Calorimetry and Q=mcat
Identify what every variable stands for in q=mcAt
*bonus 50: what does the delta T stand for?
q= heat or energy
m= mass
delta T= change in temperature (final - initial)
What is enthalpy?
The heat of a reaction
What is one word to describe entropy?
Disorder, randomness, possibilities
What does in mean if a reaction is spontaneous? Be as specific as possible.
A reaction which occurs in a given set of conditions without external intervention.
When heat is released, q has a (negative or positive) sign?
negative
If more cold water was mixed with less hot water in an insulated container, the final temperature (thermal equilibrium) of the mixture should be
a. closer to the cold
b. closer to the hot
c. exactly in the middle
d. not enough information to determine
closer to the cold water since there is more of it
The cold water will have a greater impact since there is more of it. If there is more of it, it has a larger heat capacity. It will take more energy to change the temperature of the cold water.
When energy is absorbed in a reaction, we the reaction _________________.
*Bonus 50: Does this type of reaction have a positive or negative value for enthalpy/heat?
endothermic
*positive
Which state of matter has the least entropy?
Solid since it is the most structured.
Entropy is a measure of disorder, randomness, or the number of possibilities. Solids are very structured, not random and there aren't many ways the atoms could arrange themselves. They are "stuck" in their positions.
Name an example of a spontaneous and a non spontaneous reaction. (Give the conditions for spontaneity if applicable)
Ice melting at room temp is spontaneous, while ice freezing at room temp is non spontaneous.
Thermal equilibrium is when:
a. the average temperature of all the molecules in the final mixture are the same
b. the temperature of the final mixture is exactly in between the temperatures of the starting substances
A. thermal equilibrium does not always have to be at the temperature exactly in between the two substances mixed together. Remember when the hot metal was put into the room temperature water. The final temperature was closer to the water, not exactly in between the temperature of the water and the metal.
Heat can only be measured when it is ___________
In other words, heat is a(an) _____________ of energy.
transferred, or exchanged
Are exothermic or endothermic reaction favorable? Why?
Exothermic, since energy is released. Endothermic reactions are not favored since energy needs to be put in to the reaction for it to occur.
Predict the entropy change for the following reaction. In other words, will entropy increase or decrease? Why?
2H2 (g) + O2 (g) ---> 2H2O (l)
Given the following, calculate the enthalpy of the reaction at 298 K.
G= -0.9 kJ
S= -135 J/K
*Hint: (-100)
-41.2
What is the name of this chapter?
Energy
Thermochemistry is also an acceptable answer.
What is the mass of water which releases 34.3 kJ of heat when it is cooled from 48.5 C to 12.0 C? The specific heat capacity of water is 4.18 J/gC.
225 g
(-34.3)= (m)(4.18)(12-48.5)
The 34.3 is assigned a negative number because heat is being released. This negative then cancels out with the negative temperature on the other side, giving a positive value for mass.
Remember that mass cannot be a negative number.
Standard Enthalpy of Formation
CO2 (g): -394 kJ
CO (g): -110.5
H2O (l): -286 kJ
H2O (g) : -241 kJ
CH4 (g): -75 kJ
Calculate the enthalpy of reaction for
CH4 (g) + 2O2 (g) ---> CO2 (g) + 2H2O (l)
-891 kJ
Predict the entropy change for the following reaction. Will it increase or decrease? Why?
2IBr(g) ---->I2 (s) + Br2 (l)
Decrease since the liquid and solid phase of matter are both less disordered than the gas phase of matter.
What does delta G stand for in the following equation?
delta G= delta H - T(delta S)
Is a reaction spontaneous when delta G is positive or negative?
Gibbs free energy, and when it is a negative value, the reaction is spontaneous
The quiz we had last week was over what equation?
q=mcat
A 2.50 g sample of zinc is heated, then placed in a calorimeter containing 65.0 g of water. The temperature of water increases from 20.00 C to 22.50 C. The specific heat of zinc is 0.390 J/gC. The specific heat of water is 4.18 J/gC. What was the initial temperature of the zinc metal sample?
Hint: The final temperatures of zinc and water are the same because when something hot and cold are mixed together, both eventually reach the same temperature.
719 C
This is a mcat= - mcat question. We know this because we are given two values for every variable. Two masses, two heat capacities, two sets of temperatures, etc..
Find the enthalpy (H) value for the following reaction using the three reactions below.
2B ---> 2AB + A (H= ?)
B + C ----> AB (H= 5 kJ)
D + C ----> DC (H= 10 kJ)
D -----> C + DC + A (H = 15 kJ)
Delta H reaction= 15 kJ
Predict the entropy change for the following reaction. Will entropy increase, decrease, or stay the same? Why?
2NO2 (g) ---> N2 (g) + 2O2 (g)
Increase since there are more moles of gas produced, which means more disorder and more possibilities for atoms to arrange themselves.
What two factors determine whether a reaction will be spontaneous or not?
Hint* (-150 points)
enthalpy and entropy
True/False
Spontaneous reactions are reactions which are able to occur quickly in a given set of conditions.
False, spontaneous reactions happen in a given set of conditions but are not necessarily quick.