An environmental chemist is investigating the rate of weathering of limestone (calcium carbonate) by acid rain. They model this in the laboratory by reacting a large excess of limestone chippings with 100 cm3100\text{ cm}^3100 cm3 of 0.20 mol dm−30.20\text{ mol dm}^{-3}0.20 mol dm−3 nitric acid (HNO3\text{HNO}_3HNO3) in a conical flask placed on a continuous-recording electronic balance.
The equation for the reaction is:
CaCO3(s)+2HNO3(aq)→Ca(NO3)2(aq)+H2O(l)+CO2(g) \text{CaCO}_3(\text{s}) + 2\text{HNO}_3(\text{aq}) \rightarrow \text{Ca(NO}_3)_2(\text{aq}) + \text{H}_2\text{O}(\text{l}) + \text{CO}_2(\text{g}) CaCO3(s)+2HNO3(aq)→Ca(NO3)2(aq)+H2O(l)+CO2(g)A plug of cotton wool is placed in the neck of the flask, and the total mass of the flask and its contents is recorded over time.
Explain why a loose cotton wool plug is used in the neck of the flask, contrasting its function with:
Explain why using a large excess of limestone chippings ensures that the concentration of CaCO3\text{CaCO}_3CaCO3 (or its surface area) does not need to be factored in as a variable affecting the reaction rate over time.
Let mtm_tmt represent the mass of CO2\text{CO}_2CO2 lost up to time ttt, and m∞m_{\infty}m∞ represent the final mass of CO2\text{CO}_2CO2 lost when the reaction has completely stopped. Explain clearly why the quantity (m∞−mt)(m_{\infty} - m_t)(m∞−mt) is directly proportional to the concentration of nitric acid remaining in the flask at time ttt.
32 exam-style questions on AQA A Level Chemistry Rate equations (A-level only), covering Rate equations (A-level only) and Determination of rate equation (A-level only). Each one has a worked solution and a mark scheme showing where the marks go.