Thermochemistry

Question 1

What is Enthalpy

Answer 1

the heat transferred at constant pressure

Question 2

How is Enthalpy worked out
Hence what type of function is it?

Answer 2

Enthalpy is defined as:
H = U + pV
= internal energy + (pressure x Volume)
* Enthalpy is a state function since it depend on three other state function

Question 3

Enthalpy of a substance increase with…
Which depends on what?

Answer 3

The enthalpy of a substance increases with temperature by an amount which depends on the heat capacity at constant pressure

Question 4

What is an exothermic process

Answer 4

Exothermic processes have ΔH<0 so that heat is released from the system at constant pressure

Question 5

What is an Endothermic process

Answer 5

Endothermic processes have ΔH>0 so that heat is absorbed by the system at constant pressure

Question 6

What is a phase transition

Answer 6

Substances undergo changes of state, such as melting or vapourisation

Question 7

What is the Standard Molar Enthalpy of Melting

Answer 7

Is the enthalpy change which occurs when 1 mol off a pure solid melts to form a liquid at its melting point and 1 bar pressure
(ΔmeltH∘)

Question 8

What is the Standard Molar Enthalpy of Vapourisation

Answer 8

Is the enthalpy change which occurs when 1 mol of a pure liquid boils to form a gas at its boiling point and 1 bar pressure
(ΔvapH∘)

Question 9

What does Hess’s law state

Answer 9

The overall reaction enthalpy is the sum of the reaction enthalpies of the step into which the reaction can be divided
It follows from the fact that enthalpy is a state function, so its value is independent of the path chosen. Intermediate reactions do not matter

Question 10

What is the Standard Enthalpy Change of Formation (ΔrH)

Answer 10

is the enthalpy change during the formation of a compound it its standard state from its elements in their standard states

Question 11

What is standard state/conditions

Answer 11

A substances most stable form at a pressure of 1 bar
(Usually reported at 298K)

Question 12

What is the formula to work out the Standard Enthalpy Change of Formation

Answer 12

ΔrH = Σ(ΔrH products) - Σ(ΔrH reactants)

Question 13

If a reaction involves a compound for which the Standard Enthalpy of Formation is unknow, what can be used instead

Answer 13

Bond enthalpies

Question 14

What is bond enthalpy Δ(X-Y)

Answer 14

Is a measure of the strength of the X-Y chemical bond where X and Y are atoms or molecular fragments
It is equal to the standard enthalpy change for the dissociation of a molecule in the gas phase into atoms or fragments in the gas phase
XY(g) → X(g) + Y(g)

Question 15

Why is Standard Enthalpy of formation perferred over bond enthalpy

Answer 15

The bond strength between a given pair of atoms varies to a small extent between different compounds, so normally the average bond enthalpy is tabulated

Question 16

What does Kirchhoff’s equation allow you to work out

Answer 16

Kirchhoff’s equation allows you to calculate the enthalpy change for a reaction at any temperature, providing you know the value of the enthalpy change at one temperature, and the heat capacity of the reactants and products

Question 17

State Kirchhoff’s equation

Answer 17