Humboldt State University ® Department of Chemistry

Richard A. Paselk

Science 331
Fall 2004 Lecture/Activity Office: SA560a
Notes: 4 October Phone: x 5719
Home: 822-1116
e-mail: rap1

Chemical Reactions & Chemical Change

We have had two sets of activities involving Chemical reactions - Acids & Bases, and Burning. Today I want to go back and take another look at these processes and also other chemical and physical processes, looking for common features and differences.

Burning (oxidation of carbon)

Recall the reaction we saw for natural gas (methane)

CH4 + 2 O2 -> CO2 + 2 H2O

And the very similar reaction for burning methanol (wood alcohol)

CH3OH + O2 -> CO2 + 2 H2O

(Why do we draw methanol like this? Why not CH4O? Look at Lewis Structure - C must be in middle and H can't bridge C and O.)

Dissolving and Chemical Change

What happens when we dissolve something and get a solution?

How do these differ from simple mixtures such as finely ground salt and pepper, mud in water, or really shaken up oil and vinegar dressing?

Solutions are homogeneous mixtures which will NOT separate out. They have a single phase. They are clear if the solvent is clear.

How do we know they do not represent chemical reactions (that is, how do we know the moleculaes havn't cahnged)? Look at our examples:

How does making solutions compare to changes of state or phase? Look at heating/cooloing curve for water (Fig 1.24, p 38).

Types of Changes

So now we have seen three types of changes matter undergoes:

  1. Phase changes (changes of state) - Physical change
  2. Dissolving - Physical change
  3. Substance changes - Chemical change

 

Acids and bases. So what kinds of changes do we see with acids and bases. Review first.

Recall we defined acids and bases in two ways:

What does the chemical definition really mean? Let's look at some common acids:

HCl -> H+ + Cl-

H2SO4 -> H+ + HSO4-

CH3CO2H -> H+ + CH3CO2-

NaOH -> Na+ + OH-

KOH -> K+ + OH-

Ca(OH)2 -> Ca+ + 2 OH-

What happens when an acid and a base are mixed? Make "salt" water. Of course we can't see it happen, because we are working in water, and the new water is a very small fraction of the amount we already have!

Let's try hydrochloric acid and sodium hydroxide:

HCl + NaOH -> H+ + Cl- + Na+ + OH- -> H2O + Na+ + Cl-

so the net result is that water was made (salty water in this case):

H+ + OH- -> H2O

pH and pH scale (Fig 1.20, p 32)


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© R A Paselk

Last modified 5 October 2004