Lecture 2: Heat

Question 1

What are isolated, closed and open systems?

Answer 1

An isolated has no exchange of matter or energy.
A closed system has no exchange of matter, but does have exchange of energy.
An open system has exchange of both matter and energy.

Question 2

What is U?

Answer 2

Internal energy. It is constant for an isolated system.

Question 3

What can increase U in a closed system?

Answer 3

q (heat) or w (work)

dU = dq + dw

Question 4

What is H?

Answer 4

Enthalpy

Question 5

What are the units for enthalpy?

Answer 5

Joules, J

Question 6

What is enthalpy?

Answer 6

Enthalpy, H, represents the way that heat is stored in a system other than in the form of a volume change. A change in enthalpy of the system is heat entering or leaving the system.
Chemical bonds are a form of enthalpy.
H = U + PV
Enthalpy = internal energy + (pressure x volume)
dH(sys) = dq(sys)

Question 7

What is heat capacity?

Answer 7

The amount of heat required to raise the temperature of a system by one degree. It is given the symbol C and has the units joules/kelvin, J/K.
C = dq/dT

Question 8

Does water have a higher heat capacity when solid or liquid?

Answer 8

Water has a higher heat capacity when liquid. In the phase transition from solid to liquid, a lot of heat is taken in without raising the temperature, because extensive bonds have to be broken by the energy.

Question 9

What types of enthalpy are there?

Answer 9

• Molecular motions/interactions: temperature, phase transitions
• Chemical energy: covalent bonds, weak bonds

Question 10

Give two examples of chemical enthalpy.

Answer 10

• Formation of a molecule from elements

- Dissolving a molecule in water

Question 11

Relate exothermic/endothermic reactions to enthalpy.

Answer 11

An exothermic reaction is one in which heat is released into the surroundings, so the enthalpy of the system decreases, so dH = -ve.

An endothermic reaction is one in which is absorbed by the system, so the enthalpy of the system increase, so dH = +ve.