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chemistry

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Describing Chemical Reactions

On May 6, 1937, the huge airship Hindenburg erupted into a fireball. Within a short time, 210,000 cubic meters of hydrogen had burned and the airship was destroyed. The chemical reaction that occurred is “hydrogen combines with oxygen to produce water.” You will learn to represent this chemical reaction by a chemical equation.

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Writing Chemical Equations

How do you write a word equation?

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Writing Chemical Equations

Word Equations

To write a word equation, write the names of the reactants to the left of the arrow separated by plus signs; write the names of the products to the right of the arrow, also separated by plus signs.
Reactant + Reactant --> Product + Product

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Describing Chemical Reactions

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Writing Chemical Equations

iron + oxygen --> iron oxide

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Writing Chemical Equations

Chemical Equations

A chemical equation is a representation of a chemical reaction; the formulas of the reactants (on the left) are separated by an arrow from the formulas of the products (on the right).

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Describing Chemical Reactions

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Writing Chemical Equations

Here is the UNBALANCED chemical equation for rusting:
Fe + O₂ --> Fe₂O₃

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Balancing Chemical Equations

What are the steps in writing a balanced chemical equation?

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Balancing Chemical Equations

To write a balanced chemical equation, first write the formulas of the reactants and products in an equation.
Use coefficients to balance the equation so it obeys the law of conservation of mass (meaning nothing is created nor destroyed - in other words, the amount of atoms for each element on one side of the arrow must equal the amount on the other side)

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Balancing Chemical Equations

This is a balanced equation for making a bicycle. The coefficients are the whole numbers that are placed in front of the formulas in an equation in order to balance it.

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Describing Chemical Reactions

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Reactants:

1 frame, 2 wheels, 1 handlebar, 2 pedals

Products:

1 frame, 2 wheels, 1 handlebar, 2 pedals

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Coefficients & Subscripts

Coefficients and Subscripts:

5H₂0

What number is a Coefficient? ____
What number is a Subscript? ____
How many Hydrogen atoms are there? ____
How many Oxygen atoms are there? ____

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5

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5

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Coefficients & Subscripts

Coefficients and Subscripts:

H₂ + O₂ H₂O

When balancing equations…

You CANNOT change subscripts. If the product is water, for example, it will ALWAYS need 2 hydrogen atoms for every 1 oxygen atom.
You CAN change the coefficient. For example, it may be necessary to produce 2 water molecules instead of 1 to balance the given equation.

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Balancing Chemical Equations

The Goal of Balancing Equations:
A chemical reaction is described by a balanced equation when each side of the equation has the same number of atoms of each element.

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Writing Chemical Equations

A catalyst is a substance that speeds up the reaction by lowering activation energy, but is not used up in the reaction.
Without Catalyst With Catalyst

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Describing Chemical Reactions

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Writing Chemical Equations

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Reaction Vocab

Some additional equation language:

Endothermic: A reaction that uses heat (often represented as: )

Exothermic: A reaction that gives off heat

Equilibrium: The term used for a reversible reaction when the rate of its forward reaction equals the rate of its reverse reaction

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