1.6.1Physical Chemistry
Bond Energy — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 31 of 102
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.
Part 1 of 4Energy Changes in Reactions
Exothermic
- Exothermic An exothermic reaction transfers energy to the surroundings,
- so the temperature of the surroundings rises and the enthalpy change ΔH is negative.
Endothermic
- Endothermic An endothermic reaction takes in energy from the surroundings, so the temperature falls and ΔH is positive.
Exothermic examples
- Exothermic examples Combustion, neutralisation and the oxidation of metals are exothermic and release heat to the surroundings.
Endothermic examples
- Endothermic examples Thermal decomposition of calcium carbonate and the dissolving of ammonium nitrate in water are endothermic.
Conservation
- Conservation Energy is not created or destroyed, so energy lost by the chemicals is gained by the surroundings and vice versa.
Part 2 of 4Bonds and Energy
Bonds and Energy
Bond breaking
- Bond breaking Energy must be supplied to break bonds, so bond breaking is an endothermic process.
Bond making
- Bond making Energy is released when new bonds form, so bond making is an exothermic process.
Net effect
- Net effect If more energy is released making bonds than is needed to break bonds, the reaction is exothermic overall.
Activation energy
- Activation energy Even exothermic reactions need a minimum energy input to start breaking bonds, which is the activation energy.
| Type | Energy flow | Temperature of surroundings | Sign of ΔH |
|---|---|---|---|
| Exothermic | To surroundings | Rises | Negative |
| Endothermic | From surroundings | Falls | Positive |
Part 3 of 4Applying the Idea
1
Identify the bonds in the reactants that must be broken and the bonds in the products that will be formed.
2
Recognise that the energy needed for breaking is taken in and the energy for making is given out.
3
Compare the two totals to decide if the reaction is exothermic or endothermic overall.
4
Link this to an observation such as a temperature rise in the reacting mixture.
5
Remember that ΔH refers to the system, so the sign is opposite to the temperature change of the surroundings.
Part 4 of 4Exam Checklist & Tips
1.6.1 – Key Points for the Exam
- Exothermic Describes a reaction that transfers energy to the surroundings, with negative ΔH.
- Endothermic Describes a reaction that takes in energy from the surroundings, with positive ΔH.
- Enthalpy change The heat energy change at constant pressure, measured in kJ mol⁻¹.
- Activation energy The minimum energy particles need for a reaction to occur.
Common Mistakes
- Saying bond breaking releases energy; it always requires energy.
- Confusing the sign of ΔH with the temperature change; a temperature rise means ΔH is negative.
- Thinking exothermic reactions need no energy to start, when activation energy is still required.
- Writing ΔH without a sign or without the unit kJ mol⁻¹.
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