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Inorganic chemistry for students / acids and alkalis

Acids and alkalis

What are acids and bases?

  • The first acid that was known in ancient times was vinegar. In the Middle Ages, other acids were known (for example hydrochloric, nitric and sulfuric acid).
  • Hydrochloric acid and acetic acid can be recognized by their smell, but we strongly advise against an odor test!
  • The effect of acids: acids etch! They particularly attack base metals and lime. But clothing and skin are also at risk if they come into contact.
  • The "opponents of acids" are the alkalis. They are also corrosive and attack many other substances that are not necessarily strongly attacked by acids (for example hair, skin and fat).
  • Sodium hydroxide solution and potassium hydroxide solution are known alkalis.
  • Alkalis are just as dangerous, just a little less known than acids. They attack many natural substances, but i. A. no metals - aluminum is an exception. That is why a drain cleaner such as "Drain-free" also removes hair and grease, but does not damage the pipes.
  • Acids and bases can be diluted. If you add the same amount (or more) of water to acid, the effect is significantly weaker
  • Attention! Chemical burns can always happen. It is best to rinse the area immediately with water. Etched clothing must be removed. If something gets into your eyes: wash it off well and see a doctor immediately!
  • Alkalis are often in the form of a solid that still has to be dissolved in water.
  • It is a common misconception that acids and alkalis are always liquids. Rather, it is true that there are solid and gaseous acids even at room temperature. A well-known representative of a solid acid is vitamin C powder (be careful, it attacks the teeth if it is not diluted with a lot of water). A typical gaseous acid is hydrochloric acid (HCl).
Humans cannot perceive the acidity of a diluted solution, which is why we need an aid in the laboratory to determine the acidity ⇒ indicators

Indicators

Indicators are dyes that show a different color in acids and bases. In school it is mostly just Universal indicator used.

tasks

  1. What happens if you mix up the acidic universal indicator solution with the neutral one? What color will it show?
  2. Why is this happening?
  3. Read about the properties and uses of the most important acids on the internet!

Important acids

A selection of important acids for research on Wikipedia: Sulfuric acid, Hydrochloric acid, Phosphoric acid, Nitric acid, Carbonic acid, Acetic acid, citric acid

Formulas

With acids that will Hydrogen atomH colored red ( Acids have acidic hydrogen).
With caustic solutions, the Hydroxyl group OH colored blue ( Alkalis are aqueous hydroxide solutions).

Here are the most important acids and their acid residues, you should know them by heart!

The most important acids and their acid residues
acidAcid residue
HFfluorinehydrogen (acid)F.-fluorineid
HClchlorinehydrogen (acid)Cl-chlorineid
HBrbrominehydrogen (acid)Br-bromineid
HIIodinehydrogen (acid)I.-Iodineid
H2S.sulfurhydrogen (acid)S.2-Sulfid
    
ENT3SaltpetreacidNO3-Nitrat
H2SO4sulfuracidSO42-

Sulfat

H2CO3Carbon)acidCO32-Carbonat
H3PO4phosphorusacidPO43-

Phosphat

    
ENT2SalpetrigeacidNO2-Nitrit
H2SO3SulfurigeacidSO3-Sulfit
H3PO3phosphorusigeacidPO3-Phosphit


The most important alkalis in inorganic materials follow here. You should know her.

The most important alkalis
Lye
NaOHsodiumlye
KOHpotassiumlye
Ca (OH)2Calciumlye = lime water
Hints
  • instead of sodium lye, one says caustic soda.
  • instead of potassium hydroxide, one says potassium hydroxide.
Spellings of the hydroxide ion
Frequently used notations for hydroxide ions (without representation of the lone pairs of electrons).
Notations for hydroxide ions including the six free electrons (dots) or three electron pairs (dashes) on the oxygen atom (here each blue marked).

Definitions according to Svante Arrhenius (1859-1927)

The chemist Svante Arrhenius

Svante Arrhenius was born on February 19th, 1859 in Uppsala and died on October 2nd, 1927 in Stockholm. The Swedish physicist and chemist carried out research in the field of electrolytic dissociation (e.g. the decomposition of salts and acids in water). In his doctoral thesis he dealt with the conductivity of salt and acid solutions. However, it was not recognized by other chemists because of the many new ideas of the time. Only as that Chemist Ostwald commented positively, the value of his research was recognized.

He also researched the influence of carbon dioxide on the earth's climate and was the first to investigate that Greenhouse effect. In 1903 Svante Arrhenius was the first Swede to receive the Nobel Prize in Chemistry.

He gave the following definition for acids
The aqueous solutions of hydroxides are called alkalis. An acid is a substance that forms hydrogen ions in aqueous solution.
(In the 8th grade we say rather "protons" instead of "hydrogen ions")

Reactions of the oxides with water I - Acid formation in two steps

The aim of these two experiments is to produce sulfuric acid or carbonic acid. For this purpose, the respective element is burned in pure oxygen. So-called non-metal oxides are formed.

Both experiments are carried out in closed so that none of the oxides produced is lost Round bottom flask instead.

Non-metal oxides (e.g. carbon dioxide) form an acid with water.

tasks

  1. The word acid and all acids are to be underlined, the word non-metal and all non-metals are to be underlined in a different color.
  2. Name 5 non-metals. How do they differ from metals? What is the difference between non-metal oxides and non-metals?
  3. How can you form an acid?
  4. Cola contains a lot of phosphoric acid. How can a beverage company Make phosphoric acid?
  5. Why doesn't the flame in the round bottom flask go out immediately? How long does the incineration actually run?
  6. A student writes in the test: To make sulfuric acid, you take sulfur and mix it with water. Why is that wrong?

Reactions of the oxides with water II - in two steps

Metal oxides are chemical compounds of a metal with oxygen. Many metal oxides are used as ores for metal extraction. The oxygen is withdrawn from the metal oxide and the pure metal is obtained. Metals can also react again to form metal oxides. A common problem is rust (iron oxide), which arises from valuable iron.

In this experiment, the metal oxides serve as the starting material for the production of lye. But first you have to make such a metal oxide from the element:

Experimental setup 1:Burning magnesium over a beaker
Observation 1:A bright, glistening flame emerges; white solid (smoke),
Conclusion 1:The white powder magnesium oxide was created

Magnesium + oxygen Magnesium oxide + energy

Experiment setup 2:The product is then mixed with water
Observation 2:Magnesium oxide dissolves poorly in water, after adding the water you can add universal indicator paint. It shows the color blue.
Conclusion 2:Magnesium lye was formed.

Magnesium oxide + water Magnesium Lye + Energy

When metal oxides dissolve in water, they react with water to form hydroxides.
The aqueous solutions of hydroxides are called alkalis.

Tasks:

  1. The word metal oxide and all metal oxides as well as all alkalis are underlined again.
  2. What is the difference between metal oxides and metals?
  3. Where can you find in the Periodic table of the elements the metals and where the non-metals?
  4. What are metal oxides used for?
  5. Describe in general: How do you make a lye?
  6. What is the result of burning sodium?
  7. An important lye is the so-called Lime water. The correct name is calcium hydroxide. Can you describe how it can be made?
  8. What is Calcium oxide used?

The pH

The effect of an acid is not always the same. There are strong acids like H2SO4 and weak acids like citric acid. In addition, strong acids are also weak in their effect if they are diluted with plenty of water. This also applies to alkalis. In order to be able to assess the effect of an acid or lye, you need a suitable unit of measurement.

→ Definition for the 8th and 9th grade: The pH value measures the strength of the effect of acids and alkalis, i.e. how acidic or alkaline a substance reacts.

The pH value is measured with a measuring device or with so-called indicator paper. The values ​​from 0-14 can be found on its color scale. The middle value (7) is measured in neutral solutions, such as B. pure water. Values ​​less than 7 are acidic (acid), values ​​greater than 7 are alkaline (lye).

  • pH <7 corresponds to an acidic solution
  • pH ≈ 7 corresponds to a neutral solution
  • pH> 7 corresponds to an alkaline solution

Here are some examples of pH values ​​for everyday substances:

The special thing about the pH value scale is that the difference between one pH value and the next is 10 times that. This means that z. B. an acid with pH 2 is ten times as acidic as an acid with pH 3 and a hundred times as acidic as one with pH 4 is.

Note that you write the "p" of the pH value in lower case!

extra information

tasks

  1. Why do you suppose you have to measure the acidity at all? Isn't a term like sour or neutral enough?
  2. Why are aquarium lovers so interested in the pH of their water?
  3. How many times the amount of water do you have to dilute a liter of vinegar so that it is no longer acidic (pH ≈ 7)?

Acids attack base metals

magnesium

Experiment description
Magnesium is mixed with dilute hydrochloric acid (HCl)

observation
A flammable gas is created, the Mg dissolves and energy is formed in the form of heat (exothermic reaction).

conclusion
Mg reacts to a salt and hydrogen

magnesium+hydrochloric acidhydrogen+Magnesium salt+energy
Mg+2HClH2+MgCl2+energy

copper

Experiment description
The experiment is repeated with a copper cent.

observation
no reaction

conclusion
In contrast to magnesium, copper is a more noble metal. It is not attacked by dilute hydrochloric acid.

Acid starch

concentrated acid

Experiment description
Mg tape is mixed with acetic acid and concentrated HCl (teacher's experiment!)

observation
With concentrated hydrochloric acid the reaction is much more violent and flammable gas is produced

conclusion
HCl is a stronger acid, acetic acid is a weak acid

magnesium + acidsalt + hydrogen + Energy

weakly concentrated acid

Experiment description
The acid HCl is diluted with plenty of water and thrown into Mg. The experiment is repeated with acetic acid

observation
Both reactions are similarly slow

conclusion
The effect of acids can be weakened. So you can dilute strong acids with water, their effect is then less strong. Yet they remain strong acids. (Comparison with a fast sports car that also drives slowly in the city)

Not all acids are equally strong or equally corrosive.
Roughly speaking, a distinction is made between strong and weak acids.

Neutralization

Neutralization is the reaction between acids and bases. Water is formed in the process. The remaining ions form a salt. One acid and one Base. The neutralization is therefore not to be equated with the reaching of the neutral point, which corresponds to the pH value 7.

Demonstration of the universal indicator in three beakers:

Is there a way to make acids harmless?

Experiment description
Sodium hydroxide solution (= sodium hydroxide dissolved in water) is added dropwise to the hydrochloric acid solution, which is colored with a universal indicator.

observation
The indicator will gradually turn green.

conclusion
Hydrochloric acid and caustic soda have reacted to form water.

If you add a certain amount of acid to a lye, you get a neutral, non-corrosive liquid.
→ Lye and acid are antagonists. Combined in the same concentration, they cancel each other out in their effect.

The following must be observed when neutralizing:

  • dropwise addition, for example with a pipette
  • constant stirring
  • to be patient
  • watch out because acids and alkalis are corrosive

Quantitative neutralization

During the neutralization, another substance is created besides water. To see this, a neutralization must be carried out and the water evaporated. The residue is then examined.
But how do you neutralize without an indicator?

Experiment description
For 15 ml of sodium hydroxide solution, one Burette hydrochloric acid added dropwise until the color changes. The amount is noted. The experiment is repeated without an indicator.

observation
For 15 ml sodium hydroxide solution .... ml hydrochloric acid are required

Experiment description
The neutral solution (without indicator) from V1 is evaporated.

observation
A white precipitate forms, steam rises

conclusion
Sodium hydroxide and hydrochloric acid have reacted to form table salt and water

N / AOH+HClH2O + NaCl + energy
General rule for every neutralization: alkali + acid react to salt + water + E
The resulting salt is usually dissolved in the water and can be obtained pure by evaporation.

tasks

  1. Why does the experiment have to be carried out twice?
  2. What do doctors give to drink if someone accidentally drank acid?
  3. Many people suffer from Heartburn, the build-up of excess stomach acid. Common drugs contain calcium oxide. Can you explain why
  4. Can you create the reaction equations for the following neutralizations?
    a) Sodium hydroxide solution with sulfuric acid
    b) Potash with phosphoric acid
    c) Lime water with phosphoric acid
  5. Name the salts that arise during the tasks

Red cabbage, the indicator from the kitchen

In chemistry, an indicator is understood to be a substance that is used to monitor a chemical reaction or a state. Often the change is indicated by a change in color.

Everyday acid-base indicators

Red cabbage juice can also be used as an acid-base indicator. Red cabbage juice can take on colors from red = acidic to blue = alkaline (in an even more alkaline environment it turns green and even yellow at pH> 10).

To obtain red cabbage from red cabbage, an apple slice (with malic acid) is often added, which causes the red cabbage to turn red. Incidentally, the names "red cabbage" and "blue cabbage" differ from region to region, for example raw red cabbage heads are also known as red cabbage heads in southern Germany.

Tea drinkers know tea as an indicator: If lemon juice is added to the black tea, the color changes from dark brown to light reddish brown. This color change is also due to the coloring agents in the tea, which act as indicators. The source is Indicator.

Try it yourself: Take a fresh red cabbage and cut up one or two leaves with a pair of scissors or a knife. Put them in a vessel with a little water for a few minutes. You will get particularly good results if you use hot water.

With the colored solution you can then create at least 5 different colors by adding acid or lye.

The neutralization is an exothermic reaction

Experiment description
5ml 5% -10% HCl are placed in a large, vertically clamped test tube, then concentrated NaOH (aq) is added in small portions. The temperature is measured with a thermometer.

observation
Both substances react with each other under violent surges, heat and noise. A white fabric settles on the floor.

conclusion

Table salt and water are formed. The neutralization releases great energies. Such reactions are called

.

Review questions

  1. Name 5 acids and 3 bases with their formula. Then set up the reaction equation for any neutralization!
  2. Cola contains a lot of phosphoric acid.
    a) How can a beverage company manufacture phosphoric acid?
    b) Set up the reaction equations for this!
    c) How can this acid be rendered harmless? Describe exactly how to do this!
  3. Sulfur oxide is mixed with water. Set up the reaction equation!
  4. Write down the reaction equation for the combustion of magnesium
    a) Use this example to explain the terms oxidation and metal oxide!
    b) What color does the universal indicator show when the product is mixed with water?
  5. Scientists have found that rainwater also contains acids. In recent years it has now been observed that the rain is particularly acidic, especially in areas with heavy traffic. Is there a connection? (Tip: gasoline contains carbon and sulfur)
  6. What is the pH?
  7. Why do you suppose you have to measure the acidity at all? Isn't a term like sour or neutral enough?
  8. Why can strong acids sometimes appear weak? Compare with a strong sports car driving in a 30 zone.
  9. Why are aquarium lovers so interested in the pH of their water?
  10. How many times the amount of water do you have to dilute 1L vinegar with so that it is no longer acidic (pH = 7)?
  11. What is "neutralization"?
    a) Describe how an experiment was carried out and establish observation and conclusion (with reaction equation!)
    b) Set up the reaction equation for the neutralization of phosphoric acid with potassium hydroxide!
  12. A construction worker works without gloves when mixing the cement. To do this, he uses so-called "burnt lime" (CaO). After a few weeks his hands are badly damaged, torn and reddish. Find an explanation!