# Question Video: Identifying Which Reaction Will Have the Slowest Rate of Reaction Given the Temperature, Concentration, and Surface Area of Reactants Chemistry • 7th Grade

In which of the following diagrams will the rate of reaction be slowest? [A] Diagram A [B] Diagram B [C] Diagram C [D] Diagram D [E] Diagram E

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### Video Transcript

In which of the following diagrams will the rate of reaction be slowest?

The reaction rate quantifies how quickly reactants turn into products. We can use a model called collision theory to explain how reactions occur and at what rate. Collision theory states that a reaction occurs when particles collide with appropriate orientation and enough energy. Many factors affect how often reactant particles collide and, therefore, how quickly they turn into products. The temperature affects the reaction rate because it determines the kinetic energy of the reactant particles.

Particles move faster at a higher temperature and consequently collide and turn into products more often. Lower-temperature systems generally have a lower reaction rate because their particles have less kinetic energy and thus collide less frequently. The temperature of the water is a factor in the reactions of our answer choices. The reactions are occurring at either 60 degrees Celsius or 30 degrees Celsius. Since the question is asking us to determine which will have the slowest rate of reaction, we know we must select a beaker with water at a temperature of 30 degrees Celsius, not 60 degrees Celsius. So, we can eliminate answer choices (A), (C), and (D).

Let’s look now at another factor that determines the reaction rate: concentration. The concentration of a substance can be summarized as the amount of particles per volume of space. There is relatively little space between highly concentrated reactants, which causes them to collide more frequently. At a low concentration, reactant particles collide less frequently and turn into products at a slower pace. The reactant hydrochloric acid is either at a lower concentration with a molarity of 0.5 or at a higher concentration with a molarity of 2.0.

It is likely that answer choice (B) would occur at a slower reaction rate because of its lower concentration. But let’s have a look at the final factor featured in this problem. The surface area of reactant substances is a determinant of the reaction rate as well. Surface area can be equated to the size of solid reactants in a reaction. If a solid reactant is in large chunks, then only the outermost particles can initially be involved in the reaction. If the large chunks are ground into a powder, then the surface area of the reactant increases and more particles are available to react. So, for a slower rate of reaction, solid reactants generally have a lower surface area.

The solid reactant magnesium carbonate in the answer choices is either powdered or in lumps. A larger size of lumps would likely result in a slower reaction rate, as we have in answer choice (B). Therefore, the answer choice that would have the slowest rate of reaction due to lower temperature, lower concentration, and lower surface area would likely be answer choice (B).