15 Acids, Bases, and pH Concept Overview

ACIDS, BASES, AND PH | CONCEPT OVERVIEW The TOPIC of ACID, BASES, AND PH can be referenced on page 54 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing.

CONCEPT INTRO: In the study of CHEMISTRY and ENGINEERING, some of the most important reactions we will study involve the interaction of ACID-BASE REACTIONS in AQUEOUS SOLUTIONS. ACIDS and BASES are everywhere in our daily lives, and make up a fundamental component of CHEMISTRY. An ACID is defined as a SUBSTANCE that IONIZES or DISSOCIATES in an AQUEOUS SOLUTION to produce HYDROGEN IONS [𝐻 + ] when DISSOLVED in WATER. A BASE is defined as a SUBSTANCE that IONIZES or DISSOCIATES in an AQUEOUS SOLUTION to produce HYDROXIDE [𝑂𝐻 − ] ions when DISSOLVED in WATER. In our everyday lives, we are surrounded by these ACIDS and BASES in the forms of HOUSEHOLD CLEANING CHEMICALS, FOOD, and even the WATER we drink. It is commonly known that certain soft drinks and energy drinks are considered ACIDIC, which means they sometimes contain ACID STRONG enough to affect the enamel on our teeth. Made with

by Prepineer | Prepineer.com

If we look under our kitchen sink, we find the common CLEANING PRODUCTS such as BLEACH and AMMONIA are STRONG BASES, which help clean and disinfect surfaces in various applications. ACIDS are characterized as having a SOUR TASTE, as we have commonly experienced when consuming lemons, limes, vinegar, or SOUR PATCH KIDS. BASES are characterized as having a BITTER TASTE, as we have commonly experienced when consuming certain vegetables (broccoli and cauliflowers), baking soda, and certain refined dairy products. In a PHYSICAL SENSE, bases are known to feel SLIPPER or SOAPY, as characterized by most household cleaning products such as bleach and ammonia. When ACIDS or BASES are mixed with WATER, they mixed together to form AQUEOUS SOLUTIONS called ELECTROLYTES that are able to conduct ELECTRICAL CURRENT. PH OF ACIDS AND BASES: The TOPIC of PH OF ACIDS AND BASES can be referenced under the SUBJECT of CHEMISTRY on page 54 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. ACIDS and BASES are CLASSIFIED based on their PH or STRENGTH, which is determined by how many HYDROGEN IONS [𝐻 + ] or HYDROXIDE IONS [𝑂𝐻 − ] are released when mixed with WATER Made with

by Prepineer | Prepineer.com

The STRONGER the ACID or BASE, the MORE it DISSOCIATES, releasing more HYDROGEN IONS [𝐻 + ] or HYDROXIDE IONS [𝑂𝐻 − ], depending on the CHEMICAL STRUCTURE. The PH of a solution REPRESENTS the CONCENTRATION of HYDROGEN IONS [𝐻 + ] or HYDROXIDE IONS [𝑂𝐻 − ] in 1 LITER of SOLUTION. The abbreviation “pH” stands for the “POWER OF HYDROGEN”, which DEFINES the NUMERICAL VALUE for PH, as the NEGATIVE BASE 10 LOGARITHM of the MOLAR CONCENTRATION of HYDROGEN IONS. An INDICATOR is a special type of COMPOUND that is used to INDICATE the pH of a SOLUTION as it changes, indicating whether the SOLUTION is ACIDIC or BASIC. The FORMULA for PH can be referenced under the SUBJECT of CHEMISTRY on page 54 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. The RELATIONSHIP between PH and MOLAR CONCENTRATION of HYDROGEN IONS is represented on a LOGARITHMIC SCALE as shown by the expression:

𝑝𝐻 = log10 (

1 ) [𝐻 + ]

Where: • pH is the measurement of the molar concentration of hydrogen ions • [𝐻 + ] is the MOLAR CONCENTRATION of HYDROGEN IONS given in units of GRAM MOLES per LITER. Made with

by Prepineer | Prepineer.com

Looking at this formula, we may be INTIMIDATED by the use of a LOGARITHMIC FUNCTION. However, we can REARRANGE the equation, so that we can ELIMINATE the LOG BASE 10 term.

𝑝𝐻 = log10 (

1 ) = log10 ([𝐻 + ]−1 ) + [𝐻 ]

Let’s jump back to the RULES of LOGARITHMS that we learned in MATHEMATICS, so that we can re-write the formula, isolating the TERM [𝐻 + ] that represents the MOLAR CONCENTRATION of HYDROGEN IONS. The PROPERTIES AND IDENTITIES OF LOGARITHMS can be referenced under the topic of LOGARITHMS on page 23 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. The logarithm of “𝑥” to the base “𝑏” is defined by: log 𝑏 (𝑥) = 𝑐, 𝑤ℎ𝑒𝑟𝑒 𝑏 𝑐 = 𝑥 Using this FORMULA for LOGARITHMS, we can re-write the expression representing the RELATIONSHIP between PH and the MOLAR CONCENTRATION of HYDROGEN IONS as: log10 ([𝐻 + ]−1 ) = 𝑝𝐻, 𝑤ℎ𝑒𝑟𝑒 10 𝑝𝐻 = [𝐻 + ]−1

Made with

by Prepineer | Prepineer.com

Let’s go ahead and RE-WRITE this expression one more time, so we have it in a FORM where we can plug and chug to calculate the MOLAR CONCENTRATION of HYDROGEN IONS in the SOLUTION:

10 𝑝𝐻 =

1 [𝐻 + ]

Isolating the TERM for the MOLAR CONCENTRATION of HYDROGEN IONS, we find: [𝐻 + ] = 10−𝑝𝐻 The VALUES for the PH SCALE can be referenced under the SUBJECT of CHEMISTRY on page 54 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. The PH SCALE is a based on a number line from 0-14 representing the RANGE of PH VALUES possible, where 0 is the STRONGEST ACID possible, and 14 is the STRONGEST BASE POSSIBLE. A NEUTRAL SOLUTION, such as WATER, is one that has a PH of 7 and always dissociates into equal PROPORTIONS of both HYDROGEN IONS [𝐻 + ] or HYDROXIDE IONS [𝑂𝐻 − ].

Made with

by Prepineer | Prepineer.com

In pure water or any AQUEOUS SOLUTION, at a TEMPERATURE of 25°𝐶, the CONCETRATIONS of HYDROGEN IONS [𝐻 + ] and HYDROXIDE IONS [𝑂𝐻 − ] are equal and found to be: 𝐴𝑐𝑖𝑑: [𝐻 + ] = 1 × 10−7 𝑀 𝐵𝑎𝑠𝑒: [𝑂𝐻 − ] = 1 × 10−7 𝑀 The VALUE for the AUTOIONIZATION CONSTANT OF WATER can be referenced under the SUBJECT of CHEMISTRY on page 54 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. We can then MULTIPLY these two values together to calculate the AUTOIONIZATION CONSTANT or ION-PRODUCT of WATER "𝐾𝑤 " , which represents the EQUILIBRIUM CONSTANT for WATER in any ACID-BASE REACTION. The RELATIONSHIP between the CONCENTRATION of HYDROGEN IONS [𝐻 + ] and HYDROXIDE IONS [𝑂𝐻 − ] in water or an aqueous solution is given by the IONPRODUCT of water, 𝐾𝑤 : 𝐾𝑤 = [𝐻 + ][𝑂𝐻 − ] = (1 × 10−7 𝑀)(1 × 10−7 𝑀) = 1 × 10−14

Made with

by Prepineer | Prepineer.com

Using the PH SCALE to measure the STRENGTH of an ACID or BASE, a SOLUTION is CLASSIFIED as an ACID or BASE per the defined RANGES shown below:

A SOLUTION with a PH of 7 is considered NEUTRAL, such that it is neither ACIDIC nor BASIC. In a NEUTRAL SOLUTION, the concentration of HYDROGEN IONS [𝐻 + ] and HYDROXIDE IONS [𝑂𝐻 − ] are EQUIVALENT as shown by the expression: [𝐻 + ] = [𝑂𝐻 − ] = 10−7 𝑀 A SOLUTION with a PH less than 7 is considered ACIDIC, such that the SMALLER the pH, the MORE ACIDIC the SOLUTION. In an ACIDIC SOLUTION, the CONCENTRATION of HYDROGEN IONS [𝐻 + ] is GREATER than the CONCENTRATION of HYDROXIDE IONS [𝑂𝐻 − ], as shown by the expression: [𝐻 + ] > [𝑂𝐻 − ] or [𝐻 + ] < 10−7 𝑀 and [𝑂𝐻 − ] < 10−7 𝑀 Made with by Prepineer | Prepineer.com

A SOLUTION with a PH greater than 7 is considered BASIC, such that the GREATER the pH, the MORE BASIC the SOLUTION. In a BASIC SOLUTION, the CONCENTRATION of HYDROXIDE IONS [𝑂𝐻 − ] is GREATER than the CONCENTRATION of HYDROGEN IONS [𝐻 + ], as shown by the expression: [𝑂𝐻 − ] > [𝐻 + ] or [𝐻 + ] < 10−7 and [𝑂𝐻 − ] > 10−7 𝑀 Once ACIDIC or BASIC substance have been added to pure water, the concentration of HYDROGEN IONS [𝐻 + ] will increases if ACIDS are ADDED or the concentration of HYDROXIDE IONS [𝑂𝐻 − ] will increase if BASES are ADDED. In practice, we can change the concentration of either HYDROGEN IONS [𝐻 + ] or HYDROXIDE IONS [𝑂𝐻 − ] in a SOLUTION, but we cannot vary both of them independently. POH OF ACIDS AND BASES: The TOPIC of POH is not provided in the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. We must memorize this formula and understand its application independent of the NCEES Supplied Reference Handbook. As the with concentration of HYDROGEN IONS [𝐻 + ], the concentration of HYDROXIDE IONS [𝑂𝐻 − ] can be represented on a LOGARITHMIC SCALE.

Made with

by Prepineer | Prepineer.com

Similar to the pH scale, the pOH scale has a maximum of 14 integer values on a NUMBER LINE, such that the SUM of the pH value and pOH value must always add up to 14 for a given SOLUTION, as shown by the formula: 𝑝𝐻 + 𝑝𝑂𝐻 = 14.00 The POH SCALE is similar to the pH scale in that a value of 7 is indicative of a NEUTRAL SOLUTION. The FORMULA for POH is not provided in the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. We must memorize this formula and understand its application independent of the NCEES Supplied Reference Handbook. The RELATIONSHIP between POH and MOLAR CONCENTRATION of HYDROXIDE IONS [𝑂𝐻 − ] is represented on a LOGARITHMIC SCALE as shown by the expression:

𝑝𝑂𝐻 = log10 (

1 ) [𝑂𝐻 − ]

Where: • pOH is the measurement of the molar concentration of hydroxide ions • [𝑂𝐻 − ] is the MOLAR CONCENTRATION of HYDROXIDE IONS given in units of GRAM MOLES per LITER. Looking at this formula, we may be INTIMIDATED by the use of a LOGARITHMIC FUNCTION.

Made with

by Prepineer | Prepineer.com

However, we can REARRANGE the equation, so that we can ELIMINATE the LOG BASE 10 term.

𝑝𝑂𝐻 = log10 (

1 ) = log10 ([𝑂𝐻 − ]−1 ) − [𝑂𝐻 ]

Let’s jump back to the RULES of LOGARITHMS that we learned in MATHEMATICS, so that we can re-write the formula, isolating the TERM [𝑂𝐻 − ] that represents the MOLAR CONCENTRATION of HYDROXIDE IONS. The PROPERTIES AND IDENTITIES OF LOGARITHMS can be referenced under the topic of LOGARITHMS on page 23 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. The logarithm of “𝑥” to the base “𝑏” is defined by: log 𝑏 (𝑥) = 𝑐, 𝑤ℎ𝑒𝑟𝑒 𝑏 𝑐 = 𝑥

Using this FORMULA for LOGARITHMS, we can re-write the expression representing the RELATIONSHIP between POH and the MOLAR CONCENTRATION of HYDROXIDE IONS as: log10 ([𝑂𝐻 − ]−1 ) = 𝑝𝐻, 𝑤ℎ𝑒𝑟𝑒 10 𝑝𝑂𝐻 = [𝑂𝐻 − ]−1

Made with

by Prepineer | Prepineer.com

Let’s go ahead and RE-WRITE this expression one more time, so we have it in a FORM where we can plug and chug to calculate the MOLAR CONCENTRATION of HYDROXIDE IONS in the SOLUTION:

10 𝑝𝑂𝐻 =

1 [𝑂𝐻 − ]

Isolating the TERM for the MOLAR CONCENTRATION of HYDROXIDE IONS, we find: [𝑂𝐻 − ] = 10−𝑝𝑂𝐻 Using the POH SCALE to measure the STRENGTH of an ACID or BASE, a SOLUTION is CLASSIFIED as an ACID or BASE per the defined RANGES shown below:

A SOLUTION with a POH of 7 is considered NEUTRAL, such that it is neither ACIDIC nor BASIC.

Made with

by Prepineer | Prepineer.com

A SOLUTION with a POH less than 7 is considered BASIC, such that the SMALLER the pOH, the MORE BASIC the SOLUTION. A SOLUTION with a POH greater than 7 is considered ACIDIC, such that the GREATER the pOH, the MORE ACIDIC the SOLUTION. STRONG/WEAK ACIDS: The TOPIC of STRONG/WEAK ACIDS is not provided in the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. We must memorize this formula and understand its application independent of the NCEES Supplied Reference Handbook. STRONG ACIDS are ACIDS that are assumed to IONIZE completely in WATER, such that all the acid molecules break up into IONS and ATTACH to WATER MOLECULES. In a STRONG ACID, the concentration of HYDROGEN IONS [𝐻 + ] is EQUIVALENT to the concentration of the acid. STRONG ACIDS are STRONG ELECTROLYTES, and act as good conductors of ELECTRICAL CURRENT in SOLUTIONS. The MAJORITY of acids exist as WEAK ACIDS, which IONIZE to a LIMITED extent in WATER, and do not completely ionized as a strong acid would.

Made with

by Prepineer | Prepineer.com

The TABLE of COMMON ACIDS is not provided in the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. We must memorize this formula and understand its application independent of the NCEES Supplied Reference Handbook. Most STRONG ACIDS are INORGANIC ACIDS and have the letter “H” in the beginning of the formula, with the exception of acetic acid, as shown in the table below of COMMON ACIDS: Chemical Formula

Name

Strong Acid?

HCl

Hydrochloric Acid

Yes

HNO3

Nitric Acid

Yes

H2SO4

Sulfuric Acid

Yes

HBr

Hydrobromic Acid

Yes

HI

Hydroiodic Acid

Yes

HClO4

Perchloric Acid

Yes

HF

Hydrofluoric Acid

No

CH3COOH

Acetic Acid

No

H3PO4

Phosphoric Acid

No

Made with

by Prepineer | Prepineer.com

STRONG/WEAK BASES: The TOPIC of STRONG/WEAK BASES is not provided in the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. We must memorize this formula and understand its application independent of the NCEES Supplied Reference Handbook. STRONG BASES are BASES that are assumed to IONIZE completely in WATER, such that all the base molecules break up into IONS and ATTACH to WATER MOLECULES. In a STRONG BASE, the concentration of HYDROXIDE IONS [𝑂𝐻 − ] is EQUIVALENT to the concentration of the base. STRONG BASES are also STRONG ELECTROLYTES, and act as good conductors of ELECTRICAL CURRENT in SOLUTIONS. An ALKALINE is a SOLUTION that contains a BASE as the SOLUTE or SOLVENT. The TABLE of COMMON BASES is not provided in the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. We must memorize this formula and understand its application independent of the NCEES Supplied Reference Handbook. Most WEAK BASES are ANIONS of WEAK ACIDS, such they have a HYDROXIDE [𝑂𝐻 − ] term in the CHEMICAL FORMULA, as shown in the table below of COMMON BASES:

Made with

by Prepineer | Prepineer.com

Chemical Formula

Name

Strong Base?

LiOH

Lithium Hydroxide

Yes

NaOH

Sodium Hydroxide

Yes

KOH

Potassium Hydroxide

Yes

RbOH

Rubidium Hydroxide

Yes

CsOH

Cesium Hydroxide

Yes

Ca(OH)2

Calcium Hydroxide*

Yes

Sr(OH)2

Strontium Hydroxide*

Yes

Ba(OH)2

Barium Hydroxide*

Yes

NH3

Ammonia

No

(CH3CH2)2NH

Diethylamine

No

Mg(OH)2

Magnesium Hydroxide

No

Ca(OH)2

Calcium Hydroxide

No

NH3(NH4OH)

Ammonia (Ammonium Hydroxide)

No

* Completely dissociate in solutions of 0.01 M or less. Other bases make solutions of 1.0 M and are 100% dissociated at that concentration level.

Made with

by Prepineer | Prepineer.com

CONCEPT EXAMPLE: Given an acidic solution with a pH of 1.79, the molar concentration of hydrogen ions in the acidic solution is most close to: A. 1.62 × 10−2 B. 4.89 × 10−2 C. 9.21 × 10−3 D. 𝑁𝑜𝑛𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑎𝑏𝑜𝑣𝑒

Made with

by Prepineer | Prepineer.com

SOLUTION: The TOPIC of ACIDS, BASES, AND PH can be referenced under the SUBJECT of CHEMISTRY on page 54 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. In this problem we are given the PH of a SOLUTION, and asked to calculate the corresponding CONCENTRATION of HYDROGEN IONS [𝐻 + ] The PH of a solution REPRESENTS the CONCENTRATION of HYDROGEN [𝐻 + ] or HYDROXIDE IONS [𝑂𝐻 − ] in 1 LITER of SOLUTION. The abbreviation “pH” stands for POWER OF HYDROGEN, which DEFINES the NUMERICAL VALUE for PH as the NEGATIVE BASE 10 LOGARITHM of the MOLAR CONCENTRATION of HYDROGEN IONS. The FORMULA for PH can be referenced under the SUBJECT of CHEMISTRY on page 54 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. The RELATIONSHIP between PH and MOLAR CONCENTRATION of HYDROGEN IONS is represented by the expression:

𝑝𝐻 = log10 (

1 ) [𝐻 + ]

Where: Made with

by Prepineer | Prepineer.com

• pH is the measurement of the molar concentration of hydrogen ions • [𝐻 + ] Looking at this formula, we may be INTIMIDATED by the use of a LOGARITHMIC FUNCTION. However, we can REARRANGE the equation, so that we can ELIMINATE the LOG BASE 10 term.

𝑝𝐻 = log10 (

1 ) = log10 ([𝐻 + ]−1 ) [𝐻 + ]

Let’s jump back to the RULES of LOGARITHMS that we learned in MATHEMATICS, so that we can re-write the formula, isolating the TERM [𝐻 + ] that represents the MOLAR CONCENTRATION of HYDROGEN IONS. The PROPERTIES AND IDENTITIES OF LOGARITHMS can be referenced under the topic of LOGARITHMS on page 23 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. The logarithm of “𝑥” to the base “𝑏” is defined by: log 𝑏 (𝑥) = 𝑐, 𝑤ℎ𝑒𝑟𝑒 𝑏 𝑐 = 𝑥 Using this FORMULA for LOGARITHMS, we can re-write the expression representing the RELATIONSHIP between PH and the MOLAR CONCENTRATION of HYDROGEN IONS as: log10 ([𝐻 + ]−1 ) = 𝑝𝐻, 𝑤ℎ𝑒𝑟𝑒 10 𝑝𝐻 = [𝐻 + ]−1 Made with

by Prepineer | Prepineer.com

Let’s go ahead and RE-WRITE this expression one more time, so we have it in a FORM where we can plug and chug to calculate the MOLAR CONCENTRATION of HYDROGEN IONS in the SOLUTION:

10 𝑝𝐻 =

1 [𝐻 + ]

Isolating the TERM for the MOLAR CONCENTRATION of HYDROGEN IONS, we find: [𝐻 + ] = 10−𝑝𝐻 As we are told the pH of the SOLUTION, we plug in the value of 2.12 and SOLVE for the MOLAR CONCENTRATION of the HYDROGEN IONS as: [𝐻 + ] = 10−𝑝𝐻 = 10−1.79 = 1.62 × 10−2 𝑀 𝐻 +

Therefore, the correct answer choice is A. 𝟏. 𝟔𝟐 × 𝟏𝟎−𝟐

Made with

by Prepineer | Prepineer.com