Topic 6 - Energetics Flashcards
What is enthalpy change?
The heat energy change measured at constant pressure.
What are standard conditions?
Pressure - 100 kPa
Temperature - 298 K
Solutions at a concentration of 1 mol dm-3
What is an exothermic reaction?
A reaction in which energy is released.
What is an endothermic reaction?
A reaction in which energy is taken in.
What is the sign of ΔH for exothermic and endothermic reactions?
Exothermic: ΔH is negative
Endothermic: ΔH is positive
What does an enthalpy level diagram look like for an exothermic reaction?
https://alevelchemistry.co.uk/wp-content/uploads/2018/11/Enthalpy-changes-2.jpg
What does an enthalpy level diagram look like for an endothermic reaction?
https://alevelchemistry.co.uk/wp-content/uploads/2018/11/Enthalpy-changes-2.jpg
What is the definition for the standard enthalpy change of reaction, ΔrHθ?
The enthalpy change that occurs when the quantities of reactants shown in the equation react under standard conditions, with all reactants and products in their standard states.
What is the definition for the standard enthalpy change of formation, ΔfHθ?
The enthalpy change when one mole of product is formed from its constituent elements with all reactants and products in their standard states, under standard conditions.
What is the definition for the standard enthalpy change of combustion, ΔcHθ?
The enthalpy change that occurs when one mole of compound is completely reacted with excess oxygen under standard conditions with all reactants and products in their standard states.
What is the definition for the standard enthalpy change of neutralisation, ΔneutHθ?
The enthalpy change that occurs when one mole of water is formed from a neutralisation reaction between an acid and base under standard conditions, with all reactants and products in their standard states.
What is the definition for the standard enthalpy change of atomisation, ΔaHθ?
The enthalpy change that occurs when 1 mole of gaseous atoms is formed from its element in its standard state, under standard conditions.
What equation can be used to work out the energy transferred in a reaction?
q = mcΔT
q = energy released/absorbed (J)
m = mass (g)
c = specific heat capacity (J g-1 °C-1)
ΔT = temperature change (°C)
Why are energy change reactions carried out in an insulated container?
To prevent heat energy loss to the surroundings.
What is Hess’s law?
The enthalpy change of a reaction is independent of the pathway taken.
What is mean bond enthalpy?
The mean energy needed to break a specific covalent bond, averaged out across a wide variety of different compounds.
How are bond enthalpy and bond strength related?
The larger the bond enthalpy, the stronger the bond, hence the longer it takes to break this bond. Therefore bonds with lower average bond enthalpies are weaker and will break first in a reaction.
What axis needs to be labeled in an enthalpy level diagram?
Vertical axis
Is it necessary to label the horizontal axis in an enthalpy level diagram?
No
What can be used to label the horizontal axis if desired?
Extent of reaction or Progress of reaction
What must be included for both reactants and products in an enthalpy level diagram?
Formulae and state symbols
What values should be provided in the enthalpy level diagram?
Values for ΔH with the correct sign
Is it essential to show activation energy in an enthalpy level diagram?
No
Should activation energy be shown in an enthalpy profile diagram?
Yes
What is the standard enthalpy change of combustion of ethanol?
~1367 kJ/mol
This value is obtained from a data book.
What does it indicate if the value obtained from the experiment is in reasonable agreement with the standard enthalpy change?
Errors in procedure were minimal.
Name a possible source of error related to heat transfer in the experiment.
Some of the heat energy produced in burning is transferred to the air and not the water.
What issue can arise from incomplete combustion of ethanol?
It would produce less heat energy and cause soot to form on the bottom of the copper can.
What is another source of error concerning the transfer of heat energy?
Some of the heat energy produced in burning is transferred to the copper can and not to the water.
What condition might affect the accuracy of the experiment?
The conditions are not standard, such as producing water vapor instead of liquid water.
Fill in the blank: The experiment takes a long time, meaning that not all of the heat energy transferred from the water to the surroundings is _______.
compensated for.
