# 0.7 Representing chemical change

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## Introduction

As we have already mentioned, a number of changes can occur when elements react with one another. These changes may either be physical or chemical . One way of representing these changes is through balanced chemical equations . A chemical equation describes a chemical reaction by using symbols for the elements involved. For example, if we look at the reaction between iron ( $\mathrm{Fe}$ ) and sulphur ( $\mathrm{S}$ ) to form iron sulphide ( $\mathrm{FeS}$ ), we could represent these changes either in words or using chemical symbols:

$\mathrm{iron}+\mathrm{sulphur}\to \mathrm{iron sulphide}$

or

$\mathrm{Fe}+\mathrm{S}\to \mathrm{FeS}$

Another example would be:

$\mathrm{ammonia}+\mathrm{oxygen}\to \mathrm{nitric oxide}+\mathrm{water}$

or

$4{\mathrm{NH}}_{3}+5{\mathrm{O}}_{2}\to 4\mathrm{NO}+6{\mathrm{H}}_{2}\mathrm{O}$

Compounds on the left of the arrow are called the reactants and these are needed for the reaction to take place. In this equation, the reactants are ammonia and oxygen. The compounds on the right are called the products and these are what is formed from the reaction.

In order to be able to write a balanced chemical equation, there are a number of important things that need to be done:

1. Know the chemical symbols for the elements involved in the reaction
2. Be able to write the chemical formulae for different reactants and products
3. Balance chemical equations by understanding the laws that govern chemical change
4. Know the state symbols for the equation

We will look at each of these steps separately in the next sections.

## Chemical symbols

It is very important to know the chemical symbols for common elements in the Periodic Table, so that you are able to write chemical equations and to recognise different compounds.

## Revising common chemical symbols

• Write down the chemical symbols and names of all the elements that you know.
• Compare your list with another learner and add any symbols and names that you don't have.
• Spend some time, either in class or at home, learning the symbols for at least the first twenty elements in the periodic table. You should also learn the symbols for other common elements that are not in the first twenty.
• Write a short test for someone else in the class and then exchange tests with them so that you each have the chance to answer one.

## Writing chemical formulae

A chemical formula is a concise way of giving information about the atoms that make up a particular chemical compound. A chemical formula shows each element by its symbol and also shows how many atoms of each element are found in that compound. The number of atoms (if greater than one) is shown as a subscript.

## Examples:

${\mathrm{CH}}_{4}$ (methane)

Number of atoms: $\left(1×\mathrm{carbon}\right)+\left(4×\mathrm{hydrogen}\right)=5$ atoms in one methane molecule

${\mathrm{H}}_{2}{\mathrm{SO}}_{4}$ (sulphuric acid)

Number of atoms: $\left(2×\mathrm{hydrogen}\right)+\left(1×\mathrm{sulphur}\right)+\left(4×\mathrm{oxygen}\right)=7$ atoms in one molecule of sulphuric acid

A chemical formula may also give information about how the atoms are arranged in a molecule if it is written in a particular way. A molecule of ethane, for example, has the chemical formula ${\mathrm{C}}_{2}{\mathrm{H}}_{6}$ . This formula tells us how many atoms of each element are in the molecule, but doesn't tell us anything about how these atoms are arranged. In fact, each carbon atom in the ethane molecule is bonded to three hydrogen atoms. Another way of writing the formula for ethane is ${\mathrm{CH}}_{3}{\mathrm{CH}}_{3}$ . The number of atoms of each element has not changed, but this formula gives us more information about how the atoms are arranged in relation to each other.

can someone help me with some logarithmic and exponential equations.
20/(×-6^2)
Salomon
okay, so you have 6 raised to the power of 2. what is that part of your answer
I don't understand what the A with approx sign and the boxed x mean
it think it's written 20/(X-6)^2 so it's 20 divided by X-6 squared
Salomon
I'm not sure why it wrote it the other way
Salomon
I got X =-6
Salomon
ok. so take the square root of both sides, now you have plus or minus the square root of 20= x-6
oops. ignore that.
so you not have an equal sign anywhere in the original equation?
Commplementary angles
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Sherica
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Sherica
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Tamia
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or infinite solutions?
Kim
The answer is neither. The function, 2 = 0 cannot exist. Hence, the function is undefined.
Al
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Kristine 2*2*2=8
Differences Between Laspeyres and Paasche Indices
No. 7x -4y is simplified from 4x + (3y + 3x) -7y
is it 3×y ?
J, combine like terms 7x-4y
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Asali
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Need to simplify the expresin. 3/7 (x+y)-1/7 (x-1)=
. After 3 months on a diet, Lisa had lost 12% of her original weight. She lost 21 pounds. What was Lisa's original weight?
what's the easiest and fastest way to the synthesize AgNP?
China
Cied
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I start with an easy one. carbon nanotubes woven into a long filament like a string
Porter
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Porter
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Yasmin
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Cesar
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Yes, Nanotechnology has a very fast field of applications and their is always something new to do with it...
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Stotaw
In this morden time nanotechnology used in many field . 1-Electronics-manufacturad IC ,RAM,MRAM,solar panel etc 2-Helth and Medical-Nanomedicine,Drug Dilivery for cancer treatment etc 3- Atomobile -MEMS, Coating on car etc. and may other field for details you can check at Google
Azam
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Prasenjit
after 100 year this will be not nanotechnology maybe this technology name will be change . maybe aftet 100 year . we work on electron lable practically about its properties and behaviour by the different instruments
Azam
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Prasenjit
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Damian
silver nanoparticles could handle the job?
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At high concentrations (>0.01 M), the relation between absorptivity coefficient and absorbance is no longer linear. This is due to the electrostatic interactions between the quantum dots in close proximity. If the concentration of the solution is high, another effect that is seen is the scattering of light from the large number of quantum dots. This assumption only works at low concentrations of the analyte. Presence of stray light.
the Beer law works very well for dilute solutions but fails for very high concentrations. why?
how did you get the value of 2000N.What calculations are needed to arrive at it
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What makes metals better to use as wires than non-metals? (please link to bonding type)??? HELP