The rate of a reaction can be determined by measuring changes in the physical properties of the reaction mixture. Which of the following parameters is not typically monitored to obtain a measure of reaction rate? A) Temperature. B) Mass. C) Volume. D) Color. Or E) cloudiness.
In ideal circumstances, reaction rates are measured as changes in concentration over time. However, it isn’t always practical or possible to measure concentrations directly. So some other factor needs to be measured and interpreted. Some of these measures are better than others. I’m gonna give some examples of reactions where each of these physical properties can be used to infer the reaction rate.
The reaction of sodium hydroxide solution and hydrochloric acid is exothermic. The temperature rise after mixing these two solutions together could be used as an indicator of the rate of the reaction. Meanwhile, the rate of a reaction like the oxidation of magnesium could be monitored by mass using a balance. As a solid metal burns in air, the heavier metal oxide forms causing an increase in the measured mass.
Volume can be used to monitor a reaction rate, for instance, the reaction of calcium carbonate with hydrochloric acid. The volume of carbon dioxide gas produced can be measured using a gas syringe. Sodium hydroxide and phenolphthalein react together slowly. The pink color of the phenolphthalein gradually fades. This can be monitored using a colorimeter.
And lastly, we come to cloudiness, otherwise known as turbidity. This can be useful in the reaction of sodium thiosulfate and hydrochloric acid. This reaction produces a fine suspension of sulphur which makes the solution cloudy. This can be used to monitor the reaction by having some kind of object like a black cross that is gradually obscured by the suspension.
So it seems like all these parameters could be useful for measuring reaction rate. However, there is a good reason that one of them isn’t typically used. Changes in mass, volume, color, and cloudiness are generally very repeatable. The conditions for measurement can easily be controlled. And it normally is recorded.
Measurements of temperature are more vulnerable to error. Direct temperature measurements are vulnerable to local cooling effects. A thermometer in the centre of a warm beaker is going to be warmer than a thermometer at the edge because the edge is being cooled by the surrounding air. That’s not to say that temperature is never used to measure reaction rates. It’s just that, in most cases, there’s a better way of doing it. So, of the five parameters given, the one not typically monitored to obtain a measure of reaction rate is temperature.