Presentation on theme: "Chemical Reactions Physical and Chemical Change Law of Conservation of Mass Balancing Equations."— Presentation transcript:
Chemical Reactions Physical and Chemical Change Law of Conservation of Mass Balancing Equations
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
Some chemical reactions absorb energy, such as in the chemical reactions that cook food.
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 happen at different rates. Some chemical reactions are fast, such as when rocket fuel burns.
Other chemical reactions happen slowly, such as the formation of rust on a corroding bicycle chain.
The chemical reactions in your own body, which are keeping you alive, are among the fastest chemical reactions known.
Scientists are constantly working to find new kinds of chemical reactions in order to produce new substances with useful properties.
All chemical reactions involve the conversion of starting materials, called reactants, into new substances, called products. The products have different properties than the reactants.
A description of a substance that does not involve forming a new substance. Examples: Colour Texture State Density Solubility Melting point
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 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
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.
Classes of physical change: 1. Change in state (includes dissolving) 2. Change in form Example: Evaporation Cutting paper in half
A chemical change is the transformation of one or more substances into new substances with new properties
Presence of a new colour Formation of a precipitate
Release of heat or light
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
A chemical reaction is often described by writing a chemical equation
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
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
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 word equation: magnesium + hydrochloric acid magnesium chloride + hydrogen formula equation: Mg + HCl MgCl 2 + H 2 formula equation: Mg + HCl MgCl 2 + H 2
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)
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
In a chemical reaction, the mass of the products always equals the mass of the reactants. In other words, the mass is conserved.
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
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
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: 2H 2 + O 2 2H 2 O
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
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.
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!
Balance atoms of elements in any complicated looking formulas first and balance atoms of pure elements last. H2H2 H2H2
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.
You may be able to treat polyatomic ions as a unit. Example: If NO 3 - appears in the reactants and products of a skeleton equation, count the number of NO 3 - groups rather than the number of N and O atoms separately.
Example 1: Balance the following chemical equation: AlBr 3 (s) + Cl 2 (g) AlCl 3 (s) + Br 2 (g) 1.) Count the number of atoms in the reactants and products:
Example 1: 2AlBr 3 (s) + Cl 2 (g) AlCl 3 (s) + 3Br 2 (g) Balance the number of bromine atoms by adding a coefficient of 2 in front of AlBr 3 and a coefficient of 3 in front of Br 2. Count the atoms again:
Example 1: 2AlBr 3 (s) + Cl 2 (g) AlCl 3 (s) + 3Br 2 (g) The number of aluminum atoms is no longer equal.
Example 1: 2AlBr 3 (s) + Cl 2 (g) 2AlCl 3 (s) + 3Br 2 (g) Balance the number of aluminum atoms by adding a coefficient of 2 in front of AlCl 3. Count the atoms again:
Example 1: 2AlBr 3 (s) + Cl 2 (g) 2AlCl 3 (s) + 3Br 2 (g) The number of chlorine atoms is no longer balanced.
Example 1: 2AlBr 3 (s) + 3Cl 2 (g) 2AlCl 3 (s) + 3Br 2 (g) Balance the number of chlorine atoms by adding a coefficient of 3 in front of Cl 2. Count the atoms again:
Example 1: 2AlBr 3 (s) + 3Cl 2 (g) 2AlCl 3 (s) + 3Br 2 (g) The equation is balanced!
Try it! Balance the following chemical equations: 1. Al + F 2 AlF 3 2. Ca + H 2 O Ca(OH) 2 + H 2 3. CaCl 2 + Na 3 PO 4 Ca 3 (PO 4 ) 2 + NaCl
Try it! Balance the following chemical equations: 1. 2Al + 3F 2 2AlF 3 2. Ca + 2H 2 O Ca(OH) 2 + H CaCl 2 + 2Na 3 PO 4 Ca 3 (PO 4 ) 2 + 6NaCl