CHEMISTRY COMMUNITY

Cristina Sarmiento 1E

Based on the equation Matt gave, if Ecell is positive, Delta G is negative = spontaneous. If Ecell is negative, Delta G is positive = nonspontaneous.

Cristina Sarmiento 1E

You also add Pt when there is a solid that cannot conduct electricity (e.g. halogen)

Cristina Sarmiento 1E

a. Yes, there is a phase change occurring because liquefied propane was released as gaseous propane vapors.
b. Yes, the temperature remained constant, meaning the surroundings had to supplement heat to keep the temperature from changing.

Cristina Sarmiento 1E

They were both correct because complex molecules have more entropy than less complex molecules and gas has more entropy than liquid.

Cristina Sarmiento 1E

The nucleophile attacks a positively charged, for example carbon, and in the process, substitutes with the leaving group.

Cristina Sarmiento 1E

No, rate constants can never be negative.

Cristina Sarmiento 1E

When you flip the sign because you flip the equation, you must do Ecell = Ecathode + Eanode.
It is also helpful to know that for galvanic cells, the Ecell must always be positive.

Cristina Sarmiento 1E

Just adding on that with reaction mechanisms, the rate law will sometimes have the same exponents as the coefficients in the chemical equation by mere coincidence. We cannot assume that the coefficients will be the exponents. We must look at the elementary steps.

Cristina Sarmiento 1E

Catalysts can be appear in the overall rate law, but sometimes they are cancelled out (e.g. consumed in the fast step before and produced in the slow step) and therefore would not appear in the rate law.

Cristina Sarmiento 1E

If a fast step is before a slow step, we can assume the fast step is in equilibrium. I believe Lavelle called it pseudo equilibrium.
We substitute equilibrium constant K to get rid of any intermediates because intermediates cannot be in the rate law.

Cristina Sarmiento 1E

Since both reactants are first order, the rate law would be rate law = k[CHCL3][CL2]. Because you are given the instantaneous rate of reaction, 2.54 x 10^/2 mol/L and the concentrations can be solved for, you just plug those numbers into the rate law equation to find k. To find the concentration of ...

Cristina Sarmiento 1E

Someone please correct me if I am wrong, but do we multiply the unique average rate by the coefficients (for e.g. oxygen) to find its rate because the unique average rate already took into account its coefficients and now we just have to multiply it back?

Cristina Sarmiento 1E

I thought the unique average rate law took into account the stoichiometric coefficients so it's not necessarily just one mole

Cristina Sarmiento 1E

The equation given is 2A --> B + C So there is a decrease in A because it is making B (an increase in B) If we know the increase in B, we can use the coefficients from the equation to convert moles of B back to moles of A. The number we get from the conversion is how much A was used to make B. So if...

Cristina Sarmiento 1E

The textbook tends to write the answers in moles

Cristina Sarmiento 1E

Why do we multiply the unique rate by the coefficient of oxygen to find its rate and multiply the unique rate by the coefficient of water to find its rate? Why don't we divide the unique rate by the coefficient?

Cristina Sarmiento 1E

Because the reactant is being consumed, the overall concentration of the reactant is decreasing so there is a negative rate of change. However, if the question asked at what rate was A being consumed, then it will be a positive answer. Just be careful about how the question is worded! Can you pleas...

Cristina Sarmiento 1E

Yes, it only goes to #39

Cristina Sarmiento 1E

So when you are calculating the rate of reaction that is not the unique rate of reaction, you multiply the concentration by the coefficient?

Cristina Sarmiento 1E

The unique reaction rate takes into account the coefficients of the chemical equation.

Cristina Sarmiento 1E

The unique rate of reaction is the general rate of (for example) $\frac{-1}{a} \frac{\Delta [A]}{\Delta t} = \frac{1}{b}\frac{\Delta [B]}{\Delta t}$
when $A \rightarrow B$
This rate of reaction takes into account the coefficients of the reaction.

Cristina Sarmiento 1E

How do you know when a reactant is independent of the rate?

Cristina Sarmiento 1E

I put it in order of the reactants, products
I am not sure if this is completely right but I have gotten the same answers so far

Cristina Sarmiento 1E

The oxidizing agent is the substance being reduced.
The reducing agent is the substance being oxidized.

When the cell potential is more positive, it has a stronger oxidizing agent
When the cell potential is more negative, it has a stronger reducing agent

Cristina Sarmiento 1E

The commas mean that they are in the same phase.
" | " means that they are in different phases
"||" is the salt bridge that separates the anode from the cathode

Cristina Sarmiento 1E

When you need to balance out the oxygens, you add H20 to other side. Then to balance the H's from the H20, you add H+ to the side of the oxygens so everything balances out.

Cristina Sarmiento 1E

You use one | to represent different phases in contact with each other and two || to indicate the salt bridge.

Cristina Sarmiento 1E

I think we use Pt when they are no other solids to transfer electrons.

Cristina Sarmiento 1E

Can someone please explain the relationship between temperature and internal energy ?
Also, the relationship between temperature and entropy?
Thanks

Cristina Sarmiento 1E

What formulas do you use when it is a) monatomic gas b) diatomic gas?

Cristina Sarmiento 1E

How would you use the Vant Hoff Equation when there are two different temperatures given?
Do you solve for one temperature first and then the other and add them?

Cristina Sarmiento 1E

At constant pressure delta H = q, which means q would have to be 0.
q is 0 when it is an adiabatic reaction.

Cristina Sarmiento 1E

When w = 0, delta U = q.
q is only equal to delta H when it is q is at constant pressure.

Cristina Sarmiento 1E

S°m is molar entropy.
S°f is standard entropy of formation.
S°r is standard reaction entropy.

Cristina Sarmiento 1E

I think of catabolic reactions breaking down things, releasing energy = exothermic and anabolic reactions forming things, requiring energy = endothermic.
These reactions tend to go hand in hand with catabolic reactions supplying the energy for anabolic reactions.

Cristina Sarmiento 1E

Negative delta G means the reaction is spontaneous.

Cristina Sarmiento 1E

When a reaction is adiabatic, q=0

Cristina Sarmiento 1E

The isothermal reaction tells us that internal energy is equal to 0, which means q = -w. Since the temperature stayed constant while there was work being done, we know that there was a heat transfer from the surroundings into the system to maintain the temperature.

Cristina Sarmiento 1E

Reactions that do not require energy are seen as more favorable. Since exothermic reactions release energy and endothermic reactions require energy, exothermic reactions are more favorable.

Cristina Sarmiento 1E

If it was delta P instead of P in the equation, then pressure would equal 0 when it is constant.

Cristina Sarmiento 1E

Delta U = Delta H when there is constant pressure and constant volume. Remember that Delta U = q +w. Delta H = q when there is constant pressure. w= -P Delta V. So when volume is constant, Delta V = 0. Therefore when there is constant pressure and volume, you can substitute q & w and get delta U...

Cristina Sarmiento 1E

Delta H fusion = Hm (liquid) - Hm (solid) as mentioned in the book and in lecture. What does Hm stand for? Are we going to have to know how to solve for delta H fusion or for the other delta H phase changes?

Cristina Sarmiento 1E

Melting, vaporization, and sublimation all require energy (endothermic).

Condensation releases energy.

Cristina Sarmiento 1E

I would just look at the units in your equation that you are trying to solve and see if the moles cancel out. If not, then delta H is kJ/mol. If the moles cancel out, then delta H is kJ.

Cristina Sarmiento 1E

An exothermic reaction releases energy, which means that heat is also being released, causing a rise of temperature.

Cristina Sarmiento 1E

The 3 methods are just specific types of reaction enthalpies.

Cristina Sarmiento 1E

I am also confused because in my notes I have that endothermic reactions absorbs and requires energy so breaking bonds would be endothermic. Exothermic releases and in my notes I put forming bonds would be exothermic but why would you release energy when you form bonds?

Cristina Sarmiento 1E

When a reaction is endothermic, delta H is positive. Reactions that are endothermic include vaporization, melting/fusion,and sublimation because heat is being absorbed. When a reaction is exothermic, delta H is negative. A reaction that exothermic is combustion because heat is released.

Cristina Sarmiento 1E

A normal calorimeter is for constant pressure and a bomb calorimeter is for constant volume.

Cristina Sarmiento 1E

An isolated system does not come in contact with its surroundings (heat stays in thermos).
A closed system has a fixed amount of matter and can exchange energy with its surroundings (the ice pack is used on injuries to exchange energy).

Cristina Sarmiento 1E

Steam burns more because of the phase change from liquid to vapor. Water boils at 100 degrees Celsius and during the phase change of vaporization, heat continues to increase. So when the boiling water has been turned into steam, a lot more heat had been supplied and is the reason why steam burns more.

Cristina Sarmiento 1E

Heat capacity is the amount of heat over how much the temperature increased; it's a ratio. Whereas specific heat capacity is the heat capacity divided by the mass of the sample. Molar heat capacity is similar to specific heat capacity but instead of dividing the heat capacity by the mass, you're div...

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