# Help on Practical #1

See also: Practicals.htm and
chem225index.htm

For a chemical reaction A + B ®
products, the rate is related to the concentrations of
reactants by

Rate = k[A]^{m}[B]^{n}^{ }where m and n are normally
positive integers. Using the properties of logarithms, we have

ln(rate) = ln(k) + m ln[A] + n ln[B]

and if, for example the concentration of A, [A], is kept constant, so that ln(k)
and m ln[A] are constant,

ln(rate) = const + n ln([B])

which is the equation of a straight line where ln(rate) is the dependent
variable (y), ln([B]) is the independent variable (x) and n is the slope.
You must set out your results for rate and [A] in a spread sheet, calculate
ln(rate) and ln([A]) and then plot ln(rate) against ln([A]), as well as rate
against concentration. Put in the
line of best fit and display the relevant equation in each case. Indicate the value of
n, the order w.r.t. the reactant whose concentration was varied in your
experiment. You also need to find the rate constant.

Click on this link
to view student results for the same experiment. It shows the
numerical results in full, together with the two graphs required. In the
first graph, ln(rate) is plotted against ln(concentration) and in the second
one, rate is plotted against concentration, with the intercept set to zero (the
rate must be zero if the concentration is zero). Notice how
the *trendline *for the plots is displayed. The coefficient of *x* is the slope,
which, in the ln/ln plot, is equal to the order of the
reaction with respect to the reactant whose concentration is varied. In
the second plot, the slope is the rate constant for the reaction. (NB. If
you submit these results, you will be guilty of cheating!)

Note that all calculations are done by the spreadsheet program, you don't have
to do them manually. For example, entering the formula *=ln(A2) *in
cell B2 finds the natural logarithm of the value in cell A2, i.e.
-6.90776. You can then copy the formula into the other
cells in the column and it will be adjusted automatically to calculate the
appropriate value. Values can be entered as ordinary numbers or in
scientific notation. Excel does not recognise (nor show) 3.1×10^{-6},
instead it expects (and may also display) 3.1E-6.

*Excel *draws your graphs for you. To plot the rate against concentration, for example,
select the values for the concentration with the mouse, then press *Ctrl *and select the
values for the rate. You should wind up with two columns of values
selected. Then click on* insert *and then *chart *followed by *XY
(scatter)*. Then click on *next* and type in the chart title and
the labels for the axes (you can put units in at this stage when there are any).
(Note that after taking natural logarithms (ln) the result has no units.)
You can add subscripts and superscripts later. Click on *gridlines*
and check all the boxes for a proper graph look. (You can alter the
spacing between the gridlines later.) Click on *legend* and uncheck
the box *show legend*. Click *next* again and then *finish*.
When the graph shows up you can right click on each axis in turn and select*
format axis *to adjust the range of values covered by the graph, as well as
the major and minor units so as to alter the grid spacing, and various other
aspects of the appearance. Double click on a title and select a character
if you want to make it a subscript, for example. Right click on the
selected character and select *format chart title. *You can then
select subscript (or superscript).

Now select *chart *and then *add trend line*.
You can select various types of trend line, according to theory. Note that
if you are plotting* ln*(rate)* *against *ln*(concentration) you
should select *line. *Right click on the new trend line and select *
format trend line* then *options* and check the box for *display
equation on chart*. This latter is most useful for a straight line
where you want the slope or the intercept.

You can also adjust things such as the background colour,
and also the size of the chart by dragging the corners or sides. Make sure
you give the chart a title as well as each axis.

Put the names of all your group members (or just yourself
if you are submitting individually) and a title at the top of your spreadsheet,
too.