Higher Tier: count gaseous coefficients before predicting a shift
| English | 中文 | Pinyin |
|---|---|---|
| compression/kəmˈpreʃn/ | 压缩 | yā suō |
| gas-coefficient count | 气体系数计数 | qì tǐ xì shù jì shù |
What would explain this observation?
- Higher Tier: Compressing an equilibrium gas mixture favours the side with fewer gaseous particles. Count coefficients from the balanced equation before naming the favoured product.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- For gaseous equilibria, increasing pressure by compression 压缩 at fixed temperature shifts toward the side with fewer gas molecules in the balanced equation. Decreasing pressure by expansion favours the side with more gas molecules. Count gaseous coefficients, not every atom inside each formula. For N₂(g) + 3H₂(g) ⇌ 2NH₃(g), the left side has four gas molecules to the right side’s two, so compression favours ammonia.
- gas-coefficient count 气体系数计数: The sum of balanced equation coefficients for gaseous species on one side; compression: Reducing the volume of a stated gas mixture to raise its pressure at fixed temperature.
For N₂(g) + 3H₂(g) ⇌ 2NH₃(g), compression at fixed temperature favours which side?
If the gas-molecule counts are equal on both sides, changing pressure by compression gives no pressure-driven composition shift in this model. H₂(g) + I₂(g) ⇌ 2HI(g) has two on each side. If solids or liquids occur in a supplied equation, do not count their coefficients as gas molecules. Compression can also affect rates by increasing gas particle concentration; a rate effect alone is not proof of an equilibrium shift.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- If the gas-molecule counts are equal on both sides, changing pressure by compression gives no pressure-driven composition shift in this model. H₂(g) + I₂(g) ⇌ 2HI(g) has two on each side. If solids or liquids occur in a supplied equation, do not count their coefficients as gas molecules. Compression can also affect rates by increasing gas particle concentration; a rate effect alone is not proof of an equilibrium shift.
- Write state symbols and a gas-count total under each side. Specify compression or expansion of the reacting mixture at constant temperature: merely adding an unrelated gas does not justify blindly applying the same rule without a volume/pressure description. Evaluate a supplied industrial choice using yield, rate, equipment cost and operating constraints. Do not ask students to construct high-pressure reaction vessels.
Which two habits make the investigation or model in this case more defensible?
Write state symbols and a gas-count total under each side. Specify compression or expansion of the reacting mixture at constant temperature: merely adding an unrelated gas does not justify blindly applying the same rule without a volume/pressure description. Evaluate a supplied industrial choice using yield, rate, equipment cost and operating constraints. Do not ask students to construct high-pressure reaction vessels.
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 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), gas coefficients total three on the left and two on the right. Compression favours the product side; expansion favours reactants. One thousand represented stoichiometric forward sets convert 3,000 gas molecules into 2,000, a decrease of 1,000 in this counting model. The equation count predicts shift direction, not a specific percentage conversion.
For N₂ + 3H₂ ⇌ 2NH₃ with all species gaseous, find the total gaseous coefficient count on the reactant side. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
For N₂ + 3H₂ ⇌ 2NH₃ with all species gaseous, find the total gaseous coefficient count on the reactant side.
The result is 4 . Known: in 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), gas coefficients total three on the left and two on the right. Compression favours the product side; expansion favours reactants. One thousand represented stoichiometric forward sets convert 3,000 gas molecules into 2,000, a decrease of 1,000 in this counting model. The equation count predicts shift direction, not a specific percentage conversion.
Check the conclusion and its limits
- Do not count oxygen atoms to decide the gas-count rule. A higher pressure does not always favour products; it favours the smaller gaseous count. Equal counts mean no pressure-driven shift under the stipulated compression model, not that rate can never change. This entire source section is Higher-only.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Increasing pressure always shifts equilibrium toward the products. This claim is false: Do not count oxygen atoms to decide the gas-count rule. A higher pressure does not always favour products; it favours the smaller gaseous count. Equal counts mean no pressure-driven shift under the stipulated compression model, not that rate can never change. This entire source section is Higher-only.
Higher Tier: count gaseous coefficients before predicting a shift: If the gas-molecule counts are equal on both sides, changing pressure by compression gives no pressure-driven composition shift in this model. H₂(g) + I₂(g) ⇌ 2HI(g) has two on each side. If solids or liquids occur in a supplied equation, do not count their coefficients as gas molecules. Compression can also affect rates by increasing gas particle concentration; a rate effect alone is not proof of an equilibrium shift.
Increasing pressure always shifts equilibrium toward the products.
Do not count oxygen atoms to decide the gas-count rule. A higher pressure does not always favour products; it favours the smaller gaseous count. Equal counts mean no pressure-driven shift under the stipulated compression model, not that rate can never change. This entire source section is Higher-only.
The sum of balanced equation coefficients for gaseous species on one side: write the technical term.
gas-coefficient count means The sum of balanced equation coefficients for gaseous species on one side.