Reaction profiles: distinguish the barrier from the overall change
| English | 中文 | Pinyin |
|---|---|---|
| activation energy/ˌæktɪˈveɪʃn ˈenədʒi/ | 活化能 | huó huà néng |
| reaction profile | 反应能量图 | fǎn yìng néng liàng tú |
What would explain this observation?
- An exothermic reaction may still need a spark to start. Releasing energy overall does not remove the initial energy barrier that reacting particles must overcome.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- Reactions occur when particles collide with sufficient energy. Activation energy 活化能 is the minimum energy particles must have for the reaction to occur. A reaction profile 反应能量图 plots energy vertically against reaction progress horizontally. The reactant level leads up to a curved peak, then down to the product level. The activation-energy arrow runs from the reactant level to the peak; it is not measured from the graph’s arbitrary zero.
- activation energy: The minimum energy that particles must have for a reaction to occur; reaction profile: A diagram showing energy levels as a reaction progresses.
A profile has products above reactants. What does it show?
In an exothermic profile products lie below reactants, showing an overall energy decrease of the reacting chemicals and transfer to surroundings. In an endothermic profile products lie above reactants, showing energy taken from surroundings. Both profiles can have an activation barrier. The vertical difference between reactants and products is the overall change, distinct from the larger climb to the peak. Horizontal distance is not elapsed time or a measured reaction rate.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- In an exothermic profile products lie below reactants, showing an overall energy decrease of the reacting chemicals and transfer to surroundings. In an endothermic profile products lie above reactants, showing energy taken from surroundings. Both profiles can have an activation barrier. The vertical difference between reactants and products is the overall change, distinct from the larger climb to the peak. Horizontal distance is not elapsed time or a measured reaction rate.
- Draw axes first, mark clearly different reactant and product levels, then join them with a smooth curve rising above both. Label reactants, products and activation energy. Add a separate vertical arrow for the overall energy change. Compare the two diagrams using level differences, not the apparent length of the curve. Use relative energy units if numbers are supplied; they illustrate a model rather than requiring calorimetry.
Which two habits make the investigation or model in this case more defensible?
Draw axes first, mark clearly different reactant and product levels, then join them with a smooth curve rising above both. Label reactants, products and activation energy. Add a separate vertical arrow for the overall energy change. Compare the two diagrams using level differences, not the apparent length of the curve. Use relative energy units if numbers are supplied; they illustrate a model rather than requiring calorimetry.
Work from known quantities
- State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
- Known: a supplied relative-energy model has reactants at 30, peak at 85 and products at 10 units. Activation energy is 85−30=55 units; the reacting chemicals decrease by 30−10=20 units, so it is exothermic. If products instead lie at 50 with the same reactants and peak, the increase is 20 units and the reaction is endothermic. The initial barrier remains 55 units.
Relative reactant level is 25 and peak level is 90. Find the activation-energy difference. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
Relative reactant level is 25 and peak level is 90. Find the activation-energy difference.
The result is 65 relative units. Known: a supplied relative-energy model has reactants at 30, peak at 85 and products at 10 units. Activation energy is 85−30=55 units; the reacting chemicals decrease by 30−10=20 units, so it is exothermic. If products instead lie at 50 with the same reactants and peak, the increase is 20 units and the reaction is endothermic. The initial barrier remains 55 units.
Check the conclusion and its limits
- A peak is not an intermediate product that must be collected. Do not label reaction progress as time, equate activation energy to overall energy change or assume exothermic reactions always occur rapidly at room temperature. The source requires qualitative profiles; numerical relative levels are a way to practise reading their geometry.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Every exothermic reaction has zero activation energy. This claim is false: A peak is not an intermediate product that must be collected. Do not label reaction progress as time, equate activation energy to overall energy change or assume exothermic reactions always occur rapidly at room temperature. The source requires qualitative profiles; numerical relative levels are a way to practise reading their geometry.
Reaction profiles: distinguish the barrier from the overall change: In an exothermic profile products lie below reactants, showing an overall energy decrease of the reacting chemicals and transfer to surroundings. In an endothermic profile products lie above reactants, showing energy taken from surroundings. Both profiles can have an activation barrier. The vertical difference between reactants and products is the overall change, distinct from the larger climb to the peak. Horizontal distance is not elapsed time or a measured reaction rate.
Every exothermic reaction has zero activation energy.
A peak is not an intermediate product that must be collected. Do not label reaction progress as time, equate activation energy to overall energy change or assume exothermic reactions always occur rapidly at room temperature. The source requires qualitative profiles; numerical relative levels are a way to practise reading their geometry.
The minimum energy that particles must have for a reaction to occur: write the technical term.
activation energy means The minimum energy that particles must have for a reaction to occur.