Unit 2 - Energetics and Kinetics Flashcards
What directions to the arrows point in a Hess Cycle when we are given formation data to calculate combustion?
Both point up from the elements.
Why are mean bond enthalpies less useful than enthalpies worked out using Hess Cycles?
They occur across a range of compounds so are an average of bonds of differing strength due to other components of the compounds, whereas with Hess Cycles they are for that specific compound.
What is Hess’ Law?
That the enthalpy change is the same regardless of the reaction path taken.
How do you work out the mean bond enthalpies?
∆H = ∑∆Hrθ (bonds broken) - ∑∆Hfθ (bonds formed)
What are the standard conditions used for calculating bond enthalpies?
100kPa;
298K;
Substances are in their standard states.
What conditions does there have to be to make a reversible reaction?
A closed system, where no substances are lost but energy and other conditions can be changed.
What is Le Chatelier’s Principle?
Whenever a system which is in dynamic equilibrium is disturbed, it tends to respond in such a way as to oppose the disturbance and restore equilibrium.
Give an ideal answer to a question where the forward reaction is endothermic and the temperature is lowered.
If the temperature is lowered, the equilibrium position shifts so the exothermic reaction is favoured. This is in order to oppose the change and to increase the temperature. As the position of equilibrium is moved to the right, more products are formed.
What affect on the percentage yield of a dynamic equilibrium will a catalyst have?
None, as it increases the rate of reaction in both directions equally.
What is the equation used to find the heat energy released/absorbed during a calorimetry experiment?
q = mc∆T.
Which way do the arrows point when we are given combustion data and we are working out formation data?
The arrows point down, towards the CO₂ and H₂O.
Give the 2 main causes of error in calorimetry.
Heat loss and incomplete combustion.
Explain briefly the idea behind collision theory.
Particles must collide quickly head on and with enough energy (≥ their activation energy) in order to collide. Therefore, if they do not collide head on, with enough speed or enough energy then they will not react.
Give the asymptote when drawing a Maxwell-Boltzmann Distribution.
y = 0 after going through O (as particles can theoretically have infinite energy).
What are the 4 main points to remember when drawing a Maxwell-Boltzmann Distribution?
Curve passes through origin;
The peak/mode is the temperature at which most of the molecules are - the temperature given;
The curve must go through the origin;
No particles can have zero energy.
