Chapter 8 Chemical Equations and Reactions

Sect. 8-1: Describing Chemical Reactions Chemical equation – represents the identities and relative amounts of the reactants and products in a chemical reaction

Evolution of heat and light Production of a gas Formation of a precipitate (a solid produced as a result of a reaction in solution and settles out of solution) Color change Indications of a Chemical Reaction

Characteristics of Chemical Equations 1. Must represent known facts (all reactants and products listed) 2. Must contain correct formulas for all elements and compounds (remember diatomic elements and sulfur (S 8 ) and phosphorus (P 4 )) 3. Law of conservation of mass must be satisfied

Different Ways to Write Equations Word equation – reactants and products are represented by words, rather than symbols Formula equation – reactants and products are represented by their chemical formulas Balanced formula equation – uses coefficients to balance out amounts of reactants and products

Common Symbols used in Equations  yields ↔ reversible reaction (s) or ↓ solid (arrow only used for precipitate) (g) or ↑ gas (l) liquid (aq) aqueous (dissolved in water) Δ or heat (above arrow) reactants were heated

What information can a balanced equation give us??? Coefficients indicate relative amounts of atoms, molecules, or formula units of reactants and products  Ex. 2Na + Cl 2  2NaCl means 2 atoms of Na react with 1 molecule of Cl 2 to form 2 formula units of NaCl Coefficients also indicate moles of reactants and products and thus also mass

Steps for Balancing Equations 1. Write the formula equation for the reaction. 2. Make a list of elements involved in the reaction below the arrow. 3. Count the atoms of the elements in the reactants. Write number down to the left of the element below the arrow.

4. Count the atoms of the elements in the products. Write number to the right of the element below the arrow. 5. Identify coefficients to make the number of atoms of each element equal on both sides of the equation. 6. Write the coefficients in front of the substances in the equation. 7. Check your work!!!

Sect. 8-2: Types of Chemical Reactions Five types of reactions  Synthesis  Decomposition  Single-replacement  Double-replacement  combustion

Synthesis (composition) reaction – 2 or more reactants come together to form a single product  Ex.: 2Fe + O 2  2FeO  Ex.: CaO + H 2 O  Ca(OH) 2

Decomposition Reaction – a single compound breaks into 2 or more products  Usually requires heat or electricity to be added  H 2 CO 3  CO 2 + H 2 0 Electrolysis – the decomposition of a substance by an electric current

Single-replacement Reaction (displacement reaction) – one element replaces a similar element in a compound  Ex: Mg + 2HCl  H 2 + MgCl 2

Double-replacement reaction – the ions of 2 compounds exchange places in an aqueous solution to form 2 new compounds  Either water, a gas, or a precipitate will be one of the products  Ex: HCl + NaOH  NaCl + H 2 0

Combustion Reaction – a substance combines with oxygen, releasing energy in the form of heat and light  Products will typically be water and carbon dioxide

Sect.8-3: Activity Series of the Elements Activity Series – a list of elements organized according to the ease with which the elements undergo certain chemical reactions (greater ease with which to lose or gain electrons) The element at the top of the list (most active) can replace any element below it from a compound

Activity Series is used to predict whether or not a reaction will take place  Ex.: Co + NaCl  no reaction Written this way because cobalt cannot replace sodium since it is further down on the activity series