FACTORS AFFECTING THE RATE OF CHEMICAL REACTION

From previous classes we know that: Reactants------------------> Products, or a number of reactants combine to form a product / products. We can also say that the particles in the products collide in order for a reactant to be formed.

Depending on the types of reactant the time taken for a reaction to occur can vary considerably.

For example, the chemical reactions that occur in the formation of crude oil occur over huge periods of time whereas the chemical reactions betwen Sodium and Acid occur very quickly. So how do we determine / measure the rate of a reaction? We can say:

Rate of Reaction = amount of reactant(s) used or product(s) formed / Time Taken

We can look at either how much reactant is used up in the chemical reaction or the amount of product formed. Below is a simplified graph to show this relationship:

It can be seen that with higher temperatures and higher concentration and smaller pieces the line of the graph showing the relationship between the total amount of products used and the time taken is steeper, thus indicating the reaction is happening more quickly.

Equally if this graph was showing the total amount of reactants being used against time then the steeper line would, again, indicate, a faster rate of reaction.


What this graph tells us then is that there are a number of factors that determine the rate of the reaction. These are listed below:

• The temperature at which the reaction takes place
• The concentration of reactants being used
• The pressure at which the reaction is occuring
• The size of the pieces of reactants being used in the reaction
• The addition of a catalyst

In simple terms:

•  the higher the temperature the faster the rate of reaction
• the greater the concentration of dissolved reactants the faster the rate of reaction
• The higher the pressure of a reactsnt gas the faster the rate of reaction
• the smaller the pieces of a solid reactant the faster the rate of reaction
• if a catalyst is added the faster the rate of reaction

Again, keeping it simple:

• with higher temperatures the greater the energy of the particles in the products and the more likely they are to react when they collide with other particles
• with greater concentrations of particles or pressure in gases the greater number of particles there are thereby increasing the chances of collisions at high enough energy levels to create a reaction
• with smaller pieces of solids both the surface area and the number of pieces are increased, thereby increasing the number of collisions which will speed up the chemical reaction
• if a catalyst is added the activation energy needed to result in a reaction when particles collide is lowered, thereby increasing the rate of the reaction. Catalysts are added to a reaction but are not used up

The slide show below gives a greater depth of detail if you are interested:

Reaction Rates

View more presentations from wsautter.

an excellent site with more on this as well as lots of other science topics

GRAPHING TUTORIAL

When having to draw graphs and interpret data the starting point is making sure the correct axes are used. The independent variable goes on the X axis (horizontal axis) and the dependent variable goes on the y axis (vertical axis).

From designing our Fair Tests we know that the Independent Variable is the variable that we, as Scientists, change, making sure that we only use the 1 independent variable. As we change the value of the independent variable we observe what happens to the Dependent Variable. If we have designed a good Fair Test then there should be a relationship between the two; this is what we are testing. Time is often an Independent variable that is used with observations being observed for any changes in the chosen dependent variable over time.

For example we could look at changes in rate of growth of microorganisms on an agar plate over time. With all other variables being controlled, such as temperature, nutrient source, sterilisation etc then time which is obviously measurable is the independent variable and changes in growth is the dependent variable. If we changed the fair test to study the effects of different concentrations of disinfectant  on microorganism growth then the concentrations of disinfectant we have chosen, and measured, would be the independent variable.

Returning to graphs there are a number of types of graphs we can draw depending upon the type of data being analysed. Some of the most common include:

• Line Graphs
• Bar Charts
• Pie Charts

Generally the most common type of graph we will be concerned with plotting is a line graph which are particularly good for plotting data that changes over time, with time often being the independent variable, plotted on the x axis.For further information on different types of graphs press here.

The science buddies website has more useful information:
this page gives a good overview and lists key points when drawing up graphs!