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Writing Chemical Equations

You must have heard about physical changes and chemical changes. Physical changes involve changes in the physical state of matter by the application of temperature and pressure, for example, crushing of paper, melting of ice etc. In any physical change, the chemical composition of matter remains same before and after the physical change, only physical appearance gets changed.

On the contrary, chemical changes can be defined as the process which involves the change in the composition of matter such as burning of paper, baking of cake etc. There are several chemical changes which we can observe in our surrounding such as photosynthesis is the most important process for life to sustain on earth, is a chemical reaction.

Similarly, digestion of food in a living body is also a chemical change. Any chemical change forms new substances that are called as products. The chemical change or chemical reaction can be expressed with the help of chemical equation. Chemical equation is used to describe the chemical reaction with the help of atomic and molecular formulas. The molecular formulas are reactants are separated by an arrow which is headed from reactants to products. There is a certain way to write the chemical equation. Let’s discuss the rules of writing the chemical equation for chemical reactions.

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Certain conventions and symbols in addition to those for elements are used in equation writing. The following are common.

  • ? This arrow indicates "reacts to give"
  • Reactants are written on the left side separated by + signs.
  • Products are written on the right, also separated by + signs.
  • 'D' (the Greek capital letter delta) placed over or below the arrow means that “heat was added to the reactants”.
  • (g) means that the substance is gaseous.
  • (l) means that the substance is liquid.
  • (s) means that the substance is solid.
  • (aq) indicates an aqueous solution i.e. the substance is dissolved in water. The designations for the physical state appear as subscripts.
  • A number in front of the formula or symbol is a coefficient.
  • Conditions required for the reaction are specified above or below the arrow.

Eg. Catalyst, temperature, pressure, etc.

The following example illustrates these aspects of chemical equations.

1. Limestone when heated, yields solid calcium oxide and gaseous carbon dioxide. (In symbols)

CaCO3(s) $ \overset{\Delta}{\rightarrow}$ CaO(s) + CO2 (g)

2. The electrolysis of water in its liquid form yields hydrogen and oxygen gases. (In symbols)

2H2 O(I) $ \overset{electrolysis}{\rightarrow}$ 2H2(g) + O2(g)

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Conservation of Mass and Energy

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An important duty of a chemical equation is to demonstrate faithfully the law of conservation of mass, which states that mass can be neither created nor destroyed. In Dalton's atomic theory this law was explained for chemical reactions. He suggested that reactions are simply rearrangements of the same number of atoms. A close look at the equation above shows that there are two oxygen atoms on the left but only one on the right.
To conform to the law of conservation of mass, an equation must be balanced. A balanced equation has the same number and type of atoms on both sides of the equation.

Energy must also be conserved in a reacting system. However, energy balances are complicated by the fact that chemical bonds between atoms are associated with the storage of energy. Therefore the breaking and forming of chemical bonds that occurs during the chemical reaction must be specifically considered in the energy balances.

Chemical Balance

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Balances are mechanical devices used to determine the mass of objects. Because the mass to be determined ranges from kilograms to micrograms the choice of the balance to be used for any determination is governed by the total mass of the object and the sensitivity desired.

In chemical balance the objects can be placed either on one pan or on both for each weighing When the objects are placed on one pan only in a chemical balance we shall call the weighing one pan weighing and the design one pan design.

Sensitiveness of chemical balance

The change in load required to produce a perceptible change in indication. It is therefore a ratio and is not to be used to discuss the quality of a measurement. The factors influencing the sensitiveness of chemical balance are:

  1. Magnitude of the level arm error
  2. Magnitude of error in scale indication due to variable load
  3. Adjustment error of masses
  4. Uniform values of divisions throughout the optical scale
  5. Precision
  6. Environmental factors

Balanced Chemical Equation

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All chemical reactions needs to be balanced. This is because, in a chemical reaction, “matter can neither be created nor destroyed”. This means that the total mass of the reactants must be equal to the total mass of the products. This is the law of conservation of mass. Equations may be balanced by the Hit and Trial method.

Balancing equations is the technique which balances same values on both the sides of a chemical equation i.e. on Right Hand Side (RHS) and Left Hand Side (LHS). Balancing equation obtains when we get the same value on both sides. This is one of the most important concepts of chemistry. The presence of an excess of one reactant was stated or implied. The calculation were based on the substance that was used up first called the limiting reactant or reagent.

1. Balancing Under acidic conditions


To properly conserve mass and charge as you balance a redox reaction under acidic conditions, we often need to add water or hydrogen ion to the reactants or products. Here is the summary of the method for balancing a redox reaction equation for a reaction under acidic conditions
  1. Separate the reaction equation into the oxidation half-reaction and the reduction half-reaction. Use oxidation numbers to identify these component half-reactions.
  2. Balance the half-reactions separately temporary ignoring O and H atoms.
  3. Balance the half-reactions separately using H2O to add O atoms and using H+ to add H atoms.
  4. Balance the half-reactions separately for charge by adding electrons.
  5. Balance the charge of the half-reactions with respect to each other by multiplying the reactions such that the total number of electrons is the same in each half-reactions.
  6. Reunite the half-reactions into a complete redox reaction equation.
  7. Simplify the equation by cancelling items that appears on both sides of the arrows.

2. Balancing Under basic conditions

The process of balancing equation under basic condition is 90% identical to to the one used for balancing under acidic conditions. Instead of using water and hydrogen ion as balancing tools here water and hydroxide ion is used as balancing tool. The hydroxide ion is present in greater concentration in basic solutions.

  1. Perform all the steps as described in balancing under acidic condition.
  2. Observe where H+ is present in the resulting equation. Add an identical amount of OH- to both sides of the equation such that all the H+ is neutralized becoming water.
  3. Cancel any amount of water (H2O) that appears on both sides of the equation.

Half Reaction Equation

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The powerful technique for balancing redox reaction equations involves dividing these reactions into separate oxidation and reduction half-reactions. We then balance the half-reactions one at a time and combine them so that electrons are neither created nor destroyed in the reaction.

The steps involved in half-reaction method for balancing equations can be illustrated by considering the reaction used to determine the amount of tri iodide ion in a solution by titration. The steps are described below.
  1. Write a skeleton equation for the reaction.
  2. Assign oxidation numbers to atoms on both the sides of equation.
  3. Determine which atoms are oxidized and which are reduced.
  4. Divide the reaction into oxidation and reduction half-reaction and balance these reactions one at a time.
  5. Combine these half-reaction so that electrons are neither created nor destroyed.
  6. Balance the remainder of equation by inspection if necessary.

Chemical Reaction Equations

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A chemical equation, an equation of reactants and products on either side, with the temperature, pressure and other things necessary for it to occur, is called a chemical equation.
A chemical equation is a shorthand way of describing the events that occur in a chemical change or reaction.The method of representing a chemical reaction with the help of symbols and formula of the substances involved in it is known as a chemical equation.
Example
CH4 + 2O2 $\rightarrow$ CO2 + 2H2O
(Reactants) (Products)
The substances that combine or react are known as reactants. One or more reactants can combine to form products.

The new substances produced in a reaction are known as products. As the reactants, one or more products can be formed depending upon the reaction.

Examples of Chemical Equations

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Reactions can be simple, or completely complex. Some examples of chemical reactions are:
Example:1
V2O5
2SO2 + O2 $\to$ 2SO3
In the above chemical equation, sulfur dioxide combines with oxygen to form sulfur tri oxide. The V2O5 written on the top of the arrow shows that it is used as a catalyst.

Example: 2
2Al(s) + 3Cl2(g) $\to$ 2AlCl3(s)
In the above reaction, the 's' indicates solid and 'g' indicates gas. Sometimes, a chemical equation also shows the state in which the reactants and products are present.

Example: 3

25oc
2Mg + O2 $\to$ 2MgO
In the above reaction, a temperature of 25 degree Celsius is indicated above the arrow. This shows that the particular reaction takes place at room temperature.
If a higher temperature is required for a reaction to take place, it is shown above the arrow. Similarly, a high pressure is also indicated above the arrow.
More topics in Writing Chemical Equations
Oxidation Number
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