We start our recap with a recap: What are acids and bases and acid-base reactions?
We start our recap with a recap: What are acids and bases and acid-base reactions (1:03)? Diving a bit deeper, we take a closer look at the dissociation of acids (2:20), which releases H+ ions and therefore lowers the pH. Vice-versa we look at the dissociation of bases in water (3:34 ) and it’s connection to pOH and pH (3:34). Taking a closer look at water, which is an amphoteric substance (4:16), the episode introduces the autoionization of water (5:56) and the equilibrium constant: KW (6:35) and ties it to pH and pOH (7:29).
Question: At a higher temperature, will the pH of a neutral solution be higher, lower or equal to pH=7?
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Hi and welcome to the APsolute Recap: Chemistry Edition. Today’s episode will recap the basics of acids and bases.
Lets Zoom Out:
Unit 8 - Acids and Bases
Topic 8.1 - Introduction to Acids and Bases
Big idea - Structure and Properties
Introduction:
Have you taken a closer look at our APsolute RecAP Chemistry Edition logo created by our wonderful producer Brad Kingett? NO? Hit the stop button and take a look! Now! We are finally in the acids and bases unit - and there are a lot of BASIC jokes one can make. Hey, don’t get SALTY! Okay, okay, I’ll save some for later episodes. So let’s get started and introduce the BASICs.
Let’s zoom in:
We’ll start the recap with a recap: In episode 22 we’ve already introduced acids and bases. According to Brønsted-Lowry, acids are substances that donate protons - aka H+ ions. Bases are substances that accept H+ ions. An acid-base reaction therefore involves the transfer of protons between substances.
To donate a proton the substance, of course, has to have a proton. The chemical formulas of inorganic acids usually start with a H. HCl, H2SO4, … and H2O. What? Water is an acid? Well… actually water is both an acid and a base. But I am jumping ahead. Bases often, but not always, have an OH group, for example sodium hydroxide. But, as you’ve noticed with the definition of being able to accept a proton, it doesn’t have to have an OH- anion. NH3, for example, acts as a base when accepting a proton to form the ammonium anion.
Strong acids, like hydrochloric acid will dissociate in water to form H+ and Cl-. Well, technically, to form hydronium, H3O+ and Cl-. How so? Acids are aqueous solutions, so the proton will react with a water molecule to form H3O+. And there you have the first dilemma: The water molecules are ACCEPTING a proton. So they must be bases. Huh? Hang in there, we will get to it!
In AP Chemistry, using hydronium is preferred over using H+, but you will get full credit for either one. Therefore, aqueous solutions of acids have a high concentration of hydronium ions. To measure the concentration of hydronium ions, we have the pH. The pH is the negative logarithm of the hydronium-ion concentration. Acids have a pH value below 7. A low pH means you have a higher concentration of hydronium and therefore a strong acid. A pH closer to 7 means you have a lower concentration of hydronium ions and a weak acid.
Bases can be measured in terms of the concentration of hydroxide ions. But wait, where does the hydroxide come from, like when I have ammonia? Well, bases are also aqueous solutions. So the ammonia reacts with the water molecules and water donates a proton to form OH- and NH4+. Uh-oh, here we go again: Now water DONATED a proton. So it acts like a base! Water is special - and I have said that A LOT in past episodes. And this is just more evidence to prove my point. Water is amphoteric or amphiprotic, which means it can act as an acid and donate a proton as well as a base and accept a proton!
For bases that means that we can calculate the negative logarithm of the OH- concentration, aka pOH. Substances with a higher concentration of OH- have a low pOH and substances with a lower concentration of OH- have a higher pOH. The pOH concept might sound somewhat unfamiliar, since we usually express the strength of acids and bases in terms of pH only. So let’s look at the connection between pH and pOH.
And that, of course, has to do with water! If you have pure water, what will happen? Will it behave as acid or as base? Or both? So many questions! Water by itself actually autoionizes. That means, a very very small number of molecules will donate a proton and accordingly accept a proton - about 6 molecules in 100 million. The equation for this reaction is: H2O + H2O (in equilibrium with) H3O+ and OH-. Sounds ridiculous, but it matters. This process is reversible and therefore connected to an equilibrium constant: KW. The equilibrium expression is [H3O+] times [OH-] - we don’t add the reactants since they are liquid. We’ve discussed in episode 25 that the equilibrium constant is temperature dependent. At 25℃ the Kw is 1.0 x 10-14 - a value that indicates that the equilibrium lies heavily on the reactant side. If we solve for the concentrations of hydronium and hydroxide, which have to be equal, we will get 1.0 x 10-7 for each of them. And the negative log of 1.0 x 10-7 is… 7! Plug it into your calculator if you don’t believe me! Therefore, we often see the expression: pKw = pH + pOH, where the pKw is 14 at 25℃. When the pH equals the pOH, meaning the concentration of the H3O+ ions and OH- ions is equal, as it is for pure water, the solution is neutral. For water, neutral is at pH = 7 at 25℃. Again, the temperature matters, because the autoionization is an endothermic process. At higher temperatures, the equilibrium will shift to the product side, increasing the concentration of OH- and H3O+ and therefore increasing the Kw. We will talk more about pH and pOH in our next episode, stay tuned!
Now, what does all of that have to do with our logo? Our logo refers to the reaction between an acid and a base! In the notes you see the structure of sulfuric acid - to neutralize it, it needs more BASE!
To recap:
According to Brønsted-Lowry, acids are substances that donate protons - aka H+ ions. Bases are substances that accept H+ ions. An acid-base reaction involves the transfer of protons between substances. To measure the concentration of hydronium ions, we calculate the pH and to measure the concentration of hydroxide ions, we calculate the pOH. Water is amphoteric and can act as an acid or base. PH and pOH are connected via the equilibrium constant of the autoionization of water, which is temperature dependent.
Coming up next on the APsolute RecAP Chemistry Edition: pH and pOH.
Today’s Question of the day is about the equilibrium constant of water.
Question: At a higher temperature, will the pH of a neutral solution be higher, lower or equal to pH=7?