Chemical Reactions Essential Concepts



Chemical Reactions Essential Concepts

Obj 1 Define the terms "chemical formula" and "chemical equation"

• A chemical formula shows the kinds and numbers of atoms in the smallest representative unit of the substance. Examples: Ag, NaCl, CO2, MgO, MgCl2, Li2O, C6H12O6

• A chemical equation represents a chemical reaction and indicates the relative number of reactants that react to produce products. Example: 2Na + 2H2O ( 2NaOH + H2

Obj 2 Interpret chemical equations

The parts of chemical equations are:

• Yield signs (() An arrow separates the reactants (behind the arrow) and products (what the arrow points to). Individual reactants and individual products are separated by "+" signs.

• Coefficients The numbers in front of chemical formulas in a chemical equation are coefficients. Coefficients reflect the relative number of particles that react, and also indicate the mole ratios of all particles.

In the equation N2 + 3 H2 ( 2 NH3, nitrogen (N2) and hydrogen (H2) are reactants and ammonia (NH3) is the product. The equation should be read as "One mole of nitrogen reacts with 3 moles of hydrogen to yield 2 moles of ammonia."

Obj 3 Balance chemical equations

Coefficients indicate the relative number of particles of reactants and products in a chemical reaction. But how are coefficients assigned? Easy. Since matter is neither created nor destroyed (law of conservation of matter) and atom identity is not changed in reactions, there must always be the same number of atoms of each element in the reactant side of the equation as there are on the products side. Coefficients are assigned (by trial and error) until this result is achieved.

When balancing chemical equations, make sure to:

• remember coefficients affect all elements in the formula they precede The 2 in front of 2NH3, means there are two nitrogen atoms and 6 (2 x 3) hydrogen atoms.

• use the smallest multiples possible (coefficients of 2, 6, 4 can be simplified to 1, 3, 2);

• never change subscripts!!!!! (If you do, you'll change the identity of the substance!)

Example: Balance the following equation. Mg + HCl ( MgCl2 + H2

Process:

• There's 1 Mg on the reactant side, and 1 on the product side. Cool.

• There is 1 H on the reactant side and 2 on the product side. Put a coefficient "2" in front

• of HCl. Mg + 2HCl ( MgCl2 + H2

• Using the modified equation, 2 Cl's on the reactant side and 2 on the right side. Cool.

• Now go back again. 1 Mg; 1 Mg. 2 H; 2 H. 2 Cl; 2 Cl. Way cool.

• It's balanced! Mg + 2HCl ( MgCl2 + H2

Obj 4 Classify chemical reactions

There are many types of chemical reactions and several ways to describe them. The classification system we will use divides reactions into five categories.

1. Synthesis reaction: two or more substances combine to form one new substance. (Analogous to two single people meeting to form a couple.) The general form for a combination reaction is:

element or compound + element or compound ( compound

Example: 2Mg + O2 ( 2MgO

2. Decomposition reaction: a substance breaks up into simpler substances. (Analogous to a couple breaking up into two singles.) The general form for a decomposition reaction is:

compound ( element or compound + element or compound

Example: 2KClO3 ( 2KCl + 3O2

3. Single displacement reaction: one element displaces another in a compound. (Analogous to a single meeting a couple and displacing one of them.) The general form for a single displacement reaction is:

element + compound ( element + compound

Example: Cu + 2AgNO3 ( Cu(NO3)2 + 2Ag

4. Double displacement reaction: the positive and negative portions of two compounds are interchanged. (Analogous to two couples switching partners.) The general form for a double displacement reaction is:

compound + compound ( compound + compound

Example: Fe2(SO4)3 + 3Ba(OH)2 ( 2Fe(OH)3 + 3BaSO4

5. Combustion reaction: a hydrocarbon (a compound made of carbon and hydrogen) burns in oxygen to produce (if combustion is complete) carbon dioxide and water vapor. The general form for a complete combustion reaction is:

hydrocarbon + oxygen ( carbon dioxide + water

Example: 4C2H5 + 13O2 ( 8CO2 + 10H2O

If sufficient oxygen is not available, combustion will be incomplete and elemental carbon and poisonous carbon monoxide may be additional products.

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