Unit Four Chemical Reactions

Chemical Reactions Process by which the atoms of one or more substances are rearranged to form different substances

5 Types of Chemical Reactions Synthesis Combustion Decomposition Double-Replacement Single-Replacement

Synthesis Reactions What does synthesis mean? A + B AB 2 or more compounds or elements go together to build a more complex compound Which example is a synthesis equation?

Combustion Reactions What is necessary for something to burn? Oxygen combines with a substance and releases energy in the form of heat and light OXYGEN IS ALWAYS A REACTANT! Which example is a combustion equation?

Decomposition Reactions What does decompose mean? A single compound breaks down into 2 or more elements or new compounds AB A + B Which example is a decomposition equation?

Double Replacement Reactions Exchange of ions between 2 ionic compounds AX + BY AY + BX Which example is a double replacement equation?

Single Replacement Reactions One element replaces another element in a compound A + BX AX + B A metal will not always replace another metal…it depends on how active they are…most active replaces least active Which example is a single replacement equation?

Summary of Reaction Types Funny Analogies

What do all the small letters mean? (s) solid (l) liquid (g) gas (aq) dissolved in water

Terminology Word Equations: statements that describe a reaction Skeleton Equations: uses formulas not words to describe a reaction-NOT BALANCED True Chemical Equation: a balanced skeleton reaction that follows the law of conservation of matter

Writing Chemical Equations Read a description of the reaction Note what is reacted with what Note what is yielded or produced Write formulas for each compound REMEMBER TO CRISS-CROSS IF IONIC!

Balancing Chemical Equations Reflects the law of conservation of mass which says…

Steps to Balancing an Equation Write the skeleton equation. BE SURE THE FORMULAS ARE WRITTEN CORRECTLY. Inventory reactants Inventory products Add coefficients to make atoms of each element equal on both sides of the equation Reduce the coefficients if possible

Examples www.chemfiesta.com

Write and Balance the Following Magnesium sulfate + calcium hydroxide yields magnesium hydroxide + calcium sulfate Iron + silver chloride yields silver + iron (III) chloride

So….. In a lab with beakers and chemicals, how do I know that a reaction is happening?

Evidence of Chemical Reactions Temperature change exothermic, endothermic Release energy in the form of light Color change Odor Gas bubbles Formation of a solid…called a precipitate.

Reactions in Aqueous Solution More than 70% of earth is covered by water 66% of the human body is water MANY chemical reactions occur in water Often form solids called precipitates Remember: Ionic compounds dissociate in water.

Net Ionic Equations Represent reactions of ionic compounds in aqueous solution by writing complete ionic equations Remove spectator ions (those appear on both sides of the equation but aren’t in the precipitate) Leaves a net ionic equation that can be balanced

Percent Composition The relative amounts of each element in a compound are expressed in percent composition. AKA: percent by mass of each element % of element = grams of element X 100 grams of compound EXAMPLES: pg. 191

Calculating Empirical Formula Empirical formula: lowest whole number ratio of the atoms of the elements in a compound Empirical formula doesn’t have to be the same as the actual molecular formula of the compound. Ex. Hydrogen peroxide

Calculating Empirical Formula Given % composition. Assume 100 grams. Allows you to change the % to g. Convert to moles. Find lowest number of moles. Divide all moles by the lowest number. Multiply by a number if necessary to get whole numbers. Example: pg. 193

Calculating Molecular Formulas Molecular Formula= n(empirical formula) n= molecular formula mass/molar mass of empirical Examples: pg. 194

(particles= atoms, formula units, or molecules) Introducing the Mole Avogadro’s Number: 1 mole = 6.02 x 1023 particles (particles= atoms, formula units, or molecules) Molar Mass: 1 mole = ________ grams of an element or compound (can be found on the periodic table)

Chemical Quantities Groups Gather 2 sheets of paper, periodic table, calculator, notes, and long WS Instructions: Jar Lab: Number paper 1 to 5. Calculate the # of grams in each jar. Be sure to write the given, unknown, and conversion factor % Composition and Empirical Formula Cards: Do 3 cards. Be sure to write down the #’s. IF DONE: Begin writing the equations for the LONG WS 26-36.

We cannot get rid of the mole! Stoichiometry We cannot get rid of the mole!

What is Stoichiometry? Quantitative relationships between reactants used and products formed RELATES ONE COMPOUND TO A DIFFERENT COMPOUND Based on law of conservation of matter

What is Stoichiometry? CANNOT DO STOICHIOMETRY WITHOUT A BALANCED EQUATION! Why? The coefficients in a balanced equation tell us how many moles of each compound are used or produced in the reaction.

What is Stoichiometry? Mole Ratio: ratio between the number of moles of any two compounds in a balanced chemical equation COEFFICIENTS

Stoichiometric Calculations 2 relationships in stoichiometry: Molar mass Mole ratios

Stoichiometric Calculations Mole to mole conversions Mole ratio Mole to mass conversions Mole ratio and molar mass Mass to mass conversions Molar mass, mole ratio, molar mass

Limiting Reactants Limiting reactant: ends the reaction because it runs out Excess reactant: left over at the end of the reaction AMOUNT OF PRODUCT THAT YOU GET IN A REACTION DEPENDS ON THE LIMITING REACTANT…BECAUSE IT STOPS THE REACTION!

Limiting Reactants Finding which is limiting: Will be given two things Solve for unknown twice (one time using each given) Which is the smallest answer? That is the answer to the problem. The limiting reactant is the given found at the front of that solution!

Limiting Reactants Since the limiting reactant stops the reaction, USE THE LIMITING REACTANT TO SOLVE FOR THE AMOUNTS OF PRODUCTS PRODUCED.