Section 6.1: Chemical Equations

Objectives: Relate chemical changes and macroscopic properties, demonstrate how chemical equations describe chemical reactions, Illustrate how to balance chemical reactions by changing coefficients

Identify the changes that take place when a cake is made:

When a substance undergoes a chemical change

  After a chemical reacts, it no longer has the

same chemical identity

A precipitate (solid) forms Heat is produced or absorbed Odor changes Gas is given off Color change and bubbles given off

indicates a good chance a chemical change has occurred, but they are not proof positive.

◦ growth of a tree◦ melting butter◦ the use of food in the body◦ CO2 escaping from a can of soda◦ separation of the components of crude oil through

distillation◦ a lake freezing

Chemical Reaction (RXN): a process by which one or more substances are changed into one or more different substances; represented by an equation.

Chemical Equation (EQN): shows what change takes place, and shows the relative amounts of various elements/compounds that take part in these changes.

REACTANT: Substance that undergoes a reaction (change)

PRODUCT: New substance formed 

Example:Iron and oxygen (reactants) → iron (III)

oxide (Fe3+) (O2) (Fe2O3)

 Reactions can involve single/ or many

reactants or products

Reactants are placed to the left of an arrow and products are placed to the right

 Plus signs are used to separate reactants and

also to separate productsExample: hydrochloric acid + sodium hydroxide → water + sodium chloride

(reactants) (products)

Word equations can be converted into chemical equations

 Chemical formulas replace the names of

compounds or elements 

Symbols in parentheses are put after formulas to indicate the state of the substance

(s) = solid(l) = liquid(g) = gas(aq) = aqueous

Example: HCl(l) + NaOH(s) → H2O(l) + NaCl(aq)

Energy is often released or absorbed during chemical reactions

  If energy is absorbed → endothermic

reaction If energy is released → exothermic reaction 

For reactions that absorb (need) energy, the word energy is often written along with the reactants in a chemical equation

Example: 2H2O(l) + energy → 2H2(g) + O2(g)

 

For reactions where energy is released, the word energy is often written along with the products in a chemical equation

Example:

CH4(g) + 2O2(g) → CO2(g) + 2H20(g) + energy

Note: The word energy is not always written. If it is important to know if energy is absorbed or released or in some cases where the reaction would not occur without the addition of energy

The same amount of matter is contained in the products and reactants (matter is neither created nor destroyed)

Mass of reactants = Mass or products

In chemical reactions, atoms do NOT change, they rearrange!

The number and kinds of atoms present in the reactants of the chemical reaction are the same as those present in the products

 Counting atoms: The subscript after the

symbol for an element represents how many atoms of that element are present

For a chemical equation to accurately represent a reaction, the same number of each kind of atom must be on the left side of the arrow as are on the right side → It is said to be balanced

To represent more than one unit taking part or being formed in a reaction, use a

COEFFICIENT: a number is placed in front to indicate the number of units involved

To balance equations, coefficients can be added or changed on the reactants and products

Subscripts CAN’T be changed – changing subscripts changes the identity of that subs

1. Analyze2. Set Up3. Solve4. Check

1. Magnesium metal and water combine to form solid magnesium hydroxide and hydrogen gas

 Analyze: (word equation)

magnesium + water → magnesium hydroxide + hydrogen

Set up: (chemical equation)Mg(s) + H2O(l) → Mg(OH)2(s) + H2(g)

 

Solve: (count atoms and then balance)Left: Mg = 1, H = 2, O = 1Right: Mg = 1, H= 4, O = 2

 Mg(s) + 2H2O(l) → Mg(OH)2(s) + H2(g)

 

Check: (count atoms)Left: Mg = 1, H = 4, O = 2Right: Mg = 1, H= 4, O = 2

 The equation is properly balanced.

Hydrogen and oxygen combine to form gaseous water and release energy.

Word equation:Hydrogen + oxygen -> water + energy

Chemical equation:H2(g) + O2(g) -> H2O (g) + energy

Balance:2H2(g) + O2(g) -> 2H2O (g) + energy

P. 199 (#1,3) Write word equations and chemical equations 1) Magnesium and water combine to form

solid magnesium hydroxide and hydrogen gas.

3) When energy is added to solid manganese(II) sulfate heptahydrate crystals, they break down to form liquid water and solid manganese(II) sulfate monohydrate

P. 199 (#2,4) Write word equations and chemical equations 2) An aq. Solution of dihydrogen dioxide and

solid lead (II) sulfide combine to form solid lead (II) sulfate and liquid water.

 4) Solid potassium reacts with liquid water to

produce aq. Potassium hydroxide and hydrogen gas