Five rate factors: change the conditions, keep comparisons fair
| English | 中文 | Pinyin |
|---|---|---|
| surface area/ˈsɜːfɪs ˈeərɪə/ | 表面积 | biǎo miàn jī |
| rate factor | 速率影响因素 | sù lǜ yǐng xiǎng yīn sù |
What would explain this observation?
- Powdered calcium carbonate reacts with acid faster than equal-mass large pieces. More exposed surface can speed the reaction without increasing the carbonate amount.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- Reaction rates are affected by concentration of reactants in solution, pressure of reacting gases, surface area 表面积 of solid reactants, temperature and catalysts. Increasing concentration or gas pressure generally increases collision frequency under the stated comparison. Increasing exposed solid surface gives more sites for collision. Raising temperature increases collision frequency and the fraction with sufficient energy. A suitable catalyst supplies a lower-activation-energy pathway.
- rate factor 速率影响因素: A condition whose change affects how quickly a reaction proceeds; surface area: The exposed area of a solid available for contact with reacting particles.
Equal carbonate masses react with excess acid. Smaller pieces reach the same final volume sooner. What changed?
State exactly what changes and what is controlled. For equal masses of the same carbonate with acid in excess, smaller pieces can reach the same final carbon dioxide amount sooner. If increasing acid concentration also changes the limiting reactant amount, both speed and final amount can change; this is a different comparison. Higher gas pressure is relevant to reacting gases, not automatically a general explanation for every liquid reaction.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- State exactly what changes and what is controlled. For equal masses of the same carbonate with acid in excess, smaller pieces can reach the same final carbon dioxide amount sooner. If increasing acid concentration also changes the limiting reactant amount, both speed and final amount can change; this is a different comparison. Higher gas pressure is relevant to reacting gases, not automatically a general explanation for every liquid reaction.
- Choose one independent variable and measure a defensible rate response: initial curve steepness, quantity formed in a fixed time, or time to a defined comparable endpoint. Keep other factors controlled, including temperature, total liquid volume, solid mass and exposed surface where appropriate. Repeat measurements and retain observations. A catalyst must be suitable for that reaction; a biological enzyme is not a universal catalyst for unrelated chemical changes.
Which two habits make the investigation or model in this case more defensible?
Choose one independent variable and measure a defensible rate response: initial curve steepness, quantity formed in a fixed time, or time to a defined comparable endpoint. Keep other factors controlled, including temperature, total liquid volume, solid mass and exposed surface where appropriate. Repeat measurements and retain observations. A catalyst must be suitable for that reaction; a biological enzyme is not a universal catalyst for unrelated chemical changes.
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: an original comparison reaches a fixed 30 cm³ gas endpoint in 40 s for larger pieces and 20 s for smaller pieces. Endpoint mean rates are 30/40=0.75 and 30/20=1.50 $\dfrac{\text{cm}^3}{\text{s}}$, a twofold ratio. This does not prove that the instantaneous rate was exactly doubled at every time or that the smaller pieces yield twice as much gas overall.
A fixed 36 cm³ gas endpoint is reached in 24 s. Find mean rate to that endpoint. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
A fixed 36 cm³ gas endpoint is reached in 24 s. Find mean rate to that endpoint.
The result is 1.5 cm³ per s. Known: an original comparison reaches a fixed 30 cm³ gas endpoint in 40 s for larger pieces and 20 s for smaller pieces. Endpoint mean rates are 30/40=0.75 and 30/20=1.50 cm³ per second, a twofold ratio. This does not prove that the instantaneous rate was exactly doubled at every time or that the smaller pieces yield twice as much gas overall.
Check the conclusion and its limits
- Do not confuse concentration with total volume or pressure with temperature. Cutting a solid raises exposed area without changing the identity of its atoms. A hotter reaction can still have the same limiting-reactant yield. This source section is both-tier; detailed equilibrium shifts are separate Higher content.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Increasing rate always increases the final amount of product. This claim is false: Do not confuse concentration with total volume or pressure with temperature. Cutting a solid raises exposed area without changing the identity of its atoms. A hotter reaction can still have the same limiting-reactant yield. This source section is both-tier; detailed equilibrium shifts are separate Higher content.
Five rate factors: change the conditions, keep comparisons fair: State exactly what changes and what is controlled. For equal masses of the same carbonate with acid in excess, smaller pieces can reach the same final carbon dioxide amount sooner. If increasing acid concentration also changes the limiting reactant amount, both speed and final amount can change; this is a different comparison. Higher gas pressure is relevant to reacting gases, not automatically a general explanation for every liquid reaction.
Increasing rate always increases the final amount of product.
Do not confuse concentration with total volume or pressure with temperature. Cutting a solid raises exposed area without changing the identity of its atoms. A hotter reaction can still have the same limiting-reactant yield. This source section is both-tier; detailed equilibrium shifts are separate Higher content.
A condition whose change affects how quickly a reaction proceeds: write the technical term.
rate factor means A condition whose change affects how quickly a reaction proceeds.