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9.3 The acidic environment 3. Acids

Syllabus reference (October 2002 version)
3. Acids occur in many foods, drinks and even within our stomachs	Students learn to: define acids as proton donors and describe the ionisation of acids in water identify acids such as acetic (ethanoic acid), citric (2-hydroxypropane-1,2,3-tricarboxylic acid), hydrochloric and sulfuric acid describe the use of the pH scale in comparing acids and bases describe acids and their solutions with the appropriate use of the terms strong, weak, concentrated and dilute identify pH as -log10 [H+] and explain that a change in pH of 1 means a ten-fold change in [H+] compare the relative strengths of equal concentrations of citric, acetic and hydrochloric acids and explain in terms of the degree of ionisation of their molecules describe the difference between a strong and a weak acid in terms of an equilibrium between the intact molecule and its ions	Students: solve problems and perform a first-hand investigation to use pH meters/probes and indicators to distinguish between acidic, basic and neutral chemicals plan and perform a first-hand investigation to measure the pH of identical concentrations of strong and weak acids gather and process information from secondary sources to write ionic equations to represent the ionisation of acids use available evidence to model the molecular nature of acids and simulate the ionisation of strong and weak acids gather and process information from secondary sources to explain the use of acids as food additives identify data, gather and process information from secondary sources to identify examples of naturally occurring acids and bases and their chemical composition process information from secondary sources to calculate pH of strong acids given appropriate hydrogen ion concentrations

Extract from Chemistry Stage 6 Syllabus (Amended October 2002). © Board of Studies, NSW.
[Edit: 27Jun 08]

Prior Learning: Preliminary module 8.4

Background: Acids such as ascorbic acid (vitamin C) and citric acid occur in many foods. Many drinks contain carbonic acid and some contain phosphoric acid. Other acids, such as benzoic acid and acetic acid, are added to drinks and food to act as preservatives. Hydrochloric acid is secreted into the human stomach to assist in the digestion of food, especially of proteins to amino acids.

define acids as proton donors and describe the ionisation of acids in water

• An acid is a proton donor. When an acid molecule is in contact with water it can ionise, donating a proton to a water molecule.
   
• A hydrogen atom, H, consists of one proton and one electron. A hydrogen ion, H⁺ , is formed when a H loses its electron, leaving just a proton. A proton and a hydrogen ion are thus the same and can be represented by H⁺.
   
• When an acid molecule is placed in water, it can ionise, releasing a proton and forming a negative ion. The proton, H⁺, can attach to a water molecule, H₂O, forming what is called a hydrated hydrogen ion or hydronium ion, H₃O⁺.
  
  
• Sulfuric acid is called a diprotic acid because each molecule can release up to two protons.

  

• Phosphoric acid is called a triprotic acid because each molecule can release up to three protons.

  

identify acids such as acetic (ethanoic acid), citric (2-hydroxypropane-1,2,3-tricarboxylic acid), hydrochloric and sulfuric acid

• Acetic acid occurs naturally in the decomposition of biological material, such as oxidation of wine alcohol, but most acetic acid used by humanity is manufactured industrially. Citric acid occurs naturally in fruits but most added to food as preservatives is manufactured.

  

• Hydrochloric and sulfuric acid are produced industrially on a large scale.
  
  
• Most sulfuric acid is manufactured, but it can also occur naturally. For example, most sulfur dioxide released into the earth’s atmosphere is oxidised and dissolved in water to form the sulfuric acid in acid rain. If the acid rain results from volcanic eruption it could be regarded as natural, but if acid rain results from smelting of sulfide ores, it could be regarded as manufactured.

describe the use of the pH scale in comparing acids and bases

• The pH scale is used to compare the concentration of hydrogen ions in solutions of acids and bases.
   
• The following table relates pH to the hydrogen ion concentration, [H⁺], and provides examples of common aqueous solutions for each pH value.
  
  

  pH	[H+]	[OH-]	[H+] x [OH-]	Aqueous solution example
  0	100 = 1	10-14	10-14	1 M hydrochloric acid
  1	10-1	10-13	10-14	0.1 M hydrochloric acid
  2	10-2	10-12	10-14	0.01 M hydrochloric acid
  3	10-3	10-11	10-14	soda water, wine
  4	10-4	10-10	10-14	tomato juice, beer
  5	10-5	10-9	10-14	acid rain
  6	10-6	10-8	10-14	urine
  7	10-7	10-7	10-14	pure water without any dissolved gas
  8	10-8	10-6	10-14	sea water
  9	10-9	10-5	10-14	detergent solution
  10	10-10	10-4	10-14	concentrated detergent
  11	10-11	10-3	10-14	household ammonia
  12	10-12	10-2	10-14	0.01 M sodium hydroxide
  13	10-13	10-1	10-14	0.1 M sodium hydroxide
  14	10-14	100 =1	10-14	1 M sodium hydroxide

  Note that:

• The [H⁺] x [OH^-] is the same for all aqueous (water) solutions. It does not matter whether the solution is of an acid, a base, a salt or a mixture of these.
• [H⁺] x [OH^-] is the ionisation constant for water, K_w = 1.0 x 10^-14 at 25^oC.
• In pure water without any dissolved gas, [H⁺] = [OH^-] = 10^-7 mol L^-1 and so pH =7.
• In an acidic solution, [H⁺] > 10^-7 mol L^-1 and pH < 7.
• In a basic solution, [H⁺] < 10^-7 mol L^-1 and pH > 7.

identify pH as -log₁₀ [H⁺] and explain that a change in pH of 1 means a ten-fold change in [H⁺]

• You can see from the data in columns 1 and 2 of the previous table, that a change of 1 in pH means a ten-fold change in [H⁺].
• It is assumed that you will be able to use your calculator to calculate pH from a given [H⁺] or calculate [H⁺] from a given pH.
• It is assumed you know how to do this type of calculation using the calculator that you take into examinations.
• The instructions below are guidelines only. Try them on your own calculator and change them, if needed, for your calculator. It is essential to check if your calculator is a Board-approved calculator before you attempt to take it to the HSC examination.

  Scientific calculator:

  1. Type in the [H⁺], e.g. 0.0020

  2. Tap the log key (not the ln or log_ekey)

  3. Tap the +/- or – key

  4. Round off the figures you see to an appropriate number of significant figures, e.g. because 0.0020 is to two significant figures, the pH answer should be given to two significant figures as 2.7.

  
  Graphic calculator:
  

  1. Tap the +/- or – key

  2. Tap the log key (not the ln or log_e key)

  3. Type in the [H⁺], e.g. 0.0020

  4. Round off the figures you see to an appropriate number of significant figures, e.g. because 0.0020 is to two significant figures, the pH answer should be given to two significant figures as 2.7.

  You are also expected to be able to work out [H⁺] if you are given pH. You will need to use a key marked SHIFT or INV or 2^nd function before using the log key. Given pH 2.7, you should be able to work out a method to give [H⁺] = 0.0020.

  If you are using a scientific calculator you may find this summary of key actions useful:

  [H⁺] to pH: log, +/-

  pH to [H⁺]: +/-, SHIFT log

process information from secondary sources to calculate pH of strong acids given appropriate hydrogen ion concentrations
 

• In a strong acid solution, each acid molecule is assumed to fully ionise. The concentration of hydrogen ions and hence pH will depend on whether the acid is monoprotic (releases one proton per molecule), diprotic (releases two protons per molecule) or triprotic (releases three protons per molecule).
   
• Consider HCl:

  HCl  H⁺ + Cl^–          

  [H+] = concentration of HCl

• However, for H₂SO₄:

  H₂SO₄   2H⁺ + SO₄^2–       

  [H+] = 2 x concentration of H₂SO₄

Once you have checked secondary sources to see that the acid is a strong acid and to find out whether it is monoprotic, diprotic or triprotic you can process the information to calculate the hydrogen ion concentration.

pH = -log₁₀ [H⁺] can then be used to calculate the pH from the hydrogen ion concentration.

solve problems and perform a first-hand investigation to use pH meters/probes and indicators to distinguish between acidic, basic and neutral chemicals

• This syllabus point allows opportunities to use and compare different strategies to solve a problem.

  Using a pH meter or probe is a non-destructive way of testing whether a chemical solution is acidic, basic or neutral. Provided the pH meter electrode or probe is washed well with distilled water between measurements, the solutions tested should be unaffected.

  Using indicator solution or indicator paper is a destructive way of testing, as the indicator will contaminate the portion of solution tested.

• Before performing the investigation, calibrate the pH meter electrode or data logger probe before use. The electrode or probe should be placed in solutions of known pH before use to test the required acidic, basic and neutral solutions and the meter or data logger adjusted to give appropriate readings.

Operation of a pH meter Dearborn Science Learning Center, University of Michigan, USA

plan and perform a first-hand investigation to measure the pH of identical concentrations of strong and weak acids

• Plan your investigation to ensure that it is valid. It is important to control the investigation by selecting strong and weak acids of the same concentration, such as:
  • 0.1 M solutions of hydrochloric acid
  • 0.1 M solution of acetic acid.
    
    
• Exercise care when preparing and using acid solutions to perform the investigation. Always use protective eyewear and make sure that there is a continuous supply of cold water nearby, such as from a tap. This could be used to provide a continuous stream of water to acid-affected body parts for at least 15 minutes in the event of an accident, such as a splash in the eyes.

gather and process information from secondary sources to write ionic equations to represent the ionisation of acids

• Gather information about how to write ionic equations for acids from chemistry texts. Summarise the main points as shown below and process the information for accuracy by comparing it with information from other texts.

describe the difference between a strong and a weak acid in terms of an equilibrium between the intact molecule and its ions

• A strong acid is one in which the molecules present practically all ionise when placed in water, e.g. hydrochloric acid, HCl.
  
  
  
  
• A weak acid is one where only a small proportion of the molecules ionise, e.g. acetic acid (ethanoic acid), CH₃COOH.
  
  
  
  
• If you prepare 0.1 M solutions of each you should find the hydrochloric acid has a pH of 1 corresponding to a [H⁺] = 10^-1 = 0.1 M. That is, practically every HCl molecule has ionised, producing a H⁺.

  By contrast a 0.1 M solution of acetic acid will have a pH close to 3 indicating a [H⁺] close to 10^-3 = 0.001 M. Only about 0.001 / 0.1 = 1% of the acetic acid molecules have ionised producing a H⁺.

use available evidence to model the molecular nature of acids and simulate the ionisation of strong and weak acids

• Here you can build models of acid molecules and remove the appropriate H to simulate ionisation. Note that in organic acids, the H that ionises comes from a –COOH group. The two electronegative oxygen atoms attract the pair of electrons in the covalent bond between the O and the H. This weakens the bond so that sometimes the H can break away as a H⁺ ion, leaving an electron from the H on the –COO^– , thus providing the charge of –1.

describe acids and their solutions with the appropriate use of the terms strong, weak, concentrated and dilute

• A concentrated solution contains a large amount of solute in a given amount of solution. A 10 mol L^-1 solution would be called concentrated.
  
• A dilute solution contains a small amount of solute in a given amount of solution. A 0.01 mol L^-1 solution would be called dilute.
  
  

compare the relative strengths of equal concentrations of citric, acetic and hydrochloric acids and explain in terms of the degree of ionisation of their molecules

• Hydrochloric acid is a strong acid, close to 100% ionised in solution:

  

• Citric acid (2-hydroxypropane-1,2,3-tricarboxylic acid) and acetic acid (ethanoic acid) are weak acids, with typically about 1% ionised:

  

gather and process information from secondary sources to explain the use of acids as food additives

• Gather information about food additives from a variety of sources.
  
  
• Food Additives Food Standards Australia New Zealand. You can click on the Food Additives list or just read about food additives. (Web sites last checked 27 June 2008)
   
• Process your information by comparing it with the information given below and then explain the use of acids as food additives.

identify data, gather and process information from secondary sources to identify examples of naturally occurring acids and bases and their chemical composition

• Identify the data that will be needed. This will include the types of substances as well as relevant information about each substances.
   
• Gather information from a range of sources and process the information using a method that will assist you to illustrate trends and patterns, such as an appropriately designed table as shown below. Gather information for at least two naturally occurring acids and two naturally occurring bases.
  
  

  name	composition	acid or base	pH in naturally occurring form