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What is the 1st law of thermodynamics?
Energy can never be created nor destroyed, only transferred or transformed.
Energy can be BROADLY divided into:
potential energy and kinetic energy.
What is systems, surroundings and the universe?
Describe an open, closed and isolated system.
Open system: freely exchanges energy and matter with its surroundings. i.e. an open cup of coffee.
Closed system: system that only exchanges energy with its surrounding, not matter.. i.e.a paper cup with a lid on.
Isolated system: does not exchange energy or matter with surroundings. i.e. a thermos.
What is the formal definition of temperature?
Measure of the average kinetic energy of its atoms or molecules.
Intensive property (independent on amount of material)
(A 5mL sample and a 500mL sample of water that have the same kinetic energy of their
molecules will be at the same temperature)
What is heat?
Energy in transit.
Energy that flows from one object to another (hot to cold).
We cannot measure heat directly however we can calculate it using the temperature change that occurs when heat flows from one object to another.
HEAT AND TEMPERATURE IS DIFFERENT. Temp is not an energy.
Put more heat in, the temperature will increase.
What is the heat capacity?
The amount of heat requried to raise the temeperature of a substance by 1K.
q=C*∆T
What is specific heat capacity?
The amount of energy (as heat) required to raise the temperature of 1g of substance by 1K.
q=mc∆t
q= heat in J
m= mass (g)
c= specific heat capacity (J K-1g-1)
Molar heat capacity
q=ncm∆t
n= no. of mol
Wha are the ways a chemical system exchanges energy with its surrounding?
Heat and work.
The energy that is transferred as heat comes from a object’s internal energy.
What is internal energy, U, of a system?
Sum of the kinetic energy and potential energy of the collection of atoms, ions and molecules in the system.
Cannot be measured,
A state function (value only depends on the current state. Independent of how it got to that value.)
What is the formula of U?
∆U = q + w
Heat:
the transfer of heat energy in or out the system (J).
Work: the transfer energy when an object is is moved by a force (J)
When you move a weight, you need to work against gravity.
What is work?
Defined as a motion against an opposing force.
In biochemical system, there are several forms of work possible:
Electrical work required to charge nerve membranes.
Mechanical work performed by muscles.
Chemical work to synthesise molecules.
Pressure-volume (pV) work - work of compression or expansion against pressure.
w= -P*∆V
Describe pV work.
w = -P * ∆V
If volume is increasing (∆V=Vf - Vi = +∆V) the system is doing the work (w is -ve).
If volume is decreasing (∆V=Vf-Vi = -∆V) the work is being done on the system.
Describe the relationship between work and heat.
Note the sign convention preferred by
chemists:
Heat into the system: q is positive
Heat out of the system: q is negative
Work done on the system: w is positive
Work done by the system: w is negative
Describe pressure-volume work during a chemical reaction.
Chemical reactions producing or consuming a gas undergo large volume changes.
Evolution/Consumption of gases requires large volume changes. The work done on volume changes to keep constant pressure is: w=-P∆V
-P∆V=-∆nRT = w
∆n = no. of moles of gaseous product - no. of moles of gaseous reactants.
Also, if the reaction releases 5460 kJ of heat, calculate the change in internal energy.
If you are not given mass, just use stoic coefficients.
C8H18 is liquid so it is excluded.
What is calorimetry?
The process of measuring the amount of heat released or absorbed during a chemical reaction.
How do you measure the heat of a reaction?
The reaction must be an isolated system, so that no heat is lost to the environment.
Use a calorimeter which insulates the reaction, to prevent heat loss.
Describe the volume of bomb calorimeter?
System remains at constant volume.
∆U=q+w
w = =P*∆V = -P * 0 = 0 (no work in calorimeter).
∆U = qv
qv is the heat of reaction at constant volume.
How to determine heat at constant pressure (∆H)?
What is enthalpy?
The heat of reaction at constant pressure.
What is latent heat?
Latent heat is heat transferred without temperature change.
-usually associated with phase changes where energy goes into breaking intermolecular bonds.
Where the line is flat, heat is used to break intermolecular forces, so temperature does not increase.
What is STP?
Temperature: 273K
1 atm, but no longer common, as we use SATP, which is 298K and 1 bar (100kPa).
What is molar enthalpy of fusion?
The enthalpy change for 1 mol of a solid substance to be converted to a liquid at its melting point.
Endothermic process.
H2O(s) → H2O(l) ∆fusH=6.00 kJ mol-1
The reverse process is the freezing point (exothermic)
If 1 mol of ice at 0ºC, 6.00kJ of heat energy is released to its surrounding.
It is that part on the graph.
How much heat is released to the surroundings to transform 13.6 g of liquid mercury to
solid mercury at its freezing point, -38.8 ∘C. The ΔfusH = 2.3 kJ mol−1 for mercury.
What is molar enthalpy of vaporisation>
Energy required to convert 1 mol of a substance in its liquid phase to its vapour phase at its boiling point.
Endothermic process.
H2O(l) → H2O(g) ∆vapH=40.7kJ mol-1
The reverse processL condensation, ∆H is negative.
If 1 mol of gaseous water condenses at 100 ∘C, 40.7 kJ of heat energy is released to the surroundings.
How much heat must be absorbed to warm 25.0 g of liquid methanol (CH3OH) from 25.0∘C to its boiling point of (64.6 ∘C) and then to evaporate the methanol completely at that temperature? Specific heat capacity (methanol) = 2.53 J K-1 g-1.
The ΔvapH = 37.4 kJ mol−1 for methanol.
Heat required to heat methanol from 25ºC to 64.6ºC
q=mc∆T
q=35×3.53*(64.6-25)=2504J=2.50kJ
To completely evalorate the methanol at its boiling boint.
q=∆vapH*amount of methanol
q=37.4×25/32.04=29.2kJ
Add them together: 32.7kJ
What is Hess’s Law?
The change of enthalpy is the same irrespective of the amount of steps the reaction takes.
If a reaction can be written as a sum of two ro more steps, the enthalpy change of the reaction is the sum of the enthalpy change for each step.
Lets say you know the enthalpy for each step in a 2 step reaction, add them together and thats the enthalpy of the overall reaction.
What is the standard enthalpy change of reaction, ∆rHº
The enthalpy change in a chemical reaction at a temperature T, when all the reactants and products are in their standard states for that given temperature.
Can be at any temperature as long as reactants and products are in that standard state at that given temperature.
For example, hydrogen is a gas, O2 is a gas, and H2O is a liquid.
What is standard enthalpy of formation, ∆fHº?
The enthalpy change that accompanies the formation of one mole of a substance at 1 bar at the specified temperature from its elements with all substances in their standard states.
AN element is in its standard state when it is in its most stable form and physical state at 1 bar and the specified temperature.
∆fHº of any substance in its elemental form is 0 (for example Cl2 is pure chlorine, so it would be 0).
