Chapter 7 Chemical Reactions: An Introduction

***7.1 questions have difficult wording. Try to understand what the reactants and products are. 7.1-7.2 p234 # 4, 5, 7, 9, 15, 19, 20, 25

7.1 Evidence for a chemical reaction

What clues are there that there is a chemical reaction when you burn your beautiful, white table?

1.) ______

2.) ______

3.) ______

4.) ______

Remember: What other clues can indicate a chemical reaction?

  • ______
  • ______

7.2 Chemical Equations

The reactants are like ingredients in a recipe.

 Ca(s) + H2O(l) Ca(OH)2(s) + H2(g)

“Calcium reacts with water to form calcium hydroxide and hydrogen gas.

 “+” could be translated as “reacts with” or “added to”

an arrow () could be translated to “reacts to form”

Phases

K(s) + H2O(l)  H2(g) + KOH(aq)

s, l, g, and aq are abbreviations for phases.

(s) = ______

(l) = ______

(g) = ______

(aq) = ______(dissolved in water)

Remember: At room temperature, which elements are gases? ______

Liquids? ______

Solids? ______

Reactants – On the______

chemicals mixed before the chemical reaction happens(like ingredients)

Products – On the ______

The chemicals present after the reaction.

3H2SO4

Subscript –tells you how many atoms are part of the molecule

Coefficient – tells you how many molecules are involved

Circle the subscripts above. Draw an arrow to the coefficient.

Draw a picture of 3H2SO4

7.3 Balancing Chemical Equations

Conservation of Mass

VERY IMPORTANT – Define Law of Conservation of Mass

Mass is neither ______nor destroyed in a chemical reaction

Translation – the number and type of atoms that you have must be the same before and after the reaction

In other words, if I have 85 carbon atoms and 172 hydrogen atoms before a reaction, how many carbon atoms will I have after the reaction? ______carbon atoms

How many Hydrogen atoms will I have?______hydrogen atoms

This means you must “balance” the chemical equations so that the number and type of atoms are equal before and after the chemical reaction

2H2 + O2 2H2O

Rules for Balancing Equations

Rule # 1 Only change a coefficient!!!

*** Changing a subscript MAGICALLY creates new molecules. BIG NO NO!!!

Example of water and hydrogen peroxide

Rule #2 – keep track of your atoms

Rule #3 – Change coefficients to Balance the Equation

Rule #4 – Recount and continue

Example: Count the number of each type of element on each side – (Rule # 2). I recommend making a table

CH4 + O2  CO2 + H2O

 Balance 1 of the elements on both sides of the equation by changing a coefficient – Rule 3

Example: figure out the smallest # both sides will multiply evenly into

After changing the coefficient, recount all the atoms on both sides.

Continue balancing if needed. (Rule – 4)

3 Very Helpful Hints

Hint #1: If you see a group of atoms that occurs in the same order both before and after the reaction, treat them like one atom in your table

Ca(OH)2 + LiCl  CaCl2 + LiOH

Notice that “OH” is found both before the reaction and after? “Yes, I do!” Keep it together in your table

Hint # 2: If a molecule has only 1 type of atom, balance that atom last

Example:

CH4 + O2  CO2 + H2O

Notice the “O2”doesn’t balance the oxygen atoms until the very end.

Hint # 3: Simplify all coefficients to smallest #’s

Assignment:

1) __P + __O2  __P4O10

2) __KClO3  __KCl + __O2

3) __H2 + __S2 + __O2 __H2SO4

4) __Na +__Cl2  __NaCl

5) __S2 + __N2__S3N2

6) __MgCl2 + __HF __HCl + __MgF2