Properties of Acids and Bases

In short: Acids have a pH below 7 and turn litmus red, while bases have a pH above 7 and turn litmus blue. Acids react with metals to give a salt and hydrogen, with carbonates to give a salt, water and carbon dioxide, and with bases to give a salt and water (neutralisation).

	Acids	Bases
Taste (never test!)	Sour	Bitter
Feel	Sting	Slippery, soapy
Litmus	Turns red	Turns blue
pH	Below 7	Above 7
Conduct electricity in water	Yes	Yes (if soluble)
Examples	$HCl$ , vinegar, citric acid	$NaOH$ , ammonia, baking soda

A base that dissolves in water is called an alkali.

Both acids and bases are corrosive in concentrated form. Wear safety glasses.

Indicators

An indicator changes colour over a certain pH range.

Indicator	Colour in acid	Colour change range (pH)	Colour in base
Methyl orange	Red	3.1 to 4.4	Yellow
Bromothymol blue	Yellow	6.0 to 7.6	Blue
Phenolphthalein	Colourless	8.3 to 10.0	Pink
Litmus	Red	4.5 to 8.3	Blue

Universal indicator is a mixture that shows a range of colours, from red (pH 1) through green (pH 7) to purple (pH 14). Natural indicators include red cabbage, beetroot and some flower petals.

Reactions of acids

Learn these three general equations:

acid + metal → salt + hydrogen gas $Mg (s) + 2HCl (aq) \to MgC l_{2} (aq) + H_{2} (g)$
acid + carbonate → salt + water + carbon dioxide $CaC O_{3} (s) + 2HCl (aq) \to CaC l_{2} (aq) + H_{2} O (l) + C O_{2} (g)$
acid + base → salt + water (neutralisation) $HCl (aq) + NaOH (aq) \to NaCl (aq) + H_{2} O (l)$

Tests for the gases:

Hydrogen makes a "pop" with a lit splint.
Carbon dioxide turns limewater milky.

The net ionic equation for every strong acid + strong base neutralisation is the same:

$H^{+} (aq) + O H^{-} (aq) \to H_{2} O (l)$

Salts are not always neutral

A salt is the ionic compound made in a neutralisation. Its solution can be neutral, acidic or basic:

Made from	Example	Solution
Strong acid + strong base	$NaCl$ , $KN O_{3}$	Neutral (pH 7)
Strong acid + weak base	$N H_{4} Cl$	Acidic
Weak acid + strong base	$C H_{3} COONa$ , $N a_{2} C O_{3}$	Basic

So neutralisation does not always give a neutral solution. The reason (the ions react with water) is explained in the next chapter.

Enthalpy of neutralisation

Neutralisation is exothermic. The molar enthalpy of neutralisation is the heat released per mole of water formed. For a strong acid and a strong base it is about −57 kJ/mol.

Measure it with a calorimeter (a polystyrene cup with a lid and thermometer):

$q = m c Δ T Δ H = - \frac{q}{n}$

$m$ = mass of the whole mixture (1 mL of solution ≈ 1 g)
$c$ = 4.18 J g⁻¹ K⁻¹ (or $4.18 \times 1 0^{3}$ J kg⁻¹ K⁻¹)
$n$ = moles of water formed (moles of the limiting reactant)

The measured value is usually a little smaller than the accepted value because some heat escapes to the cup, thermometer and air.

How the idea of an acid changed

Scientist	Year	Idea	Problem with it
Lavoisier	1776	Acids contain oxygen	$HCl$ has no oxygen
Davy	1810	Acids contain replaceable hydrogen	Did not explain bases
Arrhenius	1887	Acids ionise in water to give $H^{+}$ ; bases give $O H^{-}$	Only works in water; cannot explain $N H_{3}$ as a base
Brønsted and Lowry	1923	Acids donate protons; bases accept protons	(next chapter)

Each model kept what worked and fixed a problem with the one before. This is how science improves.

Acids and bases

Properties

Acid: sour, litmus red
Base: slippery, litmus blue
Alkali: soluble base

Indicators

Methyl orange
Bromothymol blue
Phenolphthalein

Acid reactions

Metal: salt + H2
Carbonate: salt + water + CO2
Base: salt + water

Salts

Neutral, acidic or basic

Neutralisation

Exothermic, about −57 kJ/mol
q = mcΔT

History

Lavoisier: oxygen
Davy: hydrogen
Arrhenius: H+ and OH−

Year 12Harder neutralisation enthalpy problems

Find the limiting reactant first. 30.0 mL of 0.700 mol/L NaOH is mixed with 20.0 mL of 0.900 mol/L HCl, both at 21.5 °C. $Δ H = - 57.1$ kJ/mol.

NaOH: 0.0210 mol. HCl: 0.0180 mol. HCl is limiting, so 0.0180 mol of water forms.

$q = 0.0180 \times 57100 = 1028$ J. The mixture has a mass of 50.0 g.

$Δ T = \frac{1028}{50.0 \times 4.18} = 4.92$ °C, so the final temperature is $21.5 + 4.9 = 26.4$ °C.

Weak acids give a smaller value. Neutralising a weak acid such as acetic acid releases less than 57 kJ/mol, because some energy is used to ionise the acid first.

Diprotic acids. One mole of $H_{2} S O_{4}$ makes two moles of water with NaOH, so it releases about twice as much heat as one mole of HCl.

Excess acid does not add heat. 50.0 mL of 2.0 mol/L HCl with 50.0 mL of 1.0 mol/L NaOH makes only 0.050 mol of water, because NaOH runs out. Divide $q$ by 0.050, not 0.10.

Two heats at once. Solid NaOH gives out heat when it dissolves and when it is neutralised. 2.00 g of NaOH is 0.0500 mol, so $q = 0.0500 \times (44.5 + 57.1) = 5.08$ kJ. In 100 g of solution, $Δ T = \frac{5080}{100 \times 4.18} = 12.2$ °C.

Quick summary

Acids turn litmus red and have pH below 7. Bases turn litmus blue and have pH above 7.
Indicators change colour over set pH ranges.
Acid + metal → salt + hydrogen. Acid + carbonate → salt + water + carbon dioxide. Acid + base → salt + water.
Salts can be neutral, acidic or basic.
Neutralisation is exothermic, about −57 kJ/mol for strong acid + strong base. Use $q = m c Δ T$ and $Δ H = - q / n$ .
Lavoisier (oxygen), Davy (hydrogen), Arrhenius (ions in water), then Brønsted-Lowry (protons).

Quiz

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An acid turns blue litmus:
1. A Red
2. B Blue
3. C Green
4. D Colourless
Show answer
A: Red
Acids turn litmus red.
A base that dissolves in water is called:
1. A A salt
2. B An alkali
3. C An indicator
4. D An ester
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B: An alkali
Soluble bases are alkalis.
Phenolphthalein in a strong base is:
1. A Colourless
2. B Pink
3. C Yellow
4. D Red
Show answer
B: Pink
It is colourless in acid and pink in base.

Properties of Acids and Bases