6.1.2 Heat, q, and Work, w

Heat

energy in transition from a region of high temperature to a region of low temperature

When will heat flow between its system and its surroundings?

when the system isn’t in thermal equilibrium w/ its surroundings

Thermal Equilibrium

when the sys. and the surr. have the same temp.

What is q?

heat

When is q > 0?

When heat is transferring from surroundings → system

• rxn absorbs heat from surroundings

• endothermic

When is q < 0? What is its name?

-q

When heat is transferring from system → surroundings

• rxn releases heat into surroundings

• exothermic

What 2 things can happen when a particle is heated? How does the heat affect each scenario?

1. temp. increases

   1. energy added goes into moving the particles

2. phase change

   1. occurs at a constant temp.

   2. energy goes into breaking bonds

   3. increases potential energy of molecules

Diathermic

a boundary that permits heat to be transferred between the system and its surroundings

Adiabatic

a boundary that prohibits heat from being transferred between the system and its surroundings

If a process is adiabatic, what does it tell you about heat?

∵ no heat is transferred ∴ q = 0

Synonym for fusion?

melting!

Constant Pressure Heating Curve for a Pure Substance

By studying the graph above, we observe that:

1. temp. of system remains constant while system undergoes a phase change

2. heat of vaporization (q_vap) is typically much greater than the heat of fusion (q_fus).

What information about the substance can we obtain from heating curve?

1. The heat capacities of solid, liquid and gas can be obtained from the slopes of the “warming (diagonal) lines”

2. The heat of fusion (q_fus) and the heat of vaporization (q_vap) can be obtained from the horizontal portions of the heating curve.

C

heat capacity

C

\frac{q}{\Delta T}

C_solid

\frac{q_{solid}}{\Delta T_{solid}}

C_liquid

\frac{q_{liquid}}{\Delta T_{liquid}}

C_gas

\frac{q_{gas}}{\Delta T_{gas}}

q_solid

heat absorbed by solid

q_liquid

heat absorbed by liquid

q_gas

heat absorbed by gas

∆T_solid

T_fus - T_i

∆T_liquid

T_vap - T_fus

∆T_gas

T_f - T_vap

Heat Capacity

C

the amount of heat required to raise the temperature by 1 ^oC or 1 K

Average Heat Capacity

C=\frac{q}{\Delta T}

q = ?

C∆T

A temp. change in K is equal to the temp change in…

^oC

\frac{\Delta T_{}}{K} = what in degrees celcius?

\frac{\Delta t_{}}{^{o}C}+273.15

or add 273.15 to celcius to get kelvin

Heat Capacity of an Object Units

J K⁻¹

Heat Capacity of an Pure Substance Units

J K⁻¹ g⁻¹ or J K⁻¹ mol⁻¹

What is “c”? What are its units?

specific heat capacity

J K⁻¹ g⁻¹

What must be constant during heating occurs?

either:

1. pressure

2. volume

C_v

heat capacity for heating at constant volume

C_p

heat capacity for heating at constant pressure

Why do complex molecules tend to have higher molar heat capacities?

• when a substance is heated, the added energy is absorbed by atoms or molecules

• if you ↑ the # of ways these entities can absorb energy, molar heat capacity ↑