2.1.1Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols
Enthalpy Level Diagrams — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 6 parts Lesson 10 of 53 Unit 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.
Section 1 of 6Enthalpy and Enthalpy Change
Chemical reactions are accompanied by energy changes, usually seen as heat given out or taken in.
Enthalpy
- Enthalpy Enthalpy, H, is the heat content of a chemical system held at constant pressure, and its absolute value cannot be measured.
Enthalpy change
- Enthalpy change ΔH is the heat energy transferred at constant pressure, calculated as H(products) − H(reactants).
Units
- Units Enthalpy changes are quoted in kJ mol⁻¹, with the value referring to the amounts in the equation as written.
Standard conditions
- Standard conditions Standard enthalpy changes, shown ΔH°, are measured at 100 kPa with all substances in their standard states, usually at 298 K.
Section 2 of 6Exothermic and Endothermic
Exothermic and Endothermic
Exothermic
- Exothermic Heat energy is released to the surroundings, so the temperature rises and ΔH is negative.
Endothermic
- Endothermic Heat energy is absorbed from the surroundings, so the temperature falls and ΔH is positive.
Examples exo
- Examples exo Combustion, neutralisation and the reaction of most metals with acids are exothermic.
Examples endo
- Examples endo Thermal decomposition of calcium carbonate and the dissolving of ammonium nitrate in water are endothermic.
Energy view
- Energy view In an exothermic reaction the products are at lower enthalpy than the reactants, while the opposite holds for endothermic.
| Type | ΔH sign | Surroundings |
|---|---|---|
| Exothermic | Negative | Temperature rises |
| Endothermic | Positive | Temperature falls |
Section 3 of 6Drawing the Diagram
1
Draw a vertical axis labelled enthalpy and place reactants at their enthalpy level on the left.
2
Draw the products at a lower level for an exothermic reaction, or a higher level for an endothermic one.
3
Draw a curved hump between them to show the transition state, whose height above the reactants is the activation energy, Eₐ.
4
Draw a vertical arrow between the reactant and product levels to show ΔH, pointing down for exothermic and up for endothermic.
5
Label the reactants and products with formulae and state symbols.
Section 4 of 6Activation Energy and Stability
Activation Energy and Stability
Activation energy
- Activation energy The minimum energy colliding particles need for a successful reaction, which is needed even when the overall reaction is exothermic.
Kinetic stability
- Kinetic stability Many exothermic reactions do not happen at room temperature because Eₐ is too high, so the reactants are kinetically stable.
Thermodynamic feasibility
- Thermodynamic feasibility A negative ΔH suggests a reaction is energetically favourable but does not guarantee it will happen at a measurable rate.
Reverse reaction
- Reverse reaction The reverse of a reaction has the same size of ΔH but the opposite sign.
Section 5 of 6Worked Example
Question
- The standard enthalpy change for H₂(g) + ½O₂(g) → H₂O(l) is −286 kJ mol⁻¹.
- State ΔH for 2H₂(g) + O₂(g) → 2H₂O(l) and for the reverse of this second reaction.
- The second equation is twice the first, so ΔH is multiplied by two: 2 × (−286) = −572 kJ mol⁻¹.
- Reversing a reaction changes the sign of ΔH.
- The reverse reaction is 2H₂O(l) → 2H₂(g) + O₂(g).
Answer −572 kJ mol⁻¹ for the forward reaction and +572 kJ mol⁻¹ for the reverse.
Section 6 of 6Exam Checklist & Tips
2.1.1 – Key Points for the Exam
- Enthalpy change The heat energy change measured at constant pressure.
- Exothermic A reaction that releases heat to the surroundings, with ΔH negative.
- Endothermic A reaction that absorbs heat from the surroundings, with ΔH positive.
- Activation energy The minimum energy particles need to react.
- Standard conditions A pressure of 100 kPa, a stated temperature (usually 298 K) and substances in their standard states.
Common Mistakes
- Drawing the products below the reactants for an endothermic reaction.
- Writing ΔH as positive when the temperature of the surroundings increases.
- Forgetting the activation energy hump and drawing a straight line between reactants and products.
- Thinking an exothermic reaction must happen quickly.
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