Reversible reactions: the products can reform the reactants
| English | 中文 | Pinyin |
|---|---|---|
| reversible reaction/rɪˈvɜːsɪbl rɪˈækʃn/ | 可逆反应 | kě nì fǎn yìng |
| reversible arrow | 可逆箭头 | kě nì jiàn tóu |
What would explain this observation?
- Heating ammonium chloride can produce ammonia and hydrogen chloride. Cooling the gases together can reform the original ammonium chloride: both directions are chemical reactions.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- In a reversible reaction 可逆反应, products can react to produce the original reactants. Write A + B ⇌ C + D with two opposed half arrows. Conditions can favour a direction. The specified example is ammonium chloride ⇌ ammonia + hydrogen chloride: heating favours decomposition, while cooling the gases together reforms ammonium chloride. Each direction forms substances with different chemical identities from its starting materials.
- reversible reaction: A reaction whose products can react to reform the original reactants; reversible arrow 可逆箭头: Opposed half arrows indicating a reaction can proceed in both directions.
What makes the specified chemical reaction reversible?
The symbol does not mean that all substances must be present in equal quantities or that the reaction continually switches completely from one side to the other. A reversible reaction may reach equilibrium when substances are contained under suitable fixed conditions, but reversibility alone is not proof that an open apparatus has reached equilibrium. Melting/freezing is a reversible physical change; it does not by itself demonstrate this chemical product-to-reactant relationship.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- The symbol does not mean that all substances must be present in equal quantities or that the reaction continually switches completely from one side to the other. A reversible reaction may reach equilibrium when substances are contained under suitable fixed conditions, but reversibility alone is not proof that an open apparatus has reached equilibrium. Melting/freezing is a reversible physical change; it does not by itself demonstrate this chemical product-to-reactant relationship.
- Use supplied observations or a teacher demonstration under the approved risk assessment. Ammonia and hydrogen chloride gases are irritant; students must not generate or smell them independently. Record where gas forms and where the white solid reappears, distinguish observation from chemical explanation and retain the condition labels. Do not improvise a sealed heated vessel. Write a balanced equation with suitable state symbols only for the stated conditions.
Which two habits make the investigation or model in this case more defensible?
Use supplied observations or a teacher demonstration under the approved risk assessment. Ammonia and hydrogen chloride gases are irritant; students must not generate or smell them independently. Record where gas forms and where the white solid reappears, distinguish observation from chemical explanation and retain the condition labels. Do not improvise a sealed heated vessel. Write a balanced equation with suitable state symbols only for the stated conditions.
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: NH₄Cl(s) ⇌ NH₃(g) + HCl(g) conserves one N, four H and one Cl atom on each side. One represented ammonium chloride formula unit gives one ammonia and one hydrogen chloride molecule. Six decomposed formula units give six of each gas, twelve gas molecules total in this particle model. Cooling permits those pairs to reform six formula units.
In NH₄Cl ⇌ NH₃ + HCl, ten represented formula units decompose. Find the total number of gas molecules formed. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
In NH₄Cl ⇌ NH₃ + HCl, ten represented formula units decompose. Find the total number of gas molecules formed.
The result is 20 molecules. Known: NH₄Cl(s) ⇌ NH₃(g) + HCl(g) conserves one N, four H and one Cl atom on each side. One represented ammonium chloride formula unit gives one ammonia and one hydrogen chloride molecule. Six decomposed formula units give six of each gas, twelve gas molecules total in this particle model. Cooling permits those pairs to reform six formula units.
Check the conclusion and its limits
- The gases do not need to have the same mass merely because their molecule counts match. A reversible arrow does not give equal rates at every moment. White solid forming on cooling must be interpreted with the identified gases; colour alone cannot establish the reaction. Written interpretation does not replace safe supervised observations.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
A reversible arrow proves that forward and reverse rates are equal at every moment. This claim is false: The gases do not need to have the same mass merely because their molecule counts match. A reversible arrow does not give equal rates at every moment. White solid forming on cooling must be interpreted with the identified gases; colour alone cannot establish the reaction. Written interpretation does not replace safe supervised observations.
Reversible reactions: the products can reform the reactants: The symbol does not mean that all substances must be present in equal quantities or that the reaction continually switches completely from one side to the other. A reversible reaction may reach equilibrium when substances are contained under suitable fixed conditions, but reversibility alone is not proof that an open apparatus has reached equilibrium. Melting/freezing is a reversible physical change; it does not by itself demonstrate this chemical product-to-reactant relationship.
A reversible arrow proves that forward and reverse rates are equal at every moment.
The gases do not need to have the same mass merely because their molecule counts match. A reversible arrow does not give equal rates at every moment. White solid forming on cooling must be interpreted with the identified gases; colour alone cannot establish the reaction. Written interpretation does not replace safe supervised observations.
A reaction whose products can react to reform the original reactants: write the technical term.
reversible reaction means A reaction whose products can react to reform the original reactants.