Dynamic equilibrium: equal rates, continuing reactions
| English | 中文 | Pinyin |
|---|---|---|
| reversible reaction/rɪˈvɜːsɪbl rɪˈækʃn/ | 可逆反应 | kě nì fǎn yìng |
| equilibrium/ˌiːkwɪˈlɪbrɪəm/ | 平衡 | píng héng |
What would explain this observation?
- A closed reaction vessel can show constant composition while particles still react in both directions. Equality of rates keeps the overall amounts unchanged.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- When a reversible reaction 可逆反应 occurs in apparatus preventing escape of reactants and products, equilibrium 平衡 is reached when forward and reverse reactions occur at exactly the same rate. The state is dynamic: both directions continue. At fixed conditions the amounts of reactants and products stay constant, but they need not be equal. A closed system prevents loss of matter; it does not necessarily prevent energy transfer to maintain a set temperature.
- equilibrium: A state with equal forward and reverse reaction rates; reversible reaction: A reaction whose products can react to reform the original reactants.
Which pair must be equal at dynamic equilibrium?
Beginning with mostly reactants, the forward rate may be high while the reverse rate is initially low. As products accumulate, reverse reaction becomes possible more often until the rates match under the stated model. A reaction-rate graph should show convergence to the same non-zero rate. A composition graph can level off at different reactant and product amounts. Identify which quantity is on the axis before claiming two curves must meet.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- Beginning with mostly reactants, the forward rate may be high while the reverse rate is initially low. As products accumulate, reverse reaction becomes possible more often until the rates match under the stated model. A reaction-rate graph should show convergence to the same non-zero rate. A composition graph can level off at different reactant and product amounts. Identify which quantity is on the axis before claiming two curves must meet.
- Use an original particle-count model or supplied data with fixed conditions and no matter lost. Count forward and reverse events over equal time windows. Their difference gives net change for a one-to-one A ⇌ B model. Do not build a sealed heated chemical apparatus to demonstrate the idea; safe school demonstration arrangements require approved containment and temperature control. A count model illustrates balance without proving a particular real chemical rate law.
Which two habits make the investigation or model in this case more defensible?
Use an original particle-count model or supplied data with fixed conditions and no matter lost. Count forward and reverse events over equal time windows. Their difference gives net change for a one-to-one A ⇌ B model. Do not build a sealed heated chemical apparatus to demonstrate the idea; safe school demonstration arrangements require approved containment and temperature control. A count model illustrates balance without proving a particular real chemical rate law.
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: in a closed A ⇌ B model, 18 forward and 18 reverse events occur in one second. Net B change=18−18=0 even though 36 reaction events occur. If 70 A and 30 B particles remain on average, these unequal amounts are compatible with equilibrium. At an earlier interval with 20 forward and 12 reverse events, net B increases by eight and the model is not yet at equilibrium.
In a one-to-one A ⇌ B model, 22 forward and 22 reverse events occur in one second. Find net change in B count. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
In a one-to-one A ⇌ B model, 22 forward and 22 reverse events occur in one second. Find net change in B count.
The result is 0 particles. Known: in a closed A ⇌ B model, 18 forward and 18 reverse events occur in one second. Net B change=18−18=0 even though 36 reaction events occur. If 70 A and 30 B particles remain on average, these unequal amounts are compatible with equilibrium. At an earlier interval with 20 forward and 12 reverse events, net B increases by eight and the model is not yet at equilibrium.
Check the conclusion and its limits
- Constant amounts do not mean particles are motionless or reaction has stopped. A steady open-flow system can also have constant measured amounts, so constancy alone does not establish closed-system equilibrium. Equal mass, concentration and molecule count are not the required equality: rates are. Condition-change predictions are separately Higher-only.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
At dynamic equilibrium both chemical reactions have stopped. This claim is false: Constant amounts do not mean particles are motionless or reaction has stopped. A steady open-flow system can also have constant measured amounts, so constancy alone does not establish closed-system equilibrium. Equal mass, concentration and molecule count are not the required equality: rates are. Condition-change predictions are separately Higher-only.
Dynamic equilibrium: equal rates, continuing reactions: Beginning with mostly reactants, the forward rate may be high while the reverse rate is initially low. As products accumulate, reverse reaction becomes possible more often until the rates match under the stated model. A reaction-rate graph should show convergence to the same non-zero rate. A composition graph can level off at different reactant and product amounts. Identify which quantity is on the axis before claiming two curves must meet.
At dynamic equilibrium both chemical reactions have stopped.
Constant amounts do not mean particles are motionless or reaction has stopped. A steady open-flow system can also have constant measured amounts, so constancy alone does not establish closed-system equilibrium. Equal mass, concentration and molecule count are not the required equality: rates are. Condition-change predictions are separately Higher-only.
A state with equal forward and reverse reaction rates: write the technical term.
equilibrium means A state with equal forward and reverse reaction rates.