Rate is a measure of how fast or slow something is. In chemistry, we speak of a rate of reaction, this tells us how fast or slow a reaction is.
Why do chemists want to know the rate of a reaction?
If you are making a product, it is important to know how long the reaction takes to complete, before the product is produced.
Rate is a measure of a change that happens over a single unit time. That unit time is most often a second, a minute, or an hour.
Reaction between zinc and dilute hydrochloric acid
What we observe over time is that gradually the zinc disappears and bubbles of gas appear. After a few minutes the bubbles of gas form less and less quickly until finally no bubbles appear because all the acid has been used up, some zinc remains.
To summarise, during this reaction zinc chloride and hydrogen gas are been formed at the same time as zinc and hydrochloric acid react.
Using the reaction between zinc and hydrochloric acid as an example, the following are methods by which you could measure the rate of that reaction.
1. Measure that amount of zinc used up per minute
2. Measure the amount of hydrochloric acid used up per minute
3. Measure the amount of zinc chloride been formed per minute
4. Measure the amount of hydrogen been produced per minute
When choosing which method to measure rate always choose the most straightforward.
In the example above, by far the easiest would be to collect the bubbles of hydrogen and measure its volume.
Methods Used for Measuring Rate
Measuring volume of gas evolved:
To measure the hydrogen gas released in the above reaction we use the apparatus as shown. As the bubbles of gas are given off, the plunger in the syringe moves out as hydrogen gas fills it. After, say every 20 seconds we read the volume of gas in the syringe. The reaction is complete when the syringe no longer moves.
To find the actual rate we plot a graph of volume of hydrogen (cm3) against time (seconds).
Note:
1. The rate is not a constant throughout the reaction - it changes!
2. The reaction is fastest at the start, gradually becoming slower as the reaction proceeds.
3. From the graph, the fastest part of the reaction is shown by the steepest curve.
4. The curve on the graph goes flat when the reaction is complete. This is because, as time goes on the volume of the gas evolved does not change.
Measuring the Rate of Loss of a Gaseous Product:
In the reaction between calcium carbonate (marble chips) and hydrochloric acid we can use the apparatus below to find the rate of reaction.
Marble chips and acid are placed in the flask but separated by a piece of card - preventing the reaction from proceeding. This apparatus is placed on a balance and the mass of the flask and its contents is read.
To start the reaction, the flask is gently lent to one side, causing the card to fall and the marble chips and acid to mix.
A piece of cotton wool is placed in the neck of the flask to allow carbon dioxide gas to escape. As the gas escapes the mass of the flask reduces. Take readings of mass loss over a time interval, e.g. 30 seconds.
To find the actual rate we plot the loss in mass (grams) against time (seconds)
As with the previous experiment, the steepest part of the curve is at the start, hence the fastest part of the reaction is at the start.
Gradually the curve becomes less and less steep as the reaction slows down. Eventually a flat curve appears indicating the end of the reaction.
2. Changing the rate of a reaction
There are 4 methods by which you can increase the rate of a reaction:
1. Increase the concentration of a reactant.
2. Increase the temperature of the reactants.
3. Increase the surface area of a reactant.
4. Add a catalyst to the reaction.
Before, we discover the reasons for the above causing an increase in rate, we must first look at what is needed to cause a reaction to occur!
If we take the reaction between magnesium and hydrochloric acid, in order for them to react together:
1. They must collide with each other
2. The collision must be with sufficient energy.
The rate of a reaction depends on how many successful collisions there are in a given unit of time.
The Effect of Concentration
If the concentration of acid (a reactant) is increased, the reaction proceeds at a quicker rate.
In dilute acid there are less acid particles. This means there is less chance of an acid particle hitting a magnesium particle as compared with acid of a higher concentration.
In concentrated acid there are more acid particles, therefore there is a greater chance of an acid particle hitting a magnesium particle.
Remember: the more successful collisions there are the faster the reaction.
The graph below shows results from two experiments. Experiment A was with concentrated acid and experiment B used dilute acid.
As you can see, the greater the concentration of the acid used in a reaction the steeper the curve and the shorter the reaction time. Hence, these results show that an increase in concentration increases the rate of a reaction.
The Effect Of Temperature
At low temperatures the reacting particles have less energy. When particles are heated they gain energy. The gaining of energy enables the particles to move around quicker, this increases their chance of colliding but also, the increase in energy increases the possibility of a collision occurring with sufficient energy. Therefore rate of reaction increases with increasing temperature.
The Effect of Surface Area
The rate of reaction between magnesium and hydrochloric acid increases as you increase the surface area of the magnesium.
For example: powdered metal (greater surface area) reacts quicker with acid than strips of metal (lower surface area).
The greater the surface area of the metal the more of its particles are exposed to the acid. This increase in exposure increases the frequency of successful collisions.
The Effect of a Catalyst
Some reactions may be speeded up by using a catalyst. A catalyst reduces the energy required for the reactants to successfully collide. The result is more collisions become successful, hence the rate of a reaction increases.
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