Science 30 Unit B Chemistry and the Environment Outcome 1: Students will analyze the sources of acids and bases and their effects on the environment. Specific Outcome 1.1: Define acids and bases in terms of proton donors and proton acceptors. Specific Outcome 1.2: Differentiate among acids, bases, neutral ionic compounds, neutral molecular compounds and strong and weak acids, based on appropriate diagnostic tests. Specific Outcome 1.3: Describe the relationship between ph and hydronium ion concentration. Specific Outcome 1.6: Trace the historical use of acid-base indicators. Textbook reference pages: p. 165 187 in Science 30 1
CHEMISTRY OF ACIDS AND BASES Types of Solutions: - There are four types of solutions we will investigate: o Neutral Ionic o Neutral Molecular o Acidic o Basic Neutral Ionic: - An ionic compound that is not acidic or basic - Litmus paper will not change color as it is NEUTRAL. Neutral Molecular: - A molecular compound that is not acidic or basic - Litmus paper will not change color as it is NEUTRAL. Acidic: - Electrolytic (conducts electricity) - Corrosive - Turns blue litmus paper red - Reacts with active metals (Mg, Zn, and Fe) to produce hydrogen gas - Tastes sour - Can be neutralized by bases - ph less than 7 - proton donors Basic: - Electrolytic (conducts electricity) - Corrosive - Turns red litmus paper blue - Feels slippery - Tastes bitter - Neutralized by acids - ph greater than 7 - proton acceptors 2
Acid, Base or Neutral? HNO3(aq) acid NaOH(aq) base CH3OH(aq) neutral NH4OH(aq) base Rules for Naming Acids: Molecular Classical System Example IUPAC System Example Name Acid Name Formula Molecular Name Acid Name Acid Name hydrogen -ide hydro-ic acid HCl(aq) hydrogen chloride hydrochloric acid aqueous hydrogen chloride hydrogen -ate -ic acid H3PO4(aq) hydrogen phosphate phosphoric acid aqueous hydrogen phosphate hydrogen -ite -ous acid HNO2(aq) hydrogen nitrite nitrous acid aqueous hydrogen nitrite IUPAC Rules for Naming Inorganic Bases: Example Base Name Formula Base Name cation + anion NaOH(aq) sodium hydroxide Name the following acids and bases: 1. KOH(aq) potassium hydroxide 2. HBr(aq) hydrogen bromide hydrobromic acid 3. H2SO3(aq) hydrogen sulfite sulfurous acid 4. H2CO3(aq) hydrogen carbonate carbonic acid 5. NH4OH(aq) ammonium hydroxide 3
ACID AND BASE THEORY Acids and bases only exhibit their properties when they are dissolved as solutes in water. Because acids and bases are IONIC COMPOUNDS, they break down into separate IONS each surrounded by water molecules The breaking down of ionic compounds into separate ions is called a DISSOCIATION REACTION. ELECTROSTATIC ATTRACTION is a force that acts to pull oppositely charged objects toward each other. Water Molecules Dissolving an Ionic Compound 4
ARRHENIUS S THEORY Arrhenius and Acids - Arrhenius said that acids form aqueous solutions that contain HYDROGEN IONS, H + (aq) and a NEGATIVELY CHARGED ION o Examples: hydrochloric acid HCl(aq) H + (aq) + Cl - (aq) sulfurous acid H2SO3(aq) 2 H + (aq) + SO3 2- (aq) Arrhenius and Bases - Arrhenius said that bases for aqueous solution that contain HYDROXIDE IONS, OH - (aq) and a POSITIVELY CHARGED ION. o Examples: potassium hydroxide KOH(aq) K + (aq) + OH - (aq) calcium hydroxide Ca(OH)2(aq) Ca 2+ (aq) + 2 OH - (aq) Arrhenius and Neutral Substances: - Solutions that do not have hydrogen or hydroxide ions are called neutral o Example: CaCl2(aq) Ca 2+ (aq)+ 2Cl - (aq) LIMITATIONS TO ARRHENIUS S THEORY 1. There are some compounds that have basic properties but do NOT have an OH - ion. a. Examples: i. Ammonia NH3(aq) ii. Sodium Carbonate Na2CO3(aq) iii. Aluminum Chloride AlCl3(aq) 2. This theory has H + (aq) existing all alone but this cannot happen a. Hydrogen ions have a very strong positive charge that causes it to combine with a water molecule to form the HYDRONIUM ION H3O + (aq). This is what is believed to cause something to be acidic. b. This really doesn t change our definition of an acid because we still needed to have the H + in the first place 5
Practice questions: Page 171 #12 12. a. HNO3(aq) H + (aq) + NO3 - (aq) b. H2SO4(aq) 2 H + (aq) + SO4 2- (aq) or H2SO4(aq) H + (aq) + HSO4 - (aq) c. H2S(aq) H + (aq) + HS - (aq) d. NaOH(aq) Na + (aq) + OH - (aq) e. Na2CO3(aq) 2 Na + (aq) + CO3 2- (aq) f. Na2SO4(aq) 2 Na + (aq) + SO4 2- (aq) g. NaCl(aq) Na + (aq) + Cl - (aq) BRØNSTED-LOWRY THEORY Brønsted and Lowry didn t like that Arrhenius s theory had limitations. Their definition extended previous definitions to include more compounds. o It includes all reactions where protons are transferred o This is the definition we will use in Science 30 THEY WANTED TO UNDERSTAND THE REASON BEHIND THE LIMITATIONS. Hydrogen ion is a proton Because when hydrogen becomes an ion all that is left is a single proton Let s start with some vocabulary: ACID: An acid is a proton donor When acids dissociate in water they give up a proton (hydrogen ion) An acid is the substance that loses a proton When acids dissociate the proton (H + ) is donated to the water molecule to form a hydronium ion which is H3O + (aq). H + (aq) + H2O(l) H3O + (aq) BASE: A base is a proton acceptor When bases dissociate in water they pull protons off other molecules (especially acids) A base is the substance that gains a proton CONJUGATE ACID (CA): acid formed when a base accepts the proton (H + ) 6
CONJUGATE BASE (CB): base formed when an acid donates the proton (H + ) Example: Loses Hydrogen ion H + HF(aq) + OH - (aq) F - (aq) + H2O(l) Acid Base CB CA According to this theory, during acid base reactions, a hydrogen ion is transferred from the acid to a base. o o The Loss of the H + converts the acid into a CONJUGATE BASE The gain of the H + converts the base into a CONJUGATE ACID Strength of Acids The strength of an acid based on its concentration and how easily it dissociates. o o Strong acids = highly dissociated Weak acids = <1% dissociation Strong acid - concentration of the hydronium ion equal concentration of the acid Strong base concentration of hydroxide ion will equal the concentration of the base Concentration is measured in moles/litre, mol/l 7
Relative strength of selected acids and bases (Page 12 of data booklet) Name: The information from this table can be used to identify the reactants and predict the products of an acid-base reaction between certain substances. 8
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Examples: 1. Hydrofluoric acid (HF(aq)) can be neutralized by a reaction with the hydroxide ion OH - (aq) of aqueous sodium hydroxide. Write the chemical equation for this neutralization reaction. HF (aq) + OH - (aq) F - (aq) + H 2O (l) acid base conjugate conjugate base acid 2. Arrhenius theory could not explain bases like sodium carbonate. Brønsted-Lowry can. Write the chemical equation for the carbonate ion CO3 2- (aq) and water. H 2O (l) + CO 3 2- (aq) + OH - (aq) + HCO 3 - (aq) acid base conjugate conjugate base acid Emissions can react! - Emissions from combustion reactions REACT WITH WATER IN THE ATMOSPHERE. - Most of these emissions are ANTHROPOGENIC (coming from human activity) - Rain is naturally acidic because of CO2, that is naturally in the atmosphere. - ACID DEPOSITION occurs when precipitation has lower than normal ph - The following table shows various oxides we have discussed react with water to form acids. Practice Problems: Page 175, #14, Page 176, #15-17 and Page 178, #18-20 10
Practice questions: page 175 #14 14. a. HNO 3(aq) + H 2O (l) NO 3 - (aq) + H 3O + (aq) acid base conjugate conjugate base acid b. H 2CO 3(aq) + H 2O (l) HCO 3 - (aq) + H 3O + (aq) acid base conjugate conjugate base acid Practice, page 176 15. HOOCCOOH (aq) + OH - (aq) HOOCCOO - (aq) + H 2O (l) acid base conjugate conjugate base acid 16. a. H 2SO 4(aq) + HCO 3 - (aq) HSO 4 - (aq) + H 2CO 3(aq) acid base conjugate conjugate base acid b. NH 3(aq) + H 3PO 4(aq) NH 4 + (aq) + H 2PO 4 - (aq) acid base conjugate conjugate base acid 17. The neutralization of an acid by a base occurs by the transfer of a hydrogen ion. By losing a hydrogen ion, the acid loses its acidic properties. The base becomes neutralized by gaining a hydrogen ion. Practice, page 178 18. carbon dioxide: CO2(g) + H2O (l) H2CO3(aq) already balanced sulfur dioxide: SO2(g) + H2O (l) H2SO3(aq) already balanced sulfur trioxide: 2 SO2(g) + O2(g) 2 SO3(aq) SO3(g) + H2O (l) H2SO4(aq) already balanced nitrogen monoxide: 2 NO (g) + O2(g) 2 NO2(aq) nitrogen dioxide: 2 NO2(g) + H2O (l) HNO2(aq) + HNO3(aq) 4 NO2(g) + 2 H2O (l) + O2(g) 4 HNO3(aq) 19. carbonic acid, H2CO3(aq) sulfurous acid, H2SO3(aq) sulfuric acid, H2SO4(aq) nitrous acid, HNO2(aq) nitric acid, HNO3(aq) 20. All the products will react with water to produce hydronium ions, H3O + (aq), and make the precipitation acidic. Consider the following equation: H2CO3(aq) + H2O (l) HCO3 - (aq) + H3O + (aq) 11
ph - A value that represents the concentration of dissolved hydronium ions (H3O + ) in a solution - Concentrated acidic solutions contain a larger number of moles of hydronium ions within each millilitre or litre of solution than dilute acidic solutions. - The concentration of H3O + influences reactions involving acids in the following ways: o How quickly the solution will begin to react o How much change the acid may cause o Amount of base required to neutralize the acid o Amount of base of metal it will react with - Strength of an acid or base is indicated by ph on a scale from 0 14-7 indicates neutral - Acids have ph less than 7 - Bases have ph greater than 7 - ph stands for power of hydrogen - Related to the concentration of hydrogen ions - ph is a measure of the acidity or alkalinity of a solution - Solutions with ph less than seven are said to be acidic (contains H3O+) - Solutions with ph of seven are said to be neutral - Solutions with ph greater than sever are said to be alkaline or basic (contains OH-) - ph scale was developed in 1909 as a means to communicate the acidity of a solution. 12
- ph is a logarithmic scale this means a change in ph of 1 is equal to a 10X change in H3O + concentration. - ph is the negative of the logarithm to base 10 of the hydronium ion concentration. - A 10 times increase or decrease in the concentration of hydrogen ions changes the ph by 1 - A solution of ph 3.0 is 10 times the hydrodium ion concentration than a ph of 4.0 How many times more acidic is ph 2 vs. ph 7? = (10 5 ) = 100,000 13
Calculating ph from H3O + concentration: ph = -log10[h3o + ] Example 1: Determine the ph of a sample of rain water that has a hydronium ion concentration, [H3O + (aq)], of 1.00 10-4 mol/l. Substitute the hydronium ion concentration. Example 2: A sample of lake water has a hydronium ion concentration of 2.27 x 10-7 mol/l. Determine the ph of the lake. ph = -log10[2.27 x 10-7 ] = 6.644 Example 3: What is the ph if the concentration of sulfuric acid is 1.64 x 10-2 mol/l? ph = -log [H3O + ] ph = -log [1.64 x10-2 ] ph = 1.785156 Sig dig s: ph= 1.785 SIGNIFICANT DIGITS AND ph CALCULATIONS A ph of 1.80 is 2 sig digs because the 1 is the power of 10 The exponent provides info about the size of the number, not how precisely the measurement was made. For ph, the significant digits are after the decimal point. This indicates how precise the measurement was. 14
Calculating H3O + concentration from ph [H3O + ] = 10 (-ph) Example 4: Calculate the H3O + (aq) concentration in a shampoo with a ph of 5.72. [H3O + ] = 10 (-ph) [H3O + ] = 10 (-5.72) [H3O + ] = 1.9 10-6 mol/l Example 4: The ph of a strong acid is 2.567. What is the hydronium concentration? ph problems [H3O + ] = 10-2.567 [H3O + ] = 0.00271 mol/l On calculator: 10^-2.567 [H3O + ] = 3.92 x 10-2 mol/l, ph =? ph = 1.407, [H3O + ] =? ph = 11.45, [H3O + ] =? [H3O + ] = 3.5 x 10-12 mol/l, ph =? 15
Practice Problems: Page 183, # 22 and 23 16
USE OF NATURAL INDICATORS USED BY FIRST NATIONS Many prairie plants were used as sources of colours to dye leather, cloth or porcupine quills. Naturally occurring acids were used by many First Nations to adjust the dyes of colours. Ash from fire pits was combined with water to create a basic solution which could be used to change the colours used for dyes. Indicators - Acid-base indicators are compounds that are different colours depending on the ph of the solution - They come in solution form or paper form Using indicators to determine ph - Use the indicator to determine the ph range - Use an additional indicator to narrow down the range of values 17
Example A drop of phenolphthalein is added to a solution and turns pink. Determine the ph of the solution. Since the indicator is pink, ph from 10 to 14 Example A solution has thymol blue added to it. The indicator turns from red to yellow. Estimate the ph of the solution. ph is between 1.2 and 2.8 18
Example: - What would the ph of a solution be that is: colorless in phenolphthalein, yellow in phenol red, orange in chlorophenol red and blue in indigo carmine. Between 6.0-6.6 Practice A solution is tested with 3 indicators. Determine the ph range of the solution. ph between 1.2 and 1.4 Practice Problems: page 186, #25-28 and Page 187, #1-5 and 9-11 Practice, page 186 25. Indicators are molecules that change colour as a response to different ph conditions. The ph at which an indicator undergoes a colour change is characteristic of that indicator. Generally, more than one indicator must be used to determine the ph of a solution. Matching indicator colours to ph ranges enables you to estimate a solution s ph. 26. Methyl orange appears red at ph values below 3.2. Since a ph below 7 is considered to be acidic, the statement is incorrect. 19
28. Bromocresol green is blue when the ph is 5.6, and thymol blue is yellow when the ph is 5.6. Mixing these two colours should produce a green solution. page 187 Knowledge 1. a. An acid is a substance that reacts with water to produce hydronium ions. It is a substance that transfers a hydrogen ion to a base during an acid-base reaction. b. A base is a substance that reacts with water to produce hydroxide ions. It is a substance that accepts a hydrogen ion during an acid-base reaction. c. Dissociation is the separation of ions in an ionic compound as it dissolves in water. d. A hydrogen ion, H+(aq), is the particle transferred from an acid to a base during an acid-base reaction, or it is a single proton. e. A hydronium ion, H3O+(aq), is the particle produced when acids react with water. f. The ph scale is a scale from 0 to 14 that ranks solutions according to their hydronium-ion concentration, where 0 represents the highest concentration (1.0 mol/l) and 14 represents the lowest concentration of hydronium ions (1.0 10-4 mol/l). Note: Highly concentrated solutions of some acids can have ph values that are negative. g. Wet deposition consists of gases or particles removed from the atmosphere that dissolve in water (liquid or solid). h. Acid deposition consists of airborne particles containing acids or acid-forming substances contained within precipitation (wet deposition) or that absorb directly into parts of Earth s surface (dry deposition). i. Acid rain is any form of precipitation with a ph of 5.6 or less. 20
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9. If thymol blue is yellow, the ph of the solution is between 2.8 and 8.0. If bromocresol green is blue, the ph of the solution is greater than 5.4. Therefore, the ph of the solution is between 5.4 and 8.0. a. Methyl violet is blue between 5.4 and 8.0. b. Indigo carmine is blue between 5.4 and 8.0. c. Methyl orange is yellow between 5.4 and 8.0. d. Alizarin yellow R is yellow between 5.4 and 8.0. 10. The statement is correct. Acids can be deposited as wet or dry deposition. 11. a. Repeating an experiment and obtaining the same result demonstrates the reliability of the procedure. b. Having a similar result to others who performed the same experiment using the same method demonstrates reliability. c. Arriving at the same answer even though different approaches were taken demonstrates that the approaches are valid methods to use. 22