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)

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

(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?

• Salt
• Sugar
• Alcohol
• Carbon dioxide

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:

• Salt
  • evaporate, get salt back
  • taste
• Sugar
  • evaporate, get sugar back
  • taste
• Alcohol
  • taste
• Carbon dioxide
  • reversible bubbles

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:

• Empirically (observationally) - acids turn litmus red (cabbage juice red), bases turn litmus blue (cabbage juice green).
• Theoretically (molecular), Arrhenius definition - acids provide protons (H⁺ ions) in aqueous solution, bases provide hydroxide (OH^-) ions in aqueous solution.

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

• Hydrochloric acid, HCl;

• Sulfuric acid, H₂SO₄;

• the Acetic acid in vinegar, CH₃CO₂H

• Note the different behaviors of the H's in acetic acid.
• Examples
  • Benzoic acid
  • Glycine
  • Ethanol
• Note that for "organic" acids (acids witrh carbon) we will see a common element = -COOH, a carbon with two oxygens, one of which has a hydrogen attached. The hydrogen on the oxygen is then the acidic hydrogen.
  • Alcohol, with a -COH is not acidic.
• Sodium hydroxide, NaOH

• Potassium hydroxide, KOH

• Calcium hydroxide Ca(OH)₂

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:

pH and pH scale (Fig 1.20, p 32)

• Note that acids have low pH (< 7), bases high pH (>7) and neutral solutions have a pH of 7 (in reality a range of pH values near 7 are generally considered neutral).

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© R A Paselk

Last modified 5 October 2004