R2.3Reactivity 2 — How much, how fast, how far?
How far? The extent of chemical change — IB Chemistry SL Revision Notes & Topical Questions
5 min read 11 parts Lesson 3 of 3 1 quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Lesson summary
R2.3 How far? The extent of chemical change — IB CHEMISTRY SL notes: Describe dynamic equilibrium, write equilibrium expressions and interpret K, and apply Le Chatelier's principle.Reversible reaction — products can react to re-form the reactants, shown with two opposing half arr…
- Constant, not equal
- Macroscopic properties
- Physical equilibria
- Closed system
- Open system
- CaCO3 example
Part 1 of 9Reversible reactions and dynamic equilibrium
Reversible reaction
- Reversible reaction products can react to re-form the reactants, shown with two opposing half arrows ⇌.
Dynamic equilibrium
Dynamic equilibrium the state in a closed system where the forward rate equals the reverse rate, so concentrations stay constant.
Dynamic
- Dynamic reactions continue at the molecular level.
Constant, not equal
- Constant, not equal concentrations of reactants and products are constant, not equal.
Macroscopic properties
- Macroscopic properties such as colour and density do not change.
Direction
- Direction the same equilibrium is reached starting from reactants or from products.
Physical equilibria
- Physical equilibria evaporation equals condensation in a sealed flask, e.g. Br2(l) ⇌ Br2(g) and C2H5OH(l) ⇌ C2H5OH(g).
Part 2 of 9Closed and open systems
Closed and open systems
Closed system
- Closed system no reactants or products escape, which is required for equilibrium.
Open system
- Open system matter is lost, so if a gaseous product escapes the reverse reaction cannot keep up and the reaction goes to completion.
CaCO3 example
- CaCO3 example strongly heated in an open tube, CO2 escapes and decomposition goes to completion.
Solutions
- Solutions reactions entirely in solution can reach equilibrium in an open flask.
Part 3 of 9The equilibrium law
1
Expression
- Expression for aA + bB ⇌ cC + dD, K = [C]c[D]d ÷ [A]a[B]b at equilibrium.
2
Brackets
- Brackets square brackets mean equilibrium concentration in mol dm−3, and round brackets lose the mark.
3
Omissions
- Omissions pure solids and liquids are left out of the expression.
4
Specificity
- K belongs to the equation as written.
5
Reverse reaction
- K′ = 1/K.
6
Multiplied equation
- Multiplied equation doubling the equation squares K.
7
Haber process example
- K = [NH3]2 ÷ ([N2][H2]3).
Part 4 of 9Interpreting the size of K
| Value of K | Meaning |
|---|---|
| K ≪ 1 | Hardly any reaction; mostly reactants |
| K < 1 | Reactants favoured; equilibrium to the left |
| K ≈ 1 | Significant amounts of both reactants and products |
| K > 1 | Products favoured; equilibrium to the right |
| K ≫ 1 | Reaction almost complete; mostly products |
Temperature
- K is constant at a specified temperature and depends only on temperature.
Acids
- Acids stronger acids dissociate more, so they have a higher K than weaker acids.
Part 5 of 9Le Chatelier's principle
Statement
Statement if a change is made to a system at equilibrium, the position of equilibrium shifts to partially oppose the change.
Position of equilibrium
- Position of equilibrium the relative amounts of reactants and products; a shift right increases the products.
1
The system is at equilibrium.
2
A change is imposed.
3
Identify the direction that opposes the change.
4
The position shifts that way.
5
A new equilibrium is established.
6
Check whether K changed (only temperature does this).
Part 6 of 9Effects of concentration and pressure
Concentration
- Concentration adding a reactant or removing a product shifts the position right;
- removing a reactant or adding a product shifts it left.
K and concentration
- K is unchanged because the ratio of concentrations returns to K.
Pressure
- Pressure only gases are affected, and an increase shifts the equilibrium towards the side with fewer gas moles.
Equal gas moles
- Equal gas moles if both sides have equal gas moles, pressure has no effect.
K and pressure
- K is unchanged.
Counting gas moles
- Counting gas moles only gaseous species count, so CaCO3(s) ⇌ CaO(s) + CO2(g) shifts left at higher pressure.
Dilution
- Dilution adding water shifts the position towards the side with more dissolved particles,
- so Fe3+(aq) + SCN−(aq) ⇌ FeSCN2+(aq) shifts left,
- while equal numbers of particles give no shift.
Part 7 of 9Effects of temperature and catalysts
Temperature
- Temperature the only factor that changes K.
Increase in temperature
- Increase in temperature shifts the position in the endothermic direction.
Decrease in temperature
- Decrease in temperature shifts the position in the exothermic direction.
| Forward reaction | Raise T | Effect on K |
|---|---|---|
| Exothermic | Shifts left | K decreases |
| Endothermic | Shifts right | K increases |
Deducing ΔH
- Deducing ΔH if raising T increases the products at equilibrium, the forward reaction is endothermic;
- if products decrease, it is exothermic.
Catalyst
- Catalyst speeds the forward and reverse reactions equally, so equilibrium is reached faster, but the position and K are unchanged.
Part 8 of 9Industrial compromise
1Haber process
- N2 + 3H2 ⇌ 2NH3, ΔH = −92 kJ mol−1, with an iron catalyst, about 200 atm and about 450 °C.
2Contact process
- 2SO2 + O2 ⇌ 2SO3 with a V2O5 catalyst.
3Exothermic reactions
- Exothermic reactions lower temperature gives a higher yield but higher temperature gives a faster rate, so a compromise is used.
4Kinetics
- Kinetics the reaction must also be fast enough to be useful.
5Heterogeneous equilibrium
- Heterogeneous equilibrium in a fizzy drink CO2(g) ⇌ CO2(aq), and opening the bottle shifts the position left so bubbles form.
Part 9 of 9More from the specification
More from the specification
Dynamic equilibrium conditions
- A closed reaction mixture where forward and reverse reaction rates are equal, resulting in constant macroscopic concentrations of all species.
Le Chatelier's principle
- When external conditions are altered, the equilibrium moves in the direction that acts to reduce the imposed change.
- (temperature, pressure, or concentration)
Gaseous equilibrium systems
- In the synthesis of hydrogen iodide (H2 + I2 2HI), changing pressure does not shift equilibrium position because total moles of gas remain equal on both sides.
Esterification equilibrium mixture
- Reversible condensation between ethanol and ethanoic acid forming ethyl ethanoate and water, yielding an equilibrium mixture containing all four components.
Mixture
- Two or more substances mixed but not chemically combined.
Hydrogen
- A gas that gives a squeaky pop with a lighted splint.
Carbon dioxide
- A gas that turns limewater cloudy.
Ethyl ethanoate
- The ester CH₃COOC₂H₅.
Key terms
| Term | Meaning |
|---|---|
| Reversible reaction | A reaction that can proceed in both directions, shown by ⇌. |
| Dynamic equilibrium | A closed-system state where forward and reverse rates are equal and concentrations are constant. |
| Equilibrium constant, K | The ratio of product to reactant concentrations at equilibrium, each raised to its coefficient. |
| Le Chatelier's principle | The position of equilibrium shifts to partially oppose a change imposed on the system. |
| Position of equilibrium | The relative amounts of reactants and products at equilibrium. |
| Closed system | A system in which no matter is lost to the surroundings. |
Avoid these mistakes
1
Saying the concentrations are equal at equilibrium — they are constant, not equal.
2
Saying changes in concentration or pressure alter K — only temperature does.
3
Using round brackets in K expressions — square brackets are required.
4
Counting solids or liquids in the pressure argument — only gas moles count.
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