CHEMICAL REACTIONS

Unit 4 (Day 1)

Ms. Kondra & Mr. Olnhoff

CHEMICAL EQUATIONS Statement conveying information about

a chemical reaction. Two ways of describing chemical

equations:Word equationsFormula equations

CHEMICAL EQUATIONSI. Word Equations Verbally describe chemical reactions. Have the following pattern:

reactant 1(state) + reactant 2(state) product 1(state) + product 2(state)

PARTS OF AN EQUATIONI. 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.

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

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 precipitateGas produced/bubblesColor changeEnergy changeNot reversible

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.

2. FORMATION OF A GAS A new substance is formed which is a

gas. May see bubbles.

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.

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.

EXOTHERMIC

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

ENDOTHERMIC

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

5. NOT REVERSIBLE Reaction happens and cannot be

undone. Ex. Toasting bread, explosions

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}

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.

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)

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)

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

LAW OF CONSERVATION OF MASS No new atoms are added. The atoms of the reactants rearrange to

form the products.

ExampleCo(NO3)2 (aq) + NaOH (aq) --> Co(OH)2(s) +

NaNO3(aq)

213.33g 213.34g

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’.

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

BALANCING EQUATIONS ASSIGNMENT Handout: Word equations practice 1 Textbook: P. 133 #1-5