Hydrogen Ions and Acidity Annotated Reading

Transcription

1 Name: Key Hydrogen Ions and Acidity Annotated Reading As you read Underline key ideas. Box vocabulary words. Write a question mark next to things you don t understand or want to know more about. Draw an arrow next to things that create a personal connection. Briefly write what they connect to. Connecting to Your World A patient is brought to a hospital unconscious and with a fruity odor on his breath. The doctor suspects the patient has fallen into a diabetic coma. To confirm her diagnosis, she orders several tests, including one of the acidity of the patient s blood. The results from this test will be expressed in units of ph, not molar concentration. In this section, you will learn how the ph scale is used to indicate the acidity of a solution and why the ph scale is used. Hydrogen Ions from Water As you already know, water molecules are highly polar and are in continuous motion, even at room temperature. Occasionally, the collusions between water molecules are energetic enough to transfer a hydrogen ion from one water molecule to another. A water molecule that loses a hydrogen ion becomes a negatively charged hydroxide ion (OH - ). A water molecule that gains a Hydrogen ion becomes a positively charged hydronium ion (H 3O + ). The reaction in which water molecules produce ions is called the self-ionization of water. This reaction can be written as a simple dissociation. H 2O (l) H + (aq) + OH - (aq) Hydrogen Ion Hydronium Ion In water or aqueous solution, hydrogen ions (H + ) are always joined to water molecules as hydronium ions (H 3O + ). Hydrogen ions in aqueous solution have several names. Some chemists call them protons. Others prefer to call them hydrogen ions or hydronium ions. In this reading, either H + or H 3O + is used to represent hydrogen ions in aqueous solution. The figure below shows how two water molecules react to form one hydronium ion and one hydroxide ion. The self-ionization of water occurs to a very small extent. In pure water at 25⁰C, The equilibrium concentration of hydrogen ions ([H + ]) and the equilibrium concentration of hydroxide ions ([OH - ]) are

2 each only 1 X 10-7 M. This means that the concentrations of H + and OH - are equal in pure water. Any aqueous solution in which [H + ] and [OH - ] are equal is described as a neutral solution. 1. What is a hydronium ion? H 3O + It is a water molecule that has bonded to a hydrogen ion that came from another split water molecule. 2. How are hydroxide ions [OH - ] and hydronium ions [H 3O + ] formed? Water naturally splits into a hydroxide (OH - ) ion and a hydronium ion (H 3O + ). 3. How many water molecules does it take to form 1 hydronium ion and 1 hydroxide ion? Two water molecules are needed to form one hydronium ion and one hydroxide ion. 4. Draw the reaction here. 2H 2O H 3O + + OH - 5. What is a neutral solution? A neutral solution is when the concentration of H 3O + equals the concentration of OH -. Ion Product Constant for Water In any aqueous solution, when [H + ] increases, [OH - ] decreases. When [H + ] decreases, [OH - ] increases. If additional ions 9either hydrogen ions or hydroxide ions) are added to a solution, the equilibrium shifts. The concentration of the other types of ion decreases. More water molecules are formed in the process. H + (aq) + OH - (aq) H 2O (l) For aqueous solutions, the product of the hydrogen-ion concentration and the hydroxide-ion concentration equals 1.0 X [H + ] X [OH - ] = 1.0 X This equation is true for all dilute aqueous solutions at 25⁰C. As you will see, the concentrations of H + and OH - may change when substances are added to water. However, the product of [H + ] and [OH - ] is always 1 X The product of the concentrations of the hydrogen ions and hydroxide ions in water is called the ion-product constant for water (K w). K w = [H + ] X [OH - ] = 1.0 X Not all solutions are neutral. When some substances dissolve in water, they release hydrogen ions. For example, when hydrogen chloride dissolves in water, if forms hydrochloric acid. HCl (g) H + (aq) + Cl - (aq) In such a solution, the hydrogen-ion concentration is greater than the hydroxide-ion concentration. The hydroxide ions are present from the self-ionization of water. An acidic solution is one in which [H + ] is greater than [OH - ]. The [H + ] of an acidic solution is greater than 1 X 10-7 M. When sodium hydroxide dissolves in water, it forms hydroxide ions in solution. NaOH (s) Na + (aq) + OH - (aq)

3 In such a solution, the hydrogen-ion concentration is less than the hydroxide-ion concentration. Remember, the hydrogen ions are present from the self-ionization of water. A basic solution is one in which [H + ] is less than [OH - ]. The [H + ] of a basic solution is less than 1 X 10-7 M. Basic solutions are also known as alkaline solutions. 1. What happens if you increase [H + ] in a solution? [OH - ] in a solution? If you increase the hydrogen concentration in solution, the hydroxide concentration will decrease. 2. What is the ion-product constant for water (kw)? K w = [H 3O + ] X [OH - ] = 1.0 X What is an acidic solution? What cause a solution to be acidic? An acidic solution is one in which the hydrogen ion concentration is greater than the hydroxide concentration. An acidic solution is created when a substance causes the hydrogen ions in solution to increase. 4. What is a basic solution? What causes a solution to be basic? A basic solution is one in which the hydrogen ion concentration is less than the hydroxide ion concentration. A basic solution is created when a substance causes the hydroxide ions in solution to increase. The ph Concept Expressing hydrogen-ion concentration in molarity is cumbersome. A more widely used system for expressing [H + ] is the ph scale, proposed in 1909 by the Danish scientist Soren Sorensen ( ). On the ph scale, which ranges from 0 to 14, neutral solutions have a ph of 7. A ph of 0 is strongly acidic. A solution with a ph of 14 is strongly basic. Calculating ph The ph of a solution is the negative logarithm of the hydrogen-ion concentration. The ph may be represented mathematically using the following equation. ph = -log[h + ] In a neutral solution, the [H + ] = 1 X 10-7 M. The ph of a neutral solution is 7. ph = -log(1 X 10-7 ) = -(log 1 + log 10-7 ) = -(0.0 + (-7)) = 7.0 You can calculate the ph of a solution using the log function key on a calculator. The figure below shows how the hydrogen-ion concentration of a solution is used to classify the solution as neutral, acidic, or basic. A solution in which [H + ] is greater than 1 X 10-7 M has a ph less than 7.0 and is acidic. The ph of pure water or a neutral aqueous solution is 7.0. A solution with a ph greater than 7 is basic and has a [H + ] of less than 1 X 10-7 M.

4 Acidic solution: ph < 7 [H + ] greater than 1 X 10-7 M Neutral solution: ph = 7 [H + ] equals than 1 X 10-7 M Basic solution: ph > 7 [H + ] less than 1 X 10-7 M Use the graph to answer the following questions: 1. What is the concentration of [H 3O + ] in a neutral solution? 2. How does [H 3O + ] compare with [OH - ] in an acidic solution? 3. In terms of ion concentrations, how are basic solutions different from acidic solutions? The ph values of several common aqueous solutions are listed in the table below. The table also summarizes the relationship among [H + ], [OH - ], and ph. You many notice that ph can sometimes be read from the value of [H + ]. If [H + ] is written in scientific notation and has a coefficient of 1, then the ph of the solution equals the exponent, with the sign changed from minus to plus. For example, a solution with [H + ] = 1 X 10-2 M has a ph of 2.0 and a solution with [H + ] = 1 X M has a ph of If the ph is an integer number, it is also possible to directly write the value of [H + ]. A solution with a ph of 9.0 has [H + ] = 1 X 10-9 M. A ph of 4 indicates a [H + ] = 1 X 10-4 M. 1. What does the ph scale range from? The ph scale ranges from 0 to What is the ph of a neutral solution? Acidic? Basic?

5 The ph of a neutral solution is 7. The ph of an acidic solution is less than 7. The ph of a basic solution is greater than What equation do we use to calculate ph? ph = -log[h 3O + ] 4. What type of solution is it when the hydrogen ion [H + ] concentration is greater than 1 x 10-7 M? The solution is acidic when the hydrogen ion concentration is greater than 1 X 10-7 M.