Why are the physical properties, such boiling point and solubility, of an alkanenitrile so similar to that of a primary alkanol with the same number of carbon atoms in the hydrocarbon chain?
Why do alkanenitriles react with so many different reactants?
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Sunday, March 22, 2020
Wednesday, March 11, 2020
Neutral pH
What is a neutral pH?
An aqueous solution is neutral when the concentration of hydrogen ions in solution is equal to the concentration of hydroxide ions in solution:
For example, an aqueous solution will be neutral if [H+(aq)] = [OH-(aq)] = 10-3 mol L-1
IF the temperature is less than 25°C, the pH will be greater than 7
Suggested Study Questions:
An aqueous solution is neutral when the concentration of hydrogen ions in solution is equal to the concentration of hydroxide ions in solution:
Neutral solution: [H+(aq)] = [OH-(aq)]
The pH of a solution is a measure of the concentration of hydrogen ions in solution:
pH = -log10[H+(aq)]
For example, an aqueous solution will be neutral if [H+(aq)] = [OH-(aq)] = 10-3 mol L-1
For this solution the pH will be: pH = -log10[H+(aq)] = -log10[10-3] = 3
For example, an aqueous solution will be neutral if [H+(aq)] = [OH-(aq)] = 10-6 mol L-1
For example, an aqueous solution will be neutral if [H+(aq)] = [OH-(aq)] = 10-6 mol L-1
For this solution the pH will be: pH = -log10[H+(aq)] = -log10[10-6] =6
"Neutral" is NOT a pH.
Why do so many (non- Chemists!) believe a solution is neutral if the pH=7 ?
An aqueous solution can have a pH of about 7 IF the temperature of the aqueous solution is about 25°C.
"Neutral" is NOT a pH.
Why do so many (non- Chemists!) believe a solution is neutral if the pH=7 ?
An aqueous solution can have a pH of about 7 IF the temperature of the aqueous solution is about 25°C.
At 25°C, a neutral aqueous solution has pH≈7
Temperature <25°C, a neutral aqueous solution has pH>7
IF the temperature is higher than 25°C, the pH will be less than 7
Temperature >25°C, a neutral aqueous solution has pH<7
The pH of a neutral aqueous solution depends on the temperature of the solution!!!
Suggested Study Questions:
- In an aqueous solution the concentration of hydrogen ions is the same as the concentration of hydroxide ions. Is this solution acidic, basic, or, neutral?
- In an aqueous solution, the concentration of hydrogen ions is 2.7 x 10-6 mol L-1 and the concentration of hydroxide ions is 2.7 x 10-6 mol L-1. Is this solution acidic, basic, or, neutral. Explain your answer.
- The concentration of hydroxide ions in a neutral aqueous solution is 5.8 x 10-6 mol L-1. What is the concentration of hydrogen ions in this solution?
- The concentration of hydrogen ions in a neutral aqueous solution is 7.9 x 10-7 mol L-1 What is the concentration of hydroxide ions in this solution?
- Calculate the pH of the solution in question 4.
- Calculate the pH of the solution in question 3.
- Calculate the pH o the solution in question 2.
- Explain how you would calculate the pH of the solution in question 1, justifying any assumptions you need to make.
- Critically evaluate this statement, "At pH 7 a solution is neutral".
- Chris the Chemist tested the pH of the water in the local pool. Monday was a hot day, about 40°C, and the pH of water was 6.82. That night the temperature dropped dramatically and the next day was a chilly 10°C. When Chris tested the pH of the pool water it was found to be 7.59. Is the pool water acidic, basic, or neutral? Justify your answer.
Saturday, March 7, 2020
Indicators for Strong Acid - Strong Base Titrations
When a strong acid is added to a strong base the products are water and a salt.
Water is neutral, that is [H+(aq)] = [OH-(aq)]
(or [H3O+(aq)] = [OH-(aq)] if you prefer)
The salt of a strong acid and base is made up of a cation that will not react with water to any appreciable extent, and an anion that will not react with water to any appreciable extent, so this salt does not affect the [H+(aq)] and [OH-(aq)] in the water, that is, the aqueous solution remains neutral.
At 25°C, Kw = [H+(aq)] × [OH-(aq)] = 10-14
Since [H+(aq)] = [OH-(aq)]
Kw = [H+(aq)]2 = 10-14
√[H+(aq)]2 = √10-14
[H+(aq)] = 10-7 mol L-1
So, at 25°C the pH of this salt solution will be pH = -log10[H+(aq)] = -log10[10-7 ] = 7.0
A suitable indicator is one that changes colour at around pH = 7.00
Suitable indicators, for example, are bromothymol blue (colour change between 6.7 and 7.6) or phenol red (colour change between 6.8 and 8.4)
Phenolphthalein changes colour between pH 8.3 and 10. Phenolphthalein is NOT an appropriate indicator for a strong acid - strong base titration.
If we add a drop of phenolphthalein indicator to an aqueous solution of strong acid, the pH will be less than 7 and the solution will remain colourless. As we add strong base, hydrogen ions react with excess hydroxide ions to produce salt and water, so the pH increases. At pH = 7.0 all the strong acid will have been neutralised by the addition of strong base, BUT the phenolphthalein indicator will not have changed colour!
Phenolphthalein will not change colour until an excess of strong base (hydroxide ions) has been added and we have overshot the equivalence point for the reaction. The volume of strong base we record in this experiment will be too large!.
In strong base such as an aqueous solution of sodium hydroxide, the pH will be high and a drop of phenolphthalein indicator will turn the solution pink.
As we add a strong acid such hydrochloric acid, we will be consuming some of the hydroxide ions, and decreasing the pH. Somewhere between pH 8.3 and 10 we will decide that all our base has been neutralised by the acid because the indicator is now colourless instead of pink. But the reality will be that there is still excess hydroxide ions in solution waiting to be neutralised by the addition of more acid, so the volume of acid we have added, as indicated by the colour change of the indicator will be too low!
Learn all about how to choose an appropriate indicator for different types of acid-base titrations ar
https://www.ausetute.com.au/indicata.html
Water is neutral, that is [H+(aq)] = [OH-(aq)]
(or [H3O+(aq)] = [OH-(aq)] if you prefer)
The salt of a strong acid and base is made up of a cation that will not react with water to any appreciable extent, and an anion that will not react with water to any appreciable extent, so this salt does not affect the [H+(aq)] and [OH-(aq)] in the water, that is, the aqueous solution remains neutral.
At 25°C, Kw = [H+(aq)] × [OH-(aq)] = 10-14
Since [H+(aq)] = [OH-(aq)]
Kw = [H+(aq)]2 = 10-14
√[H+(aq)]2 = √10-14
[H+(aq)] = 10-7 mol L-1
So, at 25°C the pH of this salt solution will be pH = -log10[H+(aq)] = -log10[10-7 ] = 7.0
A suitable indicator is one that changes colour at around pH = 7.00
Suitable indicators, for example, are bromothymol blue (colour change between 6.7 and 7.6) or phenol red (colour change between 6.8 and 8.4)
Phenolphthalein changes colour between pH 8.3 and 10. Phenolphthalein is NOT an appropriate indicator for a strong acid - strong base titration.
If we add a drop of phenolphthalein indicator to an aqueous solution of strong acid, the pH will be less than 7 and the solution will remain colourless. As we add strong base, hydrogen ions react with excess hydroxide ions to produce salt and water, so the pH increases. At pH = 7.0 all the strong acid will have been neutralised by the addition of strong base, BUT the phenolphthalein indicator will not have changed colour!
Phenolphthalein will not change colour until an excess of strong base (hydroxide ions) has been added and we have overshot the equivalence point for the reaction. The volume of strong base we record in this experiment will be too large!.
In strong base such as an aqueous solution of sodium hydroxide, the pH will be high and a drop of phenolphthalein indicator will turn the solution pink.
As we add a strong acid such hydrochloric acid, we will be consuming some of the hydroxide ions, and decreasing the pH. Somewhere between pH 8.3 and 10 we will decide that all our base has been neutralised by the acid because the indicator is now colourless instead of pink. But the reality will be that there is still excess hydroxide ions in solution waiting to be neutralised by the addition of more acid, so the volume of acid we have added, as indicated by the colour change of the indicator will be too low!
Learn all about how to choose an appropriate indicator for different types of acid-base titrations ar
https://www.ausetute.com.au/indicata.html
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