GCSE Chemistry: Understanding pH Titration Curves for Neutralisation Reactions

Summary

This educational video explores the pH changes during acid-alkali neutralization, suitable for GCSE students. It explains the practical setup involving a conical flask and a burette for controlled addition of reactants. The video describes the ionic reaction between acids (H+ ions) and alkalis (OH- ions) forming water and a salt, which neutralizes the solution. It illustrates the concept of titration curves, showing how pH starts low with acid, rises sharply at the equivalence point, and then increases or decreases depending on whether more alkali or acid is added, respectively. The video effectively uses visual aids to demonstrate the transition from acidic to neutral to alkaline pH levels.

Takeaways

• 🧪 The video discusses the concept of pH changes during the neutralization process when acids and alkalis are mixed.
• 🔬 The practical setup involves using a conical flask and a burette to add solutions drop by drop, allowing for precise pH measurement.
• 🌈 The use of an indicator is mentioned, which changes color at the point of neutralization, marking the end of a titration.
• 📈 The video explains that the pH curve will show a gradual change as acid or alkali is added, with a steep rise or drop at the equivalence point.
• 🌀 The reaction between acids (supplying H+ ions) and alkalis (supplying OH- ions) produces water, which is neutral and has a pH of 7.
• 📉 Starting with an acid, the pH begins low and increases as alkali is added, with a sharp rise at the equivalence point where H+ and OH- ions neutralize each other.
• 📈 Starting with an alkali, the pH begins high and decreases as acid is added, with a sharp drop at the equivalence point where OH- and H+ ions neutralize each other.
• 🔄 The video emphasizes that water can dissociate into H+ and OH- ions, but this is a minor effect and doesn't significantly impact the pH beyond the equivalence point.
• 🏁 The equivalence point is a critical moment in the neutralization reaction where the concentrations of H+ and OH- ions are equal, resulting in a neutral pH.
• 📊 The final pH of the solution depends on which reactant is in excess after the equivalence point: if more acid is added, the pH will be acidic; if more alkali is added, the pH will be alkaline.

Q & A

What is the main focus of the video?

The main focus of the video is to explain how pH changes during the neutralization process when acids and alkalis are mixed.

What is the practical setup used in the experiment described in the video?

The practical setup includes a conical flask or Erlenmeyer flask containing a known volume and concentration of acid or alkali, and a burette used to add the other reactant drop by drop.

Why is the burette important in this experiment?

The burette is important because it allows for the controlled addition of the reactant, even drop by drop, which is crucial for accurately tracking the pH changes during the neutralization process.

What is the purpose of using an indicator in the experiment?

An indicator is used to visually detect the end point of the neutralization reaction by changing color when the pH changes significantly.

What is the ionic equation for the reaction between a strong acid and a strong alkali?

The ionic equation is H+ + OH- → H2O, where H+ ions from the acid and OH- ions from the alkali react to form water.

Why does the pH change during the neutralization process?

The pH changes because the OH- ions from the alkali neutralize the H+ ions from the acid, forming water, which has a neutral pH, thus reducing the concentration of H+ ions in the solution.

What is the equivalence point in the context of this experiment?

The equivalence point is the point during the neutralization process where the concentrations of H+ and OH- ions are equal, resulting in a neutral pH of 7.

How does the pH curve change when starting with an acid in the conical flask?

The pH starts low, increases gradually as alkali is added, rises steeply at the equivalence point, and then continues to rise as more alkali is added, resulting in an alkaline pH.

How does the pH curve change when starting with an alkali in the conical flask?

The pH starts high, decreases as acid is added, drops steeply at the equivalence point, and then continues to drop as more acid is added, resulting in an acidic pH.

What is the significance of the steep rise or drop in the pH curve at the equivalence point?

The steep rise or drop in the pH curve at the equivalence point signifies that all the H+ ions have been neutralized by OH- ions (or vice versa), and any further addition of the reactant will start to dominate the pH of the solution.

Outlines

00:00

🔬 Introduction to Neutralization and pH Changes

This video introduces the concept of how pH changes during the neutralization process when acids and alkalis are mixed. Aimed at GCSE level, it explains the practical setup involving a conical flask and a burette to control the addition of acid or alkali. The video discusses the use of an indicator for titration and the theoretical placement of a pH probe to monitor changes. It sets the stage for understanding the reaction between a strong alkali, typically a metal hydroxide, and an acid to form a salt and water. The ionic equation is highlighted, showing the supply of H+ ions by acids and OH- ions by alkalis, leading to the formation of water, which has a neutral pH. The expectation is that the pH will change as OH- and H+ ions react to form water, and the video aims to illustrate this through a visual representation of the pH curve.

05:01

📈 Understanding the pH Curve and Equivalence Point

The second paragraph delves into the specifics of the pH curve during an acid-alkali reaction. It explains the concept of the equivalence point, where the pH rises sharply as H+ ions are neutralized by OH- ions, leading to the formation of water and a neutral pH of 7. The video clarifies that while water can dissociate into H+ and OH- ions, the concentrations of these ions will always be equal, ensuring that the pH does not become acidic. As more alkali is added beyond the equivalence point, the pH continues to rise due to the excess of OH- ions, making the solution alkaline. The paragraph also considers the scenario where the experiment begins with an alkali in the flask, and how the pH curve would reflect the neutralization process in reverse, ending with an acidic pH as more acid is added. The video concludes by emphasizing the definition of a neutral solution as having equal concentrations of H+ and OH- ions.

Keywords

💡pH

pH is a scale used to specify the acidity or basicity of a solution. It is measured on a scale from 0 to 14, with 7 being neutral. In the video, the concept of pH is central to understanding how the acidity or alkalinity of a solution changes during a neutralization reaction. The video explains how starting with either an acid or a base, the addition of the opposite (an alkali or an acid, respectively) will gradually shift the pH towards neutrality, indicated by a pH of 7. The video uses the example of adding alkali to an acidic solution, where the pH starts low and increases as the OH- ions neutralize the H+ ions, forming water.

💡Neutralization

Neutralization is a chemical reaction in which an acid and a base react to form water and a salt. This process is the main focus of the video, where the addition of an alkali to an acid or vice versa is demonstrated. The video script describes how the pH changes as the H+ ions from the acid and the OH- ions from the base react to form water (H2O), which has a neutral pH. The concept is illustrated through the use of a conical flask and a burette to add the reactants in a controlled manner.

💡Acid

An acid is a substance that donates hydrogen ions (H+) when dissolved in water. In the video, the reaction of an acid with a base is discussed, where the acid provides the H+ ions that react with OH- ions from the base. The video explains that the presence of H+ ions in a solution results in a low pH, indicating acidity. The practical setup involves adding an acid to a conical flask and then neutralizing it with a base from a burette.

💡Alkali

An alkali is a substance that donates hydroxide ions (OH-) when dissolved in water. The video script describes how a strong alkali, typically a metal hydroxide, reacts with an acid to form a salt and water. Alkalis are essential in the neutralization process, as they provide the OH- ions needed to neutralize the H+ ions from the acid. The video uses the example of adding an alkali to an acidic solution to demonstrate how the pH changes during the reaction.

💡Conical Flask

A conical flask is a type of laboratory glassware with a narrow neck and a conical shape, often used for mixing and reacting chemicals. In the video, a conical flask is used to contain the acid or alkali solution during the neutralization experiment. The script mentions how the flask is used to hold a known volume and concentration of the reactant solution before the other reactant is added from the burette.

💡Burette

A burette is a piece of laboratory equipment used to measure and dispense precise volumes of liquid. It has a tap that allows for the controlled addition of liquid, drop by drop if necessary. In the video, a burette is used to add the second reactant (either acid or alkali) to the solution in the conical flask. The script highlights the importance of the burette's tap in controlling the flow of the liquid, which is crucial for accurately observing the pH changes during the neutralization reaction.

💡Indicator

An indicator is a substance that changes color in response to changes in pH, often used in acid-base titrations to determine the endpoint of the reaction. The video script mentions the use of an indicator in the experiment, where the color change of the indicator signals the equivalence point, indicating that the reaction is complete. The indicator is used to visually confirm when the pH has reached a neutral state during the neutralization process.

💡Titration

Titration is a laboratory method of determining the concentration of an unknown solution by gradually adding a solution of known concentration until the reaction is complete. The video script refers to titration as the process of adding an alkali to an acid or vice versa, with the endpoint indicated by a color change in the indicator. The video explains that the titration curve, or the plot of pH change during the titration, can be used to visualize the progress of the neutralization reaction.

💡Equivalence Point

The equivalence point in a titration is the point at which the moles of acid and base are exactly equal, resulting in a neutral solution. The video script describes the equivalence point as a sharp rise or drop in the pH curve, indicating that all the H+ ions have been neutralized by the OH- ions, resulting in a pH of 7. The script explains that before reaching the equivalence point, the solution is either acidic or basic, depending on which reactant is in excess, and after the point, the solution becomes increasingly alkaline as more base is added.

💡Ionic Equation

An ionic equation is a chemical equation that shows the ions involved in a reaction, rather than the complete neutral formulas of the reactants. The video script mentions the ionic equation for the reaction between an acid and a base, where H+ ions from the acid react with OH- ions from the base to form water (H2O). This equation is important for understanding the neutralization reaction at a molecular level, as it shows the direct interaction between the ions that leads to the formation of water and the change in pH.

Highlights

Introduction to the concept of pH changes during neutralization reactions.

Explanation of the practical setup involving a conical flask and a burette.

Description of how the burette's tap allows for controlled addition of liquids.

Mention of using an indicator to signal the end of a titration.

Discussion of the theoretical pH probe to monitor pH changes in real-time.

Explanation of the chemical reaction between acids and alkalis to form salt and water.

Clarification that H+ ions from acids make the solution acidic and lower the pH.

Detail on how OH- ions from alkalis neutralize H+ ions to form water with a neutral pH.

Illustration of the initial low pH when starting with an acid in the conical flask.

Description of the gradual pH increase as alkali is added and H+ ions are neutralized.

Identification of the equivalence point where pH rises sharply as H+ ions are depleted.

Explanation of the continuation of pH rise after the equivalence point due to excess OH- ions.

Discussion of the autoionization of water and its effect on pH at the equivalence point.

Prediction of the pH curve's shape when starting with an alkali in the conical flask.

Description of the pH drop as H+ ions are added to an initially alkaline solution.

Final explanation of the pH curve's shape, starting alkaline, reaching neutral at the equivalence point, and then becoming acidic.

Transcripts

welcome to this video looking at how pH

changes during neutralization we're

looking at mixing acids and alkalis and

having a look at how the pH changes when

that happens and this is really aimed at

GCSE level so let's have a think about

what we're doing here practically so the

practical setup that we're using here is

that we've got this item here on the top

here could it be red and we've got down

here a conical flask or sometimes known

as an Erlenmeyer flask and the idea is

that we're going to put some of our acid

or our alkali into the conical flask and

known volume and concentration and then

we're going to add a the other so if

you've got the alkali in here it's the

acid in there or vice-versa from the

burette and the beautiful thing about a

burette here is you've got this tap that

allows you to control the flow of the

liquid out of the burette it just allows

you to add this even drop by drop so

that you can follow the pH very easily

now there is one way of doing this where

you actually add an indicator and I

think in this photo there's an indicator

here and then when the indicator changes

color you stop that's called a titration

and so sometimes these curves are called

titration curves but what we're

basically doing in this experiment is

that we're imagining that inside here

were able to put some kind of pH probe

in there and what we're then able to do

is as we add the acid or alkali from the

burette we're able to look and see how

does the pH change over the course of

this experiment now why should the pH

change let's think about the reaction

taking place so if we've got here an

acid reacting with an alkali alkali

typical strong alkali will be a metal

hydroxide and that's going to react to

make a salt or a metal salt and water

and it's important that you know that

then the ionic equation for this acids

always supply H+ ions metal hydroxides

supply o h minus ions and

the only product that actually forms

here by direct new bonds forming is

actually water so this is the reaction

that's actually taking place and h+ ions

make the solution acidic and so you'd

expect there to give a low ph h minus

ions are going to neutralize those h+

ions and remove them from the solution

and produce water which has a neutral pH

m and therefore we'd expect that the ph

will change as the o h and h minus and h

plus ions react together to make water

well let's see how that actually happens

so i've put the reaction up here and

what i've gone for here is to try to

picture what the ph curve looks like

down here and in this box here i'm going

to show you what's actually happening in

the solution so you can imagine this

isn't in the contents of my conical

flask so I'm gonna start with acid in

the conical flask so I'm gonna have H+

ions and I've put it in red here because

acids give red colors with universal

indicator so we've got all these h+ ions

and as you might expect the ph is going

to start pretty low and when we we do

this so i've put it down there at pH 1

now as you adding in more of the alkali

you're gonna add some Oh H minus ions in

here and what's going to happen is these

are H minus ions are going to react with

the H+ and they're going to together

form water so we've now got these H+ now

H minus and I'll just sort of scrub them

out as best I can

just to show you though they're actually

gone we don't have them anymore we've

just got h2o s and as a result we've

still got we've got still got an excess

of h+ ions so we're still expecting a pH

below 7 but we haven't got as higher

concentration of h+ ions and so you'd

expect the ph is going to gradually

increase as you add more alkali now

there comes a point when i'll have added

enough alkali

to get rid of all of these h+ ions so

those will have turned into h2o these

will have turned into h2o these guys

here are going to become h2o and in the

meantime i'm going to basically have

gotten rid of all of these so now my

solution contains entirely h2o molecules

and as that is the case we've got a pH

of 7 and so we get a very very sharp

rise as soon as the H+ is basically it's

starting to run out the pH Rises very

very steeply until we get to this point

here where we have what's known as the

equivalence point so this point where

the pH is rising very very steeply will

be known as the equivalence point

so before we got to equivalence here we

have more h+ ions than Oh H - and where

is at the equivalence point we've got

equal concentrations of h+ + o h-

now what interesting they said equal and

it's really delving something slightly

more complicated eh but h2o can actually

break apart into H+ no h- but firstly

very very few molecules do that and

secondly you always get one h+ + one o

h- from every h2o and therefore you're

never going to have more h+ than o h- if

you've just got h2o around now here's

what happens next as we continue adding

the alkali and we're going to have more

h- ions in the solution now there's no H

pluses for them to react with and

therefore the pH will continue to rise

because now we've got more Oh H minus

than H+ and therefore our pH is alkaline

if we just have an alkali around with

our h minus ions we have an alkaline pH

above 7 so that's the reason for the

shape of this curve now let's just think

briefly about what's going to happen if

we start with the alkaline in the

conical flask so I've just shown a

picture of it up here and what's going

to happen well at the start the pH is

going to be very high they're gonna have

a very high pH because we've got lots of

Oh H minus ions and then similarly the

same sort of story is going to happen as

I add h+ ions in here i'm expecting that

these guys are going to react together

to make water and eventually and

basically getting rid of this as a

result and eventually we're going to

have so many H pluses have been added

that all of these are H minuses are

going to become neutralized and we only

have in here water

so what does that look like in terms of

the curve well the pH is going to drop

as H+ reacts with our h- and eventually

the page is really going to drop very

steeply at which point we are again at

what we call the equivalence point we've

got equal concentrations of h+ + o h-

and then finally as we add more h+ into

the solution as we add more acid into

that conical flask

there's no h- to neutralize it and so we

expect the pH is going to drop and so

that's exactly what happens in this case

so this time we start off with a

situation where we've got a higher o h

minus concentration then h plus and so

we've got a pH in the alkaline region at

the end of the experiment we have a

higher H+ concentration then Oh H minus

and in the middle we've got equal H+ and

OH h- concentrations the definition of a

neutral solution