1. ### Melting of substance with -ve slope for fusion curve

The question says that the process is melting, so temperature must increase. Hence, Delta T > 0. Also, it is given that the slope for its fusion curve is -ve, which means that as we increase temperature, the pressure will decrease. So, Delta P < 0. The question asks to prove that the substance...
2. ### Why is enthalpy of combustion 0Kjmol-1 for oxygen

Homework Statement Why is enthalpy of combustion 0KJmol-1 for oxygen? I need to know this for my exam coming up soon. Homework Equations The enthalpy change when 1 mole of a substance is burnt in excess oxygen under standard states under standard conditions. The Attempt at a Solution Oxygen...
3. ### How do I calculate the enthalpy of formation using combusion data

The enthalpy of formation equation: ΔHƒ°reaction=∑Δhƒ°(products)-ΣΔHƒ°(reactants) When using the enthalpy of formation equation you get: -2010-((-394x3)+(-286x4))= 316Kjmol-1 However, this answer is wrong. On the mark scheme, the answer is -316Kjmol-1. So I used the enthalpy of combustion...
4. ### How much energy will be released?

Homework Statement The enthalpy of neutralization for the reaction of a strong acid with a strong base is -56 kJ/mol of water produced. How much energy will be released when 195.0 mL of 0.400 M HCl is mixed with 155.5 mL of 0.500 M NaOH? Homework Equations c=n/V The Attempt at a Solution to...
5. ### I How do I find an enthalpy value if my temp. is below 50 deg. celcius?

I wanna design a curding machine (Milk to Cheese) with an open system that operate at isobaric process (contant pressure), P = 1 atm = 1,... bar the range of temperature is = 25-75 0C can i find an enthalpy of water from Mollier Diagram if my temp. is 25 to 75 degree celcius ? how ? from my...
6. ### Liquidification of Helium under constant Enthalphy Condition

Homework Statement a)Helium enters a closed system as a flow of compressed gas at a temperature of 14 K and at pressure p (enthalpy conserved). A fraction α emerges as liquid and the rest as gas at 14 K, both at atmospheric pressure p0. Find an expression for α in terms of the enthalpy H(p) of...
7. ### Enthalpy of fusion/vaporization/sublimation to a plasma?

I may be grossly generalizing the principle, but is there a way to calculate the energy needed to transform a solid/liquid/gas into plasma? Information is easily found on enthalpy of fusion/vaporization/sublimation, but I couldn't find anything regarding transforming to the 4th state of matter...
8. ### I Enthalpy in Throttle Process (Joule–Thomson expansion)

Why is it written that enthalpy (##H##) is constant for Joule-Thompson expansion? It seems the essence of this process is to convert from one pressure to another with no heat loss. How does that connect with enthalpy being constant? When I learned about enthalpy it seemed to be most relevant to...
9. ### How to calculate the efficiency of a fuel engine

Homework Statement [/B] Im trying to find the efficiency of a gasoline engine regarded as a control volume. Am I right to use the HV as total energy input into the system? Assuming the enthalpy of formation of the fuel cant be found. I am really wondering if enthalpy of formation equals the...
10. ### Thermodynamics -- Internal Energy of Water

Homework Statement Water is initially at P = 1 bar and T = 20°C. 100kg of water is pumped to a higher pressure at which P = 10 bar and T = 25°C. Find ΔU and ΔH Homework Equations H = m*h du = c*dT dh = c*dT + v*dP The Attempt at a Solution So far I have looked in my table and found that at P...
11. ### Internal Energy Thermodynamics

Homework Statement CO2 is at P=3atm, T = 295K and V=1.2m3. It is isobarically heated to T = 500K. Find ΔU and ΔH Homework Equations dU = cpdT The Attempt at a Solution I am having a hard time in general in this class. I understand that in this problem, ΔP = 0. Does this mean that there must...
12. ### I Confusion: Internal energy u vs enthalpy h

I've been given the following relations where as I understand it subindex 2 equals subindex e and subindex 1 equals subindex i: ***EDIT*** More accurately subindex 1; initial state of a control mass subindex 2: end state of a control mass (end state is simply state 2 in the problem at hand)...
13. ### Calculating heat transfer

When calculating heat transfer, how would one know when to use Q=T*m*(delta s) versus Q=m*(delta h). I'm confused when we should calculate using entropy versus enthalpy. Anything helps. Thank you!
14. ### Enthalpy of isooctane at 500 K

I have to calculate the enthalpy of isooctane at 500 K. Isooctane is a liquid at room temperature and whenever enthalpy is to be calculated, it is from the standard state (278 K temperataure, 1 atm pressure). Do I calculate enthalpy upto boiling point, then use latent heat of vaporization and...
15. ### I Enthalpy problems...

In a trasformation in which P=costant, but internal pressure is different from external pressure, ΔH=Q? I'm asking this question because I know that Q=ΔU+PΔV (where P is the external pressure) and H=U+PV (where P is the sistem pression, so the internal pressure) Am I right?
16. ### I Mass to Energy Transformation for ordinary Chemical reaction

Hi, My Modern Physics lecturer is of the opinion that the energy dissipated during exothermic reactions is due to infinitesimally small change in mass of the reactants. Similarly, he said that an infinitesimally small part of the food we eat gets converted into the energy using which we perform...
17. ### Flue gas enthalphy

Hi 1. Homework Statement I'm trying to calculate the enthalphy of flue gas leaving a gas turbine, in kJ/kg. I just want to confirm that I'm using a correct method. Homework Equations hfg = (VCO2⋅ hCO2 + VH2O⋅ hH2O + VN2⋅ hN2 + VO2⋅ hO2) / (VCO2 + VH2O + VN2 + VO2) ρfg = (p⋅ M) / (R⋅ Tfg)...
18. ### Why don't we include VdP in U equation ?

Homework Statement I was trying to obtain Tds=dh-Vdp by differentiating the internal energy equation U=Q-PV and doing some arrangements but at the end I couldn't achieve my goal.The part that I don't understand is when we differentiate H=U+PV we obtain dH=dU + PdV + VdP and then from there we...
19. ### Enthalpy at the throat of the nozzle

Hi, This is my first post in this forum :woot: I have a total enthalpy ht=3000 kj/kg with velocity at inlet v1=20 ms-1, Speed of sound a1=562.5 ms-1 and outlet pressure P2 = 1 bar With the formula ht=h*+(a*2/2) how do i calculate actual value at throat h* ? NOTE: (by trial and error) I can...
20. ### Enthalpy Change and Bond Energies

Homework Statement Homework Equations The Attempt at a Solution 6C-H bonds= 410 x 6 = 2460 kJ/mol 3C-C bonds H of formation= +53.3 kJ/mol H of atomisation = +717 kJ/mol H-H = 436 kJ/mol as H of formation is 53.3, can't I just do: 2460+3x=53.3? But then I am not using a lot of the values...
21. ### Acetone in water influence its enthalpy of vaporization

Hi Everyone, I'm currently developing a waste heat recovery device as a side project. The system I've created uses a solution of acetone and water. One step in the process involves vaporizing acetone from the water at a low grade heat temperature (note: acetone does not form an azeotrope with...
22. ### How to calculate amount of vapor given q(t) and p(t)?

Hi everybody, I am trying to build a small model which basically should be able output "mass flow of water vapor as a function of time" given following inputs: - initial mass liquid water m_l_0 [kg] - initial temperature of liquid water T_l_0 [°C] - initial pressure p_0 [Pa] - heat added as...
23. ### Heat of Combustion, Enthalpy

Dear PF Forum, I have read this link in Wiki. But I don't understand what it means. Can someone help me? https://en.wikipedia.org/wiki/Heat_of_combustion A: Heat of combustion of CH4 is 50.09 MJ/kg https://en.wikipedia.org/wiki/Standard_enthalpy_change_of_formation_(data_table) B: Enthalpy of...