Enthalpy Calculations

This is where The Mole makes a guest appearance... and then stays!

As we mentioned before, ΔH is the energy change in the reaction in KJ/mol . 

Fun example (not) :

Measurement of enthalpies of combustion.

This is basically a method identical to that alcohol's investigation!

• Known mass of water placed in beaker (150g) 
• Temperature noted (23 ºC) 
• Final temperature noted (43ºC) 
• Mass of ethanol needed to obtain heat increase (0.9g)

Therefore 

Heat gained by water in calorimeter = m x c x DT 

= 150g x 4.2 J g^-1 K^-1 x 20ºC 

= 12,600 J Heat produced by burning 0.9g of ethanol = 12.6kJ

A simple calorimeter.

Since 0.9g is 0.9/46 mol ethanol:

Heat produced by burning 1 mol of ethanol = 12.6 kJ / (109/46) kJ mol^-1= 644kJ
Therefore DH_c [CH₃CH₂OH] = -644kJ mol^-1

^...

^{In simpler terms, it is how much energy is produced or absorbed in a chemical reaction per 1 mole of the substance.}  

^Example:

^{If you had 3 moles of Carbon, and the energy produced per one mole is -478 KJ. Then how much energy is produced in total?}

^{-478 KJ  X   3 moles =  -1434 KJ}

^{1 mole} 

^{Notice how the energy is negative; because it was produced, not absorbed!}

^{Example 2:}

^{If you had 349 g of oxygen, then how much energy is absorbed per one mole of oxygen? (energy from the reaction is +234 KJ/mol)}

^{(gotta do some mole calculations here)}

^{grams ---> moles ---> energy}

^{349 g  X   1 mole    X   234  KJ   =   5104 KJ} 

              ^{16.0  g         1 mole}

Endothermic & Exothermic Reactions

We learnt about endothermic and exothermic reactions.. again! (YAAY)

So what is an Endothermic reaction?

It is simply a reaction that needs heat energy to give off products.. This heat energy is called Enthalpy. 

In an endothermic reaction, the energy is absorbed; therefore it is positive.

Few reactions are endothermic. Examples are:

1) Photosynthesis:
Chlorophyll is a very clever catalyst,
which allows plants to make sugar from carbon dioxide in the air.
The energy needed for the reaction comes from sunlight.

 2) Melting, boiling and evaporation
are all endothermic processes (not reactions).

 

This is how the graph should look like in an endothermic reaction. The activated complex is how high the energy should be to produce the products. 

To determine if it is an endothermic reaction, ΔH (energy of the products - the energy of the reactants) should be positive. 

What is an Exothermic reaction then?

That's right! It is the exact opposite of that.

An exothermic reaction is a reaction that gives off energy in the form of heat. Just like endothermic reactions, this energy released is called Enthalpy. 

 

Most reactions are exothermic. Examples are:

 

1) Adding water to anhydrous copper(II) sulfate is exothermic.
Anhydrous copper(II) sulfate is white.
Anhydrous means "without water"

Anhydrous copper(II) sulfate is copper(II) sulfate which is completely dry.
When water is added to it, it turns into the familiar hydrated blue crystals.
Hydrated is pronounced "high-dray-tid", it means 'with water'.
This is used as a test for water.

If blue (hydrated) copper(II) sulfate crystals are heated,
an endothermic reaction occurs, they lose their water,
turn white and become anhydrous copper(II) sulfate crystals again.
The reaction is reversible.
Blue copper(II) sulfate crystals will also turn white
in the presence of  concentrated sulfuric acid.

 2) Neutralisation.

3) Combustion of methane (natural gas).

To determine if it is an exothermic reaction, ΔH (energy of the products - the energy of the reactants) should be negative.

Exothermic reaction

 

Endothermic Reaction

 

Net Ionic equations

Hello,

Net ionic equations can be viewed as like a simplified form of  a balanced DOUBLE REPLACEMENT equation.

This is needed when the one of the products in the equation changes states to solid.

For example

Take this equation,

NH4Cl(aq) + AgNO3(aq) → AgCl(s) + NH4NO3(aq)

The product AgCl is a solid, in this case, you need a "net ionic equation".

First,

Separate all elements that are not solid

NH4(aq) + Cl(aq) + Ag(aq) + NO3(aq) → AgCl(s) + NH4(aq) + NO3(aq)

Then,

Cancel out elements that are repeated in both products and reactants.

The equation will become like this:

Ag(aq) + Cl(aq) → AgCl(s)

This is net ionic equation.

Lab 5B

There were a lot of chemical reactions/experiments that we were supposed to do. The objective  of this lab was to observe a number of chemical reactions. and learn how to differentiate between them. In these reactions there were synthesis, decomposition, single replacement, and double replacement reactions. We did a total of about 7 reactions in this lab.

Here are all of them:

1)  We burned a pice of copper (with O2).
     2Cu + O2 -> 2CuO
     Synthesis reaction

2)  We inserted a nail in a tube with copper solution, and the nail turned red-brownish.
     Fe + CuSo4 -> FeSo4 + Cu
     Single Replacement reaction.

3) We heated a blue crystal like substance CuSO4 ˙ 5H2O, the substance then turned into powder.
CuSO4 ˙ 5H2O -> CuSo4 + H2O
Decomposition.

4) We added a few drops of water to that substance, which made it go back to its original state.
CuSo4 + H2O -> CuSO4 ˙ 5H2O
Synthesis reaction.

5) We added two liquids together, and they had no color. after a while, they formed a percipitate.
CaCl2 + Na2Co3 -> 2NaCl + CaCO3
Double Replacement reaction.

6) We added zinc to a clear solution (HCl), and bubbles were formed.
Zn + HCl -> ZnCl + H2
Single Replacement reaction

7) We added a liquid substance, no color, to a powder like material. When burned, bubbles went up to the surface and gas evaporated.
H2O2 -> O2 +H2
Decomposition. 

Different Types of Chemical Equation Part II

Hi, We are back.

Today we are going to talk about the other types of chemical equation:

• Double replacement
• Combustion
• Neutralization

Double replacement:

Before we talk about chemistry stuff, imagine a school prom. (even though there is practically no dances in Magee...)

Let's say that Bill and Becca are partners and Jack and Jill are partners.

After few rounds of dancing, they decide to switch partners

So Bill dances with Jill and Jack dances with Becca

This situation basically outlines the double replacement reaction in chemistry.

The general formula is:

AB + CD → AC + BD

Note:

There are situations in double replacement where there is a reaction or not.

The key is very simple:

If there is a change in state, (e.g. aqueous to solid) there is a reaction. If there is no change, there is no reaction.

Use the table of solubility determine the states

Next:

Combustion:

This is a reaction where burning with oxygen is involved. In my opinion, I think that this is the easiest.

Basically, when the reactants are organic (carbon, oxygen and hydrogen), the outcome will always be

carbon dioxide and water.

So the formula is

AB + O2 → AO + BO

Lastly,

Neutralization:

This is a special type of double replacement. This deals with only acids and bases.

This is also very simple,

the product will be water and the aqueous solution from metal and non-metal from the acid and base.

So the formula becomes:

HA + BOH → H2O + BA

H and OH are from acids and bases

This is it for today.

Stay tuned for more :D

Different Types of Chemical Equation Part I

Hello,

Today, I am going to talk about different types of chemical equations.

There are 6 in total:

• Synthesis
• Decomposition
• Single Replacement
• Double Replacement
• Neutralization
• Combustion

 Today, We are going to talk about the first 3.

First,

Synthesis equation means that you are adding two or more elements to make one product

For example:

If you add red and yellow, the product will be an orange colour.

In chemistry, take Sodium and Chlorine for example

Sodium plus Chlorine will produce sodium chloride

Na + Cl2 → NaCl

From this we can make a general conclusion:

A + B → C
      or
A+ B → AB

Next,

Decomposition is the exact opposite of synthesis.

For example:

Purple can be broken to red and blue

In relation to chemistry,

MgCl2 → Mg + Cl2

This means that magnesium chloride will produce Magnesium solid and Chlorine gas.

From this we can form a general formula:

AB → A+B

C   → A+B

Next,

Single replacement is when a compound and another reactants switch "partners".

For example,

Lets say Bob, Jill and Jack when to a prom.

For one song, Bob and Jill are dancing, Jack is dancing alone.

For the next song, Jill and Jack are dancing while Bob is dancing alone.

Back to chemistry:

In the equation,

MgCl2 + Na → NaCl + Mg

In this case, Mg and Na switched spots.

There is another case where there is 2 non-metals instead of 2 metals like the equation before.

It would look like this:

NaCl + Br2 → NaBr + Cl2

It is exactly the same.

There is a general formula:

AB + C → AC + B

That's it for today, see you later.