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Chemistry & Chemical Reactivity

10th Edition
John C. Kotz + 3 others
ISBN: 9781337399074

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BuyFindarrow_forward

Chemistry & Chemical Reactivity

10th Edition
John C. Kotz + 3 others
ISBN: 9781337399074
Textbook Problem

Peanuts and peanut oil are organic materials and bum in air. How many burning peanuts does it take to provide the energy to boil a cup of water (250 mL of water)? To solve this problem, we assume each peanut, with an average mass of 0.73 g, is 49% peanut oil and 21% starch; the remainder is noncombustible We further assume peanut oil is palmitic acid, C16H32O2, with an enthalpy of formation of −848.4 kJ/mol. Starch is a long chain of C6H10O5 units, each unit having an enthalpy of formation of −960 kJ.

Interpretation Introduction

Interpretation:

To identify how many burning peanuts are required to boil 250mL of water

Concept Introduction:

Heat energy required to raise the temperature of 1g of substance by 1K.Energy gained or lost can be calculated using the below equation.

  q=C×m×ΔT

Where, q= energy gained or lost for a given mass of substance (m), C =specific heat capacity,ΔT= change in temperature.

The enthalpy of combustion can be calculated as

                                               ΔcH0imagesΣnΔfH0(reactants)-ΣnΔfH0(products)

Explanation

Given 49% of the peanut is palmitic acid.

  0.73g×49100 = 0.3577g of palmitic acid.

Given 21% of the starch is palmitic acid.

  0.73g×21100=0.153g of starch

Molecular weight of starch=162g/mol

So we have 0.15162 = 0.000925 moles of starch.

Enthalpy of combustion of palmitic acid.is calculated as,

  C16H32O2+23O216O2+16H2O

Substitute in ΔcH0ΣnΔfH0(reactants)-ΣnΔfH0(products) as

  ΔcH0=(848.4+0)-(16× -393.5+16×241.8)=-9315.6kJ/mol

Enthalpy of combustion of starch.is calculated as

  C6H10O5+6O26O2+5H2O

  ΔcH0=(-960+0)-(6× -393.5+-241.8)=-2610kJ/mol

Since we have 0.000925 moles of starch = 0.000925 moles × -2610kJ/mol

                                                                =-2

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