Thermodynamics

Question 1

What is the First law of thermodynamics

Answer 1

Law of conservation energy
States - that energy cannot be be created nor destroyed but may change from one form into another. Energy can be transferred between a system and its surroundings as either heat or work but can never be created nor destoryed.

Question 2

What is the difference between open system and closed system?

Answer 2

Open system can gain or lose mass and energy across their boundaries - example the human body

Closed system - can absorb or release energy, but NOT mass across the boundary.

Question 3

What units are sued in chemical thermodynamics

Answer 3

Joules (J)
Kilojoule (kJ)

Question 4

Define the term ‘heat’ and how does it differ from temperature?

Answer 4

Heat is the transfer of energy due to a temperature difference. Heat flow from the hotter object to the colder object when bought in contact.

Temperature - if there is thermal equilibrium between two objects, then there is no heat transfer

Not the same thing.

Question 5

What is internal energy?

Answer 5

The energy transferred as heat, comes from an object’s internal energy. The sum of energies for all individual particles in a sample of matter (due to the random motion of atoms, molecules or particles in a system)

Is represented by the symbol U.

Question 6

how do you measure heat?

Answer 6

Cannot be achieved directly, but can calculate it using the temperature change that occurs when heat flows from one body to another. A linear relationship between heat and temperature

Heat capacity equation can help determine change in temperature

Question 7

What is heat capacity? and specific heat capacity?

How do they differ

Answer 7

the amount of heat required to increase the temperature of an object by a particular amount

Specific heat capacity is the amount of heat required
to raise the temperature of 1g of the substance by
1°C (1K). Allows us to compare values for the same mass of different substances by inspection

the specific heat of water is
approximately nine times that of iron, meaning that it takes nine times as much heat to raise the temper-
ature of water by a specified amount as it does for the same mass of iron. The heat required to raise the
theunitsofJg K .
temperature of 1 mol of a substance by 1 K is called the molar heat capacity and has units of J mol
−1 −1 K .

Question 8

How does the concept “work” apply to thermodynamics?

Answer 8

Energy can also be exchanged if the chemical system does work on the surroundings or the surroundings do work on it.

Example: Compression or expansion of gas produced in the reaction. When gas is produced in a reaction, and it expands, the system does work on the surroundings.
Conversely - when a gas is compressed at a constant temperature, the surroundings are doing work on the system called Pressure-Volume or pV work

If Pext = Pint, the system is at equilibrium; the
piston does not move, and no work is done.

If Pext < Pint, the gas will expand, performing
work on its surroundings then Vf > Vi = change in volume is positive

If Pext > Pint, the gas will be compressed,
and the surroundings will perform work on
the gas then Vf < Vi change in volume is negative
.

Question 9

What will be the outcome of the gas if heat enters the gas, or if gas is compressed?

Positive or negative

Heat enters gas
Heat exits gas

Gas is compressed
Gas expands

Answer 9

Heat enters gas = gas absorbance will be positive
Heat exits gas= gas absorbance is negative

Gas is compressed = work done is positive
Gas expands = work done is negative

Question 10

What is isothermal

Answer 10

Temperature is constant

Question 11

What is Adiabatic

Answer 11

is defined as a process in which no heat transfer takes place. This does not mean that the temperature is constant, but rather that no heat is transferred into or out from the system.

Question 12

what is Isochoric

Answer 12

process taking place at constant volume

Question 13

What is isobaric

Answer 13

having the same pressures, masses, or densities. (physics) (of a thermodynamic process) having a constant pressure throughout.

Question 14

What is enthalpy?

Answer 14

Symbol H

a substance is its total energy content held at a constant temperature

is the sum of the internal energy (U) and the product of pressure and volume (PV) given by the equation

enthalphy change during a chemical reaction can be measured by;

Change in H = Hproducts - Hreactants

Question 15

Why can Enthalpy be measured directly?

Answer 15

• number and nature of particles
• kinetic energy of the particles
• space occupied by the particles

Question 16

What is the difference between endothermic and exothermic

Answer 16

If final enthalpy of a system is greater than the initial - the system has absorbed heat from its surroundings - the Change in enthalpy will be positive (endothermic)

BUT if system has lost what to the surroundings the enthalpy of the system has decreased - so change in enthalpy will be negative (exothermic)

Question 17

Why is exothermic more energetically more favourable?

Answer 17

In an exothermic reaction, the reactants have higher energy compared to the
products.

As the reaction proceeds, energy is
released into the surroundings.

Lower energy can be thought of as
providing a greater degree of stability to a chemical system.

Since the energy of the system
decreases during an exothermic
reaction, the products of the system are more stable than the reactants.

Question 18

When does standard enthalpy change

Answer 18

Enthalpy changes when molar quantities of reactants as stated in the chemical equation react under
the standard conditions of 298K and 105Pa pressure.

• The physical states of reactants and products affect the enthalpy change of a reaction.
• Hence physical states and +ve and –ve sign of ∆Hθ MUST always BE STATED in all thermochemical
  equations.

Question 19

How is enthalpy measured?

Answer 19

calorimeter

The initial and final temperatures are measured using a
thermometer

Question 20

What is hess’s Law?

Answer 20

States that the overall enthalpy change for any chemical reaction is constant - regardless of how the reaction is carried out

Used to find enthalpy changes which cannot be measured directly in a lab

Question 21

What is entropy? (S)

Answer 21

is a measure of disorder of a system

Question 22

What is the 2nd law of thermodynamics

Answer 22

started from
the idea that heat can flow spontaneously from a
hot object to a cold object; heat will not flow
spontaneously from a cold object to a hot object.

states that as
energy is transferred or transformed, more and
more of it is wasted.
The “wasted” energy goes towards Entropy

also states that there is a natural
tendency of any isolated system to degenerate
into a more disordered state.

Question 23

What factors that affect entropy

Answer 23

Volume - entropy increases with volume
Temperature - higher the temperature, higher the entropy

Question 24

In regards to the comparison of the entropies in a physical state, which is contains higher amount of kinetic energy? explain.

Answer 24

Liquid has higher entropy because molecules can move more freely and therefore more way to distribute kinetic energy among them.

Question 25

What is Gibbs free energy (G)? and what does the change in gibbs energy provide?

Answer 25

a measurement of the mount of usable energy in that system.

During a reaction gibbs free energy provides information about the reactions energetics and spontaneity.

Question 26

In relation to Gibbs energy, are endergonic reactions spontaneous?

Answer 26

no