# The useful energy that comes out of an energy transfer process is related to the efficiency of the process by the following equation: total consumed × efficiency=useful energy where the efficiency is in decimal (not percent) form. If a process is 30% efficient, how much useful energy can be derived if 455 kJ are consumed? A person eats approximately 2200 kcal/day. How much of that energy is available to do physical work? If a car needs 5.0 × 10 3 kJ to go a particular distance, how much energy will be consumed if the car is 20% efficient? If an electrical power plant produces 1.0 × 10 9 J of electrical energy, how much energy will be consumed by the plant if it is 34% efficient?

### Chemistry In Focus

7th Edition
Tro + 1 other
Publisher: Cengage Learning,
ISBN: 9781337399692

Chapter
Section

### Chemistry In Focus

7th Edition
Tro + 1 other
Publisher: Cengage Learning,
ISBN: 9781337399692
Chapter 9, Problem 43E
Textbook Problem
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## The useful energy that comes out of an energy transfer process is related to the efficiency of the process by the following equation: total consumed  ×  efficiency=useful energy where the efficiency is in decimal (not percent) form.If a process is 30% efficient, how much useful energy can be derived if 455 kJ are consumed?A person eats approximately 2200 kcal/day. How much of that energy is available to do physical work?If a car needs 5.0 × 10 3 kJ to go a particular distance, how much energy will be consumed if the car is 20% efficient?If an electrical power plant produces 1.0 × 10 9 J of electrical energy, how much energy will be consumed by the plant if it is 34% efficient?

Interpretation Introduction

Interpretation:

The useful energy (in kilojoules) derived and the energy (in kilojoules) consumed in the given energy transfer processes are to be calculated.

Concept introduction:

The power derived from the utilization of chemical or physical resources to provide heat and light or to carry out various processes is known as energy. The SI unit of energy is joule.

Unit conversion is a multiple-step process that is used to convert the unit of measurement of a given quantity. It is determined by multiplication with a conversion factor.

The useful energy derived can be calculated by using the equation:

Total consumed×efficency=useful energy

### Explanation of Solution

a) Amount of useful energy derived if 455 kJ are consumed and the process is 30% efficient.

The efficiency and the energy consumed in a process are given.

Energy = 455 kJ

Efficiency = 30%

The useful energy is calculated as follows:

The equation used to calculate useful energy is represented as follows:

Total consumed×efficency=useful energy

Conversion of percentage into decimal:

Efficiency=30%Efficiency=30100Efficiency=0.3

Substitute the values in the given equation:

Useful energy=455 kJ×0.3Useful energy=137 kJ

Therefore, the useful energy derived is 137 kJ.

b) The amount of energy that is available to do physical work when a person eats approximately 2200 kcal/day.

The efficiency of and the energy consumed (in kcal) of a process are given.

Energy = 2200 kcal/day

Efficiency = 45%

The useful energy is calculated as follows:

The equation used to calculate useful energy is represented as follows:

Total consumed×efficency=useful energy

The efficiency of a person is 45% (given in figure 9.4)

Conversion of percentage into decimal:

Efficiency=45%Efficiency=45100Efficiency=0.45

Substitute the values in the given equation:

Useful energy=2200 kJ×0.45Useful energy=990 kcal

Therefore, the useful energy derived is 990 kcal.

c) The amount of energy will be consumed if the car is 20% efficient and needed 5.0 ×10 kJ  to go a particular distance.

The efficiency and useful energy (in kJ) are given.

Energy = 5.0 ×10 kJ  Efficiency = 20%

The total energy consumed is calculated as follows:

The equation used to calculate the total energy consumed is represented as follows:

Total consumed×efficency=useful energy

The efficiency of a person is 20%

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