C10. Thermodynamics

Question 1

what is thermodynamics/

Answer 1

study of the transfer of energy- ALLOWS US TO PREDICT IF REACTION WILL HAPPEN

Question 2

what is energy?

Answer 2

capacity to do work

Question 3

what is work

Answer 3

motion against an opposing force

Question 4

what is kinetic energy and potential energy?

Answer 4

-Kinetic energy the energy something possess due to its motion.
-Potential energy the energy something possess due to orientations / positions of its components (gravitational / coulombic).

Question 5

what are the four laws of thermodynamics?

Answer 5

-The Zeroth Law: Thermodynamic equilibrium
-The First Law: The conservation of energy
-The Second Law: Principles of entropy
-The Third Law: Absolute zero temperature

Question 6

what is a system and surrounding?

Answer 6

-System the bit we’re interested in (e.g. chemical
reaction).
-Surroundings everything else and where
we make our observations (e.g. in air / in a
water bath)

Question 7

system+ surrounding=

Answer 7

universe

Question 8

Describe open system

Answer 8

In an open system, both matter and energy
can be transferred between the system and
the surroundings

Question 9

Describe closed system

Answer 9

In a closed system, only energy can be
transferred between the system and the
surroundings (i.e. only energy changes)

Question 10

Describe isolated system

Answer 10

In an isolated system, no matter and no
energy can be transferred

Question 11

What is the internal energy, U?

Answer 11

The Internal Energy, U is the sum of all the kinetic energy (KE) and
potential energy (PE) contributions of all the components in the
system. This cannot be calculated as it is impossible to measure individual KE and PE contributions from
atoms, electrons etc

Question 12

change in internal energy equation?

Answer 12

triangleU= U final - U initial

Question 13

what is the internal energy of a system changed by?

Answer 13

-heat or work
-Work (w) and heat (q) are done on or done by the system
-Work and heat are what transfer energy into and out of our system.

Question 14

Equation to calculate change in internal energy from work and heat?

Answer 14

triangleU = q + w
-q is +ve if heat is absorbed by the system, and -ve if given off.
-w is +ve if work is done on, and -ve if work is done by the system

Question 15

when does change in internal energy = q?

Answer 15

At constant volume, where there is no expansion work ΔU = q.

Question 16

Describe oxidation of fat, work of expansion example

Answer 16

-The oxidation of fat is exothermic (q = -ve).
-But work (w) has been done by the atmosphere on the system due to the reduction in volume (compression).
-ΔU > q because some of the total energy of the system has been
gained by the atmosphere doing work on the system
EXAMPLES ON ONE NOTE

Question 17

Describe enthalpy, EQUATIONS?

Answer 17

-Enthalpy, H takes in account pressure and volume
-Equation for change in internal energy: ΔU = q + w
-Substitute definition of w: ΔU = q – PΔV as
w=-PatmΔV
qp= H
Rearrange: qp = ΔU + PΔV
ΔH = ΔU + PΔV
ONE NOTE

Question 18

What is enthalpy?

Answer 18

Enthalpy is the heat content of an open system at constant pressure.
-Exothermic processes, heat is released ΔH < 0.
-Endothermic processes, heat is absorbed ΔH > 0

Question 19

what is enthalpy change of reactions?

Answer 19

-The enthalpy change of a reaction, ΔHrxn is
-used to refer to energy changes during
chemical reactions.
-It is the enthalpy change between the
products and the reactants

Question 20

what are the different types of enthalpy changes of reaction?

Answer 20

ΔHcom = enthalpy change of combustion.
ΔHfus = enthalpy change of melting.
ΔHvap = enthalpy change of vaporisation.
ΔHsol = enthalpy change of solubilisation

Question 21

enthalpies are reported in?

Answer 21

kJ mol-1. The
total enthalpy change is governed by the
stoichiometry of the reaction

Question 22

what does ° mean

Answer 22

° means “at
standard conditions”
(298.15 K, 1 atm)

Question 23

What does Hess’s state?

Answer 23

Hess’s law states that the
standard reaction enthalpy is the sum of all the standard reaction enthalpies into which
the overall reaction is divided.
ΔH°rxn = ΔH°1 + ΔH°2 +ΔH°3
ONE NOTE

Question 24

what is enthalpy of formation?

Answer 24

Standard enthalpy of formation ΔH°f of a substance is standard enthalpy (per mol)
for its formation from its elements in their
reference state

Question 25

ΔH°rxn= ?

Answer 25

ΔH°rxn = ∑ v ΔH°
f (products) - ∑ v ΔH°
f (reactants)
ONE NOTE

Question 26

v?

Answer 26

v = stoichiometry

Question 27

How to calculate enthalpy of reaction using bond enthalpies?

Answer 27

ΔH°rxn = ∑ ΔΗ°
bonds broken (reactants) - ∑ ΔH°bonds formed (products)
-Bond breaking is endothermic (energy must be put in)
-Bond making is exothermic (energy is released)

Question 28

Describe second law of thermodynamics

Answer 28

-In an isolated system entropy increase is a spontaneous change
-In an open/closed system, you have to take into account the entropy
of the universe i.e the system and its surroundings.
-Spontaneous process are irreversible

Question 29

What is entropy?

Answer 29

-The disorder of a system
is defined as entropy, S
-Systems tend to move from
states of order to states of
disorder / chaos.

Question 30

Describe entropy of surroundings

Answer 30

-Upon transitioning from liquid à solid, the chemical process gives out
a large amount of heat (exothermic)
-The surroundings become hotter (q increases) and thermal motion is
greater
-Dispersion of energy is greater therefore the entropy of the
surroundings increases.
-The total entropy of the surroundings and the system (i.e. the
universe) is increasing.
-ΔS of the surroundings ∝ q surroundings

Question 31

Describe changes in entropy

Answer 31

-From cold ->hot ΔS&raquo_space; 0
-From warm -> hot ΔS >0
-Entropy is still increasing, but the increase in entropy is inversely proportional to the temperature.
-Therefore ΔS proportional to 1/T
ONE NOTE