6.1.5 Heat

Question 1

note

Answer 1

• Heat is a form by which energy is transferred, and is measured by change in temperature.
• Heat capacity (C) measures the amount of energy needed to raise the temperature of a material.
• With a constant volume the change in internal energy (ΔE) can be calculated by knowing the heat evolved or absorbed by the system, since the amount of work done is zero.

Question 2

note

Answer 2

• Heat (q) is a form by which energy is transferred. In an
  endothermic process, heat moves into the system from the surroundings (q > 0). In an exothermic process, heat moves into the surroundings from the system (q < 0).
• Heat capacity (C) is an extensive property of a substance. It is a measure of the amount of heat required to raise the temperature of a substance.
• The molar heat capacity (C p ) is the heat required to raise one mole of a substance one degree Celcius (°C), and is measured in units of J/mol · °C. The specific heat is the heat required to raise one gram of a substance one °C, and is measured in units of J/g · °C.
• In the example, the change in heat is calculated from
  knowledge of the mass of the substance (coffee), its molar heat capacity (using the value for water), and the change in temperature. The mass of the substance is converted to moles using the molar mass for water. The change in heat is calculated using q = n · C p · T. The value is negative because heat is leaving the system.
• With a constant volume, the change in internal energy of a system can be calculated by knowing the heat evolved or absorbed by the system since the amount of work is zero.
• A reaction carried out in a constant volume allows the
  measurement of the change in internal energy simply by
  measuring the heat lost or gained by the system.

Question 3

What is the sign of q when heat moves from the surroundings to the system?

Answer 3

Positive

Question 4

What is the sign of w when the system does work on the surroundings?

Answer 4

Negative

Question 5

How much heat (in kJ) is absorbed by a 500. g sample of water when it is heated from 44.0°C to 67.0°C?
The specific heat capacity of water is 4.184 J / g°C.

Answer 5

48.1 kJ

Question 6

What is the final temperature of 15.0 g of water when 444.7 joules of heat are added? The initial temperature of the water is 22.0°C.
The molar heat capacity of water is 75.3 J / mol°C.

Answer 6

29.1°C

Question 7

The heat required to raise one gram of a substance by one degree Celsius is the definition of ______.

Answer 7

the specific heat

Question 8

What is the correct definition of heat?

Answer 8

Heat is the energy that flows because of difference in temperature between the system and its surroundings.

Question 9

The first law of thermodynamics is written as, ΔE = q + w. What would ΔE be equal to when imposing a constant volume restriction to a system?

Answer 9

ΔE = qV

Question 10

If 478.5 joules of heat are added to a 10.0 g container of ethanol (specific heat capacity = 2.43 J / g°C), by how many degrees would the temperature increase?

Answer 10

19.7°C

Question 11

For the given specific heat capacities (cp ), which would heat up faster when placed in a bowl of hot soup; an aluminum, copper, gold or silver spoon of the same mass?

Answer 11

Gold (0.129 J / g K)

Question 12

Which of the following is a statement of the first law of thermodynamics?

Answer 12

Δ E = q + w