Reaction Rate: Videos & Practice Problems

The reaction rate, symbolized as v, measures the speed of a chemical reaction, defined as the change in product concentration over time. It can be expressed as $\frac{∆[\mathrm{Product}]]}{∆t}$. Typically, the focus is on product concentration, as reactants decrease over time. The slope of a concentration vs. time graph indicates the reaction velocity, which decreases as equilibrium is reached, highlighting the role of enzymes in accelerating reactions without altering equilibrium.

Reaction Rate

The reaction rate, also known as reaction velocity and denoted by the lowercase letter v, refers to the speed at which a chemical reaction occurs, typically expressed as the change in product concentration over a specified time interval. Mathematically, the reaction rate can be represented as:

\( v = \frac{\Delta [\text{Products}]}{\Delta t} \)

Here, \(\Delta [\text{Products}]\) signifies the change in concentration of the products, while \(\Delta t\) represents the change in time. Although it is possible to calculate the reaction rate using the change in reactant concentration, it is essential to note that the concentration of reactants decreases as the reaction progresses, necessitating a negative sign in that context. However, biochemists primarily focus on the change in product concentration when discussing reaction velocity.

The units for reaction rate are typically expressed in terms of concentration per time, with standard units being molarity (M) for concentration and seconds (s) for time. Therefore, the reaction rate is often expressed in units of M/s.

When graphing the concentration of products against time, the slope of the line connecting any two points on the curve represents the reaction velocity. This relationship can be understood through the equation of a line, \(y = mx + b\), where \(m\) denotes the slope. In this context, the slope can be calculated as:

\( \text{slope} = \frac{\text{rise}}{\text{run}} = \frac{\Delta [\text{Products}]}{\Delta t} \)

As a reaction progresses, particularly in enzyme-catalyzed reactions, the reaction rate typically decreases over time. This is illustrated in a graph where the slope of the line diminishes as the reaction approaches equilibrium. Eventually, the slope may reach zero, indicating that equilibrium has been established. It is crucial to remember that while enzymes accelerate the rate of reaching equilibrium, they do not alter the equilibrium position itself.

Understanding these concepts is vital for analyzing reaction kinetics and the factors influencing the speed of chemical reactions, particularly in biochemical contexts.

Reaction Rate Example 1

To calculate the reaction rate for the conversion of reactant A into product B, we start by recalling that the reaction rate, often denoted as v, is defined as the change in concentration of the product over the change in time. The formula for this is:

v = \frac{\Delta [B]}{\Delta t}

In this scenario, we are given the initial concentration of A as 1 molar and the initial concentration of B as 0 molar. After 2 seconds, the final concentration of B is 0.02 molar. To find the change in concentration of B, we subtract the initial concentration from the final concentration:

\Delta [B] = [B]_{final} - [B]_{initial} = 0.02 \, \text{M} - 0 \, \text{M} = 0.02 \, \text{M}

Next, we calculate the change in time, which is given as 2 seconds. Now we can substitute these values into the reaction rate formula:

v = \frac{0.02 \, \text{M}}{2 \, \text{s}} = 0.01 \, \text{M/s}

Thus, the reaction rate is 0.01 molarity per second (M/s). This matches with option A from the provided answer choices, confirming it as the correct answer. Understanding how to calculate reaction rates is essential for analyzing chemical kinetics and will be further explored in practice problems.

Calculate the reaction rate for A → B, given that [A] _i = 6.3 M, [B] _i = 0 & [A] _f after 4.8 seconds = 1.14 M.

• A. 1.69 M/s
• B. 2.53 M/s
• C. 0.24 M/s
• D. 1.08 M/s

Answer: (D)

Here’s what students ask on this topic:

What is the definition of reaction rate in biochemistry?

The reaction rate, symbolized as v, measures the speed at which a chemical reaction proceeds. It is defined as the change in the concentration of products over a given time interval. Mathematically, it can be expressed as:

$\frac{∆\left[\mathrm{Product}\right]}{∆t}$

This means that the reaction rate is the ratio of the change in product concentration to the change in time. It is typically expressed in units of molarity per second (M/s).

How is reaction rate related to the concentration of reactants and products?

The reaction rate is directly related to the concentration of reactants and products. As the reaction proceeds, the concentration of reactants decreases while the concentration of products increases. The reaction rate can be determined by measuring the change in product concentration over time. For reactants, the rate is often expressed with a negative sign to indicate the decrease in concentration:

$\frac{∆\left[\mathrm{Reactant}\right]}{∆t}$

However, biochemists typically focus on the change in product concentration to determine the reaction rate.

What units are used to express reaction rate?

The reaction rate is expressed in units of concentration over time. The standard unit of concentration is molarity (M), and the standard unit of time is seconds (s). Therefore, the reaction rate is typically expressed in molarity per second (M/s).

How does the slope of a concentration vs. time graph relate to reaction rate?

The slope of a concentration vs. time graph represents the reaction rate. In such a graph, the concentration of the product is plotted on the y-axis, and time is plotted on the x-axis. The slope of the line between any two points on the curve is equal to the reaction rate. Mathematically, the slope (m) is given by:

$\frac{∆\left[\mathrm{Product}\right]}{∆t}$

As the reaction progresses, the slope decreases, indicating a decrease in reaction rate until equilibrium is reached.

Why does the reaction rate decrease over time in enzyme-catalyzed reactions?

In enzyme-catalyzed reactions, the reaction rate decreases over time because the concentration of reactants decreases as they are converted into products. Additionally, as the reaction approaches equilibrium, the rate of the forward reaction decreases. Enzymes speed up the reaction to reach equilibrium faster but do not change the equilibrium position. Therefore, the reaction rate decreases until it reaches zero when equilibrium is achieved.