AS Chemistry- helping each other out!

thats the balanced equation to show you how MgO is formed, the 2 at the front just shows that theres 2 MgO's not the empirical formula :smile:

In other words: the number 2 is too much. So every single number at the front has nothing to do with formulas, its merely needed for balanced equation? Got it.

Reply 1101

username1426305

2

Original post by Kallisto

In other words: the number 2 is too much. So every single number at the front has nothing to do with formulas, its merely needed for balanced equation? Got it.

yeah it just tells you that there are two moles of magnesium oxide if you're asked to work out the Mr make sure you don't multiply it by 2 if its 2MgO

i.e if you have a substance such as MgCl2 then you multiply the Cl Mr by 2

hope this helps

Reply 1102

username1646063

17

Original post by chzm

erm I do AQA so you may not have learnt this but
CHEM: What is the definition of electronegativity?
Describe the type of bonding in metals

BIO: Describe how substances can move across the cell surface membrane in different ways

How can a change in the primary structure of an enzyme cause it to become non-functioning.

Okay I'll give this a try but keep in mind I haven't revised or learnt some off these.

1) electronegativity is
2) There are different types of binding in metals but the two most common ones are ionic and metallic. Ionic bonding is where electrons from other metals can be transferred in order to become an alloy or to change charges if the ion which mostly involves redox reaction. Metallic bonds are between atoms which allows the metals to have high melting and boiling point. Metallic bonds are only between metals.

3) substances can move across cell surface membranes through diffusion. The fluid mosaic model exposes that only small molecules can travel through the cell membrane due to its small surface area.

4) A change in the primary structure of an enzyme can cause it to become non functioning as the primary structure determines it's main role. If the structure it's changed its role would change and therefore the original subtrate won't fit in the enzyme.

Okay I gave my best but I believe I made no sense and most is prob wrong :frown:

Reply 1103

Kallisto

Entertainment Forum Helper

22

Original post by ThatMadClown

yeah it just tells you that there are two moles of magnesium oxide if you're asked to work out the Mr make sure you don't multiply it by 2 if its 2MgO

i.e if you have a substance such as MgCl2 then you multiply the Cl Mr by 2

hope this helps

I think you do. I am not stupid. In your example Cl2 stands for two Cl-Atoms in MgCl2, that is to say this number has nothing to do with a balanced equation, and that is why this number has to be considered in calculation.

Reply 1104

Dylann

21

Original post by Kallisto

I think you do. I am not stupid. In your example Cl2 stands for two Cl-Atoms in MgCl2, that is to say this number has nothing to do with a balanced equation, and that is why this number has to be considered in calculation.

The coefficient of any substance in a balanced equation is simply the molar ratio. Do not use it in Mr calculations.

The only exception is hydrated compounds, such as:

ZnSO4.7H2O

You have to include the 7 waters in the Mr calculation for hydrated zinc sulfate :smile:

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Reply 1105

lukejoshjames

7

Original post by Kallisto

I think you do. I am not stupid. In your example Cl2 stands for two Cl-Atoms in MgCl2, that is to say this number has nothing to do with a balanced equation, and that is why this number has to be considered in calculation.

Mr means molecular mass, which is defined as the mass per mole of a molecule. If you numbers within the empirical formula itself, for example in MgCl2, then you would add the Mr of Magnesium to twice the Mr of Chlorine, and that would give the you the Mr of MgCl2.
However, if there is a number before the empirical formula itself, for example 2MgCl2, that is the same as 2(MgCl2), which is 2 moles of MgCl2. As Mr is mass per mole, finding the mass of 2 moles of the substance would be incorrect unless you divided that by whatever number of moles there are of the substance.

Reply 1106

emilita.nxn

1

I need to do a "preparation" practical. Can someone give me an idea on what I have to do?
I'm doing edexcel chemistry 😊

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Reply 1107

KingThomas

2

Have you completed the evaluative controlled assessment on preparing a ketone. Any help or tips.

Thanks very much

Reply 1108

Neurotoxicity

1

Original post by chzm

erm I do AQA so you may not have learnt this but
CHEM: What is the definition of electronegativity?
Describe the type of bonding in metals

BIO: Describe how substances can move across the cell surface membrane in different ways

How can a change in the primary structure of an enzyme cause it to become non-functioning.

If primary structure changes => order of amino acids changes => secondary structure changes => 3D/tertiary structure changes => different shaped actives site => substrate cannot fit enzyme => enzyme-substrate complex cannot form => (so enzyme non functioning)

Reply 1109

ohnanailikenanas

2

When an excess of iron filings are added to 25cm(3) of 0.2 mol dm(-3) copper sulphate solution the temperate rises by 7 degrees Celsius. Write an equation for the reaction and calculate the enthalpy change in kJ mol(-1) of FeSO4 formed.

Please help!

Thank you.

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Reply 1110

kyraaalm

7

Original post by ohnanailikenanas

When an excess of iron filings are added to 25cm(3) of 0.2 mol dm(-3) copper sulphate solution the temperate rises by 7 degrees Celsius. Write an equation for the reaction and calculate the enthalpy change in kJ mol(-1) of FeSO4 formed.

Please help!

Thank you.

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You need to use q=mct where m is the mass, c is specific heat capacity (usually 4.18) and t is the change in temperature.
then divide this by the number of moles to get the enthalpy change per mole. Remember the sign, the temperature rose so it is exothermic so it should be a negative sign.

Reply 1111

ohnanailikenanas

2

Okay, so q = m x c x t

q = ?
m = 0.025
c = 4.18
t = 7

q = 0.025 x 4.18 x 7
q = 0.7315 kJ

n = mass / Mr
n = 25 / 151.9
n = 0.1645819618
n = 0.165

Delta H = q/n

-0.7315 / 0.165

-4.43 kJ mol(-1)

Is this right?

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Reply 1112

frozo123

12

Original post by ohnanailikenanas

When an excess of iron filings are added to 25cm(3) of 0.2 mol dm(-3) copper sulphate solution the temperate rises by 7 degrees Celsius. Write an equation for the reaction and calculate the enthalpy change in kJ mol(-1) of FeSO4 formed.

Please help!

Thank you.

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right putting it into simple terms..
In the reactivity series, iron is more reactive than copper, therefore the iron will displace the copper (2) ions.

The iron is acting as a reducing agent so it will have a loss of electrons.
Fe- ----> Fe2+ + 2e-
the copper is being reduced so it gains electrons:
Cu2+ + 2e- -----> Cu
add them upand you get:
Fe + Cu2+ -----> Fe2+ +Cu
so the stoichiometric equation of the reaction is:

Fe (s) + CuSO4(aq) ----> FeSO4(aq) + Cu(s)

now q=m x c x delta T
q= 25 x 4.18 x (final temp- inital temp)
q= 25 x4.18 x 7
q= 721.5 J
( assuming specific heat capacity of the solution is the same as raising the temperature of 1cm^3 of water by 1'c )
now the number of moles of copper sulphate solution is:
moles = concentration x volume (dm^3)
moles= 5x10^-3
now to work out the enthalpy change it will be the negative energy in joules/ divided by the number of moles x 1/1000
so Enthalpy Change is: -721.5/5x10^-3 x 1/1000
so delta H= -144 kJmol^-1 (3sf)
Hope that helps ( A2 student A at AS )

(edited 9 years ago)

Reply 1113

ohnanailikenanas

2

But isn't the mass of water supposed to be in kg? 25cm(3) = 25g = 0.025kg? But thanks for your help!

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Reply 1114

ohnanailikenanas

2

Thanks (:

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Reply 1115

frozo123

12

Original post by ohnanailikenanas

But isn't the mass of water supposed to be in kg? 25cm(3) = 25g = 0.025kg? But thanks for your help!

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what do you mean? In the working out moles equation? if it gives you the concentration and volume, you use that equation
if it gives you the grams of something you use moles=mass/mr equation and the mass is in grams