lecture 4

Question 1

what is a standard state

Answer 1

the most usual form at 1 bar pressure and a specified temperature (298.15K)

Question 2

how is enthalpy in non standard conditions calculated

Answer 2

H = H* + correction

Question 3

what is H*

Answer 3

enthalpy at standard conditions

1 bar pressure
298.15K temp

Question 4

what is standard enthalpy change

Answer 4

triangle H*

enthalpy change for a process where the initial and final substances are in their standard states

Question 5

standard state for o2

Answer 5

gas

Question 6

standard state for h2o

Answer 6

liquid

Question 7

standard state for C

Answer 7

solid graphite

Question 8

what is the enthalpy of formation

Answer 8

triangle f H*

enthapy change associated with forming a molecule from its constituent elements in their standard states

Question 9

what is the enthalpy of formation of ‘x’ in their standard states

Answer 9

0

Question 10

what is the enthalpy of formation when smt is formed but their constituent elements were not in their standard states

Answer 10

not 0

it has a value

Question 11

what is enthalpy of vaporization

Answer 11

triangle vH*

enthalpy change when smt goes from liquid to gas under standard conditions

Question 12

what is enthalpy of combustion

Answer 12

triangle cH*

oxidation of ‘x’ with o2 gas to give co2(g) and h20 (l)

Question 13

what is enthalpy of solution

Answer 13

triangle sH*
separation of + and - ions in water

infinite dilution limit

Question 14

hess law definition

Answer 14

standard enthalpy of a given reaction can be obtained as the sum of standard enthalpies for a sequence of reactions when we start with the same rea and end with the same pro.

Question 15

what is standard enthalpy change of a reaction

Answer 15

pro - rea

Question 16

hess law broken down

Answer 16

rea

standard state elements

pro

Question 17

what is a born haber cycle

Answer 17

closed path of transformations starting and ending at the same point

Question 18

what temp were enthalpy changes recorded at

Answer 18

a couple but definitely not all

Question 19

how can we determine the enthalpy of a substance at any given temp

Answer 19

use the substances specific heat capacity

Question 20

equation to find enthalpy of substance at temp 1 when we know enthalpy at temp 0

Answer 20

H (T1) =
H (T0) + integral of Cp dT

integral limits =
T1 top
T0 bottom

Question 21

what is kirchhoffs law

Answer 21

enthalpy change of reaction at temp 1 = enthalpy change of reaction at temp 0 + intgral of the specific heat capacity multiplied by dT)

integral: t1 top t0 bottom

Question 22

the change in specific heat capacity of a reaction is

Answer 22

total specific heat capacity of products - total specific heat capacity of reactants

including the stoichiometry (molar ratio)

Question 23

what does kirchhoffs law help us do

Answer 23

helps us calculate the enthalpy change for a reaction at any temp when we know the enthalpy change at one temp + the heat capacities of the rea and products

Question 24

change in Cp

Answer 24

specific heat of pro - specific heat of rea

Question 25

temp change

Answer 25

final temp - intial temp

Question 26

actual heat capacity variations equation

Answer 26

H(T1) = H(T0) + integral Cp dT

integral upper limit = t1
integral lower limit = t0

area under graph = enthalpy change of the substance

plot of cp against temp

diff phases = diff heat capacities

Question 27

how can we measure enthalpy change

Answer 27

use differential scanning calorimetry

DSC

Question 28

how does DSC work

Answer 28

differential scanning calorimetry

• increase temp in a sample with a reference that doesnt change
• measure changes in heat capacity at a constant pressure
• enthalpy change is equal to the area under the curve when Cp is plotted against temp