Thursday, 22 March 2018

Global Warming

This is a YouTube video showing the Scientific Eye programme on Global Warming, some useful ideas and explanations!


Limestone

Limestone is a sedimentary rock found in the earth.  It is formed from the shells of dead sea creatures which become crushed and buried at the bottom of the sea, when these creatures die.


Limestone is made up of the chemical calcium carbonate, CaCO3.  It is a metal carbonate. The following YouTube video is from the Scientific Eye series and explains the importance of limestone and the issues related to it.  You might be expected to discuss the social, economic and environmental of extracting and using limestone.



Limestone can undergo reactions as part of the 'Limestone cycle'.  The first step is a thermal decomposition, this is where a substance is broken down into smaller compounds using heat. The ease with which they decompose varies within a group. The higher up the reactivity series a metal is, the more stable its carbonate will be, so the least reactive metals have the carbonates that decompose most readily. Looking at group 2 magnesium carbonate decomposes relatively easily, calcium carbonate requires significant heating to decompose. As you go further down the group the metal carbonates become more stable and less easily decomposed by heat.





Calculating Reacting Masses 2

The earlier 'Calculating Reacting Masses' blog, talked about how you could use factors between the molecular masses of substances and their actual masses in order to find the mass of an unknown in a chemical reaction.  A more useful way of doing this is to look at the moles of substances in reactions, the following slideshow explains this, pause and repeat to get a better understanding. 

Look at the original 'Reacting Masses' blog post. Does the moles method give the same result as the worked example?



Wednesday, 8 November 2017

Rates of reaction revision

Some videos to help with revision of rates of reaction:

 
Where the following video talks about a fruitful collision it means a successful collision (one that has the energy to over come the activation energy).
 
 

Thursday, 4 May 2017

New WJEC Science Revision Guides

Bangor University have produced new Unit 1 revision guides for Biology, Chemistry and Physics to complement the new WJEC Double Award Science (from 2016). Click here to access and download the guides.

Tuesday, 17 May 2016

Making Ammonia - The Haber Process

In this reaction nitrogen-N2 (from the air) and hydrogen-H2 (from methane) are reacted together in the ratio of 1:3 to give ammonia-NH3.




The double arrow means that the reaction is reversible.  This has an impact on the yield of product (amount of product) obtained.  In general where a reaction is reversible you will not obtain a high yield without controlling the conditions of the reaction.


In industrial processes yield and atom economy are important considerations.


% yield
=
Mass of product obtained
x 100


Mass of product expected

The greater the yield the more product a company has to sell and therefore make money.

Atom economy
=
Molecular mass of useful products
x 100


Molecular mass of all product

Ideally the greater the atom economy the better, as there are no waste atoms (molecules) which have to be dealt with.  If there are waste molecules it could be the case that they are toxic for example hydrogen chloride or contribute to the green house effect for example carbon dioxide  and will need to be treated carefully to remove them.  This will be at the expense of the company.

Yield V's Atom Economy




Low Yield
High Yield
Low Atom Economy
Undesirable - too much waste not enough product.
Not ideal as the by-products still have to be dealt with.  Producing by-product with some other industrial use would help in this situation.
High Atom Economy
Not ideal as companies need to produce useful product to make profit.  If the reaction is reversible any unreacted product could be fed back into the start of the process to help increase yield.
Most desirable – maximum product with little waste.

With only 1 product formed in the Haber process it has a 100% atom economy.

Conditions for the Haber Process

This reaction is exothermic (gives off energy (heat)) in the forward direction.

The following graph shows the % yield for ammonia production under different conditions of pressure and temperature:

High pressures (reducing the volume the gases move in) force the reaction in the forward direction (making product).  This is because there are fewer gaseous molecules in the products so they will take up less volume.  So high pressures are favoured, the graph shows that 400 atmospheres (pressure 400 times greater than normal atmospheric pressure) produces the greater yield. However it can be expensive to build machinery to produce and maintain high pressures.

The reaction is exothermic in the forward direction and endothermic in the reverse direction.  Increasing the temperature of the reaction will increase the rate of reaction, but as you are increasing energy it will favour the reverse (endothermic) reaction, so the yield will be lower.  This is a difficult condition, to get a high yield temperature must be low however the reaction will be too slow.

A compromise is used:

Temperature: 450ÂșC
Pressure: 200 atmospheres
With an iron catalyst to speed up the reaction.

The process occurs in the following system:

To try and increase the yield any unreacted hydrogen and nitrogen are recycled to the beginning of the process.