Why would a mixture of gases react faster when the volume they occupy is decreased?

Reaction Rates (Chemical Kinetics) and Collision Theory Tutorial

Key Concepts

• The rate of a chemical reaction is the speed with which reactants are converted to products.
• Collision Theory is used to explain why chemical reactions occur at different rates.
• Collision Theory states that in order for a reaction to proceed, the reactant particles must collide.
  The more successful collisions there are per unit of time, the faster the reaction will be.
• In order for a reaction to proceed, the reactant particles must:

  (a) collide with sufficient energy to break any bonds in the reactant particles.
  The activation energy is the minimum amount of energy the colliding reactant particles must have in order for products to form.

  (b) be in an orientation favourable for breaking those bonds.

• Factors which can affect the rate of a reaction include:

  ⚛ concentration

  ⚛ temperature

  ⚛ particle size

  ⚛ catalysts

  ⚛ stirring rate

  ⚛ light intensity

Please do not block ads on this website.
No ads = no money for us = no free stuff for you!

Factors that Affect Reaction Rates

• Concentration of Reactants

  Increasing the concentration of reactants in solution increases the number of reactant particles which increases the number of collisions so the reaction rate increases.

  Increasing the partial pressure of a gaseous reactant by adding more reactant gas particles increases the concentration of this gaseous reactant and therefore increases the number of collisions so the reaction rate increases.

  Increasing the pressure of a gaseous reaction by reducing the volume of the reaction vessel increases the concentration of reactants and products and increases the number of collisions.

• Temperature (see also Kinetic Energy (Maxwell-Boltzman) Distribution)

  Increasing the temperature of a reaction increases the kinetic energy of the particles which increases the number of collisions so the reaction rate increases.

  Increasing the kinetic energy of reactant particles also means more of the reactant particles will have the minimum amount of energy required to form products (ie, activation energy) which leads to more successful collisions and therefore increases the reaction rate.

  Increasing the temperature will increase the reaction rates of both endothermic and exothermic reactions, it will also, by Le Chetalier's Principle, affect the equilibrium position.

• Particle Size

  Smaller reactant particles provide a greater surface area1 which increases the chances for particle collisions so the reaction rate increases.

• Presence of a Catalyst

  A catalyst lowers the activation energy for the reaction so more reactant particles will have the minimum amount of energy required to form products so the reaction rate increases.

• Rate of Stirring

  Stirring keeps reactant particles in motion increasing the chances of collision and increasing the rate of reaction.

• Intensity of Light affects some reactions

  Some reactions occur very slowly in the dark but much more quickly in light.
  eg, methane reacts very slowly with chlorine in the dark, but the rate of reaction is much faster in the presence of ultraviolet light.
  (see Halogenation of Hydrocarbons)

Measuring Reaction Rates

How a reaction rate is measured depends on the nature of the reactants and products.

Some measurable quanitities are:

• the volume of gas evolved per unit time
• the mass of solid formed per unit time
• the intensity of colour per unit time
• the change in pH per unit time
• the change in temperature per unit time

Imagine an experiment in which we can measure the amount of product being formed in a closed vessel.2
At the start of the experiment, only reactants are present, there are no products yet, so the amount of product is zero.
But after that, measurable amounts of product are produced, and we record the amount of product produced every minute (for example).
Then, imagine we repeat the experiment, keeping all the variables the same, EXCEPT that we heat the whole system.

When we plot the points of amount of product produced vs time, for the same reaction at two different temperatures, we get a graph like the one shown below:

____ high temperature ____ low temperature

The initial slope of the graph provides you with the initial rate of reaction.
A fast reaction will have a steeper slope than a slower reaction.
On the graph above, the red line represents a faster initial reaction rate than the blue line because the slope of the red line is greater than the slope of the blue line in the early (initial) stages of the reaction.

Reaction rates slow down as the reaction approaches equilibrium.
As products are formed, there are fewer reactant particles to react which means there will be fewer successful collisions, so, the reaction rate decreases.

At equilibrium, the concentration of a reactant, or the concentration of a product, does not change with time.
At equilibrium each line in the graph above will be straight horizontal line.

Worked Example: Zinc and Hydrochloric Acid Reaction Rate

Consider an experiment in which we add hydrochloric acid, HCl(aq), to a clean piece of metallic zinc, Zn(s), in an open beaker.
The products of this reaction will be water soluble zinc chloride, ZnCl2(aq), and hydrogen gas, H2(g).
Because this is an "open system" in which the hydrogen gas can escape, we will use a single-headed arrow, →, to show the reaction goes to completion in one direction as shown in the balanced chemical reaction below:

Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)

In order to determine the rate of the reaction we could:

• Measure the rate at which a reactant disappears:

  For example, we could remove the piece of zinc from the beaker every minute and weigh it separately, recording its mass before placing it back into the beaker and starting the timer again.
  Over time, the mass of zinc would decrease.

• Measure the rate at which a product appears:

  For example, we could use the "water displacement" method to collect the hydrogen gas and measure its volume every minute.
  Over time, the volume of gas would increase.

We could then investigate the factors that influence the rate of reaction by changing one variable at a time and repeating the experiment.
For example, we could study the effect of changing the concentration of the acid on the reaction by rate by repeating the experiment several times using the same volume of acid BUT each time using a different concentration of acid, while keeping all other variables constant, that is,

• same volume of hydrochloric acid in all experiments
• same temperature for all experiments
• same atmospheric pressure for all experiments
• same clean, dry beaker for all experiments
• same mass of zinc for all experiments
• same dimensions (width, height and breadth) for the piece of zinc

(see Experimental Design Tutorial for more details of how to design your own experiments.)

If we were to perform a set of experiments to test each of the variables that we think would effect the rate at which zinc metal reacts with hydrochloric acid, we would find results such as those given in the table below:

1. "Surface area to volume ratios" crop up all the time in chemistry. As the size of a particle (as measured by its radius or diameter) decreases, the amount of substance at the surface of the particle compared to the amount of substance occupying the body (or volume) of the particle increases dramatically.
See the Nanoparticles and Nanotechnology Tutorial for an indepth discussion of how this happens.

2. In a closed system our reaction can reach equilibrium, and at this point the concentrations of reactants and products that we measure will not change with time.
However, in an open system it might be possible for some of our reactants and/or products to escape from the system.
In this case the system we are studying will not reach equilibrium.

3. For the purposes of this discussion it doesn't matter whether you think of the "acid particles" as HCl(aq) particles or as H+(aq) or H3O+(aq) particles.
The point is that increasing the concentration of the acid increases the number of "acid particles" irrespective of their identity.