Presentation on theme: "Chemical Reactions Physical and Chemical Change"— Presentation transcript:
1Chemical Reactions Physical and Chemical Change Law of Conservation of MassBalancing EquationsChemical Reactions
2Chemical ReactionsChemical 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
3Chemical ReactionsSome chemical reactions absorb energy, such as in the chemical reactions that cook food.
4Chemical ReactionsOther chemical reactions release energy in the form of heat, light and/or sound, such as the burning of wood in a campfire
5Chemical ReactionsChemical reactions happen at different rates. Some chemical reactions are fast, such as when rocket fuel burns.
6Chemical ReactionsOther chemical reactions happen slowly, such as the formation of rust on a corroding bicycle chain.
7Chemical ReactionsThe chemical reactions in your own body, which are keeping you alive, are among the fastest chemical reactions known.
8Chemical ReactionsScientists are constantly working to find new kinds of chemical reactions in order to produce new substances with useful properties.
9Chemical ReactionsAll chemical reactions involve the conversion of starting materials, called reactants, into new substances, called products. The products have different properties than the reactants.
10Physical PropertiesA description of a substance that does not involve forming a new substance.Examples:ColourTextureStateDensitySolubilityMelting point
11Chemical PropertiesA description of what a substance does as it changes into one or more new substances.Examples:CombustibilityCorrosionReaction with acidBleaching ability
12Properties & ChangeProperties 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
13Physical ChangeA 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.
14Physical Change Classes of physical change: Change in state (includes dissolving)Change in formExample:EvaporationCutting paper in half
15Chemical changeA chemical change is the transformation of one or more substances into new substances with new properties
16Visual Clues to a Chemical Change Presence of a new colourFormation of a precipitate
17Visual Clues to a Chemical Change Release of heat or light
18Visual Clues to a Chemical Change Production of gas or bubblesExampleReactants: Solid magnesium metal placed into a solution of hydrochloric acidClue: bubbles / gasesProduct: hydrogen gas and magnesium chloride
19Chemical EquationsA chemical reaction is often described by writing a chemical equation
20Chemical EquationsA chemical equation uses either words or symbols and formulas to describe the changes that occur during a chemical reaction.ExamplesWord equation:Hydrogen gas + oxygen gas waterFormula equation:H2 + O2 H2O
21Chemical Equations Every chemical equation must have: One or more reactantsOne or more productsAn arrow directing reactant to productIf there are more than one reactants or products, the chemical names/formulas are separated by a ‘+’ sign
22Chemical EquationsFor 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 equationsword equation:magnesium + hydrochloric acid magnesium chloride + hydrogenformula equation:Mg + HCl MgCl2 + H2
23States 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)
24Coefficients 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 reactionExample: Mg + HCl MgCl2 + H2Mg + 2HCl MgCl2 + H2A coefficient of 2 is in front of the formula HClThis means that Mg and HCl combine in a ratio of 1:2
25Law 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.
26Conservation of MassNo 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 productsChemical bonds between atoms are broken and new ones are formed, and the atom simply reconnect in new ways
27Conservation of MassThe 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 waterformula equation: H2 + O2 H2O
28Conservation of MassWhen 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
30Skeleton Chemical Equations A chemical equation that is complete except for coefficients is called an unbalanced equation or skeleton equation.Example:Skeleton equation: H2 + O2 H2OBalanced equation: 2H2 + O2 2H2O
31Balancing 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.
32Rules 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!
33Hints to help balance equations Balance atoms of elements in any complicated looking formulas first and balance atoms of pure elements last.H2
34Hints 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.
35Hints 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.
36Balancing 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:
37Balancing 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:
38Balancing Chemical Equations Example 1:2AlBr3(s) + Cl2(g) AlCl3(s) + 3Br2(g)The number of aluminum atoms is no longer equal.
39Balancing 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:
40Balancing Chemical Equations Example 1:2AlBr3(s) + Cl2(g) 2AlCl3(s) + 3Br2(g)The number of chlorine atoms is no longer balanced.
41Balancing 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:
42Balancing Chemical Equations Example 1:2AlBr3(s) + 3Cl2(g) 2AlCl3(s) + 3Br2(g)The equation is balanced!
43Balancing Chemical Equations Try it!Balance the following chemical equations:Al + F2 AlF3Ca + H2O Ca(OH)2 + H2CaCl2 + Na3PO4 Ca3(PO4)2 + NaCl
44Balancing Chemical Equations Try it!Balance the following chemical equations:2Al + 3F2 2AlF3Ca + 2H2O Ca(OH)2 + H23CaCl2 + 2Na3PO4 Ca3(PO4)2 + 6NaCl