Comprehensive Guide to Acids, Bases, and pH Concepts Explained

hello everyone this lesson is going to be on acids and bases it will introduce you to

some new information and hopefully some of the information you will recall from previous science classes

so acids and bases um important molecules and ions that we'll start off with here

um hydrogen kind of unique it can end up with a negative charge looking like this that's called hydride

so normally hydrogen well always has one proton starts off with one single electron as well which would mean it's

neutral so if it picks up an extra electron it has one more electron that is this one here the

hydride in the case of the one that's important to us though

it is the hydrogen that has the positive charge so in this case there are no electrons

it has lost its only electron all we're left with is a proton which of course has a positive charge

and this one is referred to as a proton because there is no electron that is only the

proton in the nucleus so just realize here that when i do talk about a proton from now on i'm

not talking about the subatomic particle in the nucleus i am actually talking about this

positively charged ion of hydrogen so that as we'll see is very important in fact it leads to the formation of

this one here which is hydronium so you might kind of notice

that it well sort of looks like h2o water only well we have an extra hydrogen

and we have a positive charge so yeah if we take the proton that we were talking about

previously and if it's added to the water well this is what we end up with is the hydronium ion

that's going to be very important for acids the next one here important for bases is

the hydroxide so you may recognize this from the table of polyatomic ions this is the hydroxide

ion and the last one water written in may be a different form than what we've seen in

the past formally of course you would have seen it in this form here

h2o but for many purposes it's easier and better to write it as hoh and even if you think of the

structure of water we do have the hydrogen kind of on an angle anyway

sandwiched between the two hydrogen and so that is our hoh for

water so we'll kind of see why it's easier in some cases when we talk about acid and base chemistry

to write it as hoh rather than as h2o for science 30 what we use are the

bronsted-lowry definitions of acids and bases so there are various different definitions

but it is the bronsted-lowry definition that we do go with for science 30. so what is the

bronsted-lowry definition of acid c well it says that proton or that says that acids will donate

protons or they will lose they will give up their protons

so when they give up their protons what they're giving them up to is water and as we just saw if water

picks up the proton then it just forms hydronium ion and that's going to be on the

product side whenever you have more hydronium that's forming that is what leads to an acidic solution

the more acidic a solution is the more hydronium ions there are and that's what's making it more acidic

and of course acidic solutions have lower ph values a ph lower than 7. the more hydronium the

lower that ph value is going to be bronsted-lowry definition of a base is bases will accept

protons so acids donate protons bases except or they gain the protons so if we have a base that's gaining

protons typically where it's gaining it from is water so if we take water and now

strip a proton away what we're left with is this here on the product side the more hydroxide

ion that you have the more basic the solution is going to be and exactly the opposite with

hydronium instead of lowering the ph it's going to increase the ph

the more hydroxide the lower the ph sorry the more hydronium the lower the ph the more hydroxide

the higher the ph is going to be let's take a look at a few examples of reactions here

so in the first example are two reactants that we have so a couple of things to go by to

determine if something is going to be an acid first of all there has to be a hydrogen there has to

be a hydrogen because acids again are proton donors if we take a look at our two reactants

we have hydrogen here and we have hydrogen here so both of these could be acting as an

acid and you might say wait a minute this is water water can't be an acid well it actually can according to the

bronsted-lowry definition depending upon what it is reacting with and depending upon whether it donates

or accepts electrons so how is water behaving in this case well if we take a look at

the product side we see cl minus which well kind of looks like this hcl only

it's missing the hydrogen in fact not only is it missing the hydrogen

it's missing a hydrogen and a positive charge so we have taken away a hydrogen and a

positive charge from hcl and if we take a look at the other product

well it's the water that has picked up that proton so we have a proton that's being donated

from hcl to the water proton donor is your acid and in fact this is hydrochloric acid

hcl and the water is going to be acting as a base

because it is accepting the proton so in this case here because we do have hydronium ions that are forming

what we're going to get is it's going to become a acidic solution and we're going to see a decrease in ph

something else to go by so if it is an acid it has to be hydrogen and also there will be hq

so whenever it is an acid it will have the subscript aq but don't just go by that because we see

this one down here that's also eq but this one is not going to be an acid

so you need to look for a little bit more it needs to have the aq it needs to have a hydrogen

and you may need to look at some other things as well like is the hydronium being formed

the next reaction that we have here we once again have water and i'm just going to write this a

different way now i'm going to write it as the hoh just to show it a little bit differently

and this one here um you'll find all of this on the table of polyatomic ions

what's going to happen with this reaction is very similar to what's happening at the top only now

we do have this one that is aq we do have a hydrogen but the hydrogen is not in front this here is a special kind of

acid if you see this cooh this is what we call an

organic acid or a carboxylic acid sometimes it's written like this the carboxylic acid oops this should be

an oh that we have right here and what's going to happen is it's going to be this hydrogen right here

that's going to be donated over to the water so if we add the hydrogen with the

charge the proton taking it from this one that means that this is going to be the acid

and water once again is going to be accepting the proton so it's going to be the base

so what do we end up with on the right hand side we get all of this benzoic acid

without the proton which is benzoate and again you can find this on your table of polyatomic ions

what we also have is our water which picked up the extra proton so we have the hydronium so once again

because we're forming hydronium ions this is also going to be an acidic solution that we do have

and it's going to lower the ph we'll talk about this in another lesson but there is in fact a difference

between these reactions so in the case of the first one hydrochloric acid is what we call a

strong acid every single one 100 of these protons here in the hcl

they are completely stripped away from the chloride and they're donated to the water

in order to form a whole bunch of hydronium ions so we get really a big big drop in the

ph in the case of the benzoic acid down here

for this one we do have the protons that are donated to form the hydronium but some are

actually donated back as well so there's a little bit of an equilibrium that is formed so we do have

the reaction that's going to the right but we also have the reaction that's going back in the opposite

direction to the left so what that means is that because this reaction can reverse itself

we can say that well now the hydronium is donating a proton so it is actually also acting as an acid

and the benzoate is acting as a base in this case because they're going in the opposite direction

we call them the conjugate i'll just put c for conjugate acid and the conjugate base

on the other side it's just the straight acid index going on to the third example here so

here we have naoh and aq indicated for both of them these are not

acids though so a big big clue here whenever you have a metal in this case it's sodium but it could also be

magnesium it could be potassium it could be barium attached to the o-h that is a pretty good clue that this is

a base and it in fact is a base which means that the other reactant is going to be

an acid so up above we saw that water in both cases

was acting as a base now it's going to be acting as an acid so what's going to happen

well it's going to donate its proton in fact when you take this naoh it is also an ionic compound so

when you dissolve it in water this is what happens first is the ion simply split apart

so we get in a plus dissolved in solution hydroxide dissolved in our solution

which is the h2o so why do we now talk about the donation of protons because in reality what we're going to

have is protons from here that are being donated over it turns out being exactly the same

in a plus and this is now our h2o here and this is our o h minus so it doesn't change anything in terms of what i

previously wrote but it does just show that because we have the water that's donating the

protons that's what makes it the acid here though we're not forming hydronium we're

forming oh which means now instead of having a dropping ph

we're going to have an increase in ph which means that this is a basic or an alkaline solution the last

reaction here again we already know that this one here is a base and we already know from the first

reaction that this one here is an acid so sodium hydroxide reacting with hydrochloric acid

we know then that this one is going to be donating the proton it's going to be donating it well

somewhere over here but when we do have these solutions in water again

first of all if i just rewrite this just like i did up above and then if i write the hcl

now we can see a little bit more clearly that we have the hcl that's being donated over here

so what we end up with is an a plus in water along with cl minus dissolved in the water we don't have any

hydronium we don't have any oh in other words there's nothing to cause this solution to be acidic

there's nothing to cause this solution to be basic so what we end up with is something that

has a ph of 7 and this is a special kind of reaction that is called a neutralization reaction

so we do have an acid we do have a base but they're kind of canceling general and resulting in the ph being

neutral having a ph of seven the ph scale you've all seen the ph scale before i'll show you

several different versions of the ph scale so this is a fairly simple and straightforward one

it shows the range from a ph of zero very acidic going up to a ph of 14. here it has

alkaline it doesn't matter if we call it alkaline or basically we treat it as the same thing

right in the middle of course ph of 7 that is neutral so that would be distilled water

okay not just any water not river water tap water ocean water but distilled water that has h2o molecules and nothing

else that has a ph of seven as you go lower it becomes more and more and more

acidic the further you go with the lower numbers to the left in this case

the more acidic the further you go to the right with the bigger numbers the more basic it is going to be and i

should also point out that this scale can extend beyond 14 that would be what we call a super base

and it can in fact go into negative values like negative two and those are what are

referred to as super acids but for the most part we just talked about the basic range

which is from zero to fourteen so let's bump it up a little bit and talk about more science third and

stuff so this one's kind of nice it shows um the same arrangement going from the low

ph values on the left the upper ph values on the right

again the scale is 0 to 14 0 right in the middle being neutral and it also has a number

of different examples here samples of different fluids and what their phs are so your body

fluids blood slightly basic very close to neutral but ever so slightly basic a ph of 7.4

if we take a look at uh something like vinegar that you might put on your french fries

usual ph is typically somewhere around three for vinegar stomach acid considerably more acidic a

ph of around one here it has one to three but it's usually fairly low and fairly acidic

in your house baking soda ph about eight and a half uh cleaning solutions that contain

ammonia ph around 11 or 12 so quite basic for those but this ph scale shows

something else that's really important for science 30 and that is this right here hydronium ion

and the square brackets means concentration the concentration of hydronium ions

and the units for concentration are moles per meter so all of these numbers here 10 to

the negative 2 10 to the negative 4 10 to the negative 7 those are all of the concentrations

in moles per liter so let's start with our neutral solution 10 to the negative 7 what does that

really mean whenever you have an exponent and it's negative

that's a small number it's a very small number if it's negative the bigger this number is the smaller

the number is going to be so this number here is actually equal to zero decimal zero

zero zero zero zero zero one so how does this turn into ten to the negative seven well initially

the decimal is here and it's moved over one two three four five six

seven places to the right and that's where we get the seven from it's negative because it's a really

really small number so if we had a big number like 10 to the four now the decimal is

in the opposite direction so now we would move it over one

two three four places and this is actually ten thousand okay so we're talking about really really

small numbers in terms of the concentration so if we were to take a look at this one

here ten to the negative four this one here is the same as zero

decimal zero zero zero one again we move the decimal over one

two three four places it's a small number so it's the negative now let's compare this number here to

this number here this one here is actually much much smaller and this

one here is bigger so as you go away from the neutral solution to more

acidic solutions you get a bigger concentration of hydronium ions

and again that was one of the definitions of an acid as it does produce and increase the

amount of hydronium ions as we go further and further away in the opposite direction

we get fewer and fewer of the hydronium ions we get smaller and smaller numbers for

the concentration and that means a less acidic or more basic solution

just as a little bit of an aside i did mention the hydroxide ions as well we don't have it on this picture and

it's not so much science 30 but as we go to the right hand side here more basic

there would be fewer hydronium but there would be more hydroxide

and way over here there would be more hydronium but there would be fewer of the

hydroxide so kind of an inverse relationship between the amount of hydronium

increasing the acidity and the amount of hydroxide ions one more picture of the ph scale and

notice that the orientation on this one is different so pay careful attention to that

so here it does have acidic solutions at the top and again some examples of them neutral ph right in the middle here and

our basic or alkaline solutions down at the bottom it's using this legend here to show the

well here it just has the protons but in reality it should be the h3o plus that they are

talking about so as we do go to more acidic solutions we have more of these red spheres

so this would be an acidic solution as we go to more basic solutions we have more and more of the hydroxide

which are the blue circles or spheres according to this diagram

so the other really important thing is on the left hand side and again here they're saying

the hydrogen ion concentration or the proton concentration but once again it really is the

hydronium ion concentration so what this does show is what is referred to as a logarithmic scale

the ph scale is a log scale and this is what the picture is shown as we go from a ph of 7 to a ph of

6 it's a change of only one ph value but it's ten times more acidic if we take a look at

lemon juice which has a ph of two compared to distilled water for each change of one ph value

it is 10 times more acidic so 10 100 1000 10 000 lemon juice is 100 times more acidic

than distilled water same pattern as we go in the opposite direction so if we take a look at

ammonia here we're going 10 100 1000 10 000. so ammonia is

ten thousand times more basic now ten thousand times more basic than distilled water

or one ten thousand as acidic as distilled water okay that is the logarithmic scale

and in the future lesson we'll be doing some calculations using the log scale to calculate

hydronium ion concentrations and the ph of solutions just one final slide and this is one thing that

hopefully is a review for you the properties of acids and bases so both acids and bases when they do

dissolve in water they do form ions so if we take a look at hydrochloric acid hcl very common acid

when we take it and we dissolve it in water even though protons are donated let's just write it down as this for now

there are protons formed there are chloride ions formed whenever there are charges that are

formed it's going to conduct electricity same thing within a

cl or sorry we'll do naoh for a base dissolve it and we get sodium ion we get the hydroxide ion

again the key is there are charges so they will conduct electricity litmus there is a

red litmus paper and there is blue little paper so if we use blue litmus paper

and put an acid on it it will turn red in color if we take a base it turns red litmus

blue in color okay what about if you use red litmus for acid well it just stays red and blue

litmus stays blue when it's a base so the key really is

think red for acid whether it's red or blue litmus and think blue for a base

in the case of acids as well they do react with um active metals so usually one of the metals that we

talk about is magnesium if you put a magnesium strip little piece of metal

into an acid like hydrochloric acid a strong acid then there is a gas produced and that is

hydrogen gas so that's a property of acids uh bases slippery to the field typically

acids taste sour again think about common ones uh like fruit juices and lemon juice and

lime juice citrus fruits they contain citric acid vinegar is acetic acid

for bases the opposite instead of tasting sour they taste bitter we saw a neutralization reaction

so something like hydrochloric acid will neutralize a base like sodium hydroxide

through that neutralization reaction in the cell and in both cases they can be corrosive