Combustion Analysis: Determining Chemical Formulas Through Burning, Summaries of Organic Chemistry

Download Combustion Analysis: Determining Chemical Formulas Through Burning and more Summaries Organic Chemistry in PDF only on Docsity!

lOMoARcPSD|

J. McMurry,

Organic chemistry

Combustion Analysis

How can burning a substance help determine the substance’s chemical formula?

Why?

Scientists have many techniques to help them determine the chemical formula or structure of an unknown compound. One commonly used technique when working with carbon-containing compounds is combus- tion analysis. Any compound containing carbon and hydrogen will burn. With an ample oxygen supply, the products of the combustion will be carbon dioxide and water. Analyzing the mass of CO 2 and H 2 O that are produced allows chemists to determine the ratios of elements in the compound.

Model 1 – Combustion Reactions

CH 4 + O 2 → CO 2 + H 2 O

C 2 H 6 + O 2 → CO 2 + H 2 O

C 2 H 4 + O 2 → CO 2 + H 2 O

C 3 H 8 + O 2 → CO 2 + H 2 O

1. According to Model 1, what reactant is always required for combustion?
2. Balance the reactions in Model 1 while keeping the hydrocarbon coefficient a “1.” This may require the use of fractions in other places.
3. How is the coefficient of CO 2 in the chemical reactions in Model 1 related to the chemical for- mula of the hydrocarbon being analyzed?
4. How is the coefficient of the H 2 O in the chemical reactions in Model 1 related to the chemical formula of the hydrocarbon being analyzed? Combustion Analysis 1

6. Discuss with your group how the data from Model 2 could be used to find the lowest whole number ratio between carbon and hydrogen atoms. This will give you the empirical formulas of the sample substances. Fill in the last column of Model 2.
7. Did you need balanced chemical combustion equations to find the empirical formulas of the unknowns in Model 2?
8. Did you need to know the mass of the samples to find the empirical formulas of the unknowns in Model 2?
9. What other information would you need to determine the molecular formulas?
10. A 15.00-g sample of an unknown hydrocarbon is analyzed by combustion analysis. The sample produced 50.70 grams of carbon dioxide and 10.42 grams of water. Find the empirical formula of the unknown.
11. Calculate the total mass of carbon atoms and hydrogen atoms for each sample in Model 2. Divide the work among group members. Create a new column in Model 2 for these data. a. How does the mass of carbon and hydrogen atoms compare to the mass of the original sample? b. Name the scientific law that justifies your answer to part a. c. Would part a be true if the original sample included atoms other than carbon and hydrogen? For example: C 2 H 6 O or C 2 H 5 NH 2. Justify your reasoning. Combustion Analysis 3

Read This!

When the combustion analysis unknown is a compound containing only carbon and hydrogen, all of the atoms in the sample end up in either the CO 2 or H 2 O products. However, if the unknown contains other elements, like oxygen or nitrogen, those atoms might end up in the CO 2 and H 2 O products (in the case of oxygen) or they might form other gases that move through the apparatus without being captured. Ad- ditionally, O atoms may come from the atmosphere as opposed to the combusting sample. Moles of these atoms cannot be calculated by stoichiometry directly. Instead, we must use the law of conservation of mass.

Model 3 – Combustion Analysis of CxHyOz Unknowns (10.00-g samples)

Sample Mass of CO 2 Produced Moles of Carbon Atoms Mass of H 2 O Produced Moles of Hydrogen Atoms Total Mass of C and H Atoms Mass of O Atoms Moles of O Atoms Sample’s Empirical Formula 1 19.10 g 11.73 g 2 14.65 g 6.00 g 3 21.96 g 11.99 g 4 28.05 g 5.74 g

12. Four unknown hydrocarbons containing oxygen were analyzed by combustion analysis. The samples were 10.00 g each. The results are shown in Model 3. Discuss with your group what calculations would need to be performed to complete the table in Model 3. Divide the work among group members. Show work for your calculations below. 4 POGIL™^ Activities for AP* Chemistry

Extension Questions

16. It is critical in combustion analysis procedures that the sample be dry. Discuss what errors in data would occur if the sample contained moisture. How might this affect the final empirical formula?
17. Discuss what errors in data would occur if the sample contained a carbon-based impurity. How might this affect the final empirical formula?
18. Balance the general combustion equation below using variables for the missing coefficients. C x H y + O 2 → CO 2 + H 2 O 6 POGIL™^ Activities for AP* Chemistry