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.

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: 450ºC

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.

Calculating Reacting Masses

The slide show below shows how you can work out reacting masses in chemical equations.  The important thing to remember is that you need to have a correctly balanced equation (these will usually be given to you in an exam, you should also be given the atomic masses as part of the question, e.g. [Na = 23, O = 16], if not you will have to use the Periodic table at the back of the exam paper).  The clip is only 15 seconds long so pause it at each step and replay as often as you need.

Natural factors that have changed the Earth’s atmosphere

Theories suggest that the Earth formed 4500 million years ago.  Its first atmosphere mainly consisted of hydrogen and helium gases.  These gasses are light and would have been lost to space.  As the Earth cooled, the surface formed a crust, this crust was made up of plates containing many volcanoes.  The gasses expelled by volcanic activity formed the next (early) atmosphere of the Earth.  This atmosphere consistent of:

The Earth’s atmosphere would also have contained water vapour.  Over the next few billion years the following processes occurred:

• The surface of the Earth cooled and volcanic activity became less, the water vapour in the atmosphere condensed forming seas and oceans.  This allowed living organisms to form as the water absorbed harmful UV rays from the sun.
• Carbon dioxide dissolved in the seas, it dissolved in falling rain forming carbonic acid, and this reacted with minerals in the Earth’s crust forming carbonates.  Some of these carbonates would eventually be washed into the seas.
• The first living organisms were bacteria.  Nitrifying bacteria used the ammonia from the atmosphere in order to grow, producing nitrates on the surface and in soil as a result.  Denitrifying bacteria used the ammonia from the atmosphere in order to grow, releasing nitrogen gas into the atmosphere as a result.
• Green plants were able to grow in these nitrated soils.  Using up carbon dioxide and water in the process of photosynthesis, producing oxygen gas.
• This oxygen gas reacted with elements producing oxides.  It reacted with ammonia in the atmosphere producing nitrogen and water, it reacted with methane in the atmosphere producing carbon dioxide and water.
• As oxygen levels increased, it reacted in the upper atmosphere forming ozone, a chemical which absorbs harmful UV rays.  This allowed animals to develop on the Earth’s surface.

The current atmosphere has been relatively steady for the last 200 million years

• Nitrogen, N₂ – 78%
• Oxygen, O₂ – 21%
• Other gases, including carbon dioxide, argon, water vapour and other Noble gases make up 1%.

Whilst the atmosphere has remained constant over recent times, natural occurrences such as volcanic eruptions can alter the balance slightly.  In the last decade there have been notable volcanic eruptions in Iceland and Japan that have effected the environment and populations in the area.

Iceland - http://europe.newsweek.com/iceland-experiencing-its-biggest-continuous-volcanic-eruption-centuries-277733?rm=eu

Iceland - http://news.bbc.co.uk/1/hi/world/europe/8634944.stm

Japan - http://europe.newsweek.com/photos-rescue-mission-japanese-volcano-mount-ontake-stops-274108

Volcanic eruptions can affect the Earth's atmosphere in a number of ways:

• It increases the acidity of rain as carbon dioxide and sulfur dioxide released from the volcano dissolve in rain water,
• It can cool the Earth's atmosphere as the ash and dust particles in the atmosphere reflect some sunlight meaning the heat doesn't reach the Earth's surface.

GCSE Chemistry Unit 1 Calculations

There are a number of types of calculation you need to know, these are fundamental and will be examined in Unit 1 but could still be examined in Unit 2:


1.) Be able to calculate the relative molecular (formula) mass (Mr) of a compound from its
     formula.
    
Every element in the periodic table is represented by a symbol and numbers:

The top number is the mass number (or nucleon number), it gives the total mass of the atom. 
     
So oxygen has a mass of 16 and hydrogen has a mass of 1.

The relative molecular (formula) mass is the sum of masses of all atoms within the molecule or compound. 

So for water, H₂O the relative molecular mass is:
     

     Mr = 2 x H + 1 x O = (2 x 1) + (1 x 16) = 18

2.) Calculate the percentage composition of simple compounds.

3.) Collect experimental data, and use given data, in order to calculate the formula of 

     a binary compound e.g. magnesium oxide. (Higher Tier)

You could be asked to find the empirical formula of a compound given either the mass or percentage of the elements in that formula.  The following clip explains how the calculations are carried out (The first 2 minutes and 40 seconds are the most important):

4.)  Be able to calculate the percentage yield of a reaction.

The clip below talks through how to calculate percentage yield (the first 2 minutes and 50 seconds are the most important):

Hard Water

There are 2 types of water – Soft and Hard.  The type depends on the level of dissolved calcium (Ca²⁺) and magnesium (Mg²⁺) ions are dissolved in the water.

Water is considered to be soft if it contains less than 100mg of Ca²⁺ ions per litre.  Concentrations greater than this are consider to be hard.

How hard water forms

Rain water is naturally acidic as it contains dissolved carbon dioxide forming a weak carbonic acid.  When rainwater passes along certain rocks e.g. limestone (calcium carbonate, CaCO₃) or gypsum (calcium sulfate, CaSO₄) on its way to a reservoir, the acidic water will react with the limestone producing dissolved calcium ions – Ca²⁺:

CaCO3(s)	+	H2O(l)	+	CO2(aq)	→	Ca2+(aq)	+	2HCO3-(aq)
		Water and carbon dioxide combine to form
		carbonic acid				‘hardness’

Magnesium ions, Mg²⁺ can also collect in water in a similar way.

The following clip explains hard water:

Types of hard water – Temporary and permanent

Temporary - This water contains dissolved calcium and magnesium hydrogen carbonates – CaHCO₃ and MgHCO₃.  When boiled:

Ca²⁺(aq)  +  2HCO₃^-  →  CaCO₃(s)  +  H₂O(l)  +  CO₂(g)

The calcium ions become trapped in calcium carbonate which is insoluble, this is seen as scale on a kettle and thus removing the hardness. 

Permanent - This water contains dissolved calcium and magnesium sulfates – CaSO₄ and MgSO₄.  When boiled the calcium and magnesium ions remain in solution so the water is still hard.

The following clip explains how hard water can be turned into soft water:

Human activities that affect the Earth and its environment

In previous blog posts we have described how the Earth’s surface and atmosphere have been changed by naturally occurring events and processes.  These have occurred over billions of years with little change in the last 200 000 years until recently.  Any change that has occurred in the last couple of centuries can be directly related to human influences – especially with the demand for natural resources.  These resources such as minerals, metals and fossil fuels are found under the Earth’s surface and as such extracting them involves altering the Earth’s surface.  In doing this there are also environmental and atmospheric implications.

The demand for minerals, metals and fossil fuels have had a big impact on the Earth and its environment.

Mining

In South Wales coal has been mined both deep underground and at the surface.  Leaving large, ugly and unsightly areas on the landscape.  This is an open cast mine in Ffos Y Fran, Merthyr Tydfil.  The town and the Beacons National Park are visible in the background.

The Bingham Canyon in Utah, USA, is the biggest hole in the ground on Earth.  It has been dug to extract copper ore.

These mines have a big effect on the local environment with dust and noise pollution from the digging and blast processes, noise pollution from the transport vehicles working on site and those that leave the site to deliver product.  These mines also produce tonnes of waste rock each day (250,000 tonnes in the case of Bingham) which have to be dealt with.

Metal extraction

Once the metal ores are mined they are then processed to give the pure metal.  In the example of copper extraction:

Impurities form the leaching process and the waste products could contain trace toxic metals e.g. mercury which could wash into water supplies.  Sulfur in the ore will react to form sulfur dioxide, this will dissolve in rain clouds forming acid rain (sulfuric acid).  This acid rain can fall on trees and plants damaging leaves and in extreme cases killing plants, or into lakes killing aquatic life such as fish.

The images below show Port Talbot Steel works during the day and at night.  Greenhouse gases being emitted, excess fuel being burnt at night producing carbon dioxide.

Each step in the process of extracting a metal like copper or processing iron into steel requires energy – either electricity or heat.  This essentially comes from burning fossil fuels.  Burning fossil fuels releases carbon dioxide into the atmosphere.  This gas is a greenhouse gas and contributes to global warming.  This can cause the Earth to become warmer which in turn effects many ecosystems:

• Polar ice caps melt, causing sea levels to rise, low lying coastal lands could become flooded.
• Areas suffering from drought will increase leading to famine.
• Some areas will experience greater rainfall leading to flooding.

These are just a few examples where human activities have impacted the Earth and its environment.