Chemical Reactions

Signs of a Chemical Reaction Evolution of heat and light Formation of a gas Formation of a precipitate Color change

Symbols in Chemical Reactions Many symbols are used when writing a chemical reaction

A Balancing Act

Law of Conservation of Mass mass is neither created nor destroyed in a chemical reaction total mass stays the same atoms can only rearrange 4 H 2 O 4 H 2 O 36 g 32 g 4 g

Chemical Reaction Vocabulary Chemical Reaction: One or more substances, the reactants change into one or more new substances, the products Reactant: A substance present at the start of a reaction Product: A substance produced in a chemical reaction Chemical Equation: An expression representing a chemical reaction; the formulas of the reactants (on the left) are connected by an arrow with the formulas for the products (on the right). Example: Reactants Products

Practice: Write the equation for… Copper can be produced by heating copper (II) sulfide with oxygen gas. Sulfur dioxide is also produced. Water is formed by the explosive reaction between hydrogen and oxygen

Answers CuS + O2 → Cu + SO2 2 H2 + O2 → 2 H2O

Meaning of Chemical Formula Symbol Meaning Composition H2O One molecule of water: Two H atoms and one O atom 2 H2O Two molecules of water: Four H atoms and two O atoms H2O2 One molecule of hydrogen peroxide: Two H atoms and two O atoms

Meaning of Chemical Formula When a __________ is written after the chemical symbol for an element it indicates the___________________. When a coefficient is written in front of an atom or molecule it indicates the ____________ of that atom or molecule that is present. If there are subscripts in that atom/molecule they need to be ____________ by the coefficient. subscript # of that element number multiplied

Unbalanced and Balanced Equations H Cl Cl H H Cl H Cl Cl Cl H Cl H H H2 + Cl2  HCl (unbalanced) H2 + Cl2  2 HCl (balanced) reactants products reactants products H 2 1 H 2 2 Cl 2 1 Cl 2 2 Timberlake, Chemistry 7th Edition, page 167

How do we go from Unbalanced to Balanced Equations? You ____________ change the formula of your atom/ion/molecule! This means you may not change the ___________ ! Instead, you may put a coefficient __________ of your atom/ion/molecule to balance the number of atoms after you have ___________ it by the subscript MAY NOT subscript in front multiplied

Coefficient  subscript = # of atoms Balancing Steps 1. Write the unbalanced equation. 2. Count atoms on each side. 3. Add coefficients as needed to make #s equal on both sides of the equation. Coefficient  subscript = # of atoms 4. Reduce coefficients to lowest possible ratio, if necessary. 5. Double check atom balance!!!

Helpful Tips Balance one element at a time. Update ALL atom counts after adding a coefficient. If an element appears more than once per side, balance it last. Balance polyatomic ions as single units. “1 SO4” instead of “1 S” and “4 O”

Balancing Examples P + O2 → P4O10 H2 + O2 → H2O AgNO3 + CuSO4 Ag2SO4 + Cu(NO3)2

Balancing Example Aluminum and copper(II)chloride react to form copper and aluminum chloride. 2 Al + CuCl2  Cu + AlCl3 Al Cu Cl 3 3 2 2  1 Al 1 1 Cu 1 2 Cl 3  2  6 3  6   3

Writing Chemical Reactions First we are going to start with what is considered a skeleton equation Skeleton equation: a chemical equation that does not indicate the relative amounts of the reactants and products. Example: Rusting Fe + O2 Fe203

Writing Chemical equations Next we want to add in what “state” each of the atoms/molecules is in Solid (s) Liquid (l) Gas (g) Aqueous solution (aq): a solution in which the solvent is water You also will indicate if you are adding anything usually either heat or a catalyst (substance that speeds up a reaction, but is not used up or part of either the products or reactants) ∆ or heat MnO2 Example: Fe(s) + O2(g) Fe2O2(s)

Balancing Chemical Equations A chemical equation in which mass is conserved; each side of the equation has the same number of atoms of each element Is this equation balanced? Fe(s) + O2(g) Fe2O2(s) On the left side: 1 Fe On the right side: 2 Fe 2 O 2 O The iron is not balanced to conserve mass. We have 1 on the left and 2 on the right, so we need to at a coefficient (small whole number that are placed in front of the formulas in an equation in order to balance it). So the equation would look like this: 2Fe(s) + O2(g) Fe2O2(s)

Rules for Writing and Balancing Equations Step 1: Determine the correct formulas for all the reactants and products Step 2: Write the skeleton equation by placing the formulas for the reactants on the left and the formulas for the products on the right with an arrow in between. If two or more reactants or products are involved, separate their formulas with plus signs. Step 3: Determine the number of atoms of each element in the reactants and products. Count a polyatomic ion as a single unit if it appears unchanged on both sides of the equation. Step 4: Balance the elements one at a time by using coefficients. When no coefficients is written, it is assumed to be 1. Begin by balancing elements that appear only once on each side of the equation. Never balance an equation by changing the subscripts in a chemical formula. Each substance has only one correct formula. Step 5: Check each atom or polyatomic ion to be sure they are equal on both sides of the equation. Step 6: Make sure all the coefficients are in the lowest possible ratio.

Balancing Example Aluminum and copper(II)chloride react to form copper and aluminum chloride. Step 1. Determine the Correct Formula Aluminum = Al Copper(II)chloride = CuCl2 Copper = Cu Aluminum chloride = AlCl3 Step 2. Write the skeleton equation Al + CuCl2 Cu + AlCl3 Step 3. Determine the # of atoms in reactants and products Reactants: 1 Al, 1 Cu, 2 Cl Products: 1 Al, 1 Cu, 3 Cl Step 4. Balance the elements one at a time by using coefficients. Start with Cl since it is the only one that isn’t balanced. To make both sides match we use coefficients to make both sides match. When we make that change it then effects the others so we have to use coefficients to balance those as well. 2Al + 3CuCl2 3Cu + 2AlCl3 Step 5. Check each atom or polyatomic ion to be sure they are equal on both sides of the equation. Reactants: 2 Al, 3 Cu, 6 Cl Products: 2 Al, 3 Cu, 6 Cl Step 6: Make sure all the coefficients are in the lowest possible ratio.