Energetics Flashcards
(20 cards)
Define activation energy.
The minimum amount of energy needed for a reaction to start.
Where is the energy transferred in an exothermic reaction?
The surroundings.
Where is the energy transferred in an endothermic reaction?
From the surroundings to the reaction.
Give three examples of exothermic reactions.
Respiration
Neutralisation
Nuclear fission
Give three examples of endothermic reactions.
Photosynthesis
Thermal decompostition
Melting ice
What is enthalpy change?
Change in energy.
What is the standard enthalpy change of formation, Δ HꝊf?
The enthalpy change that occurs when one mole of the compound is formed from its constituent element under standard conditions, and with everything in its standard state.
What is the standard enthalpy change of combustion, Δ HꝊc?
The enthalpy change that occurs when one mole of the compound is burned completely in oxygen under standard conditions, and with everything in its standard state.
What are the standard conditions?
Temperature - 298K
Pressure - 100kPa
What is the enthalpy change of formation of an element?
0kJ/mol
What is Hess’ law?
The enthalpy change at constant pressure in a reaction is independent of the route by which the chemical change occurs (and it only depends on
its the initial and final states).
Give three reasons why values obtained by a student are lower than the values using the enthalpy of formation data.
Heat loss/ heat absorbed by apparatus
incomplete combustion
loss of mass as some compounds may turn to their gaseous state.
How would the enthalpy change of combustion of water vapour differ to that of the formation of water in its liquid state?
The value will be more negative as the reaction will be more exothermic because heat energy is released when water vapour condenses.
What are mean bond enthalpies?
Different covalent bonds require different amounts of energy to be broken. Bond enthalpies calculated using calorimetry methods differ from the data book values (which are average values) as they are not exact and vary in each situation.
What is calorimetry?
Its is an experimental method for finding enthalpy change by measuring temperature change over time. When observed and plotted on a graph, data can be extrapolated to give an accurate value for the change in temperature.
What is the equation for energy change (q)?
q=mcΔT
Define specific heat capacity.
The energy required to raise 1g of a substance by 1K without a change of state.
How do you calculate the enthalpy change per mole?
ΔH=q/moles
What is Hess’s law?
Energy in a reaction must be conserved, as it cannot be created nor destroyed. Therefore, the overall enthalpy change for a reaction is the same, regardless of the route taken.
Define bond enthalpy.
The energy required to break one moles of the stated bond in a gaseous state, under standard conditions.
