chem assignment 10

REACTION RATES – Hands-On Experiment

Purpose: To investigate the effects of temperature changes on the progression of a chemical reaction.

Introduction Chemical reactions occur to different extents depending on the conditions of the reaction and on the thermodynamics of the chemical reaction itself. Thermodynamics is the science of the relationship between heat and other forms of energy. In this experiment, we will be investigating how temperature change affects the rate of a chemical reaction. If you have ever used effervescent antacid tablets, like Alka-Seltzer, you know that when you drop them into a glass of water, as instructed, they fizz. The fizz, or effervescence, is due to the formation of gas bubbles. If you recall from the Properties and Changes lab, the evolution of a gas, or effervescence, when you mix substances together is evidence of a chemical reaction occurring. The chemical reaction that is occurring when you drop an effervescent antacid tablet into water is as follows:

3NaHCO3(s) + C6H8O7(s) –® Na3C6H5O7(aq) + 3CO2(g) + 3H2O(l)

(sodium bicarbonate + citric acid –® sodium acetate + carbon dioxide + water) When the sodium bicarbonate and citric acid are in the solid tablet form, they cannot react with one another because the particles are unable to move and collide with one another. Collision of particles is required for chemical reactions to take place. The water breaks the solid bonds of the tablet, allowing the particles to collide and react with one another. The bubbles we see (and/or hear) are the formation of carbon dioxide gas, which is the evidence that lets us know a chemical reaction is occurring. According to collision theory, as temperature increases, more particles in a reaction will acquire the minimum amount of kinetic energy that is required for the reaction to occur, which then leads to a larger number of effective collisions and a higher rate for the reaction. If the temperature of a reaction is changed, the rate will change accordingly. It is generally accepted that the rate of a reaction approximately doubles when the temperature is increased by about 10℃ (50°F). *Formula to change °F to °C: °F – 32 = (1.8) x °C Equipment/Materials Thermometer (30-200°F range), effervescent antacid tablets, timer, 3 clear cups/glasses, ½ cup (4 oz) measuring device, access to hot and cold water

Experiment 13: REACTION RATES

Procedure (Using a pen or pencil, record by hand all of your data and results on the Data Collection and Results Pages.)

Ø You will need to have access to water of different temperatures: hot water, room temperature water, and ice cold water. You will run three trials of the experiment at each temperature, for a total of 9 trials.

Ø To get hot water, use the hot setting from a water faucet. To get room temperature water, leave lukewarm tap water out on the counter for about an hour so that it can stabilize to the temperature of the room. Make sure you leave out enough for a few trials so that you do not have to wait another hour. To get cold water, leave cold tap water in your refrigerator for about an hour OR put cold tap water into a bowl of ice and let sit for a few minutes.

Ø If you do not have access to tap water, you can use bottled water and follow the above process for room temperature and cold. For hot, you can use a microwave to heat the water.

Ø Do not mix and match the water sources. Use the same water type for each trial; either use only tap or only bottled water.

1. Add ½ cup (4 ounces) of hot water to a clear glass or cup. Record the temperature in °F.

2. Drop an effervescent antacid tablet into the water and begin the timer. Record what

you see. How vigorously is it bubbling?

3. Stop the timer when the tablet has disappeared. Record the time it took to do so.

4. Repeat two more times with fresh samples of hot water. If you are re-using the same glass or cup, be sure to rinse it out between trials.

5. Convert °F into °C for each trial.

6. Repeat steps 1 - 5 for the other two water temperatures. Be sure that your times are recorded in seconds. (1 minute = 60 seconds)

7. Calculate the averages of the temperature readings and times.

8. Using the graph area provided, create a line graph of the average temperature (y-axis) vs. the average time (x-axis) in order to see the change in the rate of reaction based on the change in temperature. (Just connect the dots. We do not need a best fit straight line for this experiment.) Be sure to give your graph a title and label your axes.