CHEMISTRY COMMUNITY

Dina Marchenko 2J

k=A*e^-Ea/RT This formula helps determine the temperature dependence of reaction rates. k is the reaction rate of the reaction. A is a pre-exponential factor and has to do with the collision of the molecule (and if it is properly oriented when it collides in order to react) Ea is the activation ener...

Dina Marchenko 2J

A is found by the arrhenius equation k=Ae^-Ea/RT. If you are asked to find A, you will be provided the information to plug in the values for all other variables in the equation. So basically, you'll end up dividing k by the term e^-Ea/RT (or the subsequent number value you get) to isolate A on one s...

Dina Marchenko 2J

The value of Ea will either be given or you will be able to find it by plugging in the values of the rest of the variables in the arrhenius equation.

Dina Marchenko 2J

Half-lives in general are also useful ways of measuring how old a substance is. Real-world applications include figuring out how old a fossil is or how long it will take a substance to decay.

Dina Marchenko 2J

I believe only the stoichiometric coefficient of the reaction would impact the rate (ie- if 2 or 1 or 3 molecules are colliding, etc.)

Dina Marchenko 2J

Yea I think the important part is knowing what to label the axes for each order. Beyond that just draw a linear function; I don't think we'd be expected to use data and graph it specifically.

Dina Marchenko 2J

Lavelle said we will never be asked to determine what the intermediates of a reaction are. We'll be provided the intermediates and enough information to balance the reactions/determine which intermediate reaction is accurate, etc.

Dina Marchenko 2J

Madeline Motamedi 4I wrote:Yeah I believe for our purpose we are usually assuming A is just 1.

Does this mean that A is always a whole number?

Dina Marchenko 2J

Graphically, activation energy is the hump in between the reactants and products that must be overcome in order for the reaction to proceed. Both exo and endo reactions have an activation energy, and catalysts help to minimize this activation energy. It is a numerical value, yes.

Dina Marchenko 2J

Why does a difference in concentrations cause a flow of electrons? And why do the electrons flow from low concentration to high concentration? Is there still a redox reaction taking place? Or is it something else?

Dina Marchenko 2J

An oxidizing agent is one that causes a substance to be oxidized (meaning that the oxidizing agent is reduced).

The reducing agent causes a substance (the other element/molecule in the reaction) to be reduced (so the reducing agent is oxidized).

Dina Marchenko 2J

The way I took this answer was that the Ag(s) on the left is serving as the metal on the anode side that conducts electricity for electron flow. Since there is no solid pure metal on the left side, the silver is necessary to conduct electricity (much like Pt does). This is why it's separated by a |

Dina Marchenko 2J

When writing half reactions to balance a redox reaction, does it matter what you put as the reactants or products? In the textbook, they always put the oxidized molecule on the left and reduced on the right (they do the same for cell diagram). Does it matter? Is it proper to keep oxidized on the lef...

Dina Marchenko 2J

Section 6K has a useful table that explains how to balance reactions in acidic and basic solutions. When it comes to balancing a reaction in a basic solution, you balance oxygen first with water on either side and then to balance hydrogens, you add water to the side that needs more hydrogens and ad...

Dina Marchenko 2J

I'm assuming that we'll be given any cell potential not values we need and from that we can calculate the cell potential. Any quantitative value will either be provided or we'll be given the equation needed to calculate it.

Dina Marchenko 2J

For acidic reactions, we have to add water to the reduction and oxidation reactions to balance out the oxygens. Do we have to add water to balance oxygens in basic conditions, or do we only add OH- to balance oxygen molecules? You use water to balance in both acidic and basic solutions. In both, yo...

Dina Marchenko 2J

You're best off memorizing the main rules (ex: O2 is always -2 for this class, group 1 are +1, etc.) When looking at a compound, first look at the elements that have oxidation number rules and figure out what the oxidation number of a single atom would be. Then multiply that by any subscript or stoi...

Dina Marchenko 2J

Section 6K has a useful table that explains how to balance reactions in acidic and basic solutions. When it comes to balancing a reaction in a basic solution, you balance oxygen first with water on either side and then to balance hydrogens, you add water to the side that needs more hydrogens and add...

Dina Marchenko 2J

I believe the subscript r just stands for reaction, so if you're using r then you're accounting for the stoichiometric coefficients (ie moles) within the reaction. I don't think that you need to worry much about which of the two equations to pick, I think the r is more symbolic and reminding you wha...

Dina Marchenko 2J

Cell potential is not a state function. Cathode and anode has to do with a single cell at a particular point in time so combining their values gives the cell potential by definition. However, you can't simply find a new cell potential by subtracting some initial value from a final value, so it is no...

Dina Marchenko 2J

This equation is related to the internal energy of an ideal gas and the heat capacities at constant pressure and volume for various gas molecules. I don't think we necessarily need to use it for this class.

Dina Marchenko 2J

When there is a constant volume or pressure, no expansion or compression work is being done by the system so w=0. When heat is at constant pressure, it equals the enthalpy change, so at constant pressure, delta U is equal to delta H.

Dina Marchenko 2J

Why is the change in internal energy of an ideal gas 0 at constant temperature? What about a constant temperature and it being an ideal gas makes the internal energy not change?

Dina Marchenko 2J

It's important to remember than delta U is the change in internal energy rather than just the internal energy of a system. It's a state function that looks at how the internal energy of a system changes over the course of a process (if a system releases or absorbs energy).

Dina Marchenko 2J

With constant pressure or volume, there is no expansion or compression work being done so w can be ignored. You don't really ignore q unless it's specifically stated that there is no heat transfer.

Dina Marchenko 2J

I view state functions as part of the definition of a variable- what it represents. We just defined internal energy as the inner store of kinetic and potential energy of a system which only cares about the current or previous states and not the paths. So yes, it just has to do with the definition of...

Dina Marchenko 2J

When a system is isothermal, it means that the temperature is constant so only work (expansion/compression) is considered. Internal energy is not 0 because a system is isothermal; it's 0 if the system is isolated. If a system is isolated, none of its energy is transferred to the surroundings, making...

Dina Marchenko 2J

(at least in this class, as far as I know) A double bond isn't 2 separate bonds but rather 1 stronger bond. So when you break a single double bond, you're breaking that entire bond. There aren't 2 bonds within a double bond.

Dina Marchenko 2J

In an exothermic reaction, the system loses energy in the form of heat, so the value of delta H and U is negative. When the system undergoes an endothermic reaction, it gains energy in the form of heat, so delta H and U is positive.

Dina Marchenko 2J

Jamie Lee 1F wrote:
GFolk_1D wrote:Are deltaH and q interchangeable?

At constant pressure, qp = deltaH. Otherwise, they are not interchangeable.

How do delta H and q relate if pressure isn't constant? Is there a formula?

Dina Marchenko 2J

How does enthalpy or change in enthalpy relate to q when pressure isn't constant?

Dina Marchenko 2J

What about when we are calculating the change in internal energy in a system under constant pressure? Work is involved, do we always use the negative sign in the expression for work? You don't necessarily need to worry about when to include the negative sign or not within -P delta V. If work is bei...

Dina Marchenko 2J

It's just the way we calculate things and what we care about looking at based on the definition of the word. Internal energy only looks at the initial and final states and doesn't care about the path taken. Heat and work, on the other hand, we look at the path taken to get the value of both. If the ...

Dina Marchenko 2J

delta H is plugged in to replace q and -PΔV is plugged in to replace w. q=ΔH because ΔH represents the heat gain or loss of a system quantitatively. w=-PΔV because: you start with force= P(external) x area -This is the formula for the pressure being exerted by the system also, the volume of the area...

Dina Marchenko 2J

Work done on a system can indicate that pressure is being put on the system and decreasing its volume (compression)/heat is being transferred into the system through an endothermic reaction/ a variation of both. This will indicate that delta H is positive. When a system does work on the surroundings...

Dina Marchenko 2J

I followed your math and got the same answer. The only thing that I can guess is that maybe the book did prematurely round and keep their calculations to 4 sig figs or something. I think that your answers are close enough that you wouldn't be penalized if this were on a test.

Dina Marchenko 2J

A positive delta H means energy is put in (endothermic) which is what is necessary to break the bonds that holds a solid together into a liquid. That's why change in enthalpy is positive.

Dina Marchenko 2J

The whole point of a reversible process is that the tiniest change results in work being done. I think overall reversible is a confusing term. I just think of irreversible as doesn't change easily. If a system is reversible it means that all surrounding and system variables are basically the same (a...

Dina Marchenko 2J

In chapter 4A of the book we learn about reversible processes. I think we're going to discuss it in class later on but what I gathered from lecture today and from the reading is that when the system is at equilibrium and stable, it is reversible. A reversible process is one where the tiniest change ...

Dina Marchenko 2J

State properties are properties where the value is determined by the current state, not the "path" taken to reach the state. For internal energy, to find the change you just subtract initial(U) from final(U) without thinking about the path taken in between (for example, if U jumped up and ...

Dina Marchenko 2J

Especially if you want to know how a system changes over time (ex- thru a reaction) without being influences by any surroundings or outside factors, isolated systems are incredibly useful. Measuring equipment can be put inside of the system to properly gauge a change in some characteristic of the sy...

Dina Marchenko 2J

Bond Enthalpy is the enthalpy (essentially the energy) needed to break a bond. All bonds require an input of energy in order to be broken (see bond potential energy graph) and a positive enthalpy signifies that energy must be put in. This is why all bond enthalpies are positive. The bond enthalpy of...

Dina Marchenko 2J

A high pKa means a low Ka which means that a small amount of the acid molecules are deprotonated, making it a weak acid.
pKa=-log(Ka)

Dina Marchenko 2J

What does it mean that when you add heat to a system, its overall temperature remains constant? Isn't ice colder than boiling water (ie they have different temperatures)? Or does water as a solid have a uniform temperature, then liquid water has its own uniform temperature, then water vapor has its ...

Dina Marchenko 2J

ICE is also generally used for weak acids and bases. With strong acids and bases, you can assume full protonation/deprotonation and assume that the concentration of the original strong acid or base becomes 0 and the concentration of the products is equal to the intitial acid/base concentration.

Dina Marchenko 2J

When you mix any acid and base, the result is water and a salt. Generally, I don't think we're going to be asked to find equilibrium constants or the pH of such reactions (at least not yet) but treat it as you would any other equation. Given the information provided, think about how much of the orig...

Dina Marchenko 2J

6B3 asks you to find the pH of 0.025 M HCl first in a flask of volume 200.0 mL (part a) and then one of 250.0 mL (part b). I found the new molarity within each new volume and then found the pH of that but in the answers for part a, the book just found the pH of 0.025 M. I don't really get why; don't...

Dina Marchenko 2J

When finding Ka or Kb, do you follow the same rules as we learned from last unit with Kp or Kc? Specifically, if one of the products of an acid reaction has a 2 coefficient, do you square the product concentration when finding Ka?

Dina Marchenko 2J

When it comes to gases, partial pressure (Kp) is more accurate for finding the equilibrium constant than Kc. For aqueous solutions obviously you can't use partial pressures so here you can use Kc. If concentrations are provided for gases rather than partial pressures, it's fine to use Kc and deal wi...

Dina Marchenko 2J

2HCl(g)->H2(g) + Cl2(g) 0.22 bar 0 0 initial -2x +x +x change 0.22-2x +x +x final Partial P(H2) x Partial P (Cl2) K=------------------------------------ Partial P (HCl)^2 3.2x10^-34= x^2/(0.22-2x)^2 (square root both sides) 1.8x10^-17=x/0.22-2x (multiply both sides by denominator, distribute 1.8x10^...

Dina Marchenko 2J

When it comes to water, I believe the two terms can be used interchangeably. Kw is referred to as either the autoprotolysis constant of water or the autoionization constant of water. They basically mean the same thing.

Dina Marchenko 2J

When finding Kc, why do we ignore liquids and solids? What values do we assume the concentrations of liquids or solids to be once the reaction goes through? (ie- when we see a liquid in an equation, do we assume its molarity will be almost equal to 1 and that's why we ignore it? and why assume that ...

Dina Marchenko 2J

The value of K itself will not be changed by anything except temperature. Other parameters such as pressure or concentration will cause the position of equilibrium to change but not the value of the equilibrium constant for that chemical equation for that temperature.

Dina Marchenko 2J

Can someone explain the process of combining two equations and determining how to combine their K's? How do you know which reactants will cancel each other out when you combine two equations? Also how do stoichiometric coefficients tie into this?

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