Unit 2 - Chapter 5

Question 1

Chemical systems

Answer 1

system = a defined part of the universe that is to be studied
chemical system = a studied chemical reaction
surroundings = the rest of the universe
universe = system plus the surroundings

Question 2

Energy

Answer 2

energy = the ability to do work, measured in Joules
work = when a force acts to move an object
kinetic energy = the energy possessed by an object in motion
potential energy = the energy possessed by an object due to its position in an attractive or repulsive force field (stored energy)

Question 3

Chemical kinetic energy

Answer 3

vibrational energy = atoms vibrating along bond length
rotational energy = particles rotating around their centre of mass
translational energy = linear movement of gas particles in space
changes in kinetic energy affect the temperature, not the state of the substance

Question 4

Chemical potential energy

Answer 4

Van der Waals forces = attraction forces between molecules
bond energy = attraction forces between electrons and nuclei involved in chemical bonds (or lattice energy = attraction between ions)
nuclear energy = strong forces between protons/neutrons in atomic nuclei
potential energy decreases as particles get closer together, so changes in potential energy occur when states are changing

Question 5

Internal energy

Answer 5

internal energy (U) = sum of all kinetic and potential energies of a system
it is impossible to measure, however we can attempt to measure the change in internal energy that occurs during a chemical process

Question 6

Laws of thermodynamics

Answer 6

energy cannot be created or destroyed, but can be converted from one form to another
energy can be transferred from one system to another
all forms of energy can be converted to heat (q)

Question 7

Thermal energy

Answer 7

thermal energy = kinetic energy of particles that results in the system’s temperature (T = average kinetic energy)
heat = energy that moves from a system of higher thermal energy to one of lower thermal energy

Question 8

Enthalpy

Answer 8

enthalpy (H) = the energy of a system at a specific pressure and volume
it is impossible to measure
when a chemical process is carried out under constant atmospheric pressure, the heat transferred between the system and the surroundings = the change in enthalpy of the system

Question 9

Change in enthalpy

Answer 9

change in enthalpy = the change in internal energy plus the work done by the system on its surroundings
at a constant pressure, work results in a change of volume of the system, when there is no change in volume, then no work is done
this is true for systems that do not involve gases or have no change in the number of moles of gas during the reaction

Question 10

State dependent property

Answer 10

internal energy, enthalpy, and change in enthalpy are all state dependent properties
they depend on the conditions of the system including : amount (moles) of reactant/product, physical states of substances, temperature, pressure conditions
change in enthalpy data is therefore often reported at SATP, and is referred to as the standard molar enthalpy of reaction

Question 11

Exothermic reactions

Answer 11

heat energy is transferred from the chemical system to the surroundings, resulting in an increase in the temperature of the surroundings
the products of the reaction have less enthalpy than the reactants
less energy is required to break bonds in reactants than is released as new bonds form in products
products are thermodynamically more stable
change in enthalpy is assigned a negative value

Question 12

Endothermic reactions

Answer 12

heat energy is transferred from the surroundings into the chemical system, resulting in a decrease in the temperature of the surrounding
the products of the reaction have more enthalpy that the reactants
more energy is required to break bonds in reactants than is released as new bonds form in products
products are thermodynamically less stable
change in enthalpy is assigned a positive value

Question 13

Enthalpy change and physical change

Answer 13

when a pure substance is heated (from solid) or cooled (from gas) and temperature is measured and plotted against time, a characteristic curve results (heating/cooling curve)
when the heat causes a change in temperature = the average kinetic energy increases, added heat is causing particles to move faster
when the heat does not cause a change in temperature = a change of state is occurring, particles are overcoming attraction forces to move further apart, potential energy increases

Question 14

Physical properties of pure substances

Answer 14

specific heat capacity (c) = amount of energy in Joules required to change the temperature of 1g of a substance by 1 degree Celsius
latent heat of fusion (Lf) = amount of energy in Joules involved to convert 1g of a solid to liquid (or liquid to solid) at constant temperature
latent heat of vapourization (Lv) = amount of energy in Joules involved to convert 1g of a liquid to gas (or gas to liquid ) at constant temperature

Question 15

Enthalpy change and chemical change

Answer 15

in chemical reactions, the majority of the change in enthalpy results from changes in potential energy as bonds are broken and new bonds form
we cannot directly measure this change in enthalpy of the system, but we can measure how it affects the surroundings
by measuring the change in temperature, we can infer how the enthalpy of a system is changing

Question 16

Calorimetry

Answer 16

measuring the change in enthalpy of a chemical reaction can be done using a calorimeter
calorimeter = insulated reaction vessel which isolates the system, and a small, defined surrounding
we can measure the change in temperature of the isolated surroundings as chemical change occurs

Question 17

Molar enthalpy of solution

Answer 17

enthalpy change associated with the dissolving of 1 mole of solute into solution

Question 18

Application of thermochemical equations

Answer 18

change in enthalpy information is present as kJ/mol of reaction, and is therefore proportional to molar coefficients in balanced chemical equation
states of reactants and products affects the change in enthalpy of reaction, and therefore must be included
gas has a higher initial enthalpy than a liquid of the same substance

Question 19

Hess’s Law

Answer 19

law = if a target reaction can be expressed as the algebraic sum of a series of equations, then the change in enthalpy of the target reaction will equal the sum of the changes in enthalpy of the series of reactions
Hess’s law therefore allows us to determine the change in enthalpy of reactions that may be difficult to perform experimentally

Question 20

Standard molar enthalpy of formation

Answer 20

the enthalpy change associated with the synthesis reaction in which one mole of a compound is produced directly from its elements in their standard state at SATP
elements in their standard state at SATP have been assigned a standard molar enthalpy of formation of 0

Question 21

Magnitude of change in enthalpy for various changes

Answer 21

physical change = change in enthalpy ranges from 10 to 100 kJ/mol
chemical change = change in enthalpy ranges from 100 to 1000 kJ/mol
nuclear change = change in enthalpy ranges from 10^9 to 10^12 kJ/mol