Chemical Reactions

1. Chemical Reactions Unit 4 (Day 1) Ms. Kondra & Mr. Olnhoff

2. Chemical equations • Statement conveying information about a chemical reaction. • Two ways of describing chemical equations: • Word equations • Formula equations

3. Chemical equations I. Word Equations • Verbally describe chemical reactions. • Have the following pattern: reactant 1(state)+ reactant 2(state) product 1(state) + product 2(state)

4. Parts of an equation I. Reactants • Are the substances with which you begin the reaction. • The names of the reactants are listed before the arrow in a word equation. • A plus sign is used between the reactants when there is more than one reactant. II. Products • Are the substances made as a result of the reaction. • The names of the products are listed after the arrow in a word equation. • A plus sign is used between the products when there is more than one product.

5. State of matter • Behind the name of each reactant and product is the state of the substance in brackets using the short form: • Solid (s) • Liquid (l) • Gas (g) • Aqueous (aq) **dissolved in water or solution form

6. Chemical Reactions • Involve the breaking and forming of bonds to create new and different substances. • Since bonds can’t be observed directly, we instead look for evidence that a new substance formed (5 different ways) • Formation of a precipitate • Gas produced/bubbles • Color change • Energy change • Not reversible

7. 1. Formation of a Precipitate • (Out of a solution) • A new substance is formed with a much lower solubility than the original. • It will not stay dissolved so a solid appears.

8. 2. Formation of a gas • A new substance is formed which is a gas. • May see bubbles.

9. 3. Color Change • A new substance is formed which is a different color than the original. • The energy of the electrons influences the color of substances.

10. 4. Energy Change (entropy) • (heat and/or light is absorbed) • New substances are formed which contain a different amount of chemical energy than the reactants. • If the new bonds have less energy than the original, then some energy must be released; called EXOTHERMIC. • If the new bonds have more energy than the original, then some energy must have been absorbed; called ENDOTHERMIC.

11. EXOTHERMIC • Energy is released • Written on products side • Ex. Water formation hydrogen(g) + oxygen(g) water(l)+ heat

12. endothermic • Energy is absorbed • Written on reactants side • Ex. Oxygen formation water(l) + heat  hydrogen(g) + oxygen(g)

13. 5. Not reversible • Reaction happens and cannot be undone. • Ex. Toasting bread, explosions

14. Writing Chemical equations • Methane burns in air (oxygen), giving off carbon dioxide and water vapour. • A. Word equation: • Methane + oxygen  carbon dioxide + water {reactants} {products}

15. Word equations • Ex. 1: Magnesium metal burns in oxygen gas with a bright white light to make a white powder called magnesium oxide. • Ex. 2: A solution of sodium iodide is added to a solution of potassium nitrate to make a potassium iodide precipitate and a sodium nitrate solution. • Ex. 3: Gaseous hydrogen and gaseous oxygen react explosively to form water vapor.

16. Writing Chemical equations • B. Molecular or Formula equation • Instead of a verbal description of the chemical reaction, the words are exchanged for chemical symbols. • Shorthand method of representing chemical reactions. • May include state of each species (s, l, g, aq). • All chemical equations should be written in a BALANCED form. • In a chemical reaction, the mass of the products MUST equal the mass of the reactants. • CH4(g) + 2O2(g) CO2(g) + 2H2O(g)

17. Formula equations • The equation must represent fact. • The equation must be represented with the correct formulas and symbols. • The equation must obey the Law of Conservation of Mass • Diatomic elements must be shown as such. • If the phase of a substance is given, it must be included in the equation in abbreviated form (s, l, g, aq)

18. Balancing equations • We need to follow the Law of Conservation of Mass when balancing equations: • “In any chemical reaction, there is no detectable difference between the total mass of the reactants and the total mass of the products” • http://phet.colorado.edu/en/simulation/balancing-chemical-equations

19. Law of Conservation of Mass • No new atoms are added. • The atoms of the reactants rearrange to form the products. Example • Co(NO3)2 (aq) + NaOH (aq) --> Co(OH)2(s) + NaNO3(aq) 213.33g213.34g

20. Balancing equations • To balance chemical equations we use ratios. • Coefficients in front of each formula must be used when balancing chemical equations. • These apply to each element in that particular formula. • You cannot change any of the subscripts of any formulas or you will change the compound involved. • To understand which coefficients to use we must be able to count atoms • If there is no coefficient in a formula we assume its coefficient is ‘1’.

21. Balancing equations rules • Balance 1 type of atom/ion at a time • Balance the atom/ion in the greatest abundance first • Balance H and O last • Examples • __H2 + __O2 __H2O • __Na + __Cl2 __NaCl • __Zn + __HCl __ZnCl2 + __H2 • __Cu + __AgNO3 __Ag + __Cu(NO3)2

22. Balancing Equations Assignment • Handout: Word equations practice 1 • Textbook: P. 133 #1-5