Equation Station
What is the equation for specific heat?
q = mcΔT
What must ΔH and ΔS be in a combustion reaction? Is this spontaneous?
-, +, always spontaneous
For the reaction
H2(g)+1/2 O2(g)→H2O(l)
the enthalpy change is –285.8 kJ.
If 2 moles of water are produced, what is ΔH?
–571.6 kJ?
If a reaction goes from solid reactants to gaseous products, the entropy change ΔS is usually this sign.
+
This symbol represents specific heat capacity in the formula q=mcΔTq = m c \Delta Tq=mcΔT
c
What is ΔE and how do you find it?
change in internal energy of a system; ΔE = q + w
What must ΔH and ΔS be when something is freezing? Is this spontaneous?
-, -, spontaneous at low temperature
When using Hess’s Law, if you multiply a reaction by 2, you must also multiply this by 2.
enthalpy change (ΔH)
The Second Law of Thermodynamics states that for a spontaneous process, the total entropy of the universe must do this.
ΔS > 0
A 50 g metal sample is heated from 25°C to 75°C. If it absorbs 10,000 J of heat, the specific heat capacity c is calculated using this formula.
c = q / (m ΔT) = 10,000 ÷ (50 × 50) = 4.0 J/g·°C?
What equation is derived from the 2nd law of thermodynamics regarding entropy at equillibrium?
ΔS = q/T
What must ΔH and ΔS be when something is melting? Is this spontaneous?
+, +, spontaneous at high temperature
If
C(s)+O2(g)→CO2(g) ΔH=−393.5
and
C(s)+1/2 O2(g)→CO(g) ΔH=−110.5
find ΔH for
CO(g)+1/2 O2(g)→CO2(g)
–283.0 kJ? (–393.5 – (–110.5))
1 mole of ice melts at 0°C (273 K). The heat of fusion is 6.01 kJ/mol.
Calculate the entropy change ΔS for the melting.
22.0 J/K
A 100 g piece of copper (c = 0.385 J/g·°C) is heated from 25°C to 75°C. How much heat is absorbed?
q = 100 × 0.385 × (75–25) = 1,925 J?
What equation do you use for work done by or on a gas? What do all variables stand for and their units?
w = -P ΔV
w = work in joules
P = external pressure in atm or Pa
ΔV = change in volume usually in liters
What must ΔH and ΔS be in non-natural endothermic reactions? Is this spontaneous?
+, -, never spontaneous
Use Hess’s Law to find ΔH for:
C(s)+2H2(g)→CH4(g)
Given:
1️⃣ C(s)+O2(g)→CO2(g) ΔH = –393.5 kJ
2️⃣ H2(g)+12O2(g)→H2O(l) ΔH = -285.8 kJ
3️⃣ CH4(g)+2O2(g)→CO2(g)+2H2O(l) ΔH = –890.3 kJ
–74.9 kJ
(By reversing Reaction 3 and combining with 1 and 2)
1 mole of water vaporizes at 100°C (373 K). The heat of vaporization is 40.7 kJ/mol.
Calculate ΔS for the phase change.
109 J/K
A gas absorbs 400 J of heat and 100 J of work is done on it. What is ΔE for the system?
ΔE = q + w = 400 + 100 = 500 J?
What does the 1st law of thermodynamics say? What equation is symbolic of this?
Energy cannot be created or destroyed, it can only be transferred or transformed.
ΔE=q+w
This thermodynamic quantity determines spontaneity when both enthalpy and entropy are known.
Gibbs free energy (ΔG)
Find ΔH for
2NO(g)+O2(g)→2NO2(g)
Given:
1️⃣ N2(g)+O2(g)→2NO(g) ΔH = +180 kJ
2️⃣ N2(g)+2O2(g)→2NO2(g) ΔH = +66 kJ
–114 kJ
(ΔH = 66 – 180)
A reaction absorbs 15 kJ of heat reversibly at 298 K.
Calculate the entropy change ΔS of the system.
50.3 J/K
A gas expands against a pressure of 2 atm from 3 L to 8 L. Calculate the work done (w) in J. (1 L·atm = 101.3 J)
w = –PΔV = –2(8–3) = –10 L·atm = –1,013 J?