17.4 Why do chemical reactions occur Flashcards
What do the terms feasible and spontaneous mean in terms of reactions
Reactions that take place on their own accord
Majority of these are exothermic so ∆H is negative
What is entropy ∆S
How does this effect particles
When is there a positive value for entropy
A measure of disorder in a system
Particles have a tendency to become disordered or chaotic
When the products in a reaction are more disordered than the reactants there is a positive value for entropy
How does the value of entropy change between solids to gases
Gases have the highest entropy,
Then liquids
Solids have the lowest value for it
This is because gases have more randomness as the particles can move more and energy can be distributed in more different ways in them.
How do you calculate entropy changes ∆S
What is the unit for entropy
Add all entropies of products, and subtract the sum of entropies of reactants
JK-1mol-1
How can you tell from an equation if the entropy change is positive or negative
. Look at states, if state changes from liquid to gas or from solid to liquid there is a positive entropy change
. Look at number of moles on each side, so if there are more products than reactants it is a positive entropy change
What factors help us tell if a reaction is feasible or not
. Entropy change
. Enthalpy change
. Temperature in kelvin
What is Gibbs free energy equation
What is the temperature measured in
What is the enthalpy change and entropy change made into?
∆G = ∆H - ∆ST
Gibbs energy = Enthalpy change - (entropy change X temp in kelvin)
Enthalpy change is from KJ into Joules
How does Gibbs free energy tell us if a reaction if feasible or not
When using the equation, if the answer is negative or zero, the reaction is feasible
If the answer is positive, the reaction is not
How does temperature effect feasibility of a reaction
Some reactions may be feasible at one temperature but not at another
If a reaction is endothermic, but has a very high temperature that makes the (temp X ∆S) bigger than the ∆H, then there will be a large enough positive entropy change.
How can you calculate the temperature where a reaction is just feasible, so when ∆G is zero?
When ∆G = 0, you can use equation
T = ∆H / ∆S which will give you the temperature in kelvin
How can you calculate an entropy change using Gibbs free energy equation
Make ∆G = 0 and then change the equation so
∆S = ∆H / T
Although Gibbs energy tells us if a reaction is feasible or not, how can we find out the rate of reaction
You might predict if it occurs spontaneously because of enthalpy or entropy changes, but its possible that a reaction takes place so slowly that it basically doesn’t react
Use kinetics to find out rate
If you find out that a reaction is feasible from Gibbs free energy, why might the reaction still not occur
If the activation energy is too high
