Reactivity of Group 7 elements

These elements are found in the same group as they have the same number of electrons in their outer shells. They have similar properties as they all need one electron to get a full outer shell. Trends: Atomic Size – Increases down the group, as the atoms have more shells Density – Increases down the group, as the atomic mass increases (more protons and neutrons). Reactivity – Decreases down the group, as it is harder for the nucleus to attract an outer electron, as the outer shell is further from the nucleus and the inner shells shield the attractive force of the nucleus.

It is important to know the reactivity of the halogens with iron wool and with halide salt solutions in displacement reactions:

Reactivity of Group 1 metals

These elements are found in the same group as they have the same number of electrons in their outer shells. They have similar properties as they all need to lose one electron to get a full outer shell. Trends: Atomic Size – Increases down the group, as the atoms have more shells Density – Increases down the group, as the atomic mass increases (more protons and neutrons). Reactivity – Increases down the group, as it is harder for the nucleus to attract the outer electron, as the outer shell is further from the nucleus and the inner shells shield the attractive force of the nucleus. So the outer electron is more easily lost

The Royal Society of Chemistry video below shows the reactivities of some of the Group 1 metals:

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?

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).

 

 

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.

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.

Natural factors that have changed the surface of the Earth

The Surface of the Earth

The Earth’s crust is broken up into smaller pieces called tectonic plates.  These plates can move around by a few centimetres a year.

The crust and the very top of the mantle (just below and touching the underside of the crust) make up an area called the lithosphere.

Convection currents within the lithosphere cause the crust to move.

Heat released by radioactive decay in the mantle causes convection currents.  The hot rock rises then cools and sinks again.

The hot rock will rise and cause friction on the underside of the crust, moving it by a few cm a year. 

From the diagram of the lithosphere the plates are moving apart at point 1 and together at point 2.

Billions of year ago the Earth was thought to be made up of one land mass called ‘Pangea’.  Over time the plates moved due to convection currents to form the continents as we know them now.

What happens at plate boundaries?

1.) Plates colliding

When the plates collide the more dense plate sinks forcing the less dense plate upwards, making a fold mountain, sometime volcanoes form if the more dense plate melts and the magma rises.  If the volcano erupts is causes damage to the surface of the Earth and the habitats that are found there.

This is known as a destructive plate boundary.

An example of where this is occurring is on the west coast of South America, where the Nazca plate is colliding with the South American plate forming the Andes Mountains.

2.) Plates moving apart

When the plates move apart magma rises and fills the gap.  This cools forming new plate.  This process will happen continually.

This is known as a constructive plate boundary.

An example of where this is occurring is under the Atlantic Ocean, where the new plate that forms makes up the Mid Atlantic ridge.  So America and the UK are moving further apart from each other.

3.) Plates sliding past each other

This happens when plates are moving side by side, sometimes they get caught together until the pressure is too great and they move suddenly.  This causes an earthquake. 

This is known as a conservative plate boundary.

An example of where this is occurring is along the Californian coast.  The San Andreas Fault in San Francisco.

If movements occur suddenly at plate boundaries earthquakes and/or volcanic eruptions can occur. This can cause severe devastation to the surface of the Earth and to habitats in the area.

The surface of the earth has been changed naturally over time by processes such as:

• Asteroid impact - This would create large craters in the surface of the Earth, along with destroying objects and habitats and has been attributed to be the cause for certain mass extinctions in the planet's history, including the dinosaurs.  The effects of asteroid collisions have been shown in the films 'Armageddon' and 'Deep Impact'. 
• Volcanic eruption - In 1980 Mount St Helens erupted changing the landscape of the mountain and surrounding areas.  Soils become more fertile as the lava and ash contain vital nutrient elements such as sulfur and phosphorus.
• Earthquakes - These have the ability to change the landscape.  In 1958 at Lituya Bay, Alaska, an earthquake caused a landslide that shifted so much material into the bay that it caused a mega-tsunami (500m high wave).  This wave destroyed the whole shoreline of the bay. Another example of an earthquake that caused a tsunami in 2011 was one that happened off the north-east coast of Japan (http://www.bbc.co.uk/news/world-asia-pacific-12709598).  This caused devastation and loss of life, also causing critical damage to a nuclear power plant. 
• Erosion - this can happen by air, water (ice) or biological action.
• During a number of ice ages, large ice masses called glaciers moved from high in mountain regions to lower regions over millions of years, carving deep channels into the surface and breaking up and depositing rocky masses along the way.

Read more by clicking the following link:

 http://www.ehow.co.uk/info_8047828_natural-factors-affect-surface-earth.html

Mendeleev and the development of the Periodic table

Over time as elements have been discovered scientists have tried to observe patterns in their properties and behaviours, looking to list and categorise them accordingly. 

Döbereiner spotted patterns in sets of 3 elements, he called them triads. Newland arranged elements in order of atomic mass, he found that every 8th element had similar properties (although this did not work exclusively for all elements).  It was Dmitri Mendeleev who really got to grips with categorising elements into a table.  His ideas eventually led to the modern Periodic Table that we have now.   The video below explains the development of the Periodic Table:

So Mendeleev arranged his table in the following way:

• elements were arranged in rows of increasing atomic mass and columns of elements with similar properties,
• gaps were left for yet undiscovered elements, Mendeleev even predicted the atomic masses and properties of these elements. (Once discovered, it was found that he was very accurate with his predictions),
• where elements were in the wrong place when sorted by atomic mass, he swapped then so that they would be grouped by their properties e.g. tellurium and iodine were swapped so that iodine would be grouped with fluorine, chlorine and bromine,
• some elements were paired in the same space.

Mendeleev's table did not include a transition metal block or the Noble gases. As more was discovered about the atom and new elements found the Mendeleev's table evolved into the modern table that we now know.  In recognition of his role the 101st element was named after him - Mendelevium!

Reducing Our Impact on Global Warming - Carbon Capture and Storage

Evidence for the impact of carbon dioxide on global warming

There is much debate about the causes of global warming, some argue that it is just part of the Earths' natural cycle and that there have been periods of extreme heat and ice ages throughout the millennia. Whilst others say that increasing carbon dioxide in our time is contributing directly to global warming.  There are plenty of websites on the internet giving argue for and against these theories.  The Houston Chronicle blog typifies this showing that data can be used to argue for and against the impact of carbon dioxide. Click here.

The graph below (from http://www.discoveringantarctica.org.uk/alevel_2_3.html) shows a link between carbon dioxide levels and temperature over the last 600,000 years.  This data has been gathered by looking at ice core samples from Antarctica.  Click on the graph for more information.

It suggests that there is a link between carbon dioxide and temperature levels.  The greater the carbon dioxide the greater the temperature.  It also suggests that these temperature changes are cyclic.  At present the levels of carbon dioxide in our atmosphere are greater than at any time during the period of the graph above, it is also thought in some areas that the rise in global temperature is probably greater than would be expected in these normal temperature cycles.

The Artic Climate Impact Assessment (website click here) has shown that since the onset of the Industrial Revolution (1800s onwards) where the use of fossil fuels has increased, carbon dioxide emissions have increased rapidly as has global temperature.

Carbon-dioxide levels in the air closely track temperature records for the past 10 centuries. If they continue to parallel each other, global warming will only get worse. From: http://whyfiles.org/211warm_arctic/ Courtesy Arctic Climate Impact Assessment

How can carbon dioxide emissions be reduced?

Scientists believe that reducing carbon dioxide emissions could help reduce levels of global warming.  One method of doing this is by carbon capture and storage, the video below explains how this is being undertaken in Australia by the Australian Coal Association:

To summarise:

• carbon dioxide can be captured at the point of production,
• condensed into liquid form,
• then stored deep under sea beds in geological formation such as old oil fields, or underground in geological rock formations such as old coal seams.

For more information visit the Carbon Capture & Storage Association website. Click here.

Global Warming

What is Global Warming?

Carbon dioxide is one of a number of gases in our atmosphere that are known as 'greenhouse gases'.  The sun’s rays (made up of UV and Infra-red (IR) rays) pass through the atmosphere and warm the surface of the Earth.  These rays are reflected out into space again. Greenhouse gases such as carbon dioxide absorb some IR rays from the sun, and reflect them back towards the Earth, warming up the atmosphere and the Earth's surface.  These gases are needed as without them the surface temperature would be approximately 33^oC colder than it is now making it highly unlikely that plant and animal life would survive.

The Carbon Cycle

 

The carbon cycle is a naturally occurring process where carbon dioxide is removed from the atmosphere by organisms (such as plants) undergoing photosynthesis:

 

 

carbon dioxide  +    water     ---------->    glucose            +   oxygen

6CO₂(g)       +    6H₂O(l)   ---------->    C₆H₁₂O₆(s)         +   6O₂(g)

The carbon is trapped in the plant as glucose, this carbon can be transferred to animals when plants are eaten.

 Both plants and animals use glucose as a source of energy in the process of respiration:

glucose        +     oxygen  ---------->    carbon dioxide      +   water

C₆H₁₂O₆(s)    +     6O₂(g)    ---------->    6CO₂(g)                  +   6H₂O(l)

This (along with decomposition of plants and animals when they die) releases carbon dioxide back into the atmosphere.  This is not a problem as the amount carbon dioxide released is the same as the amount taken in when the plants grew originally, so the process can be considered ‘carbon neutral’.

 

On this basis carbon dioxide levels in the atmosphere would be sufficient to maintain a temperate climate.

Why is Global Warming an issue?

The problem exists in the combustion (burning) of fossil fuels.  Fossil Fuels contain carbon that was trapped millions of years ago.  This releases extra carbon dioxide into the atmosphere, which is not part of the natural carbon cycle.

 

This carbon dioxide absorbs more IR rays from the sun reflecting them back towards the Earth's surface, increasing the surface temperature.  This process is known as global warming, the video below describes the process:

 

 

Issues with Global Warming

As the surface temperature of the Earth increases a number of problems occur, many of which are linked together:

• Polar ice caps melt more quickly – sea levels rise, causing flooding in low lying coastal areas. Destroying habitats in both of these regions.

• Climate changes – Temperature rises will cause increased rainfall in some areas, tropical storms and flooding, yet in other areas would cause drought.