- Study of rates of reactions
- Study of mechanisms of reaction
- Factors affecting rate of reaction
Reaction rate is a measure of how fast a reaction occurs, or how something changes during a given time period. The speed of a chemical reaction or its reaction the is expressed as the change in concentration of a reactant or product per unit time. It is usually expressed in molarity per second (M/s).
We often define the rate of a chemical reaction as:
Δ[concentration]
Δ[time]
A graph illustrating this equation is shown below. There is an explanation included in the graph.
Therefore, if we wanted to express the rate of the following reaction:
SO2(g) + NO2(g) → SO3(g) + NO(g)
We can write it as:
rate = -Δ[SO2] = -Δ[NO2] = Δ[SO3] = Δ[NO]
Δt Δt Δt Δt
Rate = k[SO2]x[NO2]y
A Rate Law is an equation that tells us how fast the reaction proceeds and how the reaction rate depends on the concentrations of the chemical species involved.
1.2 Collision Theory
- The collision theory states that for a chemical reaction to occur the reacting particles must collide with one another.
- The rate of the reaction depends on the frequency of collisions
- The theory also tells us that reacting particles often collide without reacting. This is called ineffective collision.
- In order for collisions to be successful or be an effective collision, reacting particles must collide
- with sufficient energy, and
- with the proper orientation
Activation Energy Video
1.3 Factors Affecting Rates of Reactions
1. Nature of Reactants
A chemical reaction involves the breaking of old bonds and the formation of new bonds. The rate of a reaction depends on the particular reactants and the number of bonds that have to be broken and formed.
If the two reactants have the same bond or state, then they will react easily to each other. If the two reactants have different bond or state, then they will not react easily to each other.
2. Concentration
The collision theory explains the effect of change in concentration of reactants on the rates. An increase in concentration means an increase in the number of molecules or particles per volume, and thus a decrease in spaces between the reacting particles. With less distance to travel inside the vessel, the more frequent the collision, the faster the rate of reaction.
3. Temperature
The average kinetic and hence, the number of collisions increase with absolute temperature. Hence, the rate of reaction increases with increase in temperature.
The rate of reaction is usually doubled when the temperature is increased by 10oC.
4. Surface Area
The surface area of a solid reactant has an important effect on the rate of reaction. The smaller the size of particles, the larger the surface area exposed. A larger surface area increases the frequency of collision, hence the rate of reaction increases.
5. Effect of Catalysts
Another way to speed up a reaction is to use catalyst. A catalyst provides an alternate pathway of lower activation energy. It increases the rate of reaction, but is not used up in the reaction. Catalysts in the body, such as those involved in breaking down sugar or protein, are called enzymes. Enzymes increase the speed of reactions essential to life that would otherwise be very slow.
Sample Problems
1. Given the below initial rate data, determine the rate law and rate constant for the following reaction:
Solution:
Using the method of initial rates, we take the ratio of rates between reactions 2 and 1 to determine the order of the reaction in permanganate.
By taking the ratios of the rates of experiments 3 and 1 we can obtain the order of the reaction in chlorite.
Finally, by taking the ratios of the rates of experiments 4 and 1 we can obtain the order of the reaction in H+.
Now that we know the order of the reaction in permanganate is 2, chlorite is 1, and H+ is 1/2, we can use the rate and concentration data in experiment 1 to calculate the rate constant.
2. Consider the following reaction:
Solution:
From the balanced equation we see that there are 2 moles NH3 produced for every 3 moles H2 used. Thus:
3. In the following decomposition reaction,
oxygen gas is produced at the average rate of 9.1 × 10-4 mol · L-1 · s-1. Over the same period, what is the average rate of the following:
4. Given the following experimental data, find the rate law and the rate constant for the reaction:
N2(g) + 3 H2(g) → 2 NH3(g)
If the rate of loss of hydrogen gas is 0.03 mol · L-1· s-1, what is the rate of production of ammonia?Solution:
From the balanced equation we see that there are 2 moles NH3 produced for every 3 moles H2 used. Thus:
| rate NH3 production | = | 2 3 | × (0.03 mol · L-1· s-1) |
| = 0.02 mol · L-1· s-1 | |||
3. In the following decomposition reaction,
2 N2O5 → 4 NO2 + O2
- the production of NO2
- the loss of N2O5
- From the equation we see that for every 1 mole of oxygen formed, four moles of nitrogen dioxide are produced. Thus, the rate of production of nitrogen dioxide is four times that of oxygen:
| rate NO2 production | = 4 × (9.1 × 10-4 mol · L-1· s-1) |
| = 3.6 × 10-3 mol · L-1· s-1 |
- N2O5 is consumed at twice the rate that oxygen is produced:
| rate loss of N2O5 | = 2× (9.1 × 10-4 mol · L-1· s-1) |
| = 1.8 × 10-3 mol · L-1· s-1 |
NO (g) + NO2 (g) + O2 (g) ž N2O5 (g)
Run [NO]o , M [NO2[o , M [O2]o , M Initial Rate, Ms-1
1 0.10 M 0.10 M 0.10 M 2.1 x 10-2
2 0.20 M 0.10 M 0.10 M 4.2 x 10-2
3 0.20 M 0.30 M 0.20 M 1.26 x 10-1
4 0.10 M 0.10 M 0.20 M 2.1 x 10-2
Solution:
From the equation Rate = k[NO] [NO2], we can derive the equation for k:
k = Rate
[NO][NO2]
Substituting,
k = 2.1 x 10-2 M/s
[0.10 M] [0.10 M]
k = 2.1 1/M·s
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