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What does it mean for a reaction to reach equilibrium, and how do acids and bases react?

Reversible reactions and dynamic equilibrium, Le Chatelier's principle, the Bronsted-Lowry theory of acids and bases, strong and weak acids, and the reactions of acids.

An Eduqas A-Level Chemistry C2.1 answer on dynamic equilibrium, Le Chatelier's principle, the Bronsted-Lowry theory, strong and weak acids, and the typical reactions of acids.

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  1. What this topic is asking
  2. Dynamic equilibrium
  3. Le Chatelier's principle
  4. Bronsted-Lowry acids and bases
  5. Strong and weak acids
  6. Reactions of acids
  7. Examples in context
  8. Try this

What this topic is asking

Eduqas topic C2.1 covers reversible reactions and the idea of dynamic equilibrium, how Le Chatelier's principle predicts the response to changes in conditions, the Bronsted-Lowry theory of acids and bases, the distinction between strong and weak acids, and the characteristic reactions of acids. The quantitative treatment of KcK_c, KpK_p and pH comes later in the Physical and Inorganic section; here the treatment is qualitative.

Dynamic equilibrium

The reaction is written with the reversible arrow β‡Œ\rightleftharpoons. The position of equilibrium describes whether reactants or products are favoured; it can be shifted by changing conditions, but at a fixed temperature it always returns to the same balance.

Le Chatelier's principle

Bronsted-Lowry acids and bases

A Bronsted-Lowry acid donates a proton; a base accepts one. Every acid has a conjugate base (what remains after losing H+\text{H}^+), and every base has a conjugate acid. Water is amphoteric, acting as an acid with bases and a base with acids.

Strong and weak acids

A strong acid (such as HCl or HNO3\text{HNO}_3) dissociates completely in water. A weak acid (such as ethanoic acid) only partially dissociates, setting up an equilibrium CH3COOHβ‡ŒCH3COOβˆ’+H+\text{CH}_3\text{COOH} \rightleftharpoons \text{CH}_3\text{COO}^- + \text{H}^+. Strong and weak describe the extent of dissociation, which is different from concentration (how much acid is dissolved).

Reactions of acids

Acids react with metals (giving a salt and hydrogen), with metal carbonates (salt, water and carbon dioxide) and with bases or alkalis (salt and water, a neutralisation). These reactions return throughout inorganic chemistry and in the practical work.

Examples in context

Example 1. Industrial compromise in the Haber process. High pressure raises the ammonia yield, but is expensive and hazardous; a low temperature raises the yield but slows the rate. Industry uses a moderate temperature (about 450 ∘C450\ ^{\circ}\text{C}), a high pressure and an iron catalyst as a compromise between yield and rate.

Example 2. Carbonated drinks fizz. Dissolved CO2\text{CO}_2 is in equilibrium with the gas above it under pressure; opening the bottle lowers the pressure, so by Le Chatelier the equilibrium shifts to release CO2\text{CO}_2 gas, and the drink fizzes.

Try this

Q1. Predict the effect on the position of equilibrium of removing a product from a reversible reaction at constant temperature. [1 mark]

  • Cue. The equilibrium shifts toward the products to replace what was removed, so more product forms.

Q2. Write an equation for the partial dissociation of the weak acid ethanoic acid in water. [1 mark]

  • Cue. CH3COOHβ‡ŒCH3COOβˆ’+H+\text{CH}_3\text{COOH} \rightleftharpoons \text{CH}_3\text{COO}^- + \text{H}^+ (the reversible arrow shows only partial dissociation).

Exam-style practice questions

Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Eduqas 20194 marksThe Haber process reaches equilibrium: N2(g)+3H2(g)β‡Œ2NH3(g)\text{N}_2(\text{g}) + 3\text{H}_2(\text{g}) \rightleftharpoons 2\text{NH}_3(\text{g}), Ξ”H=βˆ’92Β kJΒ molβˆ’1\Delta H = -92\ \text{kJ mol}^{-1}. Use Le Chatelier's principle to predict and explain the effect on the equilibrium yield of ammonia of (a) increasing the pressure and (b) increasing the temperature.
Show worked answer β†’

(a) Increasing the pressure shifts the equilibrium to the side with fewer gas molecules (4 moles to 2 moles), which is the products, so the yield of ammonia increases (2).

(b) The forward reaction is exothermic. Increasing temperature shifts the equilibrium in the endothermic (backward) direction to oppose the change, so the yield of ammonia decreases (2).

Markers reward the correct direction of shift and an explanation referring to moles of gas (pressure) and the sign of Ξ”H\Delta H (temperature).

Eduqas 20213 marks(a) State the Bronsted-Lowry definitions of an acid and a base. (b) In the reaction HNO3+H2Oβ†’H3O++NO3βˆ’\text{HNO}_3 + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+ + \text{NO}_3^-, identify the conjugate base of nitric acid and explain your choice.
Show worked answer β†’

(a) A Bronsted-Lowry acid is a proton (H+\text{H}^+) donor; a base is a proton acceptor (1).

(b) The conjugate base of HNO3\text{HNO}_3 is NO3βˆ’\text{NO}_3^- (1). It is what remains after the acid donates its proton, so the acid and its conjugate base differ by a single H+\text{H}^+ (1).

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