Chemical Reactions :Chemical Reactions Evidence for Chemical Reactions and Balancing Chemical Reactions


Slide 3:Heated Sugar


Slide 4:Orange precipitate: copper


Evidence for Chemical Reactions :Evidence for Chemical Reactions 1.Changes in Properties Color change Precipitate- a solid that forms from solution during a chemical change Gas given off One or more NEW substances are produced


Evidence for Chemical Reaction :Evidence for Chemical Reaction 2. Changes in Energy Endothermic reaction- energy is absorbed during the reaction and the temperature goes down Exothermic reaction- energy is released by the reaction and the temperature rises


Changes in Energy :Changes in Energy Exothermic Reaction Al + Fe 2 O 3  Al2 O3 + Fe Endothermic Reaction 6CO2 + 6H20  C6H12O6 + 6O2


Chemical Equations use Symbols as a shorthand way to communicate :Chemical Equations use Symbols as a shorthand way to communicate


Parts of a Chemical Equation :Parts of a Chemical Equation Reactants- the substances at the beginning before the reaction occurs Products- the substances that form after the reaction is complete


Writing Chemical Equations :Writing Chemical Equations Make sure all of the formulas for the reactants and the products are correct FOR EXAMPLE: iron(III) oxide is written as : Fe2O3 Because iron has a +3 oxidation state and oxygen has a -2 oxidation state REMEMBER THE CRISS-CROSS METHOD FOR WRITING FORMULAS Fe+3 O -2 = Fe203 when you multiply the +3 X 2= +6 and the -2 X 3 = -6 add them together and they = 0. Then you know it is correct. Check all of the formulas first before you write the equation. When you are balancing an equation, you cannot change subscripts!! H2O


Law of conservation of Mass :Law of conservation of Mass The law of conservation of mass states that matter is never created or destroyed, just changed from one form to another.


: Chemical equations must reflect the law of conservation of mass: The products cannot have any more mass than the reactants


Writing chemical Equations :Writing chemical Equations Write the formulas for the reactants and the products Fe + O2  Fe2O 3 Count up the # of atoms of each kind on both sides 1 Fe(iron) 2 O(oxygen)  2 Fe and 3 O The Fe is not balanced and neither is the O


Writing Chemical Equations :Writing Chemical Equations Use coefficients to balance the equation 4Fe + 3O2  2Fe2O3 The coefficients multiply all of the elements that follow by that number so now the reactants have 4 Fe and 6 O atoms and the products have 4Fe and 6 O atoms


Balancing Chemical Equations :Balancing Chemical Equations Write the equation Count the atoms Use coefficients to balance atoms Look back and check


Classifying Chemical Reactions :Classifying Chemical Reactions SYNTHESIS-When 2 or more elements or compounds combine to make a more complex substance DECOMPOSITION- breaks down compounds into simpler substances.


Synthesis :Synthesis 2Na + Cl2  2NaCl


Decomposition :Decomposition 2H2O2 2H2O + O2 H2O2 H2O2


Classifying Chemical Reactions :Classifying Chemical Reactions REPLACEMENT-when one element replaces another in a compound, or when 2 elements in different compounds trade places


Replacement reaction :Replacement reaction 2Al + Fe2O3  Al2O3 +2 Fe (rust) Al and Fe switched places


Controlling Chemical Reactions :Controlling Chemical Reactions Energy and Reactions 1. Activation Energy: the energy needed to start a chemical reaction


Activation Energy :Activation Energy


Rates of Chemical Reactions :Rates of Chemical Reactions Factors that affect the rate of reaction: 1. Surface area- if surface area is increased, the rate of reaction increases.


Ways to increase surface Area :Ways to increase surface Area Crush the solid into smaller pieces


Rate of Chem Reaction (Cont) :Rate of Chem Reaction (Cont) Temperature – when temperature increases, the rate of reaction also increases Concentration- the amount of substance in a given volume. When concentration increases, The rate of reaction increases.


Rates of Chem Reactions (cont) :Rates of Chem Reactions (cont) Catalysts- a chemical that increases the rate of reaction by lowering the activation energy. Example: enzymes in your body allow reactions to occur at much lower temperatures.


Activation energy with a catalyst :Activation energy with a catalyst Activation energy is less With the catalyst =Energy


Rate of Reaction(cont) :Rate of Reaction(cont) Inhibitor- a chemical that decreases the rate of reaction EXAMPLE: preservatives in food