Understanding Equations

Transcription

1 Chemical Reactions Chemical reaction: a process of chemically changing both the physical and chemical properties of a substance to a new substance with different physical and chemical properties.

2 Video Examples Rust forming Photosynthesis Wood burning or Gasoline combustion Note: chemical reactions change the energy of a substance as well as its physical and chemical properties. E.g. wood burning produces infrared light (heat)

3 Understanding Equations 1. Reactant - substance that enters the chemical reaction. 2. Product - a substance that is produced by a chemical reaction 2Mg + O 2 2MgO + Light energy Reactants Yield Products

4 Understanding Equations Chemical equations are an easy way of writing a chemical reaction. It is a chemical sentence. (I.e. using symbols and formulas to represent a chemical reaction.) H 2 + O 2 H 2 O 2 The reaction of a diatomic hydrogen molecule with a diatomic oxygen molecule yields a hydrogen peroxide molecule. 2H 2 + O 2 2H 2 O The reaction of two diatomic hydrogen molecules with a diatomic oxygen molecule yields two water molecules.

5 Understanding Equations Q: How do we know that the atoms of reactants will be present in the products of a chemical reaction? A: The Law of Conservation of Mass Mass can be neither created nor destroyed

6 Understanding Equations The number of atoms of each element must be the same before and after a chemical reaction. i.e. It s why we Balance Chemical Equations!! 2Mg + O 2 2MgO + Light energy

7 Balancing Equations review (When balancing equations, coefficients are the only change that can be made.) BaCl 2 + H 2 SO 4 BaSO 4 + HCl KClO 3 KCl + O 2 C 3 H 8 + O 2 CO 2 + H 2 O

8 Understanding Equations Measuring the amounts of reactants and products is part of understanding equations. What do we measure and how do we measure? What - You can measure mass, - or volume - or you can count pieces. How - measure mass in grams. - measure volume in liters. - count pieces in MOLES.

9 What is the mole? Pretty cute, right??? These are moles, but not the right kind.

10 The Mole (mol): A unit to count numbers of particles Just like... Dozen = 12 or Pair = 2 The mole (mol) is the amount of a substance that contains as many pieces as there are atoms in exactly grams of 12 C 1 mol = x = Avogadro s number (N A )

11 Molar mass is the mass of 1 mole of eggs shoes marbles atoms in grams. 1 mole 12 C atoms = x atoms = g 1 12 C atom = amu 1 mole 12 C atoms = g 12 C 1 mole lithium atoms = g of Li For any element atomic mass (amu) = molar mass (grams)

12 How many atoms are in g of potassium (K)? 1 mol K = g K 1 mol K = x atoms K Mass (g) x Molar Mass (mol/g) x N A = # atoms K g K 1 mol K g K x 1023 atoms K 1 mol K x x = 8.49 x atoms K

13 Molecular mass (or molecular weight) is the sum of the atomic masses (in amu) in a molecule. SO 2 1S 2O SO amu + 2(16.00 amu) amu For any molecule molecular mass (amu) = molar mass (grams) 1 molecule SO 2 = amu 1 mole SO 2 = g SO 2

14 Types of Reactions 1. Synthesis reaction occurs when 2 or more substances combine to form 1 new substance. A + B AB E.g. 2Na + Cl 2 2NaCl sodium atoms and a diatomic chlorine molecule yields sodium chloride.

15 Decomposition reaction Decomposition reaction occurs when one substance degrades to 2 new substances; complex simple. AB A + B I.e. it is the reverse of a synthesis reaction. E.g. H 2 CO 3 H 2 O + CO 2 Carbonic acid yields water and carbon dioxide

16 Single replacement Single replacement reaction occurs when a free element replaces another element of a compound. AB + C AC + B E.g. 2Na + 2H 2 O 2NaOH + H 2 2 sodium atoms and 2 water molecules yields 2 sodium hydroxide molecules and a diatomic hydrogen molecule.

17 Double replacement Double replacement reaction occurs when 2 compounds react to form 2 new compounds. AB + CD AC + BD E.g. MgCO 3 + 2HCl MgCl 2 + H 2 CO 3

18 Combustion reaction Combustion reaction occurs when a compound (fuel) reacts with an oxidizing element (O 2 or F 2 ) and releases one or more new compounds and heat. Examples: CH 4 + 2O 2 CO 2 + 2H 2 O + energy CH 2 S + 6F 2 CF 4 + 2HF + SF 6 + energy

19 More Combustion Reactions Combustion reactions usually result in additional energy released in the form of visible light and flame (Fire) The fuel is also rarely completely consumed. In the previous reactions, CO or C (soot) is often formed. Atmospheric gases are also associated with the reactants forming NO x.

20 Activation Energy All reactions require activation energy to start the reaction... Fireworks (A firework has to be started with a match and a fuse to explode)

21 Capacity to React Reactants must be able to react for a chemical reaction to occur. (Water will not burn but gasoline will) An element s capacity to react is determined by it s arrangement of electrons. (I.e. chemical reactions involve the bonding of valence electrons.) Bonding capacity is the ease with which an atom will form chemical bonds. What elements are NOT likely to be involved in a chemical reaction? Why?

22 Energy of Chemical Reaction All chemical reactions require a change in energy. Activation energy is the energy needed to start a reaction.

23 Exothermic reaction Exothermic reaction results in energy released as heat or light. All combustion reactions are exothermic. exo = outside thermic = heat 2H 2 + O 2 2H 2 O + heat Note: Energy is stored in molecules of H 2. The products have less energy than the reactants.

24 Endothermic reaction Endothermic reaction results when energy is absorbed in the reaction. endo = within thermic = heat E.g. 2NaCl + energy 2Na + Cl 2 In endothermic reactions, the products have more energy than the reactants.

25 Rates of Chemical Reactions The study of reaction rates or how quickly reactants turn into products is called kinetics. Kinetics is based on the collision theory reacting molecules must collide with sufficient energy if they are to form products.

26 Factors that influence reaction rates: Concentration is the amount of a substance in a given unit of volume High concentrations of a reactant = a high rate of reaction. Ex: a high concentration of K in H 2 O = an explosion (quick reaction)

27 Factors that influence reaction rates: Surface area of a reactant increases the rate of reaction. E.g. dried grass burns quicker than firewood increased surface area to O 2 increases the collisions with the fuel and O 2 An increase in temperature increases the motion of molecules and increases the energy of collision.

28 Catalyst A catalyst is a substance that increases the reaction rate but is not changed by the reaction. Catalysts form a temporary low-energy bond with reactants. Inhibitors are substances used to slow down the reaction rate or to keep them from happening. Ex. food preservatives prevent the reactions that spoil or decompose food.

29 Equilibrium A state in which forward and reverse reactions proceed at equal rates. Reversible reaction is a reaction that can occur in both directions forward and reverse. the yield symbol for that is Le Châtelier s Principle: a stress added at equilibrium will shift the equilibrium in the direction that opposses the stress. Examples: changing concentrations, adding heat or removing heat, changing volume or pressure.