Acid Base Reactions

General Chemistry Index

Where are we going with this? This page will assist in developing the ability to classify solutions as acids or bases and describe their characteristic properties and/or compare and contrast the strength of acids and bases in solutions and/or, given the hydronium ion and/or the hydroxide ion concentration, calculate the pH and/or the pOH of a solution. Explain the meanings of these values.

Acid-Base Reactions

https://www.britannica.com/science/acid-base-reaction

Okay, so this one sort of defines itself! An acid-base reaction is a reaction between an acid and a base. 

There has to be more…

The definition can be expanded to explain that, in an acid-base reaction, the H⁺ and OH^- ions will "find each other" and form water. Then, the "other guys" will combine to form a salt.

To make this into a fancy definition, we might have something like…

In and acid-base reaction, the H⁺ ions separate from their constituent anions and the OH^- ions separate from their constituent cations resulting in H2O and a salt made of the anion and cation.

So… salt? Not "salt" but "a salt." We need a definition.

In chemistry…

A salt, when the result of an acid-base reaction, is a substance that consists of the ionic assembly of a positive ion (cation) from a base and the negative ion (anion) from an acid. 

So… not table salt. But, yes, NaCl (table salt) is a salt according to chemistry. But it's not the only salt.

There are lots of different salts. Pretty much anything from the periodic table columns 1 and 2 with anything from column 17… And a whole lot more! A few include:

• sodium chromate
• potassium dichromate
• cobalt nitrate
• copper sulfate
• potassium
• nickel chloride

That is NOT a complete list by far!

More info…

So… back to that acid base reaction…

Mixing an acid and a base typically results in water and a salt. For instance…

NaOH + HCl ---> NaCl + H2O

KOH + HBr --->  KBr + H2O

Because the properties of both the acid and the base are eliminated, acid-base reactions are sometimes called acid-base neutralization reactions.

Acid/Base Concepts, Definitions, and Other Word-type Things

General Chemistry Index

Where are we going with this? This page will assist in developing the ability to classify solutions as acids or bases and describe their characteristic properties and/or compare and contrast the strength of acids and bases in solutions and/or, given the hydronium ion and/or the hydroxide ion concentration, calculate the pH and/or the pOH of a solution. Explain the meanings of these values.

Acid/Base Concepts, Definitions, and Other Word-type Things

Most people reading this have some idea of what an acid is. It is fictionalized to be that stuff in a beaker that eats holes in everything. Usually instantly. And dramatically.

Likewise, most people know the word "base" but… possibly not within the context of chemistry. 

Looks like we have some concepts to clear up!

Definitions and Concepts

Acid: A substance that, when dissolved in water, produces H⁺ ions. More generally, a proton donor when in a solution.

Generally…

• Conduct electricity
• Acidic foods like lemons have a sour taste
• React with bases to neutralize their properties
• React with active metals to liberate hydrogen

Base: A substance that, when dissolved in water, produces OH^- ions. More generally, a proton acceptor when in a solution.

Generally…

• Conduct electricity
• Basic foods have a bitter taste
• React with acids to neutralize their properties

Oh, well, now… that certainly cleared things up!

There's a lot in those definitions. First off, we have to think about that  H⁺ ion. That's a hydrogen ion with a positive charge. It's a hydrogen (H) atom missing an electron. That means that the  H⁺ ion is essentially a proton… hence, the "more generally" part of the definitions.

Next, acids and bases are very frequently considered within solutions where their ions are present. This is common, but NOT exclusive.

What happens is this… You have an acid. Let's say you have some HCl (since that's easy to type). So, this is an ionic bond, meaning that the Cl attracts the electron from the H, giving each of the atoms a charge and making them cling to each other (ionic bond).

When you put them into water that whole solute, solvent, solution thing happens. So, in solution the H and the Cl keep their changes, but stop clinging to each other. <Complicated explanation about water being a polar molecule and stuff omitted.>

 

There you go… that H+ ion is just roaming around in the solution, ready to connect to something else. When the something else happens to be a base, you have an acid / base reaction. 

 

But, the whole "what's an acid?" question is illustrated. An acid is a substance in solution that dissociates into an H⁺  ion and its constituent anion.
 
The whole "What happens is this…" could be rewritten with NaOH. Put it in water and it dissociates into Na⁺ and OH^-. And then if an acid comes along… magic! (Not actually magic.) 

Therefore, a base is a substance in solution that dissociates into an OH^-  ion and its constituent cation.

 

 

More on that acid / base reaction…

To get more detailed, in 1923, some dudes named J. N. Brønsted and T.M. Lowry extended the definition of the acid - base reaction to include non-aqueous reactions where the protons are transferred. 

By extending the concept with the "Brønsted-Lowry definition," reactions need not be in water. For example…

NH3 + HCl --> NH4Cl

satisfies the definition: the H from the HCL jumps over to the NH3, then the Cl bonds ionically to the NH4 "chunk."

Okay, so what?

So, the acid-base reaction involves the transfer of a proton originating  (typically) in the form of hydrogen in one of the compounds.

Examples of Acids and Bases

What are we actually talking about? Pretty much a LOT of things!

Household acids and bases…

Lemons, vinegar (and things that have vinegar in them like ketchup), aspirin, coffee, grapefruit, teas, cranberry…stuff, and lots of marinades are acidic. Foods with a sour taste… or "tangy"? are acidic. 

Baking soda, bleach, ammonia and some green vegetables are bases.  You might correctly guess that antacids are bases! Foods with a bitter taste are basic.

Some "laboratory" acids and bases…

Here are some common acids:

HCl

H2SO4

HNO3

HBr

Here are some common bases:

LiOH

NaOH

KOH

Ca(OH)2

WAZZAT? It seems like all of the acids start with hydrogen and all of the bases end with OH!

This is true as a general rule. Except… 

One exception is water (little surprise there!)

Written as HOH, it could be either and acid or a base. In fact, it is neither! Distilled water is neutral. Although it can behave in some situations as either acid or base,  it's safe to consider it neither; to consider it neutral.

Also, HLi (aka LiH) is a base… But…

But, within the realm of common classroom chemistry, the H / OH rule is pretty solid. If it starts with an H you can be pretty sure that it is acidic and if it ends on OH, you can be pretty sure that it is basic. Unless it's water.

Strengths of Acids and Bases

Strong. Weak. There… Some acids/bases are strong. Some are weak.

Not helpful.

A strong acid completely reacts with water so that ALL of its H⁺ ions separate from the corresponding anion (the other thing in the acid molecule, such as Cl or SO4). 

A strong base completely dissociates in water so that ALL of the the OH^- ions separate from the corresponding cation (the other thing in the base molecule, like Na or K).

So… weak? Yeah… Intuition works on this!

Weak acids and bases exist where only a fraction of the molecules actually break up into ions.

Because the ions are charged, the more of them in a solution, the more it will conduct electricity. Conducting electricity is a characteristic of being an electrolyte. 

So… more logic… Strong acids and bases are also very good electrolytes.

Measuring the Strengths of Acids and Bases

Why does it feel like we are about to see NUMBERS!

Kahn Academy

The strength of acids and bases is rated on a scale called the pH scale.  A scale that… (sorry about this) ranges from 0 to 14. Not 15? Not 10? Not 100? 0 to 14? Really? Really. 

This is NOT the "math" behind it but, as a mnemonic, you can think of it as this: the bigger the number, the more protons it can take in. High numbers means it can accept a lot of protons. High numbers means it is very basic. Low numbers means it is very acidic. 

AND 7 is neutral. Water. Water is, in most cases, considered to be neutral.

So… Acids range from 0 to 7, with 0 being strongest. Bases range from 7 to 14 with 14 being strongest.

Buffers

So… You can think of these (though it is only metaphorical) as "pH shock absorbers."

Buffers work to keep the pH… of, for simplicity sake, let's say a solution… stable. If the pH gets too high, they let go of some H⁺ ions to lower it. If the pH gets too low, they absorb some H⁺ ions to raise it back up.

Source

Different types of buffers stabilize pH at different levels. 

For example, human blood needs to have a pH of around 7.4. There is a buffer in blood that helps do that.

 

The compound H2CO3 and H⁺ ions in the blood sort of do a little give and take. If there needs to be more  H⁺ ions, the buffer (the H2CO3) will add one and become H⁺  + HCO3 ^-. When there are too many H⁺ ions, it will take the ion back and return to its H2CO3 form. In this way, the pH of the blood is buffered to around 7.4. 

___________

References

https://chem.libretexts.org/Bookshelves/General_Chemistry/Map%3A_Chemistry_-_The_Central_Science_(Brown_et_al.)/04._Reactions_in_Aqueous_Solution/4.3%3A_Acid-Base_Reactions

https://www.khanacademy.org/science/biology/water-acids-and-bases/acids-bases-and-ph/a/acids-bases-ph-and-bufffers

Heat Change and Phase Change Formulas and Some Constants for Water

General Chemistry Index

Where are we going with this? This page will assist in developing the ability to perform calculations involving heat flow, temperature changes, and phase changes by using known values of specific heat, phase change constants, or both.

Heat Change and Phase Change Formulas and Some Constants for Water

These equations apply to heat and phase changes in general.

Temperature Change Formula:

Q = mc∆T

Phase Change, Solid <--> Liquid

Q=m∆Hf

Phase Change, Liquid <--> Gas

Q=m∆Hv

Many questions involve water. Here are the constants related to water.

Specific Heats for H2O

Solid (Ice) = 2.09 J/g • °C

Liquid (Water) = 4.128 J/g • °C

Gas (Steam) = 1.996 J/g • °C

Phase Change Constants for H2O 

Solid <--> Liquid — m∆Hf  = 334 J/g 

Liquid <--> Gas — m∆Hf  = 2257 J/g 

Virtual Lab: Heat Exchange and Calculating Initial Temperature

General Chemistry Index

Where are we going with this? This page will assist in developing the ability to perform calculations involving heat flow, temperature changes, and phase changes by using known values of specific heat, phase change constants, or both.

Virtual Lab: Heat Exchange and Calculating Initial Temperature

Background Information: Heat Exchange

The purpose of this activity is to provide hands-on experience using laboratory techniques to observe the transfer of energy from a hot substance to a cold substance. In this experiment, a very hot sample is introduced into a calorimeter and the final temperature and final mass is used to calculate the initial temperature of the sample.

Link to activity worksheet:  CLICK HERE

The video below has three parts:

• Setup and Procedures
• Result Collection
• Calculation of Ti

The lab can be done virtually by watching the video, collecting data, and doing your own calculations.

Virtual Lab: Heat Exchange and Predicting Final Temperature

General Chemistry Index

Where are we going with this? This page will assist in developing the ability to perform calculations involving heat flow, temperature changes, and phase changes by using known values of specific heat, phase change constants, or both.

Virtual Lab: Heat Exchange and Predicting Final Temperature

Background Information: Heat Exchange

The purpose of this activity is to provide hands-on experience using laboratory techniques to observe the transfer of energy from a hot substance to a cold substance. In this video, the formulas are used with some measurements to predict the final temperature of the mixture of hot and cold water and the predicted final temperature is compared to the measured final temperature.

Link to activity worksheet:  CLICK HERE

The video below has three parts:

• Setup and Procedures
• Result Collection
• Discussion

The lab can be done virtually by watching the first two parts, recording your own data, and doing your own calculations.

Virtual Lab: Finding Specific Heat Of An Unknown Sample

 General Chemistry Index

Where are we going with this? This page will assist in developing the ability to perform calculations involving heat flow, temperature changes, and phase changes by using known values of specific heat, phase change constants, or both.

Virtual Lab: Finding Specific Heat Of An Unknown Sample

Background Information: Heat Exchange

The purpose of this activity is to provide hands-on experience using laboratory techniques to observe the transfer of energy from a hot substance to a cold substance. The energy exchange is used to calculate the specific heat of the hot substance, a metal sample.

Link to activity worksheet:  CLICK HERE

The video below has three parts:

• Setup and Procedures
• Result Collection
• Discussion

The lab can be done virtually by watching the first two parts, recording your own data, and doing your own calculations.