During the combustion of 5.000 g of octane, C8H18, 239.5 kcal is released. Part

ID: 897774 • Letter: D

Question

During the combustion of 5.000 g of octane, C8H18, 239.5 kcal is released.

Part A: Write a balanced equation for the combustion reaction.

Express your answer as a chemical equation with whole number coefficients. Include phases.

Part B: What is the sign of H for this reaction? H<0 OR H>0

Part C: How much energy is released by the combustion of 1.451 mol of C8H18 ?

Express your answer uas a positive number of kcal.

Part D: How many grams of octane must be burned to release 374.3 kcal ?

Part E: How many kilocalories are released by the combustion of 17.4 g of C8H18 ?

Express your answer as a positive number in kcal

The equation:

C8H18 + O2 ---> CO2+ H2O

Balanced equation

C8H18 + 25/2O2 ---> 8CO2 + 9H2O

For whole numebrs, multiply by 2

2C8H18(g) + 25O2(g) ---> 16CO2(g) + 18H2O(g)

PART B)

dH of reaction must be negative since this an exothermic reaction (will "free" energy)

PART C)

5 g of C8H18 --> 239.5 kcal

MW of C8H18 = 114.23 g/mol

mol = mass/MW= 5/114.23 = 0.04377 mol of Octane --> 239.5 kcal

Therefore, if we have 1.451 mol of OCtane

1.451/0.04377 * 239.5 kcal = 7939.5 kcal are released

PART D)

How many grams of octane must be burned to get 374.3 kcal

5 grams are needed for 239.5 kcal

x gram are needed for 374.3 kcal

that is

374.3/239.5 * 5 = 7.81 grams of Octane are needed

PART E)

Find calocalories when m = 17.4 g

5 grams --> 239.5 kcal

17.4 g ---> x kcal

17.4/5 * 239.5 = 833.46 kcal will be released

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