Energetics Flashcards
Define enthalpy of combustion
The enthalpy change when 1 mol of substance is completely burned in oxygen with reactant and products in their standard states, under standard conditions.
Describe enthalpy diagram for endothermic reaction
Energy at end of reaction is greater than energy at end.
What are standard conditions
100kPa
298K
Describe how to measure the enthalpy change of a reaction
Place reation mixture in polysterene cup
Measure temperature change at regular intervals
Graph results and extrapolate to find the peak temperature
Calculate temperature change
Use q=mc∆t
Describe enthalpy diagram for an exothermic reaction
Energy at end is less than energy at start of reaction.
Define endothermic
Absorbs more energy to start the reaction than is outputted by the reaction.
Define mean bond enthalpy
The energy required to break one mole of the bond averaged across all species in their gaseous statess.
What is heat?
Measure of total energy within a substance
Depends on number of particles.
What is temperature?
The average kinetic energy of all the particles in a system. Higher kinetic energy, higher temperature.
Define Exothermic
Chemical Reaction which releases heat into its surroundings
Define Hess’s law
States that enthalpy change for a chemical reaction is the same regardless of the route taken from reactants to products.
Define enthalpy of formation
Enthalpy change when 1 mol of substance is formed from its constituent elements in their standard states, under standard conditions.
How to use mean bond enthalpy to find ∆H
The products minus the reactants
Why do calculations based on mean bond enthalpy differ from those calculated via Hess’s Law
Mean bond enthalpy are approximations of the ∆H whilst values from Hess’s law are based on the actal compounds being reacted.
Hess’s law values are considered more ‘correct’.
What is the equation for heat change?
Q=mc∆t
Q = Heat change
m=mass heated
c= specfic heat capactity of substance heated
∆t= change in temperature
