Suggested languages for you:

Americas

Europe

|
|

# Reaction Rates

Lerne mit deinen Freunden und bleibe auf dem richtigen Kurs mit deinen persönlichen Lernstatistiken

Nie wieder prokastinieren mit unseren Lernerinnerungen.

In 1953, famed actress and superstar Marilyn Monroe sang, "Diamonds are a girl's best friend." The performance inspired dozens of singers and actresses for generations. In the song, Monroe sings that "these rocks don't lose their shape" while charming audiences.

However, unfortunately for her and every one of a diamond's best friends, one day, diamonds will be no more. All diamonds on this planet will eventually turn back into graphite and disappear from every wedding band on every hand. But don't worry, this won't happen for billions of years! Although this reaction is thermodynamically favored, it is limited by its kinetics, meaning it will take a very long time.

But how do we calculate reaction rates, and what affects them? Come along, and we'll learn about reaction rates in time for you to watch a diamond slowly turn into graphite.

• Today we will be looking at reaction rates
• First, we will be looking at the rate of reaction and what that means.
• Next, we will assess how the formula for reaction rate looks and which units you will be dealing with.
• Then, we will complete a calculation together to help you get some practice.
• And finally, we will see some things that affect reaction rate, like physical state, concentration, temperature, and catalysts.

## Rate of Reaction Definition

When you watch any process occur, you are simultaneously observing two fields of chemistry. The first is the field of Thermodynamics, which covers whether something is actually possible. The second field is Kinetics, which measures how quickly something will happen. The speed of a reaction is determined by a couple of factors, which constitute the rate of reaction.

Reaction rate is a measure of how quickly a chemical reaction will occur. Specifically, it measures a change in concentration versus a change in time.

The diamond example highlights perfectly how important measuring the rate of reaction is. Even if a reaction is spontaneous (thermodynamically allowed), if the rate is too slow, then it won't be observed happening. Conversely, if a reaction is quick, then it might be too fast to observe. Ionic reactions, for example, are so quick that they have to be observed with special equipment, like spectrometers. These devices use light to detect changes in a sample. Some reactions are faster than 1.0 x 10-12 s, which is 0.000000000001 seconds!

So, by now we've seen fast and slow reactions, but how is rate calculated? Well, fortunately, the rate calculations are simple.

## Rate of Reaction Formula

We said earlier that reaction rate was a measure of concentration vs. time. Well, let's visualize what the rate of reaction formula looks like with the following reaction:

$$H_2 (g) + I_2 (g) \rightarrow 2 HI (g)$$

In a forward reaction, the reactants are being consumed to form the products.

$$Rate ~ = - \frac { \Delta [H_2] } { \Delta t } = - \frac { [H_2]_{t_2} - [H_2]_{t_1} } { t_2 - t_1 }$$

The reaction rate measures the changes in concentrations at times, t1 and t2. The negative sign in the expression represents the negative change in reactant concentration because as the reaction proceeds, the reactants disappear. A negative sign in the reaction rate will make the rate of reaction a positive expression. Using the same logic, the products' concentration will be increasing as the reaction moves forward. As such, the rate expression for the products will be positive.

To ensure that the rate is the same for every species, each species must be divided by its stoichiometric coefficient.

$$+ \frac {1} {2} \frac { \Delta [HI] } { \Delta t }$$

In the formation of HI, two moles of HI are being produced for each mole of H2 and I2. In other words, HI is being produced at twice the rate as the decomposition of each reactant.

It should be noted that what we are typically calculating is the average rate of reaction. We can also calculate the instantaneous rate of reaction. However, this is done after constructing a graph of reaction rate over a given time interval. This calculation is done using the stoichiometric coefficient and the slope of the tangent line at a specific time.

To provide a general formula for the overall reaction, we will use this general reaction formula:

$$aA + bB \rightarrow cC + dD$$

In this formula a, b, c, and d, are all stoichiometric coefficients, and A, B, C, and D are the chemical species. These can be applied to a general reaction formula as follows:

$$\text {Rate} = - \frac {1} {a} \frac { \Delta [A] } { \Delta t } = - \frac {1} {b} \frac { \Delta [B] } { \Delta t } = + \frac {1} {c} \frac { \Delta [C] } { \Delta t } = + \frac {1} {d} \frac { \Delta [D] } { \Delta t }$$

This formula shows us that the concentration of one species can elucidate the concentration of any other species at that given time interval. This is all made possible through the stoichiometric coefficients. So, by knowing the balanced chemical equation, you can calculate the concentration of any other species.

## Rate of Reaction Units

If you are ever confused about the units which can be used for reaction rate, it may be helpful to look at the formula. In the formula, concentration is being divided by time. But, in reality, it's a change in concentration vs. a change in time. How does this affect the units in the rate? Well, it doesn't. Look at this example to get a better idea (note, the calculation below involves a dimensional analysis only).

\begin{align}\text {Rate} &= \frac { \Delta Conc. } { \Delta time } \\\text {Rate} &= \frac { C_{final} - C_{initial} } { t_{final} - t_{initial} } \\\text {Rate} &= \frac { mol~L^{-1}_{final} - mol~L^{-1}_{initial} } { s_{final} - s_{initial} } \\\text {Rate} &= \frac { mol~L^{-1} } { s } \\\text {Rate} &= mol ~ L^{-1} ~ s^{-1}\end{align}

The units for reaction rate are typically mol L-1 s-1, but you may see them written with some other units. The important thing to note is that reaction rate measures concentration vs. time. So, they will likely always be in this form.

## Rate of Reaction Calculation

Now that we're familiar with the general formula for reaction rate, we can do a rate of reaction calculation. Use the following balanced equation to determine the average rate of reaction.

$$H_2O_2 (aq) + 3 I^- (aq) + 2 H^+ (aq) \rightarrow I_3 ^- (aq) + 2 H_2O (l)$$

In the first 10.0 seconds, the concentration of I- decreased from 1.000 mol L-1 to 0.868 mol L-1. So, we know the concentration and the time interval for I-. Now we can input it into our rate equation to determine the rate of reaction.

\begin{align}\text {Rate} &= - \frac {1} {3} \frac { \Delta [ I^- ] } { \Delta t } \\&= - \frac {1} {3} \frac { ( 0.868 ~ mol ~ L^{-1} - 1.000 ~ mol ~ L^{-1} ) } { ( 10.0 ~ s - 0.00 ~ s ) } \\&= - \frac {1} {3} \frac { (- 0.132 ~ mol ~ L^{-1} ) } { ( 10.0 ~ s ) } \\\text {Rate} &= 4.40 \times 10 ^ {-3} ~ mol ~ L ^{-1} ~ s^{-1}\end{align}

Now that we have our rate, what if we want to figure out the concentration of some other participant in the reaction? Let's try calculating the rate of change in H+ during the first 10 seconds.

\begin{align}\text {Rate} &= - \frac {1} {2} \frac { \Delta [ H^+ ] } { \Delta t } \\-2( \text {Rate} ) &= \frac {1} {2} \frac { \Delta [ H^+ ] } { \Delta t } \\\frac {1} {2} \frac { \Delta [ H^+ ] } { \Delta t } &= -2( 4.40 \times 10 ^ {-3} ~ mol ~ L ^{-1} ~ s^{-1} ) \\\text {Rate} &= -8.80 \times 10 ^ {-3} ~ mol ~ L ^{-1} ~ s^{-1}\end{align}

You aren't given the initial concentration of H+, so all you can do is calculate the rate of change for the hydrogen cations. As an exercise, try calculating the rate of change for the other species. What trends do you notice when calculating the rate of change for the products?

## Factors Affecting Rate of Reaction

The rate of reaction is something which can vary a lot under different conditions. There are numerous factors affecting rate of reaction, but we will only discuss a few. These are some factors affecting rate that we will discuss:

### Effects of Physical State on Reaction Rate

The first factor is the physical state of the reactants.2 As you know, gas molecules diffuse quickly, while solids simply just vibrate. This means that if a molecule moves around a lot, it has a higher chance of coming into contact with something which it will react with. This can be observed when assessing homogeneous vs. heterogeneous reactions.

A homogeneous reaction has every reactant in the same state, while a heterogeneous one has reactants in different states; like a solid, and a liquid.

Heterogeneous reactions are often limited by the amount of surface area on the solid, which greatly impacts the rate.

The surface area to volume ratio of a solid will decrease with an increase in size of the chunk. This means that more molecules are in the core of the chunk than on the surface. The ones in the core are surrounded by the same molecules, and don't react with anything. The reactions that occur will happen at the surface of the solid piece. If you split the chunk in half, you expose countless molecules to the surface, increasing the surface area.

Fig 1. - With an increased surface area, the reactivity of the substance increases dramatically.

This will increase the amount of molecules capable of reacting, which will increase the reactivity. Thus, having more surface area will increase the rate of reaction. This is why, when dissolving something in water, it goes quicker if you break it up with a utensil.

### Effects of Concentration on Reaction Rate

Another factor which may affect the rate of reaction is the concentration of reactants. This may have already been obvious, since concentration is one of the variables in our rate expression. But, it is important to understand why concentration matters. If you were to add one grain of salt and one drop of water into a bucket, would the salt dissolve?

Well, it really depends on where you placed them in the bucket. Maybe if you moved it around enough, or placed each strategically, you could get them to interact. Now, what if you added 1 kg of salt and 1 L of water? Well, it's a lot more likely that they would interact with each other. This is why concentration matters when determining rate. If in a solution, there is more of one reactant, it means that there is a higher chance it will come into contact with something it's going to react with.

Fig 2. - With an increased number of molecules in solution, there is a higher chance that they will collide.

When working in the lab, determining initial concentrations is very important. This is because some reactions don't proceed if the concentrations are too low. It's not because they are thermodynamically unfavored. They just don't proceed because they are too slow to react. In reality, some will react, but it is so little that we don't really observe it.

### Effects of Temperature on Reaction Rate

Temperature is another factor which affects the rate of reaction. Typically, increasing the temperature will increase the speed at which a reaction occurs. This is due to the reactants having an increased energy. When they collide into each other, the increased energy means that it is easier to overcome the Activation Energy needed to react.

### Catalytic Effects on Reaction Rate

There is one other factor that we will briefly mention, which are catalysts. Catalysts will decrease the activation energy of a reaction, which makes it easier to overcome. Certain catalysts, like Enzymes, also bring molecules closer together, which allows them to react faster.

Catalysts are mandatory for some reactions to occur, which makes them vital for life on Earth. Without enzymes, biological processes would occur too slowly, and life would not be possible.

A lot of the artificial catalysts designed by scientists are based on the enzymes which are found in the body. Life on this planet has been evolving for billions of years. That means that there is an enzyme behind every single biological process in all life on Earth. This provides excellent blueprints which scientists can use to create catalysts in the lab. Scientists have created some groundbreaking catalysts which allows life for humans to continue. However, they still fall short of what nature can achieve. Although we try hard to match it, nature is constantly proving itself as smarter than us.

We've listed a few different factors affecting rate of reaction, but there are still many more which we didn't mention. Kinetics is a very diverse field, with numerous avenues to explore. Here, we've discussed a short overview on rates and how to calculate the rate of reaction for simple reaction equations. This quickly becomes more complicated when considering integrated rate expressions and the Rate Law. That is a topic we will save for another article, so be sure to check that out. Until then, try to observe kinetics and reaction rates in everyday life.

The next time you stir sugar into coffee, or salt into water, ask yourself: why does this dissolve faster? With your new-found knowledge on kinetics, you'll no doubt answer that quickly.

## Reaction Rates - Key takeaways

• The rate of reaction is a measure of the speed of reaction, which compares concentration vs. time.
• The rate equation for the general reaction $$aA + bB \rightarrow cC + dD$$ is as follows:$$\text {Rate} = - \frac {1} {a} \frac { \Delta [A] } { \Delta t } = - \frac {1} {b} \frac { \Delta [B] } { \Delta t } = + \frac {1} {c} \frac { \Delta [C] } { \Delta t } = + \frac {1} {d} \frac { \Delta [D] } { \Delta t }$$
• As the reaction proceeds, the reactants decrease, and the products increase. This is observed in the rate equation with reactants having a negative sign, and products having a positive sign.
• As a reaction occurs, the rate will decrease as the reactants begin to diminish. Therefore, the rate of reaction will be different depending on the given time interval.
• Several factors affect the rate of reaction, including the physical state of reactants, concentration, temperature, and whether a catalyst is present.

## References

1. Nivaldo Tro, Travis Fridgen, Lawton Shaw, Chemistry a Molecular Approach, 3rd ed., 2017
2. Theodore Brown, Eugene LeMay, Bruce Bursten, Catherine Murphy, Patrick Woodward, Matthew Stoltzfus, Chemistry the Central Science, 13th ed., 2014

The rate of a chemical reaction can be increased by the physical state of reactants, the concentration, and the temperature of reaction. It can also be influenced by whether a catalyst is involved.

Enzymes lower the activation energy of a reaction making it easier to overcome, and thus making the reaction rate faster. Enzymes are catalysts which work in living cells.

The rate of reaction is determined by comparing the change in concentration versus the change in time of a reactant (or product), multiplied by the inverse of their coefficient.
Rate = -1/a ( Δ[A] ) / ( Δt )

Generally, the rate of reaction is how quickly the reaction happens. More specifically, it is a measure of a change in concentration versus a change in time. The more the reactants change in a given time interval, the faster they are reacting.

Temperature can affect the speed of a reaction by influencing the amount of energy involved. If reactants are higher in energy (higher temperature), they may be able to overcome the reaction barrier quicker, which will speed up the rate of reaction.

## Reaction Rates Quiz - Teste dein Wissen

Question

What does the rate law model?

A reaction's speed based on the concentration of its reactants.

Show question

Question

What are the two kinds of rate law?

Differential and integrated rate law.

Show question

Question

What three reaction orders are dealt with in AP Chemistry?

Zeroth order, first order, and second order.

Show question

Question

What does k represent in the general rate law formula?

k represents the rate constant (which is covered in more detail on a separate lesson!)

Show question

Question

What is the difference between reactant and reaction order?

Reactant order describes the order that each reactant individually contributes. Reaction order describes the overall reaction order.

Show question

Question

How can the overall rate of reaction be found?

The overall rate of reaction can be found by summing the reactant orders.

Show question

Question

If a reactant rate is zeroth order, what do we do?

We ignore it, as [A] to the power of 0 only equals 1.

Show question

Question

Which integrated rate law order has a positive slope?

Second order

Show question

Question

Which integrated rate law order has a negative slope?

Zero and first order

Show question

Question

What are the units for the zero order IRL straight line test?

[A] vs t

Show question

Question

What are the units for the first order IRL straight line test?

ln[A] vs t

Show question

Question

What are the units for the second order IRL straight line test?

1/[A] vs t

Show question

Question

For the IRL straight line test, what variable does the slope represent?

The rate constant, k.

Show question

Question

What is the purpose of the straight line test for the integrated rate law? (IRL)

It can be used to determine reaction order through experimental data.

Show question

Question

What is the rate of reaction?

A measure of concentration versus time for a given reaction.

Show question

Question

In the rate equation for a reacting species, why does the expression have a positive sign?

It should have a negative sign.

Show question

Question

In a typical reaction, as the reactants are consumed, what will happen to the rate?

It will decrease.

Show question

Question

Which factors have an affect on the rate of reaction?

Temperature.

Show question

Question

What is an example of a favorable reaction with a small rate?

Milk spoiling in the fridge.

Show question

Question

How do scientists determine reaction rates for fast reactions?

With machines called spectrometers.

Show question

Question

What is a general formula for the rate of a reactant?

$$Rate ~ = - \frac {1} {a} \frac { \Delta [A] } { \Delta t }$$ .

Show question

Question

What are the correct units for reaction rate?

mol L-1 s-1.

Show question

Question

In the first 10.0 seconds of a reaction, the concentration of I- decreased from 1.000 mol L-1 to 0.868 mol L-1. What is the reaction rate for the given interval?

$$H_2O_2 (aq) + 3 I^- (aq) + 2 H^+ (aq) \rightarrow I_3 ^- (aq) + 2 H_2O (l)$$

4.40 x 10-3 mol L-1 s-1.

Show question

Question

For a given reaction, you are given a rate of 4.40 x 10-3 mol L-1 s-1. After the first 20 seconds, the final concentration of H+ is 0.646 mol L-1. What was the initial concentration?

$$H_2O_2 (aq) + 3 I^- (aq) + 2 H^+ (aq) \rightarrow I_3 ^- (aq) + 2 H_2O (l)$$

0.822 mol L-1 s-1.

Show question

Question

How does temperature affect the rate of reaction?

It gives them more energy to overcome reaction barriers.

Show question

Question

How does an enzyme affect a reaction?

It catalyzes the reaction by lowering the activation barrier.

Show question

Question

Increasing the concentration will increase the rate.

True?

Show question

Question

By ______ the size of a chunk of sugar, you _______ the rate at which it will dissolve in water.

Decreasing, increase.

Show question

Question

By keeping one reactant in the gas phase and one in the solid phase, it will increase the rate of reaction, as opposed to keeping both in the gas phase.

False?

Show question

Question

The ______  is referred to as the speed at which a specific reaction proceeds from left to right (reactants to products) in a given amount of time.

Reaction rate

Show question

Question

What happens to a reaction rate if temperature is increased?

The reaction rate increases

Show question

Question

Reaction rate is ______ to both concentration and pressure.

directly proportional

Show question

Question

True or false: The equation for a reaction rate is expressed as a change in product concentration over a time interval.

True

Show question

Question

Reactants whose exponents (reaction orders) equal _____ will not have an effect on reaction rates when their concentration is changed.

0

Show question

Question

When the reaction order is _____ , doubling the concentration of the reactant will double the reaction rate

1

Show question

Question

If the reaction order is ____ , if the concentration of that reactant gets doubled, the rate of reaction will be quadrupled.

2

Show question

Question

True or false: rate laws and rate constants are determined experimentally.

True

Show question

Question

The ______ is used by chemists to compare the speed of different reactions, as it gives the relation between the rate of reaction and the reactant concentration in the reaction.

Rate constant (k)

Show question

Question

What is the unit of rate constant in first-order reactions?

s-1

Show question

Question

What is the unit of rate constant in second order reactions?

M-1 s-1

Show question

Question

What is the equation used to calculate rate constant (k) is second-order reactions?

Since the rate law expression of a second order reaction is rate = k[A][B], we can re-arrange the equation to solve for k.

Show question

Question

True or false: In zero-order reactions, k = r

True

Show question

60%

of the users don't pass the Reaction Rates quiz! Will you pass the quiz?

Start Quiz

## Study Plan

Be perfectly prepared on time with an individual plan.

## Quizzes

Test your knowledge with gamified quizzes.

## Flashcards

Create and find flashcards in record time.

## Notes

Create beautiful notes faster than ever before.

## Study Sets

Have all your study materials in one place.

## Documents

Upload unlimited documents and save them online.

## Study Analytics

Identify your study strength and weaknesses.

## Weekly Goals

Set individual study goals and earn points reaching them.

## Smart Reminders

Stop procrastinating with our study reminders.

## Rewards

Earn points, unlock badges and level up while studying.

## Magic Marker

Create flashcards in notes completely automatically.

## Smart Formatting

Create the most beautiful study materials using our templates.