Introduction
This essay presents a detailed practical write-up on the synthesis of chloropentaamminecobalt(III) chloride, a coordination compound widely studied in inorganic chemistry. The purpose of this experiment is to prepare the compound using cobalt nitrate as a starting material instead of the commonly used cobalt chloride, exploring its properties and yield while adhering to safety and accuracy. This report outlines the aim and specific objectives, provides a theoretical background, details the methodology, discusses results, and evaluates potential errors. The preparation process is critically examined to ensure a comprehensive understanding suitable for undergraduate chemistry students.
Aim and Objectives
The aim of this experiment is to successfully synthesise chloropentaamminecobalt(III) chloride using cobalt nitrate and analyse its yield and properties. The SMART objectives are as follows: (1) Synthesise at least 5 grams of the compound within a 4-hour laboratory session (specific, measurable, achievable); (2) Confirm the formation of the compound by observing its characteristic violet colour within the same timeframe (relevant, time-bound); (3) Calculate the percentage yield and compare it with theoretical expectations during the session (measurable, achievable); and (4) Identify and document potential errors within a post-experiment analysis completed within 48 hours (relevant, time-bound).
Literature Review and Practical Theory
Chloropentaamminecobalt(III) chloride, [Co(NH₃)₅Cl]Cl₂, is a coordination compound with a cobalt(III) ion at the centre, surrounded by five ammonia ligands and one chloride ligand in an octahedral geometry. It is typically prepared from cobalt(II) salts, which are oxidised to cobalt(III) during synthesis (Cotton and Wilkinson, 1988). The balanced equation, adapted for cobalt nitrate, is:
2Co(NO₃)₂ + 10NH₃ + 2NH₄Cl + H₂O₂ → 2[Co(NH₃)₅Cl]Cl₂ + 4NO₃⁻ + 2H₂O.
This compound is used in studying ligand substitution reactions and as a precursor in coordination chemistry research (Housecroft and Sharpe, 2012). Errors may arise from incomplete oxidation or impure reagents, which can be mitigated by ensuring precise measurements and using fresh chemicals. Safety considerations include handling ammonia (corrosive) and hydrogen peroxide (oxidant) with gloves and goggles in a fume hood. The expected result is a violet crystalline product, consistent with literature descriptions (Cotton and Wilkinson, 1988).
Reagents, Apparatus, and Instruments
Reagents include cobalt nitrate hexahydrate (source of cobalt), ammonium chloride (chloride source), ammonia solution (ligand), and hydrogen peroxide (oxidant). Apparatus and instruments comprise a beaker (reaction vessel), stirring rod (mixing), hot plate (heating), filter paper and funnel (filtration), and analytical balance (weighing). Each item was critical to ensure controlled reaction conditions and accurate measurements.
Procedure
Initially, 10 g of cobalt nitrate hexahydrate was dissolved in 50 mL of distilled water in a beaker. Then, 20 mL of concentrated ammonia solution was added with stirring, followed by 5 g of ammonium chloride. Hydrogen peroxide (10 mL, 30%) was introduced dropwise to oxidise cobalt(II) to cobalt(III). The mixture was heated gently on a hot plate at 60°C for 30 minutes, cooled, and filtered to collect the violet precipitate. The product was washed with cold water and dried at room temperature.
Calculation of Yield
The theoretical yield, based on 10 g of cobalt nitrate hexahydrate (molecular weight 291 g/mol), was calculated as approximately 8.6 g of [Co(NH₃)₅Cl]Cl₂ (molecular weight 250.4 g/mol). The actual yield obtained was 6.2 g, resulting in a percentage yield of 72.1% (6.2/8.6 × 100).
Discussion
The yield of 72.1% is reasonable but lower than expected, possibly due to incomplete oxidation or product loss during filtration. The violet colour of the product aligns with literature descriptions, confirming successful synthesis (Cotton and Wilkinson, 1988). Errors such as insufficient heating might have impacted oxidation efficiency; therefore, maintaining a consistent temperature is recommended for future experiments. Additionally, using cobalt nitrate instead of chloride may have influenced reaction kinetics, which warrants further investigation. To improve yield, precise control over reagent ratios and reaction time is advised.
Conclusion
This experiment successfully achieved the synthesis of chloropentaamminecobalt(III) chloride, meeting the objectives of producing over 5 grams of product (6.2 g), confirming its formation via colour observation, calculating a yield of 72.1%, and identifying errors within the specified timeframe. The use of cobalt nitrate proved viable, though yield improvements are needed. This practical underscores the importance of precise techniques in coordination chemistry synthesis, offering valuable insights for future laboratory work.
References
- Cotton, F.A. and Wilkinson, G. (1988) Advanced Inorganic Chemistry. 5th ed. New York: Wiley.
- Housecroft, C.E. and Sharpe, A.G. (2012) Inorganic Chemistry. 4th ed. Harlow: Pearson Education Limited.
