Acids and alkalis
Acidic and alkaline solutions
Acids
Acids form acidic solutions in water.
Acids are sources of hydrogen ions, H+. For example,
Hydrochloric acid produces hydrogen ions:
HCl (aq) → H+ (aq) + Cl ⁻ (aq)
Acidic solutions have pH values less than 7
Alkalis
Alkalis form alkaline solutions in water.
Alkalis are sources of hydroxide ions, OH−.
For example, sodium hydroxide produces hydroxide ions:
NaOH(aq) → Na+(aq) + OH− (aq)
Alkaline solutions have pH values greater than 7.
Neutral solutions
A neutral solution is neither acidic nor alkaline. A neutral solution has a pH value of 7.
Indicators
Acid-alkali indicators can show whether a solution is acidic, neutral (pH 7) or alkaline.
The table shows the colours for two different common indicator solutions.
The table shows the colours for two different common indicator solutions.
The pH scale
The pH scale measures the acidity or alkalinity of a solution. The pH of a solution can be measured using a pH probe, or estimated using universal indicator and a colour chart.
Reactions of acids
Acids take part in reactions in which salts are produced.
In these reactions, the salt is formed by replacing the hydrogen ions in the acids with metal ions or ammonium ions.
In these reactions, the salt is formed by replacing the hydrogen ions in the acids with metal ions or ammonium ions.
Reactions with metals
A salt and hydrogen are produced when acids react with metals.
In general:
acid + metal → salt + hydrogen
For example:
hydrochloric acid + magnesium → magnesium chloride + hydrogen
2HCl(aq) + Mg(s) → MgCl2 (aq) + H₂(g)
Question
Hydrogen is collected in a test tube during the reaction between magnesium and hydrochloric acid. Describe the laboratory test for hydrogen.
Answer
Put a lighted splint near the mouth of the test tube. Hydrogen ignites with a squeaky pop.
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Reactions with metal oxides
A salt and water are produced when acids react with metal oxides.
Metal oxides are bases, because they neutralise acids.
In general:
acid + metal oxide → salt + water
For example:
sulfuric acid + copper oxide → copper sulfate + water
H₂SO₄(aq) + CuO(s) → CuSO₄(aq) + H₂O(l)
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Reactions with metal hydroxides
A salt and water are produced when acids react with metal hydroxides. Metal hydroxides are bases because they neutralise acids.
In general:
acid + metal hydroxide → salt + water
For example:
nitric acid + sodium hydroxide → sodium nitrate + water
HNO₃(aq) + NaOH(s) → NaNO₃(aq) + H₂O(l)
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Reactions with carbonates
A salt, water and carbon dioxide are produced when acids react with carbonates.
In general:
acid + carbonate → salt + water + carbon dioxide
For example:
hydrochloric acid + copper carbonate → copper chloride + water + carbon dioxide
2HCl(aq) + CuCO₃(s) → CuCl₂(aq) + H₂O(l) + CO₂(g)
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Question
Carbon dioxide is given off during the reaction between copper carbonate and hydrochloric acid. Describe the laboratory test for carbon dioxide.
answer
Bubble the gas through limewater. Carbon dioxide turns limewater milky or cloudy white.
Metal oxides and metal hydroxides both react with acids to produce salts and water.
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Neutralisation
Bases and alkalis
A base is any substance that reacts with an acid to form a salt and water only. This means that metal oxides and metal hydroxides are bases.
Bases that are soluble in water are called alkalis and they dissolve in water to form alkaline solutions. For example:
· copper oxide is a base, but it is not an alkali because it is insoluble in water
· sodium hydroxide is a base, and it dissolves in water so it is also an alkali
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Question
Explain why all alkalis are bases, but not all bases are alkalis.
answer
Alkalis are soluble bases, but only some bases are soluble in water. Bases that dissolve in water are alkalis.
Neutralisation reactions
A neutralisation reaction is a reaction between an acid and a base.
Remember:
· acids in solution are sources of hydrogen ions, H+
· alkalis in solution are sources of hydroxide ions, OH−
In acid-alkali neutralisation reactions, hydrogen ions from the acid react with hydroxide ions from the alkali.
Here are the ions involved in the reaction of hydrochloric acid with sodium hydroxide solution:
H+(aq) + Cl ⁻(aq) + Na+(aq) + OH-(aq) → Na+(aq) + Cl ⁻ (aq) + H₂O(l)
Now remove the spectator ions, Cl ⁻ (aq) and Na+(aq). This leaves:
H+ (aq) + OH−(aq) → H₂O(l)
Pure water is neutral (its pH is 7). A neutral solution can be produced if the correct amounts of acid and alkali react together.
The change in pH during a neutralisation reaction can be measured using a pH probe and meter, or estimated using universal indicator solution and a pH colour chart.
An acid-alkali neutralisation is the reaction between hydrogen ions and hydroxide ions, forming water.
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Core practical
Investigating neutralisation
There are a number of ways that you could investigate neutralisation in Chemistry. This is an outline of the required steps to undertake one of these methods. Calcium oxide or calcium hydroxide can be used in this practical.
Aims
To investigate the change in pH when adding powdered calcium hydroxide to a fixed volume of dilute hydrochloric acid.
Apparatus
Method
1. Use a measuring cylinder to add dilute hydrochloric acid to a beaker.
2. Dip a clean glass rod into the contents of the beaker. Use it to transfer a drop of liquid to a piece of universal indicator paper on a white tile. Wait 30 seconds, then match the colour to a pH colour chart. Record the estimated pH.
3. Add a level spatula of calcium hydroxide powder to the beaker.
4. Stir thoroughly, then estimate and record the pH of the mixture (as in step 2).
5. Repeat steps 3 and 4 until there are no more changes in pH.
Results
Record the results in a suitable table. For example:
Analysis
Plot a graph to show:
· pH on the vertical axis
· number of spatulas added on the horizontal axis
Make sure you choose suitable scales so that at least 50% of the graph area includes plotted points. Draw a curve of best fit through these points.
The diagram shows an example:
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Use the graph to estimate the number of spatulas of calcium hydroxide needed to produce a neutral solution.
answer
Six spatulas of calcium hydroxide were needed to produce a neutral solution (pH 7).
Evaluation
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Suggest two ways in which more accurate results could be obtained.
answer
You could measure the mass of calcium hydroxide powder using a balance. You could also use a pH probe and meter to measure the pH, rather than estimating it using universal indicator.
Hazards, risks and precautions
It is important in this practical activity to use appropriate apparatus and methods. This includes the safe use and careful handling of substances.
Evaluate the hazards and the precautions needed to reduce the risk of harm. For example:
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Question
Use the graph to estimate the number of spatulas of calcium hydroxide needed to produce a neutral solution.
answer
Six spatulas of calcium hydroxide were needed to produce a neutral solution (pH 7).
Evaluation
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Question
Suggest two ways in which more accurate results could be obtained.
answer
You could measure the mass of calcium hydroxide powder using a balance. You could also use a pH probe and meter to measure the pH, rather than estimating it using universal indicator.
Hazards, risks and precautions
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Evaluate the hazards and the precautions needed to reduce the risk of harm. For example:
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More about acids - Higher
Dilute and concentrated solutions
A solution forms when a solute dissolves in a solvent. The concentration of a solution is a measure of how 'crowded' the solute particles are. The more concentrated the solution, the more particles it contains in a given volume.
When solutions are described as dilute or concentrated:
· a dilute solution contains a relatively small amount of dissolved solute
· a concentrated solution contains a relatively large amount of dissolved solute
Take care to use the word 'dilute' correctly. It can be used as an adjective to describe the concentration of a solution (as here), or as a verb to describe the process of adding more water to a solution to decrease its concentration.
Strong and weak acids
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Strong acids
Strong acids completely dissociate into ions in solution. For example, hydrochloric acid is a strong acid. It completely dissociates to form hydrogen ions and chloride ions:
HCl(aq) → H+(aq) + Cl-(aq)
Nitric acid and sulfuric acid are also strong acids.
Weak acids
Weak acids only partially dissociate into ions in solution. For example, ethanoic acid is a weak acid. It only partially dissociates to form hydrogen ions and ethanoate ions:
CH3COOH(aq) ⇌ H+(aq) + CH3COO-(aq)
The ⇌ symbol is used in the equation to show that the reaction is a reversible reaction, and does not go to completion.
pH and hydrogen ion concentration
The pH of a solution is a measure of its concentration of hydrogen ions:
· the higher the concentration of H+ ions in an acidic solution, the lower the pH
· the lower the concentration of H+ ions in an acidic solution, the higher the pH
If the hydrogen ion concentration in a solution increases by a factor of 10, the pH of the solution decreases by 1.
pH of alkaline solutions
The higher the concentration of OH- ions in an alkaline solution, the higher the pH.
Example
A solution of 1 g/dm3. hydrochloric acid has a pH of 1.6. Predict its pH when it is diluted to 0.1 g/dm3.
The hydrogen ion concentration decreases by a factor of 10, so the pH increases by 1 from 1.6 to 2.6.
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Question
A solution of hydrochloric acid, with a concentration of 2 g/dm3, has a pH of 1.3. Predict its pH when it is diluted to make a solution with a concentration of 0.02 g/dm3.
answer
The hydrogen ion concentration decreases by a factor of 100, so the pH increases by 2 from 1.3 to 3.3.
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Glossary
1.
acid
Substance producing
more hydrogen ions than hydroxide ions when dissolved in water.
2.
acidic
Having a pH lower than 7.
3.
alkali
Substance producing more
hydroxide ions than hydrogen ions when dissolved in water.
4.
alkaline
Having a pH greater than 7.
5.
base
A substance that reacts with an
acid to neutralise it and produce a salt.
6.
concentration
A measure of the mass or amount
of solute dissolved in a given volume of solvent or solution.
7.
dissociation
The breaking up of a molecule
into ions when dissolved in water.
8.
dissolve
When a substance breaks up and
mixes completely with a solvent to produce a solution.
9.
indicator
A substance that has different
colours, depending upon the pH of the solution it is in.
10.
insoluble
Unable to dissolve in a
particular solvent. For example, sand is insoluble in water.
11.
ion
Electrically charged particle,
formed when an atom or molecule gains or loses electrons.
12.
limewater
Calcium hydroxide solution. It
turns milky in the presence of carbon dioxide.
13.
metal
Shiny element that is a good
conductor of electricity and heat, and which forms basic oxides.
14.
neutral
When a substance is neither
acidic nor alkaline, and has a pH of 7.
15.
neutralisation
The reaction between an acid and
a base to form a salt plus water.
16.
oxide
Compound containing oxygen or
oxide ions.
17.
pH
Scale of acidity or alkalinity.
A pH (power of hydrogen) value below 7 is acidic, a pH value above 7 is
alkaline.
18.
reversible reaction
A chemical reaction which can go
both ways.
19.
salt
The substance formed when the
hydrogen ion in an acid is replaced by a metal ion.
20.
soluble
Able to dissolve in solvent. For
example, sugar is soluble in water because it dissolves to form sugar solution.
21.
solute
The dissolved substance in a
solution.
22.
solution
Mixture formed by a solute and a
solvent.
23.
solvent
The liquid in which the solute
dissolves to form a solution.
24.
spectator ion
An ion that is exactly the same
on both sides of an ionic equation.
25.
universal indicator
A chemical solution that
produces many different colour changes corresponding to different pH levels.
26.
volume
The volume of a
three-dimensional shape is a measure of the amount of space or capacity it
occupies, eg an average can of fizzy drink has a volume of 330 ml.
Acids and alkalis
- Edexcel
1
Which of the following solutions
could have a pH of 12?
Sodium chloride
Sulfuric acid
Sodium hydroxide
2
How do the concentrations of ions
in an alkaline solution compare to pure water?
The alkali contains more hydroxide ions (OH-)
The alkali contains more hydrogen ions (H+)
The alkali and pure water contain the same concentration of
hydrogen ions (H+) and hydroxide ions (OH-)
3
How does diluting an acid with water
affect its pH?
The pH decreases towards 7
The pH increases towards 7
It has no effect on the pH
4
What colour change could occur
when hydrochloric acid is added to an alkaline solution containing universal
indicator?
Red to green
Green to purple
Purple to red
5
What is the general equation for
the reaction of a metal with an acid?
Metal + acid → salt + hydrogen
Metal + acid → salt + water
Metal + acid → salt + alkali
6
What is the name of the salt
produced when potassium hydroxide reacts with sulfuric acid?
Potassium sulfide
Potassium sulfate
Potassium sulfuric
7
What products are formed when
hydrochloric acid reacts with copper carbonate?
Copper chloride and hydrogen
Copper chloride, water and carbon dioxide
Copper chloride and water
8
What happens to the pH of a
solution of hydrochloric acid as calcium oxide is added?
It rises
It falls
It stays the same
9
A small amount of hydrochloric
acid is dissolved in a large volume of water. What is the best description of
this solution? - Higher
A dilute solution of a weak acid
A weak solution of a strong acid
A dilute solution of a strong acid
10
Natural rainwater has a pH of 5.
The most acidic rain ever recorded in Britain was 1000 times more acidic than
natural rainwater. What was its pH? - Higher
2
3
4
Next Topic
GCSE EDEXCEL - SALTS
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