pressure can also be expressed in millimeters of Mercury (mm Hg) and in this case 1 atm = 101.325 kPa = 760 mm Hg.
The final unit of pressure is the Torricelli (torr), which = 1 mm Hg.
Absolute zero is the lowest limit of the thermodynamic temperature scale, a state at which the enthalpy and entropy of a cooled ideal gas reach their minimum value, taken as zero kelvins.
In general, chemical reactions can be classified into two major categories based on the energy changes. Endothermic Chemical Reactions absorb energy from the surrounding, whereas Exothermic Chemical Reactions release energy to the surrounding. There are two general theoretical principles to follow when predicting endothermic and exothermic formations of solutions: Energy is needed to break existing bonds (endothermic), and energy is released when new bonds are formed (exothermic).
Concentration (c) is defined as the ratio of quantity of solute to quantity of solution. There are several ways in which the measure of 'quantity' can be expressed.
Percentage by Volume (aka percentage volume by volume - % v/v). These concentrations are often designated as a percentage volume by volume. For example, if a bottle contains 120 mL of pure acetic acid in a 500 mL bottle solution, what is the percentage by volume concentration of acetic acid?
Concentration Acetic acid = (120ml/500ml)*100% = 24%.
Percentage weight by volume (% W/V): Here the quantity of the solute is expressed as the mass, while the quantity of the solution is expressed as volume.
Percentage, weight by weight (%w/w): both solute and solution quantities are expressed in mass.
Parts per Million : When concentrations are very small, they can be expressed as parts per million (ppm), parts per billion (ppb) and part per trillion (ppt). Remember, 1 kg (kilogram) contains 1,000,000 mg (milligrams). So if a solution contains 1mg of the solute in 1 kg of the solution, this is equal to 1ppm. This is the same for 1ml per 1 liter of a solution. If s solution contains 0.001mg per kilogram (or 0.001ml per liter) of the solution, it can be described as 1 part per billion.
Amount Concentration: The quantity of the solute is measured in moles (n). While the solution is measured by volume in liters (V). A mole of any element or molecule, is defined as the mass of that substance, expressed in grams, equal to the atomic or molecular mass. For example, The atomic mass of oxygen is 16, the molecular mass of oxygen gas (O2) is 32. 1 mole of oxygen gas is equal to 32g.
Multiple ways to express the concentration of a solute in a solution.
There are many ways of expressing concentrations and dilution. The following is a brief explanation of some ways of calculating dilutions that are common in Chemistry
The main formula for concentration is C1V1 = C2V2 , where V1 is the Volume of stock solution needed to make the new solution, C1 is Concentration of stock solution, V2 is Final volume of new solution and C2 is the Final concentration of new solution.
For example,
Dilutions can also be expressed using dilution factors. This is expressed as a ratio of the parts of solute to the total number of parts in solution. The dilution factor (DF) can be used alone or as the denominator of the fraction, for example, a DF of 10 means a 1:10 dilution, or 1 part solute + 9 parts diluent, for a total of 10 parts. This is different than a dilution ratio, which typically refers to a ratio of the parts of solute to the parts of solvent, for example, a 1:9 using the previous example. Dilution factors are related to dilution ratios in that the DF equals the parts of solvent + 1 part.
Step dilutions: If the dilution factor is larger than the final volume needed, or the amount of stock is too small to be pipetted, one or more intermediary dilutions may be required. Use the formula: Final DF = DF1 * DF2 * DF3 etc., to choose your step dilutions such that their product is the final dilution.
For example,
Serial Dilutions: A dilution series is a succession of step dilutions, each with the same dilution factor, where the diluted material of the previous step is used to make the subsequent dilution. This is how standard curves in biology can be made. To make a dilution series, use the following formulas:
Serial dilution procedure. (Source: Wikipedia-CC BY-SA 3.0)
Dissociation is separation of ions that occurs when ionic compounds dissolve in water. Separation of ions occurs due to polar water molecules pulling ionic compounds apart.
Ionization is the process by which a neutral atom or molecule is converted to an ion. For example, in acids, Arrhenius assumed that the water solvent somehow causes the acid molecules to ionize, but he did not propose an explanation for this process. It is believed the hydrogen ions are responsible for turning litmus paper red in acidic solutions. Strong acids completely ionize (i.e., completely break up into ions). Weak acids partially ionize.
Acids and Bases can both be defined empirically and theoretically.
Acid - a substance which dissolves in water to produce a solution that:
Base - a substance which dissolves in water to produce a solution that:
Theoretical Definitions
Originally developed by Arrhenius.
Acid - a substance that forms an acidic solution by dissolving in water to produce free hydrogen ions (H+(aq)) in solution
Base - a substance that forms a basic solution by dissolving in water to produce free hydroxide ions (OH-(aq)) in solution
The above definitions were then modified into:
Acid - a species that forms an acidic solution by reacting with water to produce hydronium ions (H3O+(aq))
Base - a species that forms a basic solution by reacting with water to produce hydroxide ions (OH-(aq))
The Hydronium Ion: Theoretical chemists thought it was unlikely that a hydrogen ion, which is a tiny proton with a very high charge-to-size ratio, could exist on its own in aqueous solution. Instead they thought it would bond strongly to polar water molecules. Paul Giguère provided empirical evidence for this type of bonding when he discovered hydrated protons (H3O+(aq)); commonly called hydronium ions The modern view of acids and bases is that the hydronium ions (H3O+(aq)); are responsible for acidic properties and hydroxide ions (OH-(aq)) are responsible for basic properties.
Pure water self ionizes to produce H+(aq) and OH-(aq) ions, but their concentrations are so low that a conductivity test is negative. In pure water at SATP, the hydronium ion concentration is very low; about 1 x 10-7 mol/L. Adding acid to water adds H+(aq) ions causing the H+(aq) concentration to increase, thus it makes the solution conductive. Adding base to water adds OH-(aq) ions causing the OH-(aq) concentration to increase, thus it makes the solution conductive. Aqueous solutions exhibit a wide range of hydronium ion concentrations - from more than 10 mol/L for concentrated HCl(aq) to less than 10-15 mol/L for concentrated NaOH(aq)
pH i.e. 'power of hydrogen' is calculated as the negative of the base ten exponent for the hydronium ion concentration.
The pH scale is used to communicate a broad range of hydronium ion concentrations. Most common acids and bases have pH values between 0 and 14. Changes in pH can be deceptive. For example, adding vinegar to pure water might change the pH from 7 to 4. While this change of 3 pH units may not appear significant, the change in hydronium ion concentration is 103 or 1000 times larger.
The number of digits following the decimal point in the pH value is equal to the number of sig digs in the hydronium ion concentration. For example,
[H3O+(aq)] = 6.7 x 10 -8 (has two sig digs)
The pH = 7.17 (maintain the two sig digits).
pOH and Hydroxide ion Concentration
Although pH is used more commonly, in some applications it is more practical to describe hydroxide ion concentration.
pOH = -log [OH-(aq)] OR [OH-(aq)] =10 -pOH.
The range of pH values from 1 to 14; with 7 being Neutral, greater than 7 being basic and less than 7 being acidic.
Substances that change color when the acidity of the solution changes are known as acid-base indicators. A very common indicator used is litmus, which is obtained from lichen. Litmus paper is prepared by soaking absorbent paper with litmus solution and then drying it.
Acid-base indicators are unique chemicals because they can exist in two forms, each with a distinctly different color. The form of the chemical depends on the acidity of the solution. Their chemical formulas are complicated but can be simplified as Lt - litmus; Bb - bromothymol blue; In - indicator (generic). The two forms of any indicator depend on whether a particular hydrogen atom is present in the indicator's molecule.
In general, the lower pH form is designated HIn(aq) whereas the higher pH form is designated In-(aq).
Litmus paper: The color changes of litmus are a little more complicated because there is an indistinct region around the neutral point (pH=6-8) where the color is not easily distinguished and appears as mixtures of red and blue.
Acid base indicators have two primary uses:
| Indicator | Low pH color | Transition low end | Transition high end | High pH color |
|---|---|---|---|---|
| Alizarine Yellow R | yellow | 10.2 | 12.0 | red |
| Azolitmin (litmus) | red | 4.5 | 8.3 | blue |
| Bromocresol green | yellow | 3.8 | 5.4 | blue |
| Bromocresol purple | yellow | 5.2 | 6.8 | purple |
| Bromophenol blue | yellow | 3.0 | 4.6 | blue |
| Bromothymol blue (first transition) | magenta | less than 0 | 6.0 | yellow |
| Bromothymol blue (second transition) | yellow | 6.0 | 7.6 | blue |
| Congo red | blue-violet | 3.0 | 5.0 | red |
| Cresol red | yellow | 7.2 | 8.8 | reddish-purple |
| Cresolphthalein | colorless | 8.2 | 9.8 | purple |
| Gentian violet (Methyl violet 10B) | yellow | 0.0 | 2.0 | blue-violet |
| Indigo carmine | blue | 11.4 | 13.0 | yellow |
| Malachite green (first transition) | yellow | 0.0 | 2.0 | green |
| Malachite green (second transition) | green | 11.6 | 14.0 | colorless |
| Methyl orange | red | 3.1 | 4.4 | yellow |
| Methyl purple | purple | 4.8 | 5.4 | green |
| Methyl red | red | 4.4 | 6.2 | yellow |
| Methyl yellow | red | 2.9 | 4.0 | yellow |
| Naphtholphthalein | pale red | 7.3 | 8.7 | greenish-blue |
| Neutral red | red | 6.8 | 8.0 | yellow |
| Phenol red | yellow | 6.4 | 8.0 | red |
| Phenolphthalein (first transition) | orange-red | Less than 0 | 8.3 | colorless |
| Phenolphthalein (second transition) | colorless | 8.3 | 10.0 | purple-pink |
| Phenolphthalein (third transition) | purple-pink | 12.0 | 13.0 | colorless |
| Screened methyl orange (first transition) | red | 0.0 | 3.2 | purple-grey |
| Screened methyl orange (second transition) | purple-grey | 3.2 | 4.2 | green |
| Thymol blue (first transition) | red | 1.2 | 2.8 | yellow |
| Thymol blue (second transition) | yellow | 8.0 | 9.6 | blue |
| Thymolphthalein (first transition) | red | Less than 0 | 9.3 | colorless |
| Thymolphthalein (second transition) | colorless | 9.3 | 10.5 | blue |
Evidence clearly shows that acids with the same initial concentration can have different degrees of acidic properties. This difference can be observed in the different conductivity measurements and the different rate of reactions.
The concept of strong and weak acids was developed to describe and explain the differences in properties of acids. An acid can be described as a weak acid if its characteristic properties (under the same conditions) are less than those of a common strong acid. Some examples of strong acids include - Hydrochloric acid, sulfuric acid and nitric acid.
Strong Acids have high conductivity, high rate of reaction with metals and carbonates and a relatively low pH. Strong acids react completely (>99%) with water to form hydronium ions.
HCl + H2O -> H3O+ + Cl-. (>99% dissociation)
Weak acids on the other hand, do not dissociate completely into hydronium ions. Their dissociation rate is lower than 50%. Because of this, weak acids are much safer to handle, some can even be eaten. These include citrus acid, lactic acid etc.
Strong bases are characterized by high electrical conductivity, fast reaction rate and a very high pH greater than 7. Weak bases have a low electrical conductivity, slower reaction rate and a pH closer but greater than 7.
Like acids, strong bases dissociate almost completely into hydroxide ions, while weak bases dissociate only partially (less than 50%) into hydroxide ions.
| Strong Acids | Weak Acids | Strong Bases | Weak Bases |
|---|---|---|---|
| Weak Bases | Med to low pH | Very high pH | Med to high pH |
| High conductivity | Low conductivity | High conductivity | Low conductivity |
| Fast reaction rate | Slow reaction rate | Fast reaction rate | Slow reaction rate |
| Completely react with water to form H3O+(aq) ions | Partially react with water to form H3O+(aq) ions | Completely react with water to form OH-(aq) ions | Partially react with water to form OH-(aq) ions |
Acids that have only one acidic hydrogen atom in their compound formula (HA) are called monoprotic acids. For example, HI(aq), HCl(aq), HBr(aq), HF(aq) HCN(aq). On the other hand, acids that contain more than one acidic hydrogen and can react more than once with water (HxA) are called polyprotic acids. For example, H2SO4(aq), H3PO4(aq).
Except sulfuric acid, in general, polyprotic acids are weak acids whose reaction with water decreases with each successive step.
Some bases, like CH3COO-(aq), are monoprotic bases, meaning they can react with water only once to produce hydroxide ions. Others, like CO32- (aq) are polyprotic bases because they can react more than once with water.
Like acids, in general, polyprotic bases are weak bases, whose reaction with water decreases with each successive step.
Download a pdf copy of Unit 3: Forms of Matter for offline use.
C$0.99