2.1 Thermochemistry Flashcards
What are standard conditions
- 298K
- 101kPa
- 1moldm3
Delta H equation
-mc deltaT
—————
n
Define enthalpy change of formation
The enthalpy change when one mole of a substance is formed from its constituent elements under standard cknditioms
Enthalpy change of combustion
The enthalpy change when one mole of a substance is completely combusted under standard conditions
What does θ represent
Standard conditions
Define Hess’ law
No matter the route of a reaction, the overall enthalpy change will remain the same
Enthalpy change is
sum of anti-clockwise arrows - sum of clockwise arrows
Positive value is …
Endothermic
Negative value is …
Exothermic
Which way do arrows go when you have combustion data?
Toward element, away from equation
Where do arrows go when you have formation data
Toward equation, away from elements
How do literature enthalpy change values differ from experiment?
- lit are numerically larger
- due to there being heat loss and/or incomplete combustion in experiment
In terms of bond strengths, explain why enthalpy changes of combustion are negative
-ve enthalpy means energy in bonds broken is less than energy in bonds made
Explain why the enthalpy change of combustion of propanol is more negative than that of ethanol
More binds broken and made in propanol and therefore more energy is released
Economic and environmental factors of gasification and catalysed reactions to make glucose to ethanol
Both conserve carbon, but use non-renewable fuel sources
Gasification: lower atom economy, CO is toxic and CO2 contribute to climate change. 700°C require a lot of energy
Ethanol: higher atom economy, ethanol can be used in cars, no harm to enviro, catalyst lowers AE so cheaper
Why is it important that HCL and NaOH is at the same temp
Otherwise a temp change would occur when adding the acid which would not be related to the reaction, confounding results
State what is meant by molar mass
Mass of one mole of a compound
Why is the standard enthalpy of formation of both hydrogen and nitrogen gases 0?
High elements are in their standard states
Give a reason why the sign of enthalpy value calculated is different from the sign of the temperature change measured
Enthalpy measures chemical energy and so as heat energy increases, chemical energy must decrease
Explain what is meant by bond enthalpy
Energy required to break bonds / energy released by making 1 mole of bonds
In order to measure enthalpy change, the reaction must …
Involve a solution
Why can MgCl2 not be used to get enthalpy change
No solution present as forms a solid
Why are bond enthalpies described as average
Each individual bond will be in a different environment and therefore have different strengths
Why is the boiling temp of methanol higher than chloromethane
- methanol has hydrogen bonding between molecules
- chloromethane has VdW between molecules
- hydrogen bonding stronger than VdW
