NCSU – Dept. of Chemistry – Lecture Demonstrations Acid-Base Reactions

Determining Acid Strength Using Magnesium

Description: Reaction of an acid with Mg generates H 2 gas. Stronger acids produce more H 2 bubbles as Mg dissolves in the acidic solution.

Materials:

Mg ribbon 3 Petri dishes 1.0 M HCl 1.0 M boric acid 1.0 M acetic acid

Procedure:

1. Label the Petri dishes with each acid. Place Petri dish on overhead projector or document camera and add two or three Mg strip to each dish. Be sure that the strips have been cleaned prior to use.

2. Pour acids into respective dishes and observe H2 (g) evolution to indicate relative strength of each acid. Results show that strength of acids is as follows HCl > acetic acid > boric acid. The reaction in boric acid is too slow to observe during the lecture period.

3. For a more quantitative demonstration, the H 2 (g) produced can be collected following the setup shown in the practicalchemistry.org reference.

Discussion: Rate of dissolution of Mg is dependent upon strength of acid in solution according to the following equation (given for reaction with HCl):

Mg (s) + HCl (aq) → MgCl 2 (aq) + H 2 (g)

The stronger the acid, the faster the rate of reaction and thus the shorter amount of time that effervescence is observed.

Safety: Wear proper protective equipment including gloves and safety glasses when preparing and performing this demonstration. Concentrated solutions of acids and bases (>2 M) can irritate the skin and cause burns. Vapors of concentrated acids are extremely irritating to the eyes and respiratory system When diluting concentrated acids, add the acid to the water to avoid spattering. Avoid contacting skin with dry ice by using insulated gloves or tongs.

Disposal: Materials can be disposed of in an aqueous waste container. Excess of acid can be diluted and flushed down the drain with plenty of water.

NCSU – Dept. of Chemistry – Lecture Demonstrations Acid-Base Reactions References: http://www.creative-chemistry.org.uk/gcse/documents/Module7/N-m07-18.pdf http://www.practicalchemistry.org/experiments/the-rate-of-reaction-of- magnesium-with-hydrochloric-acid,100,EX.html

Video: http://video.google.com/videoplay?docid=1487904092116567586&ei=z3GVStaE A87XlQfFpeTbAw&q=magnesium+in+acid&hl=en&client=firefox-a (reactivity series video)