Chemistry 2: free practice, theory and problems

Many reactions go both ways. At equilibrium they go equally fast in both directions, and the concentrations no longer change.

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Chemical equilibrium
Organic chemistry
Buffers and titration
Electrochemistry

1. Chemical equilibrium

What is it about?

Many reactions go both ways. At equilibrium they go equally fast in both directions, and the concentrations no longer change.

Concepts and formulas

For $a\text{A} + b\text{B} \rightleftharpoons c\text{C} + d\text{D}$ : $K = \dfrac{[\text{C}]^c[\text{D}]^d}{[\text{A}]^a[\text{B}]^b}$ .
A large $K$ : the equilibrium lies towards the products. A small $K$ : towards the reactants.
Le Châtelier's principle: a system at equilibrium counteracts changes. If you add reactant, the equilibrium shifts towards the products.
$K$ is changed only by temperature, not by concentration, pressure or a catalyst.

Le Châtelier: the system counteracts what you do to it.

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2. Organic chemistry

What is it about?

Organic chemistry is the chemistry of carbon compounds. Carbon forms four bonds and can make long chains and rings.

Concepts

Alkanes (only single bonds) are named after the number of carbon atoms: methane (1), ethane (2), propane (3), butane (4), pentane (5), hexane (6), heptane (7), octane (8).
Alkenes have at least one double bond (-ene), alkynes a triple bond (-yne).
Functional groups: alcohol (-OH), aldehyde (-CHO), ketone (C=O inside the chain), carboxylic acid (-COOH), ester (-COO-), amine (-NH₂).
Isomers have the same molecular formula but different structures.
Reaction types: addition (typical of alkenes), substitution, elimination and condensation (for example ester formation).

The name tells the number of carbon atoms (meth-, eth-, prop-, but- …) and the ending tells the group.

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3. Buffers and titration

What is it about?

A buffer keeps the pH almost constant. Titration is used to find an unknown concentration.

Concepts and formulas

The buffer equation: $\text{pH} = \text{p}K_a + \lg\dfrac{[\text{base}]}{[\text{acid}]}$ .
When acid and base are equally concentrated, $\text{pH} = \text{p}K_a$ . The buffer is then at its best.
Titration: at the equivalence point, the added amount of base has reacted with all the acid: $c_1V_1 = c_2V_2$ (for 1:1 reactions).
An indicator changes colour near the equivalence point.

Buffer:

\text{pH} = \text{p}K_a + \lg(\text{base}/\text{acid})

. Titration:

c_1V_1 = c_2V_2

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4. Electrochemistry

What is it about?

In a galvanic cell (a battery) a spontaneous redox reaction produces a current. In electrolysis, current is used to drive a reaction that would not otherwise happen.

Concepts and formulas

The anode: oxidation. The cathode: reduction.
Cell voltage: $E^\circ_\text{cell} = E^\circ_\text{cathode} - E^\circ_\text{anode}$ (standard reduction potentials).
Examples: $\text{Ag}^+/\text{Ag}$ +0.80 V, $\text{Cu}^{2+}/\text{Cu}$ +0.34 V, $\text{Pb}^{2+}/\text{Pb}$ −0.13 V, $\text{Ni}^{2+}/\text{Ni}$ −0.26 V, $\text{Fe}^{2+}/\text{Fe}$ −0.44 V, $\text{Zn}^{2+}/\text{Zn}$ −0.76 V.
The metal with the lowest potential becomes the anode (is oxidised).

The cathode has the highest reduction potential.

E = E_\text{cathode} - E_\text{anode}

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Example problems with solutions

Here are some of the problems in chemistry 2. In the app, calculation problems get new numbers every time, so you can practise until it sticks – and take a graded practice exam before the real one.

Chemical equilibrium: What changes the value of the equilibrium constant

K

Answer: Only the temperature

A catalyst only makes the equilibrium establish faster.

Organic chemistry: Which functional group does an alcohol have?

Answer: -OH

Ethanol, C₂H₅OH, is an example.

Buffers and titration: A buffer has equal amounts of weak acid and base. What is the pH?

Answer: Equal to $\text{p}K_a$

$\lg 1 = 0$ , so $\text{pH} = \text{p}K_a$ .

Electrochemistry: What happens at the anode in a galvanic cell?

Answer: Oxidation

Mnemonic: anode–oxidation (both begin with a vowel), cathode–reduction.

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Chemistry 2: free practice, theory and problems

Contents

1. Chemical equilibrium

What is it about?

Concepts and formulas

2. Organic chemistry

What is it about?

Concepts

3. Buffers and titration

What is it about?

Concepts and formulas

4. Electrochemistry

What is it about?

Concepts and formulas

Example problems with solutions