2 Acids, alkalis and titrations Flashcards
What tells you how acidic or how alkaline a solution is?
The pH scale.
What does the pH scale range from?
0 to about 14.
What is 7 on the pH scale?
Neutral.
What is an example of a substance with a pH of 7?
Sodium chloride solution.
What are the properties of those with pH values descending from 7?
Increasingly acidic.
What are the properties of substances with values ranging from 0-3?
This is the range for hydrochloric acid. And it is strongly acidic.
What are the properties of substances with values ranging from 4-6?
This is the range for vinegar.
And it is weakly acidic.
What is another word for vinegar?
Ethanoic acid.
What are the properties of those with pH values ascending from 7?
Increasingly alkaline.
What are the properties of substances with values ranging from 8-10?
This is the range for ammonia solution.
And it is weakly alkaline.
What are the properties of substances with values ranging from 11-14?
This is the range for sodium hydroxide solution. And it is strongly alkaline.
What do you have to remember when writing pH?
It is a small p and a big H. The big H is the symbol for Hydrogen.
How can you measure pH?
- Using universal indicator.
- Acid-Alkali indicators.
What is the universal indicator?
Universal indicator is made from a mixture of indicators, which change colour in a gradual way over a range of pH values.
What are the two types of universal indicator?
It can be used as a solution or as paper.
What is the most common form of universal indicator?
It is known as full-range universal indicator.
How does the full-range universal indicator work?
It changes through a variety of colours from pH 1 right up to pH 14, but isn’t very accurate.
How do you match the specific colour to its corresponding pH?
The colour of the paper or solution is checked against a chart to find the pH.
What colours represent acidity when using the universal indicator?
Red, orange, yellow, light green.
Which colour represents neutrality when using the universal indicator?
Green.
What colour represents alkalinity when using the universal indicator?
Dark green, blue, purple.
What are acid-alkali indicators?
Any substance that has different colours depending on the pH can be used as an indicator.
How many common indicators are there?
5.
What is the first common indicator?
Litmus.
What colour does litmus turn when in presence of an acid?
Red.
What colour does litmus turn when in presence of an alkali?
Blue.
What colour is litmus when in the presence of neutrality?
Purple, which is an equal mixture of the red and blue forms.
What is the second common indicator?
Methyl orange.
What colour does methyl orange turn when in presence of an acid?
Red.
What colour does methyl orange turn when in presence of an alkali?
Yellow.
What is the third common indicator?
Phenolpthalein.
What colour does phenolphthalein turn when in presence of an acid?
Colourless.
What colour does phenolpthalein turn when in presence of an alkali?
Pink
What is the fourth common indicator?
Universal indicator
What colour does universal indicator turn when in presence of an acid?
Red.
What colour does universal indicator turn when in presence of an alkali?
Blue.
What colour does universal indicator turn when in the presence of neutrality?
Green.
What is an acid?
A substance that acts as a source of hydrogen ions, or as a proton donor.
What do all acids contain?
All acids contain hydrogen and when acids react the hydrogen is replaced with something else; all acids have a replaceable H.
What is the equation for when hydrochloric acid reacts with sodium hydroxide?
HCI + NaOH —> NaCI + H2O
What has happened in this reaction?
The H of the HCI has been replaced by an Na.
How do we find out which hydrogens in a certain formula are replaceable?
In ethanoic acid, only the H attached to the O is replaceable, not the ones joined to the C.
What is the formula for hydrochloric acid?
HCI
What is the formula for nitric acid?
HNO3
What is the formula for sulfuric acid?
H2SO4
What is the formula for ethanoic acid?
CH3COOH
What is the formula for phosphoric acid?
H3PO4
What happens when acids are in water?
They break apart to form hydrogen ions.
What is the formula for when hydrochloric acid is in water?
HCI —> H+ + CI-
What is the formula for when nitric acid is in water?
HNO3 —> H+ + NO3-
What is the formula for when sulfuric acid is in water?
H2SO4 —> 2H+ + SO42-
What actually happens when we are measuring pH?
We are actually measuring the concentration of these H+ ions in the solution. This is why pH is written with a capital letter.
What are bases?
A substance that neutralises acids by combining with the hydroxide ions in them. They are usually metal oxides, hydroxides or ammonia. A soluble base is called an alkali, and it is a substance that acts as a source of hydroxide ions in a solution or as a proton acceptor.
How can we identify alkalis?
Some bases dissolve in water to form solutions containing hydroxide ions. These are alkalis.
What are alkalis a source of?
Hydroxide ions in a solution.
What are two examples of alkalis?
- Sodium hydroxide.
- Potassium hydroxide
What happens when sodium hydroxide is submerged in water?
It breaks apart to form sodium and hydroxide ions.
What happens when ammonia is submerged in water?
It reacts with the water to form ammonium ions and hydroxide ions.
What do these alkalis all have in terms of their pH?
A pH greater than 7.
What are some other substances that react with water to form hydroxide ions?
Soluble metal carbonates.
What are some examples of soluble carbonates?
- Sodium carbonate.
- Potassium carbonate.
Both are alkalis greater than pH 7.
Why is the pH greater than 7?
It is due to the OH- ions in the solution.
Why are these solutions weakly alkaline?
Only some of the carbonate ions react with water, so these solutions are only weakly alkaline.
What happens to the acids in neutralisation reactions?
Acids react with bases or alkalis in a neutralisation reaction.
What are examples of bases?
Metal oxides, such as copper oxide and magnesium oxide.
What is an example of reacting acids with bases?
Copper oxide reacts with hot dilute sulfuric acid, in a neutralisation reaction to produce a solution of copper (II) sulfate and water.
In a neutralisation reaction, what does copper oxide react with dilute sulfuric acid to form?
A solution of copper sulfate and water.
What is the equation for this reaction?
CuO + H2SO4 —> CuSO4 + H2O
What is an explanation of what is happening in this equation?
Copper (II) oxide is an ionic compound containing the O2- ion and what has happened in this reaction is that the H+ ions from the acid have combines with the O2- ions from the base to form water (H2O).
What is an example of reacting acids with alkalis?
Sodium hydroxide solution (an alkali) reacts with dilute hydrochloric acid to form sodium chloride and water.
What is the equation that shows this reaction?
NaOH + HCI —> NACI + H2O
What kind of reaction is this?
A neutralisation reaction.
What is an explanation for why this is a neutralisation reaction?
Sodium hydroxide and sodium chloride are both ionic compounds and so will be present as ions in solution. HCI is an acid and so will dissociate into H+ and CI-. We can therefore re-write this equation showing all the ions.
What is the re-written equation showing all the ions?
Na+ + OH- + H+ + CI- —> Na+ + CI- + H2O.
What is an explanation of this ionic equation?
The Na+ and CI- ions are the same on both sides of the equation and so are spectator ions. We can leave the spectator ions out of this equation to just show the things that have changed.
What is the completed ionic equation?
OH- + H+ —> H2O
What is special about ionic equations?
All neutralisation reactions for an acid reacting with an alkali will have the same ionic equation.
Why is this applicable to ionic equations?
This is because they all involve the OH- ions from the alkali reacting with the H+ ions from the acid to form water.
When are acids a source of hydrogen ions?
When they are in aqueous solution.
When are bases a source of hydroxide ions?
When they are in aqueous solution.
What can alkalis do to acids?
Alkalis can neutralise acids.
