Thermodynamics

• What is an open system?

  An open system is one in which both matter (oxygen from air) and heat energy can be transferred.
• What is a closed system?

  A closed system is one in which only energy (heat or work) can be transferred but not matter.
• What is an isolated system?

  A system that does not exchange matter or energy with its surroundings, so energy does not leave or enter the system
• What are the 3 laws of thermodynamics?

  Energy can neither be created or destroyed, different forms of energy can interconvert but energy is conserved.

  The entropy of an isolated system always increases.

  The entropy of any pure perfect crystal can be chosen to equal zero at a temperature of 0 K.
• What's an exothermic reaction?

  A process that releases energy as heat
• What's an endothermic reaction?

  A process that absorbs energy as heat
• What's the internal energy?

  The total energy of a system

  
  

  If heat flows into the system the system gains energy or if the surroundings does work on a system both ways can increase the internal energy of the system.

  
  

  This ties in with the 1st law of thermodynamics as energy is transferred not destroyed or created.

  
  

  Symbol: ∆U
• What's the equation of internal energy?

  ∆U = Q -W

  
  

  Q: Heat added to the system

  W: work done by the system
• What is enthalpy?

  The heat transfer at constant pressure
• Define work?

  Work is the transfer of energy through a force causing motion.

  Work is done when a force is applied to an object, and the object moves in the direction of that force.

  Work=Force×Distance
• What is the energy of a system?

  Energy is the capacity or ability of a system to do work.
• What is heat?

  When there is a temperature difference between a system and its surroundings, energy will naturally transfer from the hotter object to the cooler one. This transfer of energy is called heat (q).
• If you're measuring the internal energy changes from one state to another, what equation do you use

  ΔU=UB​−UA​
• Whats the equation to calculate the total heat energy released?

  q=C×ΔT

  C= heat capacity (J/ oC)

  ΔT = change in temperature

  q = total heat released
• 0.4400 g glucose was burned in a constant volume bomb calorimeter. Temperature rose from 25.00 °C to 26.75 °C. If the heat capacity of the bomb plus water is 9203 J K-1 calculate ΔU for the combustion of glucose?

  C6H12O6 (s) + 6 O2 (g) ----> 6 CO2 (g) + 6 H2O(l)

  q = c x ΔT

  ΔT: 25 - 26.75 = -1.75

  c= 9203 x -1.75 = -15790 J

  Moles: 0.4400/180.2 = 0.002441 mol

  Molar internal energy ΔU = - 15790/0.002441 = - 6468660 J mol -1
• If the enthalpy change is negative what does this mean?

  The reaction is exothermic
• If the enthalpy change is positive what does this mean?

  the reaction is endothermic
• What's the equation for enthalpy at constant pressure?

  H=ΔU+PΔV

  • H = Enthalpy
  • U = Internal energy
  • P = Pressure
  • ΔV = Change in volume
• What's the standard enthalpy of formation?

  The standard enthalpy of formation is the change in enthalpy when one mole of a compound is formed from its elements in their standard states (most stable form at 1 atm pressure and 25°C).

  Example: For example, oxygen gas is the standard state of oxygen, and carbon is in the form of graphite.
• What's the standard enthalpy of formation of o2?

  0 because it is in its most stable form at standard conditions.
• What is Hess's law?

  Hess's Law states that the enthalpy change for a chemical reaction is the same regardless of the number of steps taken to complete the reaction. The total enthalpy change is the sum of the enthalpy changes for each step.
• Why is Hess's Law important?

  Hess's Law is important because it allows chemists to calculate the enthalpy change of reactions that cannot be easily measured directly, such as slow reactions or reactions that occur in multiple steps.
• What is the equation used in Hess’s Law?

  ΔHoverall reaction=ΔHstep 1+ΔHstep 2+ΔHstep 3+…

  This equation shows that the total enthalpy change of a reaction can be found by adding the enthalpy changes of the steps in the reaction mechanism.
• How do you use Hess formula in calculating bond energies?

  ΔH = Σ (Energy of bonds broken ) - Σ (Energy of bonds made )

  1) Determine the number (in moles) and type of bonds broken and formed from the balanced equation. This may require you to draw the molecules to identify how many of each bond type are present.

  2) Multiply the number of bonds by the average bond energy given in the table to determine the energy change.

  3) Plug values into the above formula to calculate the ΔH of the overall reaction.
• :Calculate the energy of the reaction for the burning of propane in oxygen to form carbon dioxide gas and water gas, using bond energies. The balanced equation is given below. C3H8 (g) + 5 O2 (g) ----> 3 CO2 (g) + 4 H2O(g)

  First calculate how many bonds between each atom.

  Then multiply the bond energy by the number of bonds for the reactants and the products.

  Then do bonds broken- bonds formed

  - 2,054 kJ mol-1

  
• What does it mean when q is positive or negative?

  When q is positive = means heat is absorbed by the system = endothermic

  q is negative= when heat is released by the system = exothermic

  w= work is positive when work is done on a system - increasing the internal energy of the system

  w is negative when work is done by the system - decreasing the internal energy of the system
• Calculate the change in internal energy of a system if 300j of heat energy is absorbed by the system and if 400j of work is done on the system?


  ΔU=q+w

  Q = heat absorbed by the system (positive if absorbed)

  w = work done on the system (positive if work is done on the system)

  Given:

  • q=+300 J
  • w=+400 J

  ΔU=300 J+400 J=700 

  ✅ The internal energy of the system increases by 700 J
• The system releases 700j of heat energy and 300j of work is done by the system. Calculate the change in the internal energy of the system?

  Heat is being released from the system which means it will be -700j

  work is also being done by the system which means it will be -300j

  (-700) + (-300) = -1000j
• Iron metal reacts with oxygen gas to produce Iron (III) oxide. How much heat is released when 9 moles of Fe reacts with excess O2 gas? How much heat is released when 24g of O2 is consumed in the reaction? How many grams of iron (III) oxide will be produced if 4500kJ if heat energy is released?

  4Fe + 3O2 = 2Fe2O3 Total enthalpy change for this reaction is -1652kJ

  For 9 moles you do:

  4 = -1652

  9 = -3727kJ

  b) Mass/mr = moles

  24/ (16 x2) = 0.75 moles

  3 moles = -1652kj

  0.75 moles = -413kj

  c) 2 Fe2O3

  (2 x 55.845) + (16x3) =159.69 - mr

  Mass: 159.69 x 2 = 319.38g

  319.38g = -1652kj

  870g = 4500
• Propane reacts with O2 to produce co2 and water. How much kg of propane is needed to generate 5MJ of energy if the process is 40% efficient?

  1 C3H8 + 5O2 = 3CO2 + 4H20 total enathlpy change = -2221kj

  40% of x = 5mj

  x= 12.5 MJ

  12.5MJ = 12500KJ

  1 = -2221kj

  5.628= 12500kj

  Mr of propane = 44

  Mass: 44 x -5.628 = 247g

  246g = 0.248kg
• Whats the specific heat capacity?

  Cs = q/ mass x change in temp

  q = heat absorbed

  units: J/g- oC
• Whats the molar heat capacity?

  Cm = q/ moles x change in temp

  units : J/mol - oC
• 1000j of heat energy is applied to 10g of iron metal (0.45 J/g C). Calculate the temperature change ?

  Heat is being applied therefore it will be + 100j

  q = m x c x change in temp

  1000 = 10 x 0.45 x change in temp

  Change in temp = 222.2 oC
• 293.7J of heat is removed from 5g of aluminium causing the temp to drop from 85 to 19oc. What is the specific heat capacity of aluminium?

  heat is removed so it'll be -293.7j

  q = m x c x change in temp

  -293.7 = 5 x (19-85) x c

  c= 0.89 J/g°C
• What does the enthalpy of fusion mean?

  The amount of Heat required to change a substance from solid to liquid.
• How much heat energy is required to melt 100g of ice? The enthalpy of fusion of ice is 6.01kj/mol?

  You need to find the moles first:

  Ice is water so mr of H20 is 18

  100/18 = 5.6 moles

  1 mole = 6.01 kj/mol

  5.6 moles =33.4 kj
• How much energy is required to melt 75g of ice? The heat of fusion of ice is 334 J/g?

  q = mass x enthalpy change

  q = 75 x 334=25050J
• Does heat go from hot to cold or cold to hot?

  hot to cold
• A 70g chunk of metal at 180 oc was dropped in a bucket containing 400g of water at 25oc? The final temp of the mixture was 28oc. What is the specific heat capacity of the metal? The SHC of water is 4.184

  Since heat goes from hot to cold, the metal will release heat and the water will absorb that same heat released as energy is released not destroyed.

  You can assume that the heat energy of the metal equals the heat energy of water will be equal.

  -q metal = q of water

  -m x c x change in temp = mass x c x change in temp

  -70 x cx (28-180) = 400 x 4.184 x (28-25)

  10640 c = 5020.8

  The reason why q is negative on the left side is because the temparture of h20 is going up whilst the temp of the metal is going down, thus to balance this one side of the equation has to be negative

  c= 0.472 J/g oC
• 3.36g of ethanol (c2h5oH) is burned in a bomb calorimeter with a heat capacity of 2.3kJ/c. The temperature of the calorimeter increases from 12.1c to 55.5c. Calculate the energy of combustion per mol?

  q= c x change in temp

  q = 2.3 x (55.5-12.1) = 99.82kj

  Because the value is positive this means the process was endothermic as heat was absorbed by the calorimeter (aka the surroundings).This means heat must've been released by the system so you need to make the value negative as the system is going through an exothermic reaction.

  Moles: 3.36g/ 46 = 0.073 moles

  Energy of combustion per mole:

  -99.82kj/ 0.073 = -1370kj/mol