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When a reaction at equilibrium is disturbed by changes in concentration, pressure, or temperature, the system will adjust in a way that opposes the disturbance. This response shifts the position of equilibrium to counteract the effect of the change, restoring balance.
Potassium oxide (K₂O) is an ionic compound composed of potassium ions (K⁺) and oxide ions (O²⁻). When potassium oxide is added to water, it reacts to form potassium hydroxide (KOH):
K2O+H2O→2KOH
Potassium hydroxide consists of potassium ions (K⁺) and hydroxide ions (OH⁻). The presence of a high concentration of hydroxide ions (OH⁻) in the solution makes it alkaline. As a result, the solution exhibits a high pH, typically around 14, indicating its strong alkalinity.
The major product of the reaction would be 2-chloropropane, while the minor product would be 1-chloropropane. This is due to the formation of a more stable secondary carbocation as an intermediate. The secondary carbocation is stabilised by the inductive effect, where the two adjacent carbon atoms donate electron density toward the positively charged carbon. This increased electron density helps stabilise the carbocation, making its formation more favourable and ultimately leading to the major product being 2-chloropropane. The mechanism can clearly illustrate this process.
The molecular formula of a compound represents the actual number of atoms of each element in a molecule. For example, ethane has the molecular formula C₂H₆, which indicates that it contains 2 carbon atoms and 6 hydrogen atoms in one molecule.
In contrast, the empirical formula reflects the simplest whole-number ratio of atoms in a compound. For ethane, the empirical formula is CH₃, as the ratio of carbon to hydrogen (2:6) can be simplified by dividing both numbers by 2 to give a 1:3 ratio.
It's important to note that different compounds can have the same empirical formula. For instance, the alkenes C₂H₄ and C₃H₆ have different molecular formulas but share the same empirical formula, CH₂.
Periodicity refers to the recurring patterns observed in the chemical and physical properties of elements as you move across periods (rows) and groups (columns) of the periodic table.
Across a period: All elements in a period have the same number of electron shells, but the number of protons and electrons increases as you move from left to right. As a result, the atomic radius decreases due to increasing nuclear charge, which pulls the electrons closer to the nucleus. Additionally, ionisation energy increases across a period because the electrons are more tightly bound to the nucleus.
Down a group: Elements in the same group have the same number of electrons in their outer shell, which results in similar chemical properties. The atomic radius increases as you move down a group because additional electron shells are added, making the atoms larger.
An interesting exception in periodic trends is the drop in ionisation energy between groups 2 and 3 and groups 5 and 6. This occurs due to factors like electron configuration and electron repulsion, which influence the ease with which an electron can be removed.
To understand how buffers work, let's first define what a buffer is and how it functions. A buffer is a solution that resists changes in pH. It typically consists of a weak acid and its conjugate base (salt), or sometimes a weak acid and a weak base. The weak acid, in this case, partially dissociates, allowing the buffer to resist changes when either an acid or base is added.
For example, ethanoic acid (CH₃COOH) can act as a buffer. The dissociation of ethanoic acid in water can be represented by the following equation: CH₃COOH ⇌ CH₃COO⁻ + H⁺
In a buffer system, adding more acid (e.g., ethanoic acid) shifts the equilibrium to the right, increasing the concentration of hydrogen ions (H⁺) and making the solution less acidic. On the other hand, adding an alkali (or salt) reacts with the H⁺ ions, shifting the equilibrium to the left and making the solution more acidic. In this way, the buffer resists significant pH changes.
To calculate the pH of a buffer solution, we can use the acid dissociation constant (Ka) for the weak acid involved. For example, if we prepare a buffer solution containing 0.20 mol dm⁻³ of ethanoic acid and 0.10 mol dm⁻³ of sodium ethanoate (the conjugate base), and the Ka for ethanoic acid is given as 1.74 × 10⁻⁵ mol dm⁻³, we can calculate the concentration of hydrogen ions and the pH.
We begin with the expression for Ka: Ka = [H⁺][CH₃COO⁻] / [CH₃COOH]
Substituting the known values: 1.74 × 10⁻⁵ = [H⁺] × 0.10 / 0.20
Solving for [H⁺]: [H⁺] = (1.74 × 10⁻⁵ × 0.20) / 0.10 = 3.48 × 10⁻⁵ mol dm⁻³
Finally, the pH is calculated using the formula: pH = -log([H⁺]) = -log(3.48 × 10⁻⁵) = 4.46
Therefore, the pH of the buffer solution is 4.46.
A full electron configuration describes how electrons are distributed among the shells and subshells of an atom or ion. The shells, or energy levels, are numbered from 1 to 4, with shell 1 being the closest to the nucleus and shell 4 being the furthest. Electrons are filled into these shells from the innermost to the outermost.
Electron subshells are designated as s, p, d, and f. For A Level, the highest subshell you’ll encounter is d. The electron capacities for each subshell are:
When filling electron subshells, we follow the order: s → p → d, with a few exceptions to this rule, which will be discussed later.
For example, carbon (with 6 electrons) has the following electron configuration: 1s² 2s² 2p². This means it has 2 electrons in the 1s subshell, 2 electrons in the 2s subshell, and 2 electrons in the 2p subshell.
Hydroxynitriles are synthesized through a nucleophilic addition reaction involving ketones and cyanide. The cyanide ion (CN⁻), which is negatively charged, is attracted to the electrophilic carbonyl carbon in the ketone, which has a partial positive charge. This leads to the breaking of the carbon-oxygen double bond, resulting in a negatively charged oxygen atom that possesses a lone pair of electrons. The lone pair on the oxygen then acts as a nucleophile, attacking a proton (H⁺) from the hydrochloric acid (HCl). This protonation process yields the final product: a hydroxynitrile.
When a reaction at equilibrium is disturbed by changes in concentration, pressure, or temperature, the system will adjust in a way that opposes the disturbance. This response shifts the position of equilibrium to counteract the effect of the change, restoring balance.
