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http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa_pre_2011/chemcalc/chemcalc_higherrev4.shtml

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Balancing Chemical Equations

Let's take a look at this scale. We can see that it is unbalanced, with the right (red) side, weighing more than the left (blue) side. In order for the two sides to be balanced, we need to put a little more mass on the left side until they are the same mass.

Just like we want the scale to be balanced on both sides, a chemical equation should also be balanced on both sides. A chemical equation shows us the substances involved in a chemical reaction - the substances that react ( reactants) and the substances that are produced ( products). In general, a chemical equation looks like this:

According to the law of conservation of mass, when a chemical reaction occurs, the mass of the products should be equal to the mass of the reactants. Therefore, the amount of the atoms in each element does not change in the chemical reaction. As a result, the chemical equation that shows the chemical reaction needs to be balanced. A balanced chemical equation occurs when the number of the atoms involved in the reactants side is equal to the number of atoms in the products side.

Let's take a look at an equation representing a chemical reaction:

In this chemical reaction, nitrogen (N2) reacts with hydrogen (H) to produce ammonia (NH3).The reactants are nitrogen and hydrogen, and the product is ammonia. If we look at this equation, we can see that the equation is not balanced.

The equation is not balanced because in the reactants side, there are 2 nitrogen (N) atoms and 2 hydrogen (H) atoms. In the products side, there are 1 nitrogen (N) atoms and 3 hydrogen (H) atoms. The number of the atoms is not balanced on both sides.

To balance the chemical equation above, we need to make use of coefficients. A coefficient is a number that we place in front of a chemical formula. In the chemical equation, to make the number of nitrogen (N) atoms equal on both sides, first, we place a coefficient of 2 in front of NH3.

Once we do that, the number of nitrogen (N) atoms on both sides is balanced. However, the number of hydrogen (H) atoms is not balanced on both sides. We need to make use of another coefficient in front of H2. This time, we put a coefficient of 3 in front of H2 to balance the chemical equation.

The equation above is now balanced. There are 2 nitrogen (N) atoms and 6 hydrogen (H) atoms on both the reactants and products side. Since there is no coefficient in front of N2, that means the coefficient is equal to 1.

Steps to Balance an Equation

Practice always makes perfect. In general, to balance an equation, here are the things we need to do:

Count the atoms of each element in the reactants and the products.

Use coefficients; place them in front of the compounds as needed.

The steps are simple, but it is a process of trial and error. Let's take a look at a few more example equations and techniques that can be used to balance each one.

Example 1

This is a reaction between methane (CH4) and oxygen (O2), producing carbon dioxide (CO2) and water (H2O).

The reaction shown is a combustion reaction: a compound reacts with oxygen and produces carbon dioxide and water. The technique is to balance the carbon (C) atoms first, then the hydrogen (H) atoms, and then the oxygen (O) atoms.

In this case, the carbon (C) atoms are already balanced. So now we look at the hydrogen (H) atoms. There are 4 hydrogen (H) atoms on the reactants side and 2 hydrogen (H) atoms on the products side. To balance them, we put a coefficient of 2 in front of H2O.

The hydrogen (H) atoms are now balanced. Due to the coefficient 2 in front of H2O, there are a total of 4 oxygen (O) atoms on the products side. To balance the oxygen atoms on both sides, we put a coefficient of 2 in front of O2. The chemical equation is now balanced.

Example 2

This is a reaction between ferric oxide (Fe2O3) and carbon (C), producing Iron (Fe) and carbon dioxide (CO2).

This reaction is not balanced. First, we need to balance the oxygen atoms. We do this by making it so that there are 6 oxygen atoms on each side. To do this, we need to put a coefficient of 2 in front Fe2O3 and a coefficient of 3 in front of CO2.

Now that the oxygen atoms are balanced, we need to balance the iron (Fe) atoms first. To do this, we need to put a coefficient of 4 in front of Fe in the products side. Now that the Fe atoms are balanced, we can balance the carbon atoms. We do this by putting a coefficient of 3 in front of C on the reactants side. The chemical equation is now balanced.

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balanced chemical equation: definition & examples - video & lesson transcript | study.com

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Balanced Chemical Equation: Definition & Examples - Video & Lesson Transcript | Study.com

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Let's take a look at this scale. We can see that it is unbalanced, with the right (red) side, weighing more than the left (blue) side. In order for the two sides to be balanced, we need to put a little more mass on the left side until they are the same mass.

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https://www.youtube.com/watch?v=lywmGCfIUIA

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www.videochemistrytextbook.comm

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http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa_pre_2011/chemcalc/chemcalc_bothrev4.shtml

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Percentage yield
https://m.youtube.com/watch?v=pFmCn-cwOW8
The formula for percentage yield is
Actual mass of product produced
Divided by maximum theoretical mass of product possible times by 100

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The concentration of a solution is measured in moles per cubic decimetre. The greater the concentration, the more dissolved particles there are in the solution.

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http://www.bbc.co.uk/education/guides/zbbqtfr/revision/3 you can calculate the amount of substance (in moles) if you know the volume and concentration.You can also work out the concentration of an acid reacting with an alkali.

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http://study.com/academy/lesson/balanced-chemical-equation-definition-examples.html

http://education.jlab.org/elementbalancing/

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atom econmy is a way of measure atom wasted when making a chemical

Atom economy = Mr of desired products∕sum of Mr of all products × 100

http://www.bbc.co.uk/schools/gcsebitesize/science/add_ocr_gateway/chemical_economics/atomeconomyrev2.shtml

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definition- mass percentage is one way of representing theconcentration of an element in a compound
or a component in a mixture. Mass percentage is calculated as the mass of a component divided by the total mass of the mixture, multiplied by 100%.