RI 2025 Planning Experiment 2 Notes

2025 Year 6 H2 Chemistry Planning Experiments Lecture 2 Raffles Institution 1 Raffles Institution Year 6 H2 Chemistry 2025 Planning Experiments Lecture 2 Gravimetry, Gas Collection, Energetics and Kinetics TAMP Framework When approaching planning questions, it is important to 1. know the typical steps (processes) involved in different types of experiments 2. interpret and incorporate information provided in the question in your plan. We are going to use the TAMP framework to help you organise information from the question to write a clear and coherent plan. What does it stand for: What do you have to do: Topic & Aim Identifying the Topic tested and the Aim of the experiment narrows the scope of your thinking quickly. The Process involved for the experiment should come to mind. Analyse clues Identify and Analyse the clues / information given in the question which can be found in 1. description of experiment 2. apparatus provided 3. chemicals provided 4. bulleted points in question Analyse the clues / information allows you to appropriately adapt the typical Process for that particular experiment. Measurements Decide and write down the quantities of the chemicals to be used and measured. Some of these quantities may be  assumed to take particular values, or  calculated based on Clues or your assumptions. Process TAM involves you responding the unique question presented. Each type of experiment has a typical series of steps and considerations i.e. Process, which you can / should remember. How much of the typical series of steps and considerations is used in your answer depends on what you have Analysed previously. In these lecture, we will be 1. going into the considerations in planning the different topics. 2. highlight the typical steps and considerations i.e. Process in the different topics. 3. solve worked examples using the TAMP framework to adapt the Process to situation presented in the question.

2025 Year 6 H2 Chemistry Planning Experiments Lecture 2 Raffles Institution 2 Common apparatus for gravimetric analysis using thermal decomposition  electronic mass balance  crucible / boiling tube  desiccator GRAVIMETRIC ANALYSIS Gravimetry involves the accurate measurement of mass as a means of quantifying a sample. The substance to be weighed may be formed from thermal decomposition or by precipitation. THERMAL DECOMPOSITION This method involves heating a compound until it decomposes to give a solid residue and a gaseous product. General procedure:  Record the mass of an empty boiling tube/crucible.  Weigh accurately m g of the compound in the boiling tube. Record total mass.  Heat the sample gently in the boiling tube using a Bunsen burner.  Then heat strongly for at least 10 min.  Cool and weigh the boiling tube and its contents.  The process of heating, cooling and weighing is repeated until constant mass is achieved. Points to take note:  Apparatus used for heating: dry and clean boiling tube or crucible  Heating should be gentle at first to prevent spattering of the sample. Strong heating is then carried out for about 10 min.  The crucible and its contents should be cooled to room temperature. If the crucible and its contents are weighed when they are hot, the measured mass will be less than their true mass. Why? As hot air rises above the crucible, cold air is drawn in and as it heats up and rises, it lifts the crucible very slightly from the balance pan.  Ideally, the crucible and its contents should be cooled in a desiccator (see below). The desiccator provides a dry atmosphere and allows the crucible and its contents to cool without absorbing moisture.  The procedure of reheating, cooling and weighing of the solid until constant mass is important in gravimetric analysis because this ensures that the sample is completely decomposed.  Possible source of error: Due to uneven heating, part of the sample may not have decomposed. Basic safety considerations:  Allow the crucible to cool sufficiently before handling and handle a hot crucible with crucible tongs. This is to ensure that your hands do not get burnt.  While heating a sample using a boiling tube, move the boiling tube up and down, and do not heat it at one spot. This is to prevent the contents of the boiling tube from spurting due to intensive heating at one spot.  Handle the boiling tube with a test tube holder at all times. This ensures that your hands do not get burnt.  Point the boiling tube away from yourself and others. This ensures that no one gets hurt in case of spurting.

2025 Year 6 H2 Chemistry Planning Experiments Lecture 2 Raffles Institution 3 The table below shows some substances that decompose on heating. Substance Effect on heatin g Examples Carbonate  Many carbonates decompose to produce oxide and carbon dioxide on heating.  Example: MgCO 3(s)  MgO(s) + CO2(g)  Na 2CO3 and K2CO3 are stable to heat and do not decompose on heating.  Determine the identity of M in MCO3  Determine the % composition of MgCO3 contaminated with K2CO3 (Na2CO3 and K2CO3 are stable to heat and do not decompose on heating.)  Determine the % by mass of NaHCO3 in a mixture of NaHCO3 and Na2CO3. [see worked example 2] Hydroxide  Group 2 hydroxides decompose on heating to give their respective oxides and water  Example: Mg(OH) 2(s)  MgO(s) + H2O(g)  Determine the identity of M in M(OH)2  Determine the % composition of Mg(OH)2 in a mixture of Mg(OH)2 and MgO. Salts with water of crystallisation  The water of crystallisat ion can be driven off, leaving the anhydrous salt.  Example: CuSO 4•5H2O(s)  CuSO4(s) + 5H2O(g)  Difference in the mass before and after heating allows us to find the number of water of cr ystallisation present.  Determine the value of n in CuSO4•nH2O [see worked example 1]  Determine the relative molecular mass of X in X•5H2O Gravimetry (Thermal decomposition) Topic & Aim Analyse clues Measurements Mass (see below) Process  Weighing Mass of empty container / g Mass of container + solid / g Mass of container + contents after heating / g  Heat gently, then strongly  Heat-cool-weigh until constant mass

2025 Year 6 H2 Chemistry Planning Experiments Lecture 2 Raffles Institution 4  Worked Example 1  (a) Procedure 1. Using an analytical balance, weigh and record the mass of a clean, empty and dry crucible. 2. Weigh out accurately about 5.00 g of solid CuSO 4•nH2O into the crucible. Record the total mass of the crucible and the solid CuSO4•nH2O. 3. Using a Bunsen burner, heat the crucible and its contents gently at first, and then heat strongly for 10 minutes. 4. Cool and weigh the crucible and its contents. 5. Repeat the heating-cooling-weighing process until constant mass is achieved. (b) Tabulation of results Mass of empty crucible / g A Mass of crucible and CuSO4•nH2O / g B Mass of crucible and its contents after first heating / g after second heating / g after third heatin g / g C D D (c) Analysis of results – to determine the value of n Mass of CuSO4 left after heating = (D – A) g Molar mass of CuSO4 = 159.6 g mol–1 Amount of CuSO4 = 159.6 DA mo