Arrhenius Acids and Bases

What is an acid?

What is a base?

These are both good questions and there is no simple answer.

It all depends on how you decide to define the terms "acid" and "base".

More than 100 years ago, a clever chemist by the name of Arrhenius proposed the first really useful definitions of acids and bases and AUS-e-TUTE has just added a new tutorial, game, test and exam (with worked solutions!), and even some flash cards o help you understand and apply the Arrhenius definition of an acid and of a base.

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https://www.ausetute.com.au/arrheniusab.html

pH of a Solution When Weak Acid and Strong Base are Mixed

What happens to the pH of a weak acid as you add strong base to it?

For example, aqueous sodium hydroxide solution (NaOH_(aq)) is a strong base and acetic acid (CH₃COOH_(aq)) is a weak acid.

If you add 10 mL of 0.20 mol/L NaOH_(aq) to 25 mL of 0.10 mol/L CH₃COOH_(aq), what is the pH of the resultant solution?

Find out how to work this out, and work through other examples with worked solutions at AUS-e-TUTE.

A "free-to-view" tutorial is available at https://www.ausetute.com.au/phmixwasb.html

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acid + base

When an acid and a base are mixed together they react to produce a salt and water.
The general word equation for this type of acid-base reaction is:

acid + base → salt + water

But can you write the word equation for the reaction between a specific acid, for example hydrochloric acid , and specific base, say sodium hydroxide?

AUS-e-TUTE is here to help!

We've just added new learning resources; a tutorial, game, test and exam to help our members write acid + base word equations. We even have a worksheet wizard that lets teachers make their own printable worksheet or quiz on this topic.

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pH after mixing acid and base

If you add enough strong base to a strong acid you can neutralise the solution and its pH will be 7.
But what if you don't add enough base to the acid.
Will the solution be neutral?
Will the pH of the solution be 7?
These are all very good questions which is why AUS-e-TUTE has just added a new tutorial, game, test and exam on this topic.
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Which of the following is an acid-base indicator?

Question: Which of the following is an acid-base indicator?

1. hydrochloric acid 
2. sodium hydroxide 
3. water 
4. phenolphthalein

Answer: Phenolphthalein is an acid-base indicator

1. Hydrochloric acid is an acid
2. Sodium Hydroxide is a base
3. Water is neutral

What is the best indicator for titration?

Question: What is the best indicator for titration?
Answer: That all depends on what you are titrating!

If you doing an acid-base titration, then you will need to know the relative strengths of the acid and base you going to be using.
Once you have determined an approximate pH for the equivalence point of the neutralisation reaction, you use this as the pH of the endpoint of the titration in order to determine the best acid-base indicator to use.

Salts of Drugs a Health Risk

Many studies have shown that excess salt is harmful to heart health, but many commonly prescribed medicines have sodium added to improve their absorption into the body, but the effect of doing this has not been known. For example, 1 Alka-Seltzer tablet contains 324 mg of aspirin (the drug) and 445 mg of sodium.
University of Dundee and College of London researchers have found that a person taking the maximum, daily dose of some medicines would exceed the recommended daily dietary intake limits for sodium. In Australia, the total maximum recommended limit of sodium for adults should be less than 2300 mg per day (less for children). The label on the Alka-Seltzer tablets carries a warning that you should not take more than 8 tablets per day.
In the study, the researchers found that patients taking the sodium-containing medication had a 16% increased risk of a heart attack, were 7 times more likely to develop high blood pressure, and, were 28% more likely to die, compared with patients who took the non-sodium containing versions of the same drug.

So why do we add sodium to drugs if it is potentially harmful?

We do this because many drugs are actually insoluble in water. The cells in your body are made up mostly of water, so if you want to be able to transport a drug around the body, and have it absorbed into cells, it is beneficial if the drug is soluble in water.

How do we add sodium to drugs?

If the drug is, for example, a weak acid like aspirin, then it is not very soluble in water.
Being a weak acid, though, aspirin can undergo a proton transfer (neutralisation) reaction with a base such as sodium hydroxide. The product of a neutralisation reaction are salt and water.
acid + base → salt + water

aspirin + sodium hydroxide → sodium salt of aspirin + water

The sodium salt of aspirin readily dissolves in water by dissociating into positive sodium ions and negative "aspirin" ions.


Reference:
BMJ-British Medical Journal (2013, November 26). High salt levels in medicines increase risk of cardiovascular events. ScienceDaily. Retrieved November 28, 2013, from http://www.sciencedaily.com­ /releases/2013/11/131126191557.htm 

Further Reading
aspirin
mass conversions 
ppm
molarity
neutralisation
proton transfer reactions
acid dissociation constants

Suggested Study Questions: 

1. Convert these masses in milligrams to masses in grams
• 324 mg
• 445 mg
2. The label on the Alka-Seltzer packet recommends dissolving 2 tablets in water. For these two tablets, calculate the mass in milligrams of
• aspirin
• sodium
3. If you were to take the maximum number of tablets, 8, in a day, how much of each of the following substances would you be ingesting?
• aspirin
• sodium
4. Would you be exceeding the recommended daily dietary intake limits for sodium in Australia? Explain your answer.
5. A low salt food is one that contains less than 120 mg of sodium per 100 g of food. If Alka-Seltzer were to be considered a low salt food, what would the mass of each tablet need to be?
6. Aspirin has the molecular formula C₉H₈O₄. What is the molar mass of aspirin?
7. The sodium salt of aspirin has the molecular formula C₉H₇O₄^-Na⁺. What is the molar mass of the sodium salt of aspirin?
8. Calculate the mass of sodium in 1 Alka-Seltzer tablet due to the sodium salt of aspirin.
9. Compare the mass of sodium calculated above to the actual mass of sodium present in 1 Alka-Seltzer tablet according to the package. How would you explain the difference in the two masses?
10. If a person dissolved 2 Akla-Seltzer tablets in 150 mL of water, what is the concentration of sodium ions in the water in
    • mol/L
    • mg/L
    • ppm
11. Recommendations for the daily intake of potassium are higher than those for sodium at 4700 mg day, so one way to alleviate the sodium problem in aspirin tablets might be to replace the sodium with potassium. Describe one way that you could produce the potassium salt of aspirin.

 

Heat of Neutralisation

AUS-e-TUTE has added a new tutorial, game, test and exam on the topic of heat of neutralisation.

Visit http://www.ausetute.com.au and log-in.
The new resources are in the Thermochemistry section.

AUS-e-TUTE Update

The following resources have been added to AUS-e-TUTE:

• Neutralisation Reactions (tutorial, game, test)
• Acidic, Basic, Neutral Solutions (tutorial, game, test)
• Properties of Alkanes (tutorial, game, test, exam)
• Properties of Alkanols (alcohols) (tutorial, game, test, exam)
• Properties of Alkanoic (carboxylic) acids (tutorial, game, test, exam)
• Properties of Amines (tutorial, game, test, exam)

Syllabus Study Guides have also been updated.

Neutral pH?

We hear this term a lot, often in advertising. But what does it mean?

From a Chemist's point of view, there are two different concepts involved in this seemingly harmless "neutral pH" expression. These two different concepts are:

• neutral
• pH

Let's take a look at the Chemist's definition of neutral first.
A solution is neutral if the concentration of hydrogen ions, [H⁺], is equal to the concentration of hydroxide ions, [OH^-].
Chemists often use square brackets to denote concentration, the concentration of  hydrogen ions can be written as [H⁺] and the concentration of hydroxide ions can be written as [OH^-].
So, for a neutral solution:

[H⁺] = [OH^-] = neutral solution 

Pure water is an excellent example of a neutral substance.

Some of the water molecules, actually very few of them, dissociate to form hydrogen ions and hydroxide ions:

H2O

H+ + OH-

Every time a water molecule dissociates, it produces one hydrogen ion, H⁺, and one hydroxide ion, OH^-, so that the concentration of hydrogen ions is always the same as the concentration of hydroxide ions.
Therefore, pure water is always neutral!

The pH of a solution is a measure of the hydrogen ion concentration in the solution. pH can be defined as:

pH = -log₁₀[H⁺]

This equation can be used to calculate the pH of our neutral water, but only if we know the concentration of  hydrogen ions in the water.

The concentration of hydrogen ions in water is not constant!

The concentration of hydrogen ions in water depends on the temperature of the water!
The dissociation of water molecules requires energy:

H2O + energy

H+ + OH-

If you put more energy into the system by heating it, then more water molecules dissociate, the concentration of hydrogen ions increases and the concentration of hydroxide ions also increases.
If you take energy away from the system by cooling it, then fewer water molecules dissociate, the concentration of hydrogen ions decreases and the concentration of hydroxide ions also decreases.
If we were to measure the concentration of hydrogen ions in pure water at various temperatures, we would find the following values:

Water temperature	[H+] x 10−7 M	pH
0°C	0.32	7.50
10°C	0.55	7.26
18°C	0.84	7.08
25°C	1.10	6.96
30°C	1.34	6.87
50°C	2.82	6.55
60°C	3.55	6.46
70°C	4.60	6.34
80°C	5.92	6.23
90°C	7.28	6.14
100°C	8.54	6.07

So what is the pH of water?
The pH of water is dependent on the temperature of the water.
Water is neutral for every value of pH because the concentration of hydrogen ions is always equal to the concentration of the hydroxide ions.
We can ONLY talk about the pH of water IF we state the temperature of the water.
For example, we can talk about water having a pH of approximately 7 at 25^oC, or we could say that the pH of water is approximately 6 at 100^oC.
Pure water is always neutral.
Pure water is neutral at 25^oC.
Pure water is neutral at 100^oC.

As Chemistry students, what we can't say is that water has a pH of 7, or that a neutral aqueous solution has a particular pH, unless we state the temperature of the system.

Further Reading:
Definitions of Acids and Bases
pH
Dissociation Constant for Water

Suggested Study Questions:

1. Plot a graph of temperature versus concentration of hydrogen ions in water. Describe the shape of the line, and write a generalization that links hydrogen ion concentration and temperature.
2. Plot a graph of temperature versus pH of water. Describe the shape of the line and write a generalization linking the  temperature of water and its pH.
3. Use your graph to find the pH of water at:
   • 12^oC
   • 22^oC
   • 32^oC
4. Construct a table giving the concentration of hydroxide ions in water at each of the temperatures shown above.
5. Plot a graph of temperature versus concentration of hydroxide ions in water. Describe the shape of the line, and write a generalization linking hydroxide ion concentration in water and temperature.
6. Use your graph to find the concentration of hydroxide ions in water at:
   • 12^oC
   • 22^oC
   • 32^oC
7. Explain why water is neutral at all temperatures.
8. Explain why the pH of water varies with temperature.

Bath Bombs

Fizzy bath bombs, the scented balls you place in your bath, are an example of a chemical reaction between an acid and a carbonate.
Below is a simple recipe for making your own bath bombs.

Equipment	Procedure
mixing bowl jar waxed paper 2 tablespoons citric acid 1/4 cup sodium bicarbonate 1/4 teaspoon fragrant oil (eg lavender oil) 3 tablespoons vegetable oil (eg olive oil)	1. Mix sodium bicarbonate and citric acid together in the mixing bowl. 2. Mix fragrant oil and vegetable oil together in the jar. 3. Slowly add contents of jar to mixing bowl while stirring. 4. Form small balls of mixture and place on waxed paper. 5. Allow balls to dry for 1 or 2 days before storing. 6. Add a ball to your bath water and enjoy!

When you drop your bath bomb into water, the process of dissolving the bath bomb allows the sodium bicarbonate and citric acid to react to produce soluble sodium citrate, water and carbon dioxide gas. The bubbles of carbon dioxide given off during the reaction is why the bath bomb fizzes in the water.

The vegetable oil is not soluble in water, so as the sodium bicarbonate and citric acid react, vegetable oil is released into the water forming a thin layer on your skin which can help "moisturize" it. At the same time, the fragrant oil, which is also insoluble in water, is released, so you can smell the scent.

Further Reading
Definitions and Properties of Acids and Bases
Reaction Calculations: Mass and Moles
Limiting Reagents and Reactants in Excess

Suggested Study Questions

1. Sodium bicarbonate is also known as sodium hydrogen carbonate, baking soda, bread soda and cooking soda. Write the chemical formula for sodium bicarbonate.
2. Write a word equation for the reaction between citric acid and sodium bicarbonate.
3. A balanced chemical equation for the reaction between citric acid and sodium bicarbonate is :
   C₅H₇O₅COOH + NaHCO₃ → C₅H₇O₅COO^-Na⁺ + H₂O + CO₂
   Name each of the following compounds:
   • NaHCO₃
   • C₅H₇O₅COOH
   • C₅H₇O₅COO^-Na⁺
   • CO₂
   • H₂O
   
   
4. Write a chemical equation to show citric acid acting as a Bronsted-Lowry acid.
5. Sodium bicarbonate is amphiprotic.Write a chemical equation to show sodium bicarbonate
   • accepting a proton
   • donating a proton.
6. In the reaction between citric acid and sodium bicarbonate, is sodium bicarbonate acting as an acid or a base? Explain your answer. 
7. For the bicarbonate ion, give the formula for its
   • conjugate base
   • conjugate acid 
8. The density of citric acid is about 1.5 g/mL. If a tablespoon has a volume of 15 mL, what mass of citric acid was used in preparing the bath bomb? 
9. Sodium bicarbonate has a density of about 2.2 g/mL and there are 16 tablespoons in 1 cup. What mass of sodium bicarbonate was used to make the bath bomb?
10. For the reaction between citric acid and sodium bicarbonate given the quantities used in the procedure provided above, which reactant is
    • the limiting reagent
    • the reactant in excess
11. Calculate the mass of carbon dioxide you would expect to be released during the reaction between citric acid and sodium bicarbonate as described in the above procedure. 
12. Assuming a temperature of 25^oC and a pressure of 100 kPa, what volume of carbon dioxide gas would be released using the data above? 
13. If the amount of sodium bicarbonate used to make the bath bombs as described above were doubled, what effect would that have on the amount of carbon dioxide produced when the bombs were placed in water?
14. Write an aim for the experiment described above.

Chlorine as a Chemical Weapon

World War I, saw the birth of the ANZAC legend*, and also the widespread use of "chemical warfare".

The first gas used by the German military as a killing agent was chlorine gas, also known as bertholite at this time.

Chlorine is a powerful irritant, which can damage the eyes, nose, throat, and lungs. Prolonged exposure to high concentrations, 1,000ppm, can cause death by asphyxiation.
Chlorine gas reacts with water in the tissues of the body to produce hydrochloric acid:

2Cl_2(g) + 2H2O_(l) → 4HCl_(aq) + O_2(g)

The chlorine gas was released from cylinders facing the enemy trenches in a favourable wind. The grey-green cloud of chlorine gas would then drift across the enemy positions.
The density of chlorine gas at 0^oC and 101.3kPa (1 atm) is 3.2g/L, while the density of air at the same temperature and pressure is 1.2754 g/L. Because chlorine gas is more dense than air, it would be more concentrated at the bottom of the trench, and less concentrated at the top. Those who suffered the worst effects were often the wounded lying on the ground or on stretchers.

Initially, German troops were issued with gauze pads filled with cotton, and bottles of bicarbonate solution. When the chlorine gas was to be released, the soldiers would dampen the gauze pad with the bicarbonate solution and breathe through it. The bicarbonate would neutralize the hydrochloric acid produced. If sodium bicarbonate solution were used, the reaction would be:

HCl_(aq) + NaHCO_3(aq) → NaCl_(aq) + CO_2(g) + H₂O_(l)

As other gases were being developed for use as chemical weapons, the need for better protection became important. One of the earliest devices was a hood with eyepieces. The hoods could be impregnated with sodium hyposulphite (sodium thiosulfate). Sodium thiosulfate reacts with dilute acids to produce sulfur, sulfur dioxide and water:

Na₂S₂O₃ + 2HCl → 2NaCl + S + SO₂ + H₂O

The gas mask was developed later. It was composed of an impervious mask and a box respirator or canistor. Air came through the canister which contained charcoal and granules of soda-lime, a mixture of sodium hydroxide and calcium hydroxide.

*ANZAC (Australian and New Zealand Army Corps) Day is commemorated each year on 25th April, marking the anniversary of the first major military action fought by Australian and New Zealand forces during the First World War. ANZAC Day ceremonies, which are held in towns and cities all over Australia and New Zealand, typically include an introduction, a hymn, a prayer, an address, the laying of wreaths, a recitation, the Last Post, a period of silence, and either the Rouse or the Reveille, and the national anthem. Anzac Day has evolved to acknowledge the sacrifice and service of subsequent wars.

Further Reading
Temperature Conversions
Density Calculations
Elements and Compounds
Writing Ionic Formula
Balancing Chemical Equations
Molecular Mass
Definitions of a Mole
Ideal Gas Law
Acid-Base Titration Calculations

Study Questions

1. Draw a table listing each element and each compound mentioned in the article above.
2. In April 1915, the German Army is said to have stockpiled 168 tons of chlorine which was contained in 5,370 cylinders. on average:
   • how many kilograms of chlorine gas was contained in each cylinder?
   • how many moles of chlorine gas was contained in each cylinder?
   • what volume would this moles of gas occupy at 25^oC and 101.3 kPa (1 atm)?
     
3. Using your calculations in part 2, calculate the density of chlorine gas at 25^oC and 101.3 kPa (1atm) in g/L.
4. Compare the density of chlorine gas calculate in part 3, to the density of chlorine gas given in the article above. Account for the difference in the two density values.
5. Convert 1,000ppm chlorine gas to a concentration in:
   • mg/L
   • g/L
   • mol/L
6. What mass of HCl could be produced from 1L of 1,000ppm chlorine gas?
7. What mass of sodium bicarbonate would be necessary to neutralize the amount of hydrochloric acid produced in question 6 above?
8. What mass of sodium thiosulfate would be necessary to neutralize the amount of hydrochloric acid produced in question 6?
9. Write a balanced chemical equations for:
   • the reaction between hydrochloric acid and calcium hydroxide
   • the reaction between hydrochloric acid and sodium hydroxide
10. Explain why a gas mask containing soda-lime might be preferable to one containing sodium thiosulfate.