Chemical Reactions Physical and Chemical Change Law of Conservation of Mass Balancing Equations Chemical Reactions

Chemical Reactions Chemical reactions are used in many ways in daily life. A chemical reaction is the process by which a chemical change happens. All chemical reactions are also accompanied by changes in energy

Chemical Reactions Some chemical reactions absorb energy, such as in the chemical reactions that cook food.

Chemical Reactions Other chemical reactions release energy in the form of heat, light and/or sound, such as the burning of wood in a campfire

Chemical Reactions Chemical reactions happen at different rates. Some chemical reactions are fast, such as when rocket fuel burns.

Chemical Reactions Other chemical reactions happen slowly, such as the formation of rust on a corroding bicycle chain.

Chemical Reactions The chemical reactions in your own body, which are keeping you alive, are among the fastest chemical reactions known.

Chemical Reactions Scientists are constantly working to find new kinds of chemical reactions in order to produce new substances with useful properties.

Chemical Reactions All chemical reactions involve the conversion of starting materials, called reactants, into new substances, called products. The products have different properties than the reactants.

Physical Properties A description of a substance that does not involve forming a new substance. Examples: Colour Texture State Density Solubility Melting point

Chemical Properties A description of what a substance does as it changes into one or more new substances. Examples: Combustibility Corrosion Reaction with acid Bleaching ability

Properties & Change Properties are descriptions similar to an adjective: describes what the substance is like. Change are descriptions similar to a verb: describes what the substance is doing

Physical Change A physical change is the change in the state or form of a substance that does not change the original substance. A physical change can result in new physical properties but not new chemical properties.

Physical Change Classes of physical change: Change in state (includes dissolving) Change in form Example: Evaporation Cutting paper in half

Chemical change A chemical change is the transformation of one or more substances into new substances with new properties

Visual Clues to a Chemical Change Presence of a new colour Formation of a precipitate

Visual Clues to a Chemical Change Release of heat or light

Visual Clues to a Chemical Change Production of gas or bubbles Example Reactants: Solid magnesium metal placed into a solution of hydrochloric acid Clue: bubbles / gases Product: hydrogen gas and magnesium chloride

Chemical Equations A chemical reaction is often described by writing a chemical equation

Chemical Equations A chemical equation uses either words or symbols and formulas to describe the changes that occur during a chemical reaction. Examples Word equation: Hydrogen gas + oxygen gas  water Formula equation: H2 + O2  H2O

Chemical Equations Every chemical equation must have: One or more reactants One or more products An arrow directing reactant to product If there are more than one reactants or products, the chemical names/formulas are separated by a ‘+’ sign

Chemical Equations For example, the chemical reaction between solid magnesium metal and hydrochloric acid is: Notice that hydrogen is expressed in the formula equation as H2. Recall that pure hydrogen exists as a diatomic molecule. You will need to know which elements exist as molecules when writing formula equations word equation: magnesium + hydrochloric acid  magnesium chloride + hydrogen formula equation: Mg + HCl  MgCl2 + H2

States of Matter in Chemical Equations The chemical formulas in a chemical equation will often include the state of matter of each substance (s) = solid (l) = liquid (e.g. water and oils) (g) = gas (aq) = aqueous (substance is dissolved in water, e.g. most ionic compounds) Examples: H2 (g) + O2 (g)  H2O (l) Mg(s) + 2HCl(aq)  MgCl2(aq) + H2(g)

Coefficients in Chemical Equations A coefficient is a whole number that is placed in front of the symbol of an element to show the ratios of the different substances that are present in the chemical reaction Example: Mg + HCl  MgCl2 + H2 Mg + 2HCl  MgCl2 + H2 A coefficient of 2 is in front of the formula HCl This means that Mg and HCl combine in a ratio of 1:2

Law of Conservation of Mass In a chemical reaction, the mass of the products always equals the mass of the reactants. In other words, the mass is conserved.

Conservation of Mass No atoms are destroyed and no new atoms are produced during a chemical reaction. Instead, the atoms in the reactants are simply rearranged to form the products Chemical bonds between atoms are broken and new ones are formed, and the atom simply reconnect in new ways

Conservation of Mass The rearrangement of atoms that occurs during a chemical reaction can be illustrated using models or diagrams. In this equation, there are equal numbers of hydrogen atoms (4) and equal numbers of oxygen atoms (2) on both the reactants side and the products side. word equation: hydrogen + oxygen  water formula equation: H2 + O2  H2O

Conservation of Mass When the number of each kind of atom is the same in the reactants and products, the equation is said to be balanced. balanced equation: 2H2 + O2  2H2O

Conservation of Mass

Skeleton Chemical Equations A chemical equation that is complete except for coefficients is called an unbalanced equation or skeleton equation. Example: Skeleton equation: H2 + O2  H2O Balanced equation: 2H2 + O2  2H2O

Balancing Chemical Equations To balance a chemical equation, begin by counting the number of atoms of each element in the skeleton equation. Balance by placing coefficients in front of the chemical formulas until the number of atoms in the reactants equals to the products.

Rules for using coefficients Use only whole numbers. Check that the coefficients in the equation are the lowest common factor. Never change a subscript in a formula to help make atoms balance!

Hints to help balance equations Balance atoms of elements in any complicated looking formulas first and balance atoms of pure elements last. H2

Hints to help balance equations Hydrogen atoms and/or oxygen atoms will often appear in many or all of the formulas of the reactants and products. When this is the case, balance other elements first, balance hydrogen second last and oxygen last.

Hints to help balance equations You may be able to treat polyatomic ions as a unit. Example: If NO3- appears in the reactants and products of a skeleton equation, count the number of NO3- groups rather than the number of N and O atoms separately.

Balancing Chemical Equations Example 1: Balance the following chemical equation: AlBr3(s) + Cl2(g)  AlCl3(s) + Br2(g) 1.) Count the number of atoms in the reactants and products:

Balancing Chemical Equations Example 1: 2AlBr3(s) + Cl2(g)  AlCl3(s) + 3Br2(g) Balance the number of bromine atoms by adding a coefficient of 2 in front of AlBr3 and a coefficient of 3 in front of Br2. Count the atoms again:

Balancing Chemical Equations Example 1: 2AlBr3(s) + Cl2(g)  AlCl3(s) + 3Br2(g) The number of aluminum atoms is no longer equal.

Balancing Chemical Equations Example 1: 2AlBr3(s) + Cl2(g)  2AlCl3(s) + 3Br2(g) Balance the number of aluminum atoms by adding a coefficient of 2 in front of AlCl3. Count the atoms again:

Balancing Chemical Equations Example 1: 2AlBr3(s) + Cl2(g)  2AlCl3(s) + 3Br2(g) The number of chlorine atoms is no longer balanced.

Balancing Chemical Equations Example 1: 2AlBr3(s) + 3Cl2(g)  2AlCl3(s) + 3Br2(g) Balance the number of chlorine atoms by adding a coefficient of 3 in front of Cl2. Count the atoms again:

Balancing Chemical Equations Example 1: 2AlBr3(s) + 3Cl2(g)  2AlCl3(s) + 3Br2(g) The equation is balanced!

Balancing Chemical Equations Try it! Balance the following chemical equations: Al + F2  AlF3 Ca + H2O  Ca(OH)2 + H2 CaCl2 + Na3PO4  Ca3(PO4)2 + NaCl

Balancing Chemical Equations Try it! Balance the following chemical equations: 2Al + 3F2  2AlF3 Ca + 2H2O  Ca(OH)2 + H2 3CaCl2 + 2Na3PO4  Ca3(PO4)2 + 6NaCl