Name____________________________________________Date:________ Atoms can gain or lose electrons in order to form ions in a process called ionization (compounds formed in this way are called ionic compounds). When ionic compounds dissolve in water, their ions separate from one another in a process called dissociation. One interesting feature of water and many other covalent compounds is that they too can dissociate into ions. Unlike ionic compounds, such as sodium chloride, they are not ionized before they dissociate; they accomplish ionization and dissociation at the same time. Dissociation of WaterWhen water dissociates, one of the hydrogen nuclei leaves its electron behind with the oxygen atom to become a hydrogen ion, while the oxygen and other hydrogen atoms become a hydroxide ion. Since the hydrogen ion has no electron to neutralize the positive charge on its proton, it has a full unit of positive charge and is symbolized as H+. The hydroxide ion retains the electron left behind and thus has a full unit of negative charge, symbolized by OH-. The hydrogen ion (proton) does not wander long by itself before it attaches to the oxygen atom of a second un-ionized water molecule to form a hydronium ion (H3O +) In any sample of water, very few of the molecules are dissociated at any one time: in fact, only about one in 550 million. There is, however, a constant change; as one hydrogen ion reattaches to a hydroxide ion to form a water molecule, another water molecule dissociates to replace the hydrogen ion and the hydroxide ion in solution. Hydrochloric AcidCertain molecules, ionic and covalent, dissociate in such a way that they release a hydrogen ion without releasing a hydroxide ion. These substances are called acids. Since a hydrogen ion is really just a single proton in most cases, the chemist’s definition of an acid is a “proton donor.” If very many protons (hydrogen ions) are “donated” the effect can be very profound, such as burning your skin or dissolving metal. The acid illustrated is hydrochloric acid. Pure hydrochloric acid is a gas, but it dissolves easily in water to produce a solution of hydrogen ion and chloride ion. Since nearly all of it is dissociated in water, it is called a strong acid. Acids that do not dissociate completely are called weak acids. Sodium HydroxideThe opposite of an acid is a base, also known as an alkali. A typical strong base is sodium hydroxide, the principal component of lye. Sodium hydroxide dissociates to form a sodium ion and a hydroxide ion. A base is defined as a “proton acceptor.” The most common bases produce hydroxide ion when they dissociate, and it is the hydroxide ion that accepts the proton. A strong base can give your skin a much worse burn than an acid. NeutralizationWhen a base and an acid are mixed, the hydroxide ion and the base combines with the hydrogen ion from the acid to form water. This process is called neutralization.  Questions: 1. What happens when an atom gains or loses an electron? 2. In your own words, explain why water generally has a neutral pH, even though water molecules dissociate. 3. Why are acids called proton donors? 4. What happens during neutralization? 5. Give an example of a strong base, and a strong acid.
Updated March 16, 2018 By Bert Markgraf
Whether acids are strong or weak is determined by how readily they dissociate to form ions. In water, acids dissolve to form hydrogen ions, while bases form hydroxide ions. The ions of strong acids and bases easily dissociate to completely dissolve in water, forming H hydrogen ions with a charge of plus one or OH- hydroxide ions with a charge of minus one. Weak acids and bases only partially dissociate, leaving fewer ions in solution. The hydrogen ions for acids and the hydroxide ions for bases give acids and bases their characteristics and determine their strength.
HF (hydrogen fluoride, or hydrofluoric acid) is not a strong acid. It is a weak acid because it does not make many hydrogen ions available when it dissolves in water. When HF dissolves, some of the hydrogen atoms form hydrogen ions with a positive charge, and some of the fluorine atoms form fluorine ions with a negative charge. The bond between hydrogen and fluorine is strong, so not enough HF molecules dissociate to produce the large number of ions required for a strong acid. Instead, the hydrogen atoms stay linked to the fluorine atoms, and comparatively few hydrogen ions are available to give the hydrogen fluoride solution the characteristics of an acid.
NH3 (ammonia) is not a strong base. It is considered a weak base because, in solution, it does not generate many hydroxide ions. Although ammonia has no oxygen atoms in its molecule and therefore can't dissociate into hydroxide ions directly, when dissolved in water, the NH3 molecule attracts a proton to form an ammonium ion, NH4. The proton is taken from the H2O water molecule, leaving an OH hydroxide ion with a negative charge and an ammonium ion with a positive charge. The hydroxide ions in water make NH3, a base, but only a few of the ammonia molecules take part in this process. Because there are few resulting hydroxide ions, ammonia is a weak base.
HNO3 (nitric acid) is a strong acid. This is because it dissociates completely in water. The molecule is made up of a hydrogen atom, a nitrogen atom and three oxygen atoms. In the chemical reaction that formed the nitric acid molecules, the electron from the hydrogen atom is shared by the nitrogen-oxygen atom combination. The resulting bond to the hydrogen atom is comparatively weak, and the hydrogen atom dissociates itself from the nitric acid molecule when dissolved in water. Due to the weak bond, almost all the molecules of nitric acid form hydrogen ions with a positive charge and NO3 ions with a negative charge, creating a strong acid.
NaOH (sodium hydroxide or lye), is a strong base. In NaOH, the oxygen atom has received the single electron from the outer electron shell of the sodium atom and is sharing the electron from the hydrogen atom to form the compound. As a result, the hydroxide ion has a negative charge of one, and the sodium ion with a charge of plus one is attracted to it. In solution, the polar water molecules with an oxygen atom at one end and two hydrogen atoms at the other end pull apart the NaOH ions. The hydroxide ion with a negative charge and the sodium ion with a positive charge dissociate completely, resulting in a strong base.
HCN (hydrocyanic acid) is not a strong acid. It is a weak acid. The hydrogen, carbon and nitrogen atoms are linked to form the HCN molecule by the covalent bonds of their electrons. There are a total of 10 valence electrons available for chemical reactions in the outermost electron shells of the three atoms, with hydrogen contributing one, carbon four and nitrogen five. The carbon atom shares one electron pair with the hydrogen atom and three with the nitrogen atom, while one nitrogen electron pair remains unshared. When placed in solution, the covalent bonds remain active, with the bond between the carbon and hydrogen atoms limiting hydrogen ion dissociation. As a result, only a few hydrogen ions enter the solution. Hydrocyanic acid is a weak acid.
HCL (hydrogen chloride) is a strong acid. This is because it becomes hydrochloric acid when dissolved in water. The hydrogen and chlorine atoms form a covalent bond, but the hydrogen atom is not held strongly. As a result, in water, the hydrogen atom forms a hydrogen ion, dissociating from the chlorine atom and leaving it as a chlorine ion with a negative charge. Because HCL completely dissociates when dissolved in water and all the hydrogen and chlorine atoms of HCL form hydrogen and chlorine ions, hydrochloric acid is considered a strong acid.
Let’s look at the dissociation of a common acid – hydrochloric acid or HCl. When HCl molecules dissolve they dissociate into H+ ions and Cl- ions. HCl is a strong acid because it dissociates almost completely. By contrast, a weak acid like acetic acid (CH3COOH) does not dissociate well in water – many H+ ions remain bound-up within the molecule. In summary: the stronger the acid the more free H+ ions are released into solution. The greater the number of free H+, the lower the pH value for that acid. Acids are sometimes referred to as H+ ion donors because they release H+ ions in solution for other molecules to bind to. A weak acid is one which doesn't ionize fully when it is dissolved in water. Ethanoic acid is a typical weak acid. It reacts with water to produce hydroxonium ions and ethanoate ions, but the back reaction is more successful than the forward one. The ions react very easily to reform the acid and the water. \[ CH_3COOH + H_2O \rightleftharpoons CH_3COO^- + H_3O^+ \tag{4}\] At any one time, only about 1% of the ethanoic acid molecules have converted into ions. The rest remain as simple ethanoic acid molecules. Most organic acids are weak. Hydrogen fluoride (dissolving in water to produce hydrofluoric acid) is a weak inorganic acid that you may come across elsewhere.
The position of equilibrium of the reaction between the acid and water varies from one weak acid to another. The further to the left it lies, the weaker the acid is. \[HA + H_2O \rightleftharpoons H_3O^+ + A^- \tag{5}\]
You can get a measure of the position of an equilibrium by writing an equilibrium constant for the reaction. The lower the value for the constant, the more the equilibrium lies to the left. The dissociation (ionization) of an acid is an example of a homogeneous reaction. Everything is present in the same phase - in this case, in solution in water. You can therefore write a simple expression for the equilibrium constant, Kc. Here is the equilibrium again: \[HA + H_2O \rightleftharpoons H_3O^+ + A^- \tag{5}\] You might expect the equilibrium constant to be written as: \[]K_c = \dfrac{[H_3O^+][A^-]}{[HA][H_2O]} \] However, if you think about this carefully, there is something odd about it. At the bottom of the expression, you have a term for the concentration of the water in the solution. That's not a problem - except that the number is going to be very large compared with all the other numbers. In 1 dm3 of solution, there are going to be about 55 moles of water. If you had a weak acid with a concentration of about 1 mol dm-3, and only about 1% of it reacted with the water, the number of moles of water is only going to fall by about 0.01. In other words, if the acid is weak the concentration of the water is virtually constant. In that case, there isn't a lot of point in including it in the expression as if it were a variable. Instead, a new equilibrium constant is defined which leaves it out. This new equilibrium constant is called Ka.
You may find the Ka expression written differently if you work from the simplified version of the equilibrium reaction:
This may be written with or without state symbols. It is actually exactly the same as the previous expression for Ka! Remember that although we often write H+ for hydrogen ions in solution, what we are actually talking about are hydroxonium ions. This second version of the Ka expression is not as precise as the first one. To take a specific common example, the equilibrium for the dissociation of ethanoic acid is properly written as: \[CH_3COOH + H_2O \rightleftharpoons CH_3COO^- + H_3O^+ \tag{7}\] The Ka expression is:
If you are using the simpler version of the equilibrium . . . \[CH_3COOH \rightleftharpoons CH_3COO^- + H^+ \tag{8}\] . . . the Ka expression is:
The table shows some values of Ka for some simple acids:
These are all weak acids because the values for Ka are very small. They are listed in order of decreasing acid strength - the Ka values get smaller as you go down the table. However, if you aren't very happy with numbers, that isn't immediately obvious. Because the numbers are in two parts, there is too much to think about quickly! To avoid this, the numbers are often converted into a new, easier form, called pKa. |
When a strong acid is placed in water does it dissolve
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