Thermodynamics I Flashcards
What is energy
Energy is the capacity to do work
By understanding energy changes what can we predict
Which molecules form, what they will react with and to what extent
What is the First Law of Thermodynamics
Energy can neither be created nor destroyed, merely transferred into a different form (conservation of energy)
In energy accountancy what two things is the universe made up from
They system and surroundings
What is a system in energy accountancy
The part of the universe we are interested in e.g. solutions in a beaker, chemical reactor, electrochemical cell
What are the surroundings in energy accountancy
Everything else in contact with the system which can influence its state
What type of system is this and describe it in terms of exchange of energy and matter. Give an example
Closed system. There is exchange of energy bu not matter with the surroundings. An example is making ice cubes
What type of system is this and describe it in terms of exchange of energy and matter. Give an example
It is an open system. There is exchange of both energy and matter with the surroundings. An example is a drying wet washing
What type of system is this and describe it in terms of exchange of energy and matter. Give an example
It is an isolated system. There is no exchange of energy and matter with the surroundings. An example is a thermos flask
What is work
Work is the transfer of energy causing motion against a force
Explain how this reaction is an example of work being done:
Zn(s) + HCl(aq) -> ZnCl(aq) + H2(g)
At constant pressure there is an increase in volume due to the creation of gas showing that the reaction has done work
If work = force x displacement.
But pressure = force/ area or pressure x area = force what does work equal
Work = pressure x change in volume
What does δV mean
Small change in volume
What does ΔV mean
Big change in volume
What is heat
Heat is the transfer of energy as a result of temperature difference between the system and surroundings
What is a positive change in heat or work
A change in heat or work that increases the energy of the system
What does Q stand for
Heat
What does W stand for
Work
What does a positive Q value represent
Means IN (absorbed by the system)
What does a negative Q value represent
Means OUT (absorbed by the surroundings)
What does a positive W value represent
Means DONE ON the system
What does a negative W value represent
Means DONE BY the system
How are heat changes measured
By using a calorimeter
What is a calorimeter
A reaction vessel surrounded by a water bath containing a thermometer
What is the equation for heat supplied
Q = mwater x Cwater x ΔT
m- mass, c- heat capacity
At constant pressure Q equals
Q = ΔH (enthalpy change)
At constant volume Q equals
Q = ΔU (internal energy change)
What equation is used to express the 1st Law of Thermodynamics
ΔH = ΔU + PΔV
What does a negative PΔV value show
The system is doing work
What does a positive PΔV value show
Work is being done on the system
What is reaction enthalpy
The measured heat change for a reaction at atmospheric pressure
Enthalphy is state fucntion, what does this mean
That a change in enthalpy depends only on the difference between the starting state and the final state, not the route taken
The value for ΔH depends on the number of steps taken. True/ False
False. It is the same whether the reaction is carried out in one or several steps
What is Hess’s Law
The standard enthalpy (ΔH°) of a reaction is the sum of all the standard enthalpies of the reactions into which the overall reaction may be divided, even if this division is artificial.
What is the first method for calculating Hess’s Law
What is the second method for using Hess’s Law
How would you work out enthaply when given various reactions with enthalpies and then asked to work out the enthalpy of one specific reaction
- Write out the reaction in full using the given ethaply equations
- Cancel out substances which occur on the left and right hand side of the equation
- Check if new reaction is the same as unknown enthaply reaction
- Add together the enthalpies for the stated reaction
What do you need to remember to do if you use the reverse of a stated ethaply reaction
To swap the enthaply of a reaction i.e. if it was positive it becomes negative
What is standard pressure
1 atm (atmosphere)
