Energetics

Question 1

Enthalpy change definition

Answer 1

change in heat energy during any change in the system at constant pressure

Question 2

Standard conditions?

Answer 2

100kPa
298K
solutions at 1moldm-3
all substances should be in standard states at 298K

Question 3

q = mcΔT units

Answer 3

J = g x J-1g-1K-1 x ΔK

Question 4

ΔH =

Answer 4

q/mol

Question 5

method for calculating ΔH in experimental data

Answer 5

calculate q using q=mcΔT
work out moles of reactants used
calculate ΔH using ΔH=q/mol
add a sign and unit (/1000 for Jmol-1 to kJmol-1)

Question 6

errors in calorimetry

Answer 6

• incomplete combustion of fuel
• heat capacity of calorimeter not included
• measurements not carried out under standard conditions as H2O is gas (not liquid)
• incomplete energy transfer

Question 7

Hess’ law?

Answer 7

Total enthalpy change for a reaction is the same regardless of reaction route taken

Question 8

ΔH from enthalpy changes of formation=

Answer 8

ΔfH products - ΔfH reactants

Question 9

ΔH from enthampy changes of combustion =

Answer 9

ΔcH reactants - ΔcH products
(both groups form combustion products)

Question 10

mean bond energy definition only applies when?

Answer 10

when substances start and end in the gaseous state

Question 11

ΔH from mean bond energies=

Answer 11

bond energies broken - bond energies made

Question 12

Why are ΔH values from using MBEs less accurate than using formation/combustion data?

Answer 12

Mean bond energies are not exact or specific to that bond in that molecule

Question 13

ΔfH when given an element’s MBEs/ΔatH and total MBE of a compound

Answer 13

ΔfH = ΔH to turn elements to gaseous atoms - ΔH to turn compound to gaseous atoms