Rates, Energy & Reaction Experiments: Energy Profile Diagrams From Practical Experiments (Copy)
Rates, Energy & Reaction Experiments
Energy Profile Diagrams From Practical Experiments
Purpose in ATP
- Energy profile diagrams visually represent:
- Energy changes during a chemical reaction
- Activation energy and overall enthalpy change (ΔH)
- ATP questions assess:
- Ability to draw diagrams accurately
- Correct labelling of key features
- Proper interpretation of exothermic vs endothermic reactions
Core Examiner Rule
- Diagrams must include:
- Reactants and products
- Activation energy (Ea) clearly marked
- ΔH (enthalpy change) shown
- Proper energy axis (y-axis) and reaction progress axis (x-axis)
- Examiners check:
- Relative positions of reactants and products for endothermic/exothermic
- Correct slope of curves (smooth, realistic)
Step 1: Identify Reaction Type
- Exothermic reaction
- Products have lower energy than reactants → ΔH negative
- Diagram: Products lower on energy axis than reactants
- Endothermic reaction
- Products have higher energy than reactants → ΔH positive
- Diagram: Products higher than reactants
Step 2: Drawing the Diagram
- Draw x-axis: Reaction progress
- Draw y-axis: Energy
- Mark reactants on y-axis
- Mark products on y-axis
- Draw smooth curve connecting reactants to products
- Peak of curve represents activated complex
Step 3: Labelling Key Features
- Activation energy (Ea):
- Energy difference from reactants to peak of curve
- ΔH (enthalpy change):
- Energy difference from reactants to products
- Reactants and Products clearly labelled
- Optional: Activated complex / Transition state
Step 4: Example Energy Profile
- Exothermic reaction:
- Reactants at 50 kJ
- Products at 20 kJ
- Peak at 80 kJ
- Ea = 80 − 50 = 30 kJ
- ΔH = 20 − 50 = −30 kJ
- Endothermic reaction:
- Reactants at 30 kJ
- Products at 60 kJ
- Peak at 90 kJ
- Ea = 90 − 30 = 60 kJ
- ΔH = 60 − 30 = +30 kJ
Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Chemistry Full Scale Course
Step 5: Observations From Practical Experiments
- Practical experiments provide qualitative information about:
- Reaction speed
- Exothermic/endothermic nature
- Examples:
- Temperature rise → exothermic
- Temperature fall → endothermic
- Use this to justify diagram shape in ATP
Step 6: Examiner-Traps
- Placing products incorrectly relative to reactants
- Not labelling Ea and ΔH
- Drawing peaks too sharp or flat
- Confusing x-axis and y-axis labels
- Using dotted lines for energy curve (must be smooth)
High-Yield ATP Tips
- Determine reaction type (exo/endothermic)
- Draw smooth curve from reactants to products
- Peak = activated complex
- Label Ea and ΔH clearly
- Include reactants and products labels
- Optional: mark activated complex
Core Scientific Principle
- Energy profile diagrams represent:
- Activation energy required for reaction
- Overall enthalpy change (ΔH)
- Exothermic: energy released → products lower
- Endothermic: energy absorbed → products higher
- Accurate diagram + labels → full ATP marks for energy experiments
Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Chemistry Full Scale Course