Chemical Equations But first some review

What is a chemical reaction? ► The process by which one or more substances change to produce one or more different substances. ► In a chemical reaction, the original substances are called the reactants. ► The substances that are created are called products.  Reactants  Products

To be classified as a chemical reaction, a chemical change must occur. ► What is the evidence of a chemical reaction?  Change in energy ► Release of energy as heat ► Release of energy as light ► Production of a sound ► Reduction or increase of temperature ► Absorption or release of electrical energy

 Formation of a new substance ► Formation of a gas ► Formation of a precipitate – insoluble solid ► Unexpected change in color ► Change in odor ► You must always verify to ensure that a physical change didn’t occur

Helping chemical reactions to occur ► For a chemical reaction to occur, the reactants must be brought together. ► During chemical reactions, energy is either absorbed (endothermic) or released (exothermic). ► As a result, energy can appear as a reactant (endothermic) or a product (exothermic).

Chemical equations also show the physical states and reaction information SymbolMeaning (s) (l) (g) (aq) or or heat  catalystPd Substance in the solid state Substance in the liquid state Substance in the gaseous state Substance in aqueous solution “produces” or “yields” Reversible Reaction Reactants are heated Catalyst added to speed reaction

With this information, we can now construct chemical equations. ► Chemical equations – a representation of a chemical reaction that uses symbols to show the relationship between the reactants and the products. ► Step one – write a word equation  Write the names of reactants separated by a plus sign.  Use an arrow to separate the reactants from the products.  Write the names of the products separated by a plus sign

► Step 2 - Convert the names into formulas. This is known as an unbalanced or skeleton equation. ► Step 3 - Balance the atoms by changing the coefficients only  This is how we conserve mass and obey the law of conservation of mass

Hints when balancing equations 1) Use coefficients to balance atoms involved 2) Balanced equations show mass conservation 3) Never change the subscripts when balancing an equation 4) Polyatomic ions can be balanced as a unit

Example ► When liquid bromine is mixed with aqueous sodium iodide, the result is aqueous sodium bromide and solid iodine. Br 2 + NaI  NaBr+ I 2 22 Now we have 2 Br on each side, 2 Na on each side and 2 I on each side

What if the formulas are already written? Balance the following: ___Na 2 SiF 6 + ___Na  ___Si + ___NaF ___NH 3 + ___CuO  ___N 2 + ___Cu + ___H 2 O

What if the formulas are already written? Balance the following: ___(NH 4 ) 2 Cr 2 O 7  ___Cr 2 O 3 + ___N 2 + ___H 2 O ___AgNO 3 + ___AlCl 3  ___AgCl + ___Al(NO 3 )