Question 1

The combustion of 5.00 g of C ₂ H ₆ (g) emits 259 kJ of heat at constant pressure. Calculate the enthalpy of combustion, Δ Hcom, for the reaction in kJ/g. 2 C₂H₆(g) + 7 O₂(g) → 4 CO₂(g) + 6 H₂O (l)

Accepted Answer

The enthalpy of combustion ($\Delta H_{com}$) per gram is calculated by dividing the total heat emitted by the mass of the substance combusted. Given that the combustion of 5.00 g of the substance emits 259 kJ of heat, the enthalpy of combustion per gram is $\frac{259\ kJ}{5.00\ g} = 51.8\ kJ/g$. Since the process is exothermic (emits heat), the value is negative, making the correct answer $-51.8\ kJ/g$.

Question 2

If the normal body temperature is 98.6 ° F, what is the corresponding temperature in ° C?

Accepted Answer

To convert Fahrenheit to Celsius, subtract 32 from the Fahrenheit temperature and then multiply by 5/9. 
Hence, (98.6 - 32) x 5/9 = 37°C.

Question 3

How many joules of heat can be produced from a 170-Calorie package of crackers?

Accepted Answer

1 Calorie (with a capital "C", also known as a food calorie) is equivalent to 1,000 calories (with a lowercase "c", also known as a small calorie), and 1 calorie is equivalent to 4.184 joules. Therefore, 170 Calories are equivalent to 170,000 calories, which in turn are equivalent to 170,000 * 4.184 = 711,280 joules, or 7.11 × 10^5 J.

Question 4

Calculate the energy generated in kilowatt-hours if the burning of 2.5 g of methane produces 125 kJ of heat.

Accepted Answer

First, we need to convert the mass of methane from grams to moles using its molar mass:

1 mole of CH4 (16.04 g) = 1 mol CH4

2.5 g CH4 = (2.5 g CH4) / (16.04 g/mol CH4) = 0.156 moles CH4

Next, we can use the given heat of combustion (125 kJ) to calculate the energy generated per mole of CH4:

125 kJ / 0.156 mol CH4 = 801.28 kJ/mol CH4

Now we can convert the energy generated to kilowatt-hours (kWh):

801.28 kJ/mol CH4 × (1 kWh / 3.6 × 10^6 kJ) = 0.222022 kWh/mol CH4

Finally, we can multiply by the number of moles of CH4 (0.156 mol) to get the total energy generated:

0.222022 kWh/mol CH4 × 0.156 mol CH4 = 0.034632632 kWh

Rounded to two significant figures, the answer is 3.5 × 10^-2 kWh, which corresponds to choice C.

Question 5

The amount of heat necessary to raise the temperature of a given amount of a substance by 1 ° C is known as _____.

Accepted Answer

The amount of heat necessary to raise the temperature of a given amount of a substance by 1 ° C is known as heat capacity.

Question 6

The freezing point and the boiling point of methane are − 295 ° F and − 263 ° F, respectively. The temperature at the surface of Titan, a moon of Saturn, is 93 K. If methane exists on the surface of Titan, it will be a _____.

Accepted Answer

To determine the state of methane on Titan, we need to convert the temperature from Kelvin to Fahrenheit using the formula $F = \frac{9}{5}(K - 273) + 32$. Substituting 93 K into the formula gives $F = \frac{9}{5}(93 - 273) + 32 = -290$ °F. Since -290 °F is between the freezing point (-295 °F) and boiling point (-263 °F) of methane, it means methane would be a liquid on Titan.

Question 7

If ethanol boils at a temperature of 78.5 ° C, then calculate its boiling point in ° F.

Accepted Answer

To convert from Celsius to Fahrenheit, use the formula F = (C * 9/5) + 32. For ethanol, F = (78.5 * 9/5) + 32 = 173.3 °F.

Question 8

Gold melts at 1063 ° C. Calculate its melting point in Kelvin.

Accepted Answer

To convert Celsius to Kelvin, we add 273.15 to the Celsius temperature. 
So, the melting point of gold in Kelvin can be calculated as follows:
1063 + 273.15 = 1336.15 K

Therefore, the correct choice is D) 1336 K.

Question 9

Which of the following temperature conversions is correct?

Accepted Answer

The correct conversion from Kelvin to Celsius is to subtract 273.15 from the Kelvin temperature. Therefore, at 0 K, the Celsius temperature would be -273.15°C. In option B, the negative sign before the converted temperature (-273) indicates that it is below zero on the Celsius scale. Options A and C are incorrect because they both give a positive temperature on either the Celsius or Fahrenheit scale, which is not possible at 0 K. Options D and E are also incorrect because they both give a negative temperature on the Fahrenheit scale, which is not the correct conversion from Kelvin to Fahrenheit.

Question 10

How many joules of energy does a candy bar with 250 Calories contain?

Accepted Answer

The answer of How many joules of energy does a...

Question 11

Calculate the energy released in Calories if the burning of a nut produces 15,600 J of heat.

Accepted Answer

1 Calorie (Cal) is equivalent to 4,184 Joules. Therefore, 15,600 J is equal to 15,600 J / 4,184 J/Cal = 3.73 Cal.

Question 12

Which of these statements about exothermic reactions is incorrect ?

Accepted Answer

Chemical cold packs undergo an endothermic reaction, not exothermic. They absorb heat from the surroundings to create a cooling effect, opposite to exothermic reactions which release heat.

Question 13

How many joules of heat are produced from burning methane that releases 125 kJ heat?

Accepted Answer

125 kJ is equal to 125,000 J. Therefore, the amount of heat produced from burning methane is 125,000 J.

Question 14

Calculate the energy generated in kilowatt-hours if the burning of 0.562 g of graphite produces 18.0 kJ of heat.

Accepted Answer

The answer of Calculate the energy generated in kilowatt-hours if the...

Question 15

Which of these is/are the  correct unit(s) for measuring power?
I. Joule
II. Joule/second
III. Kilowatt-hour
IV. Watt

Accepted Answer

Power is measured in watts, which is equivalent to joules per second. Kilowatt-hour is a unit of energy, not power.

Question 16

If the normal body temperature is 98.6 ° F, what is the corresponding temperature in Kelvin?

Accepted Answer

To convert from Fahrenheit to Kelvin, we use the following formula: 
K = (°F + 459.67) × 5/9 
Substituting in the temperature of 98.6 °F: 
K = (98.6 + 459.67) × 5/9 = 310. K

Therefore, the corresponding temperature in Kelvin is 310 K.

Question 17

The Voyager space probe determined that Titan, a moon of Saturn, has a temperature of 93.0 K. Calculate its temperature in ° F.

Accepted Answer

To convert Kelvin to Fahrenheit, use the formula $F = \frac{9}{5}(K - 273) + 32$. Plugging in 93 K gives $F = \frac{9}{5}(93 - 273) + 32 = -292$ °F.

Question 18

The rate of energy output or input is known as _____.

Accepted Answer

The rate of energy is described as power, expressed in watts or joules per second. Heat is a type of energy transfer, work is the energy required to move an object, enthalpy is the total heat content of a system, and entropy is a measure of the amount of disorder in a system. Only power is the rate of energy output or input.

Question 19

Which of the following is/are the correct unit(s) for measuring energy?
I. Joule
II. Quad
III. Kilowatt-hour
IV. Ampere

Accepted Answer

I. Joule is the standard unit for measuring energy, therefore it is correct.
II. Quad is not a standard unit for measuring energy, it is a unit used for measuring large quantities of energy (1 quad = 10^15 BTU), therefore it is correct.
III. Kilowatt-hour is a standard unit for measuring energy, therefore it is correct.
IV. Ampere is a unit for measuring electric current, not energy, therefore it is incorrect.

Question 20

How many joules of heat must be added to melt a 75-g block iron if it required 12.2 Calories?

Accepted Answer

First, we need to convert 12.2 Calories to joules.

1 Calorie = 4.184 J

Therefore,

12.2 Calories = 12.2 x 4.184 J = 51.02048 J ≈ 5.10 x 10^1^0 J

Next, we use the formula:

q = m x ΔHf

where q is the heat required, m is the mass of the substance, and ΔHf is the heat of fusion for the substance.

The heat of fusion for iron is 6.06 x 10^3 J/kg.

Therefore,

q = (75 g / 1000) kg x 6.06 x 10^3 J/kg = 0.4545 x 10^3 J = 4.545 J

We see that 5.10 x 10^10 J is the closest answer choice, so our final answer is:

Answer: E

Quiz 9: Energy for Today

The combustion of 5.00 g of C 2 H 6 (g) emits 259 kJ of heat at constant pressure. Calculate the enthalpy of combustion, Δ Hcom, for the reaction in kJ/g. ​ 2 C2H6(g) + 7 O2(g) → 4 CO2(g) + 6 H2O (l)