Effect of pH on solubility

Summary

This video explores how pH affects the solubility of sparingly soluble salts, focusing on the impact of acid-base reactions. It explains that salts like carbonates and hydroxides react with pH changes, using calcium hydroxide as an example. Adding hydrochloric acid to a saturated solution increases solubility by reducing hydroxide concentration, prompting more salt to dissolve. The video also discusses an exam question on iron hydroxide solubility at low pH, showing how increased hydronium ions react with hydroxide ions, shifting equilibrium and enhancing solubility.

Takeaways

• 🔬 The video discusses the impact of pH on the solubility of sparingly soluble salts, a topic often considered by the New Zealand National Certificate of Educational Achievement (NCEA).
• 🧪 The solubility of a salt is affected by pH if it undergoes an acid-base reaction, which is typical for carbonates or hydroxides, except for those of sodium, potassium, or ammonium.
• 📚 Calcium hydroxide is used as an example to illustrate how adding hydrochloric acid can increase its solubility by forming water and reducing hydroxide ion concentration.
• ⚖️ The equilibrium between calcium hydroxide and its ions is disturbed by the addition of hydrogen ions, leading to more calcium hydroxide dissolving to restore balance.
• 🌡️ Changes in pH affect the autoionization of water, which influences the solubility of sparingly soluble salts like calcium hydroxide.
• 📝 The script emphasizes the importance of including both the dissolution of the salt and the reaction with hydrogen ions in the explanation.
• 📚 An NCEA exam question is analyzed, focusing on the solubility of iron hydroxide in water at a pH less than 4, where the presence of excess hydronium ions increases solubility.
• 🧐 The video provides a methodological approach to explaining the effect of pH on solubility, using equations and chemical concepts.
• 🔍 The video concludes with a teaser for the next topic, which will cover the effect of complex ions on solubility.
• 🎓 The script is educational, aiming to help viewers understand and answer exam questions related to solubility and pH.

Q & A

What is the main focus of the video?

The video focuses on the effect of pH on the solubility of sparingly soluble salts, particularly when the salt is a carbonate or hydroxide.

Why is the pH factor considered important in solubility discussions?

pH is important because it can affect the solubility of certain salts by undergoing acid-base reactions, which in turn can either add or remove hydroxide ions in solution.

Which type of salts are typically affected by pH changes in their solubility?

Salts that are carbonates or hydroxides are typically affected by pH changes in their solubility, unless they are sodium, potassium, or ammonium salts.

What happens when hydrochloric acid is added to a saturated solution of calcium hydroxide?

When hydrochloric acid is added, the hydrogen ions react with hydroxide ions, forming water and reducing the hydroxide concentration, which causes more calcium hydroxide to dissolve, thus increasing its solubility.

How does the autoionization of water relate to the solubility of sparingly soluble salts?

The autoionization of water, which is the basis of the pH scale, affects solubility by changing the concentration of hydroxide ions in solution, which can either increase or decrease the solubility of certain salts.

What is the effect of lowering the pH on the solubility of iron hydroxide?

Lowering the pH to less than 4 results in an excess of hydronium ions, which react with hydroxide ions from the dissolving iron hydroxide. This reaction shifts the equilibrium to favor the forward reaction, increasing the solubility of iron hydroxide.

What is the significance of including equations in the explanation of pH effects on solubility?

Including equations in the explanation provides a clear scientific basis for the observed effects of pH on solubility, showing the chemical reactions that occur and how they lead to changes in solubility.

Why is it important to consider both the forward and reverse reactions when discussing pH and solubility?

Considering both the forward and reverse reactions is important because it shows how the equilibrium shifts in response to changes in pH, which directly affects the solubility of the salt.

What is the role of hydronium ions in the solubility of sparingly soluble salts at low pH?

At low pH, hydronium ions react with hydroxide ions, removing them from the equilibrium and causing the equilibrium to shift to the right, which increases the solubility of the salt.

How does the video script guide viewers in understanding the relationship between pH and solubility?

The video script guides viewers by providing clear examples, such as calcium hydroxide and iron hydroxide, and explaining the chemical reactions involved, which help to illustrate the relationship between pH and solubility.

What is the purpose of the video series that this script is a part of?

The purpose of the video series is to explore various factors that affect the solubility of sparingly soluble salts, with a focus on providing explanations supported by chemical equations and real-world examples.

Outlines

00:00

🧪 Effect of pH on Solubility of Sparingly Soluble Salts

This paragraph discusses how pH affects the solubility of sparingly soluble salts, particularly focusing on carbonates and hydroxides. It explains that these salts are typically insoluble unless they are sodium, potassium, or ammonium salts, which produce hydroxide ions in solution. The presence of hydroxide ions allows pH changes to influence solubility. The example of calcium hydroxide is used to illustrate how adding hydrochloric acid can increase solubility by reacting with hydroxide ions to form water, thus driving more calcium hydroxide to dissolve. The paragraph also touches on the autoionization of water and how changes in pH can affect the equilibrium of hydroxide ions, leading to increased solubility.

Keywords

💡pH

pH is a measure of the acidity or alkalinity of a solution. It is defined as the negative logarithm (base 10) of the concentration of hydrogen ions (H+) in a solution. In the context of the video, pH is crucial because it affects the solubility of sparingly soluble salts. The video explains how changes in pH can lead to acid-base reactions that alter the solubility of certain salts, such as calcium hydroxide and iron hydroxide.

💡Sparingly soluble salt

A sparingly soluble salt is one that does not dissolve completely in water and only partially dissociates into its constituent ions. The video focuses on how the solubility of such salts can be influenced by pH changes. For instance, calcium hydroxide is described as sparingly soluble, and its solubility can be increased by adding hydrochloric acid, which shifts the equilibrium to dissolve more salt.

💡Acid-base reaction

An acid-base reaction is a chemical process that occurs between an acid and a base, resulting in the formation of a salt and water. The video script highlights that for pH to affect the solubility of a sparingly soluble salt, the salt must be able to undergo an acid-base reaction. This is exemplified by the reaction between hydrochloric acid and calcium hydroxide, where the acid reacts with the base to form water, thus affecting solubility.

💡Hydroxide ions

Hydroxide ions (OH-) are negatively charged ions produced when a base dissolves in water. The video explains that the presence of hydroxide ions in a solution is significant because they can react with hydrogen ions (from acids), which in turn affects the solubility of salts like calcium hydroxide. The script uses the example of adding hydrochloric acid to a solution of calcium hydroxide, where the hydrogen ions react with hydroxide ions, leading to an increase in solubility.

💡Equilibrium

Equilibrium in chemistry refers to a state in which the rates of the forward and reverse reactions are equal, resulting in no net change in the concentrations of reactants and products. The video discusses the equilibrium between calcium hydroxide and its ions in water. When hydrochloric acid is added, it disrupts this equilibrium by reacting with hydroxide ions, causing more calcium hydroxide to dissolve to re-establish equilibrium.

💡Autoionization of water

The autoionization of water is the process by which water molecules dissociate into hydrogen ions (H+) and hydroxide ions (OH-). This process establishes the water's ion product (Kw) and is fundamental to the pH scale. The video script mentions that increasing the H+ concentration (by adding an acid) will decrease the OH- concentration, which can affect the solubility of sparingly soluble salts like calcium hydroxide.

💡Hydrochloric acid

Hydrochloric acid (HCl) is a strong acid that completely dissociates into hydrogen ions (H+) and chloride ions (Cl-) in water. In the video, hydrochloric acid is used to demonstrate how the addition of a strong acid can affect the solubility of sparingly soluble salts. The script explains that the hydrogen ions from HCl react with hydroxide ions, leading to an increase in the solubility of calcium hydroxide.

💡Iron hydroxide

Iron hydroxide is a sparingly soluble compound that is mentioned in the video script in the context of an exam question. The video explains that when the pH of a solution is lowered (made more acidic), the increased concentration of hydronium ions (H3O+) reacts with hydroxide ions from the dissolution of iron hydroxide. This reaction shifts the equilibrium to favor the dissolution of more iron hydroxide, thereby increasing its solubility.

💡Hydonium ions

Hydonium ions (H3O+) are the form in which hydrogen ions exist in aqueous solutions. They are formed when water molecules autoionize. The video script uses hydronium ions to explain how a lower pH (more acidic conditions) can increase the solubility of iron hydroxide. The presence of excess hydronium ions reacts with hydroxide ions, causing the equilibrium to shift and increase the solubility of iron hydroxide.

💡Complex ions

Complex ions are formed when a central metal ion is bonded to one or more ligands, which are molecules or ions that can donate a pair of electrons. The video script hints at a future discussion on how complex ions can affect the solubility of sparingly soluble salts. While not detailed in the provided script, complex ions can stabilize certain metal ions in solution, potentially increasing their solubility.

Highlights

The video explores the effect of pH on the solubility of sparingly soluble salts, a key consideration in chemistry education.

Acid-base reactions are pivotal for pH to influence solubility, suggesting a chemical interaction mechanism.

Carbonate or hydroxide salts are typically affected by pH changes, indicating specific salt types of interest.

Sodium, potassium, or ammonium salts that produce hydroxide ions are exceptions to the solubility rule.

The presence of hydroxide ions in solution can be manipulated by pH changes, leading to solubility variations.

Calcium hydroxide is used as a case study to illustrate the solubility effects of pH adjustments.

The reaction between hydrogen ions (from strong acids like HCl) and hydroxide ions is detailed.

A decrease in hydroxide ion concentration due to reaction with hydrogen ions leads to increased solubility.

The autoionization of water and its relation to pH scale are discussed to explain solubility changes.

An increase in H+ concentration decreases OH- concentration, affecting the solubility equilibrium.

The video provides a step-by-step explanation supported by chemical equations for clarity.

Iron hydroxide is used as another example to demonstrate the effect of pH < 4 on solubility.

The reaction between hydronium ions and hydroxide ions from iron hydroxide dissolving is explained.

The presence of excess hydronium ions at low pH shifts the equilibrium, increasing solubility.

The video concludes with a teaser for the next topic on the effect of complex ions on solubility.

The video aims to educate on solubility factors, preparing viewers for practical applications or exams.

Transcripts

hi and welcome to this video where we're

going to be looking at the effect of ph

on the solubility of a sparingly soluble

salt

this is one of the scenarios that ncea

likes to consider when it's looking at

factors that affect the solubility of

sparingly soluble salts

so what we need to consider is whether

or not

the salt in question would undergo an

acid-base reaction because that's really

what's

got to happen if ph is going to have an

effect on the solubility

now how do you know well

usually if ph is going to have an effect

on it your salt is going to be either a

carbonate or a hydroxide

these are typically sparingly soluble

salts unless it's sodium or potassium or

ammonium

and they produce hydroxide ions in

solution now the presence of hydroxide

ions in solution

means that changing the ph can either

add or remove those hydroxide ions which

will change the ph

okay so let's look at some examples

or a bit more detail so let's consider

calcium hydroxide for a sec

in general we consider calcium hydroxide

to be sparingly soluble

if we add some hydrochloric acid to a

saturated solution

of calcium hydroxide

what would be happening well to think

about that we have to think about what

reactions are actually occurring

okay so the first thing to do is write

the equation

okay calcium hydroxide is an

equilibrium with calcium ions and

hydroxide ions

hopefully by now this is pretty

straightforward so

if the calcium hydroxide is in

equilibrium with calcium and hydroxide

when we add some hydrogen ions from our

hcl

remember hcl is a strong acid then

those hydrogen ions will react with the

hydroxide ions

forming water this is going to have the

effect of reducing the concentration of

hydroxide

in the first equilibrium reaction

that then is going to cause more calcium

hydroxide to dissolve to replace

the calcium are the hydroxide ions that

have been removed

which is going to have the effect of

increasing the solubility of calcium

hydroxide

the other way to consider it the second

equation down there

if we write it around the other way is

the um

is considered to be the autoionization

of water

right kw which is the basis of our ph

scale so if we

increase the h plus concentration

then that's going to have the effect of

decreasing the oh minus concentration so

the calcium hydroxide is going to try

and provide more

oh minus to replace the oh minus that's

been taken away

so either way the calcium hydroxide is

going to dissolve more

increasing the amount of hydroxide ions

and therefore increasing the solubility

of the calcium hydroxide

okay so here's an ncaa exam question

looking at the solubility of

iron hydroxide in water

in a situation where the ph has been

lowered to less than four

right so the sort of answer that we're

looking at we've got to put equations in

there

so starting off the dissolving of iron

hydroxide to make

fe3 plus and hydroxide ions and then the

reaction between hydronium and hydroxide

now whether you use h

plus or whether you use h3o plus

honestly does not matter

but you must include that second

equation

okay so the sort of explanation at a ph

of less than 4

there will be an excess of hydronium

ions present in the solution

these hydronium ions will react with the

hydroxide ions formed from the

dissolving

of iron hydroxide as the hydroxide ions

are removed from the equilibrium the

equilibrium will shift to favor the

products

or the forward reaction will be favored

whatever you like to talk about the

equilibrium will move to the right

um increasing the overall solubility of

iron hydroxide

so there you have your explanation

supported by

equations and that's the sort of thing

we're looking for

when we're talking about the effect of

ph on solubility

i hope this helps come tune in for my

last video where we talk about the

effect of complex ions on solubility

and that will be where we finish off the

solubility topic other than some exam

questions

see you there