ACIDS AND BASES SAFETY PRECAUTIONS

Transcription

1 ACIDS AND BASES Mild acids and bases are used in cooking (their reaction makes biscuits and bread rise). Acids such as those in our stomachs eat away at food or digest it. Strong acids and bases are used as cleaners; they can cause burns if they touch the skin - they can be hazardous. Drain cleaners are often bases - they corrode away the clogs in our drains. Neutralized materials are no longer corrosive. In this experiment you will use ph paper, to determine if a substance is an acid, a base or a neutral solution. Then you will neutralize three acids, observe the reactions and test the ph of the neutralized solution. PURPOSE: To test if solutions are acids, bases or neutral solutions, to demonstrate the concept of neutralization and to understand the chemical reaction that occurs when acids are neutralized by bases. SAFETY PRECAUTIONS Even though many of the materials used in this experiment are found in our homes, they are potentially hazardous. Many cleaners are acids or bases and can corrode our skin and clothing and damage our eyes. Dilute sodium hydroxide and dilute acids are corrosive materials and also can corrode our skin and clothing and damage our eyes. If you use solid drain cleaner, use a very small amount. When it dissolves, it gives off a lot of heat. PROCEDURE MATERIALS NEEDED vitamin C tablets or ascorbic acid phosphoric acid cleaner ph paper small containers for solutions eye dropper or pipette 5 ml graduated cylinder or other measuring device vinegar baking soda any dilute acid dilute sodium hydroxide some common household items: ammonia, apple juice, baking soda, coffee, cola drinks, detergent, lemon juice, orange juice, saliva, tomato juice, laundry detergent, drain cleaner, milk 1. If you have a vitamin C tablet crush it, place it in a small container and dissolve it in about 5 ml of water. Depending 1

2 upon the size of your tablet, you may need more water. If you have ascorbic acid, dissolve approximately 0.5 g in 10 ml of water. Label your containers. 2. Dissolve approximately 1 g of baking soda in 5 ml of water. Label your container. 3. If you have other solid materials, dissolve a small amount of those in water also. REMEMBER, if you have solid drain cleaner, use a very, verys small amount of the material. 4. Pour very small amounts of your other materials in separate small containers. You only need enough to wet the ph paper. Label each container. 5. Test each material with the ph paper, and record your results on the DATA SHEET. Neutralization Reactions 1. Put approximately 0.5 ml of vinegar in a container. 2. Use the dropper to add your baking soda solution prepared above. Add the baking soda solution by drops, and gently swirl the vinegar after every few drops. Stop added baking soda when you no longer see a reaction occurring. Measure the ph, and record the result on the DATA SHEET. 3. Put approximately 0.5 ml of your Vitamin C or ascorbic acid solution prepared above in a container. 4. Use the dropper to add baking soda solution. Add the baking soda solution by drops, and gently swirl the Vitamin C or ascorbic acid solution after every few drops. Stop added baking soda when you no longer see a reaction occurring. Measure the ph, and record the result on the DATA SHEET. 5. Repeat steps 3. and 4. using the phosphoric acid cleaner in place of the Vitamin C or ascorbic acid solution. 6. If you run out of the baking soda solution, dissolve another 1 g of baking soda in approximately 5 ml of water. 2

3 You just completed a chemical reaction. To be more specific, you just took the base (baking soda) and neutralized the acid (vinegar). They reacted - that s what acids and bases do - and the result was a salt (solution) and carbon dioxide bubbles (foam). The liquid remains, and gas was released to the air. Your first reaction can be written as: CH 3 COOH + NaHCO 3 CH 3COONa + CO 2 + H 2 O vinegar + baking soda produces sodium acetate + carbon dioxide + water In water, vinegar dissociates, that is it looses its hydrogen ions. CH 3 COOH CH 3 COO - (aqueous) + H + (aqueous) It s the H + ions that cause this material to be an acid. The more H + ions there are the lower the ph of the solution. We neutralize this acid by forming water out of the H + ions from the vinegar (acetic acid) and oxygen (O) and hydrogen (H) from the sodium bicarbonate. The products from this reaction are neutral; they no longer corrode. Let s look at another acid-neutralization reaction with acetic acid and sodium hydroxide. CH 3 COOH + NaOH CH 3 COONa + H 2 O acetic acid + sodium hydroxide produces sodium acetate + water In this reaction carbon dioxide is not produced, only the salt, (sodium acetate) and water. 3

4 DATA SHEET - ACIDS AND BASES MEASURING THE ph OF SOLUTIONS Solution Tested ph Acid, Base or Neutral? NEUTRALIZING SOLUTIONS Solution Neutralized Final ph Neutral? 4

5 QUESTIONS 1. Try completing these reactions following the examples. a. HCOOH + NaOH formic acid + sodium hydroxide produces sodium formate + water The name for formic acid derives from the Latin word for ant. It was first isolated from stinging red ants. b. HCl + NaOH hydrochloric acid + sodium hydroxide produces sodium chloride + water c. C 6 H 7 O 6 H + NaOH ascorbic acid + sodium hydroxide produces sodium ascorbate + water d. C 6 H 7 O 6 H + NaHCO 3 ascorbic acid + baking soda produces sodium ascorbate + carbon dioxide + water (This was your second neutralization reaction.) e. HNO 3 + NaOH nitric acid + sodium hydroxide produces sodium nitrate + water 5

6 2. True or False Acids have a ph greater than 10. It's the Na + ions that make a substance acidic. You can tell by looking at a liquid if it's an acid or base. Bases have a ph greater than 7. You should always be careful when using corrosive materials. The gas given off by the vinegar/baking soda reaction was water. A neutral solution has a ph of around 7. Acids and bases are corrosive materials. You must properly dispose of waste acids and bases. Vinegar is a strong acid. Acids and bases may react violently when put together. 6