Lesson video

Hello, my name is Mrs. Collins and I'm going to be taking you through the learning today.

This lesson forms part of the unit Making salts and is called Comparing concentration with strength.

So let's get started on the lesson.

During this lesson, you'll learn how to use the pH scale to work out concentration differences between solutions relating to their pH and discuss the differences between strong and weak acids.

Here are the key words for today's lesson, hydrogen ion, pH, concentration, strong acid, weak acid.

I suggest you pause the video here, have a read through those explanations, and write down any notes you feel you need to.

Today's lesson is divided into two parts, pH and concentration and strong and weak acids.

So we're going to start with pH and concentration.

Some parts of today's lesson may be a little challenging, but I'm going to help you through it.

Now you'll remember that acids are substances that dissociate or ionise to release hydrogen ions when they're in an aqueous solution.

So in other words, when they're dissolved in water.

So if you imagine you've got a pen with a pen lid, if we were to dissociate the lid from the pen, we would remove that lid from the pen.

So think about it in a similar way.

These hydrogen ions are dissociating from the molecule when they're in an aqueous solution, when they're dissolved in water.

So for example, here we've got hydrochloric acid and it will dissociate as follows.

So the hydrochloric acid will dissociate to form hydrogen ions and chloride ions, and we can show that in a symbol equation here.

So we've got aqueous hydrochloric acid breaking down into hydrogen ions, which are aqueous and chloride ions, which are aqueous.

So remember that the concentration is the amount of a solute dissolved in a certain volume of solution, and the unit for concentration is moles per decimeter cubed, and that's the number of moles of acid in a cubic decimeter of water.

So here we've got an example for sulfuric acid.

A solution of 1.

0 moles per decimeter cubed of sulfuric acid is twice as concentrated as 0.

5 moles per decimeter cube solution.

So in the first part we've got one mole of sulfuric acid particles dissolved in a decimeter cubed of water, and in the second one we've got 0.

5 moles of sulfuric acid molecules dissolved in a decimeter cubed solution.

Now, sulfuric acid dissociates in a very similar way to hydrochloric acid.

So we've got sulfuric acid there, aqueous, breaking down to form hydrogen ions and sulphate ions both aqueous, so all dissolved in water.

Now, pH stands for potential of hydrogen or power of hydrogen, and the pH scale is numeric.

So it consists of numbers and it goes from zero to 14, as you should know from your previous learning.

So we've got a scale here, we're going to show it using a number line.

Acids dissociate to release hydrogen ions.

So we talked about that already.

The higher the concentration of hydrogen ions, the lower the pH, and that's really important.

So the more hydrogen ions there are present in the same volume of solution, the higher that hydrogen concentration is and the lower the pH is, and then the lower the concentration of hydrogen ions, the higher the pH.

So that's really important.

You'll remember from your earlier learning that if we have a pH of less than seven, that means the solution is acidic.

And if we have a pH more than seven, it tells you that that solution is alkaline.

And if it's pH seven, you've got a neutral solution.

So here's a question based on that learning.

So the pH scale goes from, so pause the video here, answer the question, and I'll see you when you're finished.

Welcome back, so the answer is naught to 14, and you should remember that from your previous learning.

So well done if you've got that correct.

Here we have a follow-up question.

So how does the concentration of hydrogen ions affect the pH of a solution? So again, pause the video here, answer the question, and I'll see you when you're finished.

Welcome back, so the answer to that question is the higher the concentration of hydrogen ions, the lower the pH.

So well done if you've got that correct.

Okay, let's deal with bases and alkalize them.

So bases are substances that can neutralise acids.

When a base is soluble in water, we call it an alkali.

So it's really important that you can differentiate between base and an alkali.

So an alkali is a base, but it's a base that is soluble in water.

Not all bases remember, are soluble in water.

Now, alkalis release hydroxide ions, that's OH minus ions in water, and that decreases the concentration of hydrogen ions and raises the pH of the solution.

So the more hydroxide ions are present in the solution, the lower the concentration of hydrogen ions and therefore the higher the pH.

So here you got an increase in concentration of hydrogen ions, basically takes you from seven through to zero, and a decrease in concentration of hydrogen ions, remember, takes you from seven up to 14.

So if we decrease the concentration of hydroxide ions, we have an increase in the concentration of hydrogen ions.

And if we increase the concentration of hydroxide ions, we have a decrease in the concentration of hydrogen ions.

Now let's think about neutral solutions now.

So neutral solutions have an equal concentration of hydrogen and hydroxide ions.

So hydroxide ions neutralise the hydrogen ions producing water.

And if you look at those two ions there, we've got hydroxide, which is OH, and we've got hydrogen, which is H.

We put those together, we end up with H2O, and we've got that in equation here.

So you've got hydrogen ions plus the hydroxide ions forming water.

Now look at the state symbol there after water, it's liquid, not aqueous.

And that's because we are not, we've not got water dissolved in water, we've got liquid water.

So this is when the concentration of hydrogen ions equals the concentration of hydroxide ions.

We'd end up with a neutral solution because all of the hydrogen ions and hydroxide ions have combined to form water.

So here's a question based on that learning.

A solution with a pH of exactly seven is what? So pause the video here, answer the question, and I'll see you when you're finished.

Welcome back, so that was a good question there based on key stage three, and that's a neutral solution.

So well done if you got that correct.

Here's a follow-up question.

Which of the following is the correct equation for neutralisation? And remember, look carefully at those state symbols when you are answering the question.

So pause the video here, answer the question, and I'll see you when you're finished.

Welcome back, so hopefully you've recognised that the correct answer is C.

If you haven't got C, look very carefully at those state symbols and just make sure you've got those correct.

So well done if you've got that correct.

Now we're going to deal with logarithmic scales, and these are a little bit complex, but we will talk through it.

So logarithmic scales simplify how large changes in measurement are represented.

A good example of this is the Richter scale, which is logarithmic.

And this is what you'll hear on the news if you hear about earthquakes that have happened.

So earthquakes are measured on the Richter scale, and that is a logarithmic scale.

Now if you look at that graph, can you see, we might be going up by one number.

So you might have an earthquake of a magnitude or a size of six is a strong earthquake, but with a magnitude of seven, you can see is major and look at the difference between a six and a seven.

There's a huge jump in the magnitude or the size of the earthquake when you go up by one number.

So a small change in value on the logarithmic scale represents a far larger change in whatever it's measuring.

And in this case, it's measuring earthquakes.

Now the pH scale is also a logarithmic scale.

So when you go up by one, you are increasing it by a larger amount than it appears.

Remember, pH is a measure of the hydrogen ion concentration in a solution.

And the logarithmic pH scale represents changes in hydrogen ion concentration.

So we did about that earlier.

So let's deal with a few hydrogen ion concentrations then.

So if you have a solution with 0.

1 moles per decimeter cubed of a hydrogen ion solution, we can also write that as one times 10 to the minus one moles per decimeter cubed.

So what we've done is we've converted 0.

1 into standard form there.

So if you want to pause the video and just have a look at how that's happened, you can do that here.

Now this would give us a pH of one.

So see we've taken that part of the standard form there to indicate that it's a pH of one.

Now, if the hydrogen ion concentration has a value of 0.

0000001 moles per decimeter cubed, then that's the same as saying one times 10 to the minus seven moles per decimeter cubed.

So if you look at that minus seven, that tells us it's a pH of seven, and then we're going to go up to the other end of the scale.

So if we have a hydrogen ion concentration of 0.

0000000000001 or one times 10 to the minus 13 moles per decimeter cubed, then that would give us a pH of 13.

So you can see how the relationship between the concentration of hydrogen ions, remember, not the concentration of the whole solution, but the concentration of hydrogen ions is related to the pH of the solution.

So one times 10 to the minus one, one times 10 to the minus seven, and one times 10 to the minus 13 relating to the pH scale there.

Again, you may want to pause the video here, just absorb that for a moment before we move on.

So a value change of one on the pH scale represents a 10 times change in the concentration of hydrogen ions.

Okay, and we're going to have a look at that.

So if we go from a pH three to a pH two, a pH two is 10 times more concentrated in hydrogen ions than a solution of pH three.

So we've increased by 10 times.

A solution of 14, pH 14 is 100 times less concentrated than a solution of pH 12.

So we've gone down two here.

So we've got two values of pH, so therefore it's 100 times difference.

Think about what it might be if it was a three pH jump.

Okay, here's a question based on that learning and this time we've got true or false.

So the pH scale goes up and down in factors of 14.

Is this true or false? And then justify your answer using the statements below.

Pause the video here, answer the question, and I'll see you when you're finished.

Welcome back, so let's go through that answer.

So that is false, and that's because a solution of pH five is 10 times more concentrated than a solution of pH six, not 14 times.

So remember about factors from your maths lessons.

Now we're going to look at the relationship between pH and concentration.

And this can be shown as follows.

So if a solution of 0.

5 moles per decimeter cubed of hydrochloric acid is a pH two, a solution of five moles per decimeter cubed would be a pH one.

And this because it's 10 is because it's 10 times more concentrated than the 0.

5 moles.

So do you see if we take 0.

5 and we multiply it by 10, we end up with 5.

0.

So here's the example on the number line.

If a 0.

1 mol per deter cubed solution is pH six, a solution of the same substance with a pH of four would be 10 moles per decimeter cubed.

In other words, 100 times more concentrated.

So 0.

1 multiplied by 100 is 10.

That takes us from pH six to pH four.

So again, you may wish to just pause the video here and just absorb that information before you move on.

So let's go through a few calculations then based on that learning.

So an acidic solution called A, has a concentration of 0.

05 moles per decimeter cubed and a pH of four.

What is the concentration and pH of solution B, which is 10 times more acidic in terms of hydrogen ions? So remember, what we need to do first of all, is multiply that concentration by 10 because it's 10 times more acidic and that gives us a value of 0.

5 moles per decimeter cubed.

And this means that we've moved up one value on the pH scale.

And remember it's a higher concentration of hydrogen ion, so we've gone down in the pH.

From pH four to pH three.

Now you are going to do one based on what I've just done.

So using my model as an example, answer this question.

An acidic solution has a concentration of 0.

0025 moles per decimeter cubed and a pH of five.

What is the concentration and pH of a different solution, which is 100 times more acidic? So it's got 100 times more hydrogen ions.

So pause the video here, use my model as an example, and I'll see you when you're finished.

Welcome back, so here's the answer to that question.

So following my modelling, we've done 0.

0025 moles per decimeter cubed, multiplied by 100 this time, which gives us the value of 0.

25 moles per decimeter cubed.

And this time we've moved up two on the pH scale because it's by 100 instead of by 10.

So we've gone from pH five up to pH three.

'Cause remember the higher the hydrogen ion concentration, the lower the pH, so we need to go down the scale.

So well done if you've got that correct.

So now we are going to go through this question.

So state whether the following solutions are acidic, neutral or alkaline, and we've got a whole series here of solutions that you're going to have a look through.

So pause the video here, state whether they're acidic, neutral or alkaline.

Welcome back, so let's go through those answers.

So a solution of pH five is acidic, a solution of pH eight is alkaline, solution of pH seven is neutral, pH 6.

5 acidic, pH three acidic, pH 10 alkaline, and pH 7.

5 is also alkaline.

So well done if you've got that correct.

Moving on to question two, what I'd like you to do is read through Alex's and Jacob's ideas here.

I want you to see if those ideas are correct.

If they're incorrect, you need to amend them.

So pause the video here, read through those statements, decide if they're correct or not, and if they're incorrect, you need to make them correct.

So I'll see you when you're finished.

Welcome back, so let's go through those answers then.

So a solution with a pH of two has more hydrogen ions than a solution of pH of seven.

That is correct.

So the more acidic it is, the more hydrogen ions are present.

Solution A has a pH of three, solution B is 100 times more concentrated.

So solution B has a pH of two that is incorrect, and we're going to correct that in a moment.

Next one, the pH scale goes from one to 14, that's also incorrect.

And then the pH scale is logarithmic, and that is correct.

So let's go through and correct those incorrect answers.

So remember a difference of pH of one, so if it's going from pH three to pH two, that's 10 times more concentrated.

So if it's 100 times more concentrated, we must be going down to pH one.

And then remember the pH scale goes from zero to 14, not one to 14.

So well done if you've got that correct.

Now we've got question three.

You're going to write down the pH of the following solutions.

So pause the video here, answer the questions, and I'll see you when you're finished.

Welcome back, so that one was a little bit more challenging.

You really had to think about it.

It does help sometimes if you've got a number line in front of you that can make it easier.

So if you did struggle a little bit, try with a number line next time.

So let's go through those answers then.

So solution A has a pH of three, and solution B is 10 times more concentrated.

It's got 10 times more hydrogen ions.

What is the pH of solution B? So it's got more hydrogen ions, that means we're going to lower the pH and it's 10 times more concentrated.

So that means we must be going down to a pH of two.

Solution C is neutral.

What is the pH of solution C? That's a pH of seven for neutral solutions at GCSE.

Solution D has a pH of eight.

And solution E is 1,000 times fewer hydrogen ions.

So if there are fewer hydrogen ions, that means the pH must be increasing, remember, and it's 1,000 times, not 10 or 100, but 1,000.

So we are moving by three pH levels.

So that gives us a pH of 11.

So 8, 9, 10, 11, moving up three.

Solution F has a pH of seven, and solution G has 100 times more hydrogen ions, so we're going down the scale.

What is the pH of solution G? So we've got 100 times more hydrogen ions, so we are going down by two.

So that should take us to pH five.

So well done if you've got those correct.

We're now moving on to question four, and we're going to complete the table below.

So we've got the pH values with the concentration for the different solutions.

So what you need to do, if you take hydrochloric acid, you know that hydrochloric acid of one mol per decimeter cubed has got a pH of one.

So you need to look at what's happened to the concentration.

We've gone down from one mol per decimeter cubed down to 0.

1.

So think about what impact that would have on the pH, what factor change has happened, and do the same with ammonia and ethanoic acid.

So pause the video here and I'll see you when you're finished.

Welcome back, so let's go through those answers then.

You really need to think about the factor change here.

So in the first example, hydrochloric acid, we are going from one mol per decimeter cubed to 0.

1 mol.

So that's a factor of 10.

We've gone down by a factor of 10.

Remember, if we decrease the concentration of hydrogen ions, then that means we increase the pH.

And if it's a factor of 10, we increase by one pH.

So we go from one up to two.

For ammonia, you've been given the pH 10 and the concentration as 0.

04 moles per decimeter cubed.

So to get from 0.

04 up to four, we've increased by a factor of 100.

So that must mean there's a two pH difference and we're increasing the concentration of hydrogen ions, remember? So we must be going down the pH scale.

So down the pH scale by two is a pH of eight.

And then for the ethanoic acid, we are going from pH five to pH three.

So we're going down the scale by two pHs.

So that's a factor of 100 change.

So that means we're going to 0.

05.

So well done if you got that correct.

Now we're moving on to part two of the lesson, strong and weak acids.

So in chemistry, the term strong and weak are used specifically to describe acids and alkalis.

And a strong acid is one where all of the hydrogen ions have dissociated.

So they're forming these hydrogen ions in aqueous solution.

So if you go back to our pen example, imagine you've got a pot with six pens in, a strong acid, all of those lids, all of the hydrogen ions will be dissociated.

So all of the lids on those pens will have come off.

So here are the three main strong acids that you need to know about.

Hydrochloric acid, nitric acid, and sulfuric acid.

We've got the ionic equations there showing the dissociation of the hydrogen ions.

And all of these acids have a pH between zero and one.

So here's a question based on that learning.

Which of the following are strong laboratory acid? So pause the video here, answer the question, and I'll see you when you're finished.

Welcome back, so the answer to that question is nitric and hydrochloric.

So well done if you've got that correct.

We now have a true or false question.

So sulfuric acid, fully dissociates to give, and you've got an equation there.

So is that true or false? And then justify your answer using the statements below.

So pause the video here, answer the question, and I'll see you when you're finished.

Welcome back, so the answer's false, and that's because the equation is actually wrong.

It fully dissociates to release both hydrogen ions.

So in the example above only one hydrogen ion is dissociated, but because this is a strong acid, all of the hydrogen ions will dissociate.

So a weak acid is an acid which only partially dissociates to form those hydrogen ions in the aqueous solution.

So if you go back to the pen example, imagine you've got that pot of six pens.

If it's a strong acid, then all of the lids are off, all of the hydrogen ions are removed.

But if it's a weak acid, then only one or two of those will be removed.

A very few hydrogen ions have dissociated, but also this dissociation is reversible.

So if we go back to that pen example, imagine there is one pen lid off and all the rest of the pen lids are on.

It's a dynamic equilibrium.

So that means imagine you are removing a pen lid from one of the other pens at the same time as returning the pen lid to the one that had the lid removed.

So it's a dynamic process.

So we can show that with an example of ethanoic acid, which is found in vinegar, and only 1% of those acid particles are ionised.

And here's the equation for that, and you can see that it's a reversible reaction or reversible dissociation.

And also you'll notice even though there are four hydrogens present, only that end hydrogen there has dissociated and only 1% of them will have done on those particles inside that solution.

Here are some other examples of weak acids.

So you've got phosphoric acid, methanoic and citric acid, and then we've got an example of a dissociation of those acids.

So remember, only a small percentage of the hydrogen ions are dissociated in these particular acids.

So here's a question based on that learning.

So which of the following equations shows the partial dissociation of ethanoic acid? So pause the video here, answer the question, and I'll see you when you're finished.

Welcome back, so the answer to that question is C.

So well done if you've got that correct.

Now we can compare strong and weak acids, and we're going to do that directly in this table so you can see the difference between the two.

So first of all, we can talk about pH.

So strong acids, remember, have got a pH between zero and one where weak acids have a pH between two and six.

Remembering that seven is neutral.

In strong acids, the hydrogen ions are fully dissociated, whereas in weak acids, they're only partially dissociated.

And we can show that in an image here.

So we've got 0.

5 moles per decimeter cubed of nitric acid in an image here, and 0.

5 moles per decimeter cubed of carbonic acid, which is an example of a weak acid in an image here.

Now look very carefully at those two different images.

So under the strong acid, we can see all of the hydrogen ions have been dissociated.

And in the second image for carbonic acid, we can see they haven't all dissociated, only three of have, but we know these have got the same concentration because they've got equal numbers of acid particles in the same volume of water.

So they've got the same concentration, but the strong acids have all dissociated, whereas in the weak acid only some of the hydrogen ions have dissociated.

And then we've got a sample equation.

So in this one, remember we've got the single arrow, and then with weak acids, you've got the double-headed arrow to show it's a reversible reaction.

So that's a really good summary of the difference between a strong acid and a weak acid.

So it might be worth pausing the video here and just absorbing that information.

Here's a question based on that learning.

Strong and weak acids of the same concentration have the same pH.

Is that true or false? Pause the video here and answer the question.

Welcome back, so that is false.

Remember, pH is related to the presence of hydrogen ions.

And weak acids have a lower level of dissociation, so they'll have fewer hydrogen ions in the same concentration.

So as strong acids fully dissociate, they release all of their hydrogen ions, and so have a lower pH than weak acids.

So well done if you got that correct.

So both strong and weak acids can be neutralised using titration.

And here an alkali can be added to an acid in a controlled manner until the acid is neutralised.

So you can see an example happening there in the image, and we use a burette as the piece of equipment to do this.

And an indicator is added to the acid, and this changes colour at the point of neutralisation.

And an indicator used in titration is an end-point in indicator.

So it has a single changing colour that happens when the solution becomes neutral.

So during titration, the pH of the solution can be measured using a pH metre.

And we can draw a graph of the data that you would get from this experiment.

So you can see at the start we've got a pH of two.

So that tells us we've started with an acid in our conical flask, and we are slowly adding alkali to it.

And as we add the alkali, the pH is changing.

So at the start, we've got an initial pH there of just under two, and the final pH there of approximately 14.

So it's gone from acid through neutral to alkaline.

Here, this is the point at which neutralisation has occurred.

We get a sudden change in pH, and what we can do is we can go down to the X-axis and we can work out what volume of alkaline neutralised the acid.

'Cause that's the point at which neutralisation has happened is where we get that sudden increase in pH.

We're now going to do task B based on that learning.

So complete the table below to identify which acids are strong and which are weak given approximate pH range for each.

So pause the video here, answer the question, and I'll see you when you're finished.

Welcome back, so let's go through that answer.

So methanoic acid is an example of a weak acid.

Hydrochloric is strong, sulfuric is strong, carbonic weak, phosphoric weak, and nitric is strong.

Now, sometimes this can be a little bit confusing and difficult to remember, but think about the strong acids.

So you've learned about three strong acids.

If other acids are introduced to you, they're likely to be weak acids.

And then the approximate pH range for those weak acids is two to six, and for strong acids is zero to one.

And we can fill in the rest of the table using that information.

Well done if you got that correct.

Moving on to question two.

Methanoic acid is a weak acid.

Complete the following questions about methanoic acid.

What is a weak acid? Give the equation for methanoic acid dissolved in water, and how can an acid be both weak and concentrated? So remember, we need to differentiate between the terms weak and strong and dilute and concentrated.

So pause the video here, answer the question, and I'll see you when you're finished.

Welcome back, so let's go through that.

So a weak acid is an acid which is partially dissociated or ionised to form hydrogen ions in aqueous solution.

The equation for methanoic acid dissolved in water is there.

And remember, you need that reversible sign.

It's a reversible dissociation.

And then how can an acid both be weak and concentrated? It can be weak because it's partially dissociated in aqueous solution, and it can be concentrated because there's a large number of acid particles per unit volume of water.

And it's, remember if you are struggling with that part C, go back to the pen example and even physically have those pens in front of you in a beaker.

And use that as an example and think about what's the difference between a dilute solution and a concentrated solution, and what's the difference between a weak acid and a strong acid? Using those pens to help you.

Question three, add the following labels to the titration curve below.

So pause the video here, sketch the graph, and add those labels.

Welcome back, so here are the labels for that diagram.

So you've got pH on the Y-axis, volume of alkali added in centimetre cubed on the X-axis, the initial pH here at the very beginning, the final pH here at the very end.

The pH of neutralisation is where we get that sudden change in pH there, and then the volume of alkali added to neutralise the acid, remember, we need to go down to the Y-axis to discover what that volume is.

So well done if you've got that correct.

Here is a summary of today's lesson.

pH is a logarithmic scale as hydrogen ion concentration increases by a factor of 10, the pH decreases by one.

For a given concentration, a stronger acid contains more hydrogen ions.

For a given concentration, a stronger acid has a lower pH.

A strong acid is completely ionised in aqueous solution.

A weak acid only partially ionises.

Titration curves show how the pH changes throughout a neutralisation reaction.

So thank you very much for joining me for today's lesson.

Some of that work was a little challenging, but hopefully I helped you through that okay.