Chemistry HL/SL help

[BustinJieber]

Hello! If you could help me with this question it would be appreciated. Our teacher threw it on us without any lecture, and my knowledge is choppy at best.

If 25.0 grams of ice at 0 degrees C are heated at a constant rate of 400.0 J/min, calculate the time needed to melt the sample completely.

Show steps and formulas please!

[ILM]

Lets start by asking ourself what is the enthalpy change of the process that you are talking about:

To change the ice to water you need 6.01 kJ per mole

on another language you need 6010 j per mole

second question: how many mole do i want to melt:

n= m/M = 25.0/ 18.02 =1.4 moles

Third question : how much energy do i need to melt this :

energy needed= no. of moles * energy needed to melt one mole = 1.4 * 6010j/mole = 8338 j

Fourth question: How many minutes do i need to melt this:

The constant rate = 400j/m

you need 8338 j

so 8338/400=20.8 minutes

Hope that you understood

Best of luck

[Drake Glau]

Le Chatlier's principle and understand what a Kc actually is. Equilibrium=equal rates...not equal concentrations.

Energetics...I'm going to assume you're HL. Know the free energy equation. Know how to tell if something is spontaneous. Know Hess's law. Know the weird mechanism thingy I don't remember the name of, it's the thing that talks about the lattice enthalpy and enthalpy of atomization and ionization and all that good stuff. Know your graphs. This is more related to kinetics in my opinion but the equation for activation energy would be good to understand and it's semi related. And by graphs I mean the little detlaH graphs, know what parts of the graph are what and how to tell if the reaction is exo/endothermic based off the graph.

There's probably more but those are the big things I remember.

[Guest KAPOWW!!]

Le Chatlier's principle and understand what a Kc actually is. Equilibrium=equal rates...not equal concentrations.

Energetics...I'm going to assume you're HL. Know the free energy equation. Know how to tell if something is spontaneous. Know Hess's law. Know the weird mechanism thingy I don't remember the name of, it's the thing that talks about the lattice enthalpy and enthalpy of atomization and ionization and all that good stuff. Know your graphs. This is more related to kinetics in my opinion but the equation for activation energy would be good to understand and it's semi related. And by graphs I mean the little detlaH graphs, know what parts of the graph are what and how to tell if the reaction is exo/endothermic based off the graph.

There's probably more but those are the big things I remember.

Thanks haha! and that's accurately said "equal rate..concentrations"

I got a new query

Doubt- Boiling point is when vapour pressure exceeds atmospheric pressure, so I'm assuming V.P. is pushing upwards and atmospheric pressure downwards, preventing its escape so far. Right eh? Pretty sure it's inaccurate since V.P. is pushing all around even at the liquid surface, so more like an 'omnidirectional force' (made that up lol) But in this context upward.

Doubt 2-The reason boiling is characterised by bubbles. When vapour pressure begins to hold of atmospheric pressure it allows for more Vapour molecules to join in with ease? or what? Enlighten me!

Edited September 7, 2011 by KAPOWW!!

[Guest KAPOWW!!]

Wowie, I spout a lot of doubts! Here's some queries on Oxidation, Reduction I have, help please?!!

Doubt 1:Drawing OH- and the electron distribution in the ion

Doubt 2: Drawing XeF4 and the symmetry. As in why not square planar why the tetrahedral shape it makes no sense. There is and extended octet I presume? ARRRGH!!!

Since Lewis structures come for around 3 marks or so and I don't get them sometimes, Most of my query concerns:

A) Is it a dative(isn't someone supposed to tell me this?), are all the electrons coming from this atom? [OH-] Is a doubt for this reason!

B) How do I form the bonds to satisfy the octet rule

C) How on earth do I know when its an extended octet??

[canadian.ib.girl]

For class, we have to calculate the seperate ΔH values for 3 equations. According to Hess' Law, ΔH1 + ΔH2 = ΔH3.

Reaction 1: NaOH(s) --> Na+(aq) + OH-(aq)

Mass of NaOH: 2g ; Volume of water= 100g

ΔT= 7 celcius

Reaction2: Na+(aq) + OH-(aq) + H+(aq) + Cl-(aq) --> Na+(aq) + Cl-(aq) + H2O(l)

Volume of 1.00M of NaOH: 50mL ; Volume of 1.00 M of HCl: 50mL

ΔT= 7.5 celcius

Reaction 3: NaOH (s) + H+(aq) + Cl-(aq) --> Na+(aq) + Cl-(aq) + H2O (l)

Mass of NaOH: 2g ; VOlume of 0.5M of HCl: 100mL

ΔT= 8 celcius

I did the mcΔT calculations using the combined mass (e.g. NaOH's mass + volume of water) however, ΔH1 + ΔH2 did not equal ΔH3. Are you supposed to only use the mass of NaOH? Because I tried that as well, and the values still didn't add up properly? What did I do wrong? What can I do to fix it?

Thanks in advance

[dessskris]

use the mass of the solution

ΔH=(mcΔT)/n

you missed the /n bit.

use the number of mol of the limiting reagent.

[Guest KAPOWW!!]

Hey guys, I hope you don't think this is selfish but if you knew me, you'd know that the KAPOWW has doubts by the second and tomorrow's my exam so I'm crucified anyway.

Doubt1: I just don't remember how to solve this!!

Use the information in the following table to calculate the enthalpy change for the

complete combustion of but-1-ene according to the following equation.

C₄H₈(g)+6O₂(g)→4CO₂ (g) + 4H₂O(g)

Compound: C₄H₈(g), CO₂(g),H₂O(g)

ΔHf/ kJ mol⁻¹ (+ 1), (– 394), (– 242)

Thanks!

Edited September 12, 2011 by KAPOWW!!

[Rigel]

Hey guys, I hope you don't think this is selfish but if you knew me, you'd know that the KAPOWW has doubts by the second and tomorrow's my exam so I'm crucified anyway.

Doubt1: I just don't remember how to solve this!!

Use the information in the following table to calculate the enthalpy change for the

complete combustion of but-1-ene according to the following equation.

C₄H₈(g)+6O₂(g)→4CO₂ (g) + 4H₂O(g)

Compound: C₄H₈(g), CO₂(g),H₂O(g)

ΔHf/ kJ mol⁻¹ (+ 1), (– 394), (– 242)

Thanks!

You're cool, dw.

First of all... you have to work with bond enthalpies. I was going to set-up the combustion reactions, lol.

You have butene here, so there are 2 C-C bonds and one C-C double bond and 8 C-H bond. In oxygen, you have a double O-O bond, and 6 molecules of oxygen, so multiply it. Then add the whole result. That would be 7598kJmol^-1.

Do the same for the products, that would be 2 C-O double bonds, then multiply it by 8. Do the same for O-H, except that it is a single bond. That would be 9680kJmol^-1.

Then: 7598-9680, the answer would be 2082kJmol^-1.

[Guest KAPOWW!!]

Hey guys, I hope you don't think this is selfish but if you knew me, you'd know that the KAPOWW has doubts by the second and tomorrow's my exam so I'm crucified anyway.

Doubt1: I just don't remember how to solve this!!

Use the information in the following table to calculate the enthalpy change for the

complete combustion of but-1-ene according to the following equation.

C₄H₈(g)+6O₂(g)→4CO₂ (g) + 4H₂O(g)

Compound: C₄H₈(g), CO₂(g),H₂O(g)

ΔHf/ kJ mol⁻¹ (+ 1), (– 394), (– 242)

Thanks!

You're cool, dw.

First of all... you have to work with bond enthalpies. I was going to set-up the combustion reactions, lol.

You have butene here, so there are 2 C-C bonds and one C-C double bond and 8 C-H bond. In oxygen, you have a double O-O bond, and 6 molecules of oxygen, so multiply it. Then add the whole result. That would be 7598kJmol^-1.

Do the same for the products, that would be 2 C-O double bonds, then multiply it by 8. Do the same for O-H, except that it is a single bond. That would be 9680kJmol^-1.

Then: 7598-9680, the answer would be 2082kJmol^-1.

Thanks!! Hey the question provided some weird 'enthalpy change of formation' values but you went about in by calculating bond enthalpies. How is that? I mean what did you use for the bond enthalpy values if none are given....you must be....Chem HL!!!

Edited September 12, 2011 by KAPOWW!!

[Guest KAPOWW!!]

Doubt2: so in the attachment I have an equation, just too much to type so I did a screen shot.

Anyway the question is such: 'State and explain the colour change when hydrochloric acid is added to the reaction

solution at equilibrium.'

Now I'm wondering, what I refer to? How do I know which halogen is released? is there a table in one of the chapters? Thanks guys!

Oops! for some weird reason only VIPs can access the image, just PM when you want it I'm always online!

Edited September 12, 2011 by KAPOWW!!

[Rigel]

Table 10 mentions average bond enthalpies. Use them.

[Rigel]

Doubt2: so in the attachment I have an equation, just too much to type so I did a screen shot.

Anyway the question is such: 'State and explain the colour change when hydrochloric acid is added to the reaction

solution at equilibrium.'

Now I'm wondering, what I refer to? How do I know which halogen is released? is there a table in one of the chapters? Thanks guys!

Oops! for some weird reason only VIPs can access the image, just PM when you want it I'm always online!

It seems that the Br is being oxidized, and it would be the one that is being released.

After that i can't help you much because i haven't done that yet.

[Rigel]

Hey guys, I hope you don't think this is selfish but if you knew me, you'd know that the KAPOWW has doubts by the second and tomorrow's my exam so I'm crucified anyway.

Doubt1: I just don't remember how to solve this!!

Use the information in the following table to calculate the enthalpy change for the

complete combustion of but-1-ene according to the following equation.

C₄H₈(g)+6O₂(g)→4CO₂ (g) + 4H₂O(g)

Compound: C₄H₈(g), CO₂(g),H₂O(g)

ΔHf/ kJ mol⁻¹ (+ 1), (– 394), (– 242)

Thanks!

You're cool, dw.

First of all... you have to work with bond enthalpies. I was going to set-up the combustion reactions, lol.

You have butene here, so there are 2 C-C bonds and one C-C double bond and 8 C-H bond. In oxygen, you have a double O-O bond, and 6 molecules of oxygen, so multiply it. Then add the whole result. That would be 7598kJmol^-1.

Do the same for the products, that would be 2 C-O double bonds, then multiply it by 8. Do the same for O-H, except that it is a single bond. That would be 9680kJmol^-1.

Then: 7598-9680, the answer would be 2082kJmol^-1.

Thanks!! Hey the question provided some weird 'enthalpy change of formation' values but you went about in by calculating bond enthalpies. How is that? I mean what did you use for the bond enthalpy values if none are given....you must be....Chem HL!!!

Lol no, i'm actually Chem SL. I'm gonna go to bed, ttyl.

[dessskris]

it's the brown bromine solution. that is the colour change.

[Guest KAPOWW!!]

Guys thanks you for all your help(*cautious* Yes I will rep you too, or ChikkyD will kill me ) But in the end what happened is that I was butchered, torn from limb to limb, impaled on equations.... It didn't work out..

I hope I pass! *fingers crossed*

[Gaby]

Hello guys!

So, my fellow IBers and I have a bit of a problem with our homework. I hope you'll be able to help, it's only SL.

1. Write a balanced equation for the dissolving of potassium chloride in water.

a. When 4.00 grams of potassium chloride dissolve, how many moles of the solid dissolve?

b. How many moles of potassium ions dissolve in the above question?

c. Determine the total number of moles of ions that are formed in the above question.

2. Determine the solubility of the following solutions:

a. 24.2 grams of KNO3 dissolve in 10.4 grams of water at 100 C degrees.

b. 28.02 grams of KNO3 dissolve in 200.1 ml of water at 0 C degrees.

In the second one with have a solubility graph for KNO3. What we basically need is to tell us what to start these exercises with, we don't really know what we should do with either of them and didn't solve any such problem during the lesson.

[dessskris]

1. KCl + H2O --> ?

a) n = mass / Mr

b) n(K+) = n(KCl)

c) count the number of ions, considering the molar ratio. if you're clueless, get back to us with answers to the previous parts first and we'll help you proceed.

2. since you have the graph, just deduce from the graph what the solubility is for each temperature. do you need to work out how many grams is dissolved or something?

[Guest KAPOWW!!]

Hello guys!

So, my fellow IBers and I have a bit of a problem with our homework. I hope you'll be able to help, it's only SL.

1. Write a balanced equation for the dissolving of potassium chloride in water.

a. When 4.00 grams of potassium chloride dissolve, how many moles of the solid dissolve?

b. How many moles of potassium ions dissolve in the above question?

c. Determine the total number of moles of ions that are formed in the above question.

2. Determine the solubility of the following solutions:

a. 24.2 grams of KNO3 dissolve in 10.4 grams of water at 100 C degrees.

b. 28.02 grams of KNO3 dissolve in 200.1 ml of water at 0 C degrees.

In the second one with have a solubility graph for KNO3. What we basically need is to tell us what to start these exercises with, we don't really know what we should do with either of them and didn't solve any such problem during the lesson.

I think you should attach a screen shot if this is virtual, cause its a world of effort and why make it, if its simplified eh?

[Keel]

6. How many oxygen atoms are contained in 3.6 moles of (NH₄)₃PO₄?

12. The sugar substitue sodium benzosulphimide (sodium saccharin) has a sweetness of about 500 times that of sucrose. You are told that a sachet of commercial saccharin contains 5.0 g. What can be deduced about the number of saccharin molecules present in this sachet? Explain.

Could anyone help me on these questions?

6. 4 x 3.6 x 6.02 x 10^23 = 8.67 x 10^24

12. This is a very poorly worded question and you're not the first one to get stuck on it. My guess would be that you are expected to make a connection between the total mass of saccharin (5.0 g), the relative molecular mass (RMM) and Avogadro's constant to reach the number of molecules present. What was the purpose of the first sentence? I have no clue.