Chapter 18 - Rates of reactions

How can you monitor rate with a colorimeter?

1. Prepare standard solutions of known concentrations of the coloured chemical 2. Select a filter with a complementary colour of the coloured chemical. 3. Zero the colorimeter with water between each reading. 4. Measure the absorbance readings of the standard solutions. 5. Plot a calibration curve of the absorbance against concentration of the standard solutions. This will allow you to convert absorbance into concentration of the chemical. 6. Carry out the desired reaction. Take absorbance readings of the reacting mixture at regular time intervals. 7. Use the calibration curve to measure the concentration of the coloured chemical against time. 8. Finally, plot a second graph of concentration of the chemical against time. Using the concentration-time graph, you can calculate order of the reaction with respect to the reactant.

How to calculate the units of rate constant k?

1. Rearrange the equation to make k the subject 2. Substitute units into the expression for k 3. Cancel common units and show the final units on a single line

What does a Rate-concentration graph look like for a zero order reactant?

A zero order gradient produces a horizontal straight line with a zero gradient. The intercept on the y-axis gives the k constant if there are no other reactants.

How accurate are clock reactions?

Clock reactions measure the average rate during the first part of the reaction. The shorter the time period over which the average rate is measured, the less the rate changes over that period. This means the average rate calculated will be more similar to the initial rate, as the initial rate wont have changed as much. Generally accurate when less than 15% of the reaction has taken place.

What is half life?

Half life is the time taken for half of a reactant to be used up. First order reactions have a constant half life with the concentration halving every half life. This pattern is called exponential decay.

What does a concentration-time graph look like for a zero order reactant?

Linear plot with a negative gradient. The reaction rate does not change at all during the course of the reaction. The value of the gradient is equal to the rate constant k.

How can you continuously monitor rate?

Monitor by gas collection Monitor by mass lost Monitor with a colorimeter

What is the Arrhenius equation?

The Arrhenius equation is an exponential relationship between temperature and the rate constant. It is made of two parts, the pre-exponential factor (the frequency factor; A) and the exponential factor (e) The exponential factor represents the proportion of molecules that exceed the activation enthalpy. The pre-exponential term takes into account the number of collisions with the correct orientation. This term increases slightly with temperature as the frequency of collisions increases, but is essentially constant over a small temperature range.

What can be deduced about a concentration-time graph with a gradual exponential curve and constant half life?

The reaction has an order of one with respect to the reactant.

What can be deduced about a rate-concentration graph with a positive straight line through the origin?

The reaction has an order of one with respect to the reactant.

What can be deduced about a concentration-time graph with a steep exponential curve?

The reaction has an order of two with respect to the reactant.

What can be deduced about a rate-concentration graph with an upwards exponential curve?

The reaction has an order of two with respect to the reactant.

What can be deduced about a rate-concentration squared graph with a positive straight line through the origin?

The reaction has an order of two with respect to the reactant.

What can be deduced about a concentration-time graph with a linear plot and a negative gradient?

The reaction has an order of zero with respect to the reactant.

What can be deduced about a rate-concentration graph with a straight horizontal line?

The reaction has an order of zero with respect to the reactant.

What is initial rate?

This is the rate when time = 0 and is the instantaneous rate at the start of a reaction. It can be found by measuring the gradient of a tangent drawn at t = 0 on a concentration-time graph. It can also be done using a clock reaction.

How can you calculate the rate constant from half life?

Use the formula k = ln 2 / half life

What is zero order?

When the concentration of a reactant has no effect on the rate, the reaction is zero order with respect to the reactant.

What is the unit for rate?

moldm-3s-1

What is a clock reaction?

A clock reaction is a more convenient way of obtaining the initial rate of reaction by taking a single measurement. The time from the start of an experiment is measured for a visual change to be observed, often a colour of a precipitate. Provided there is no significant change in rate during this time, it is assumed that the average rate of reaction over this time will be the same as the initial rate. The initial rate is then proportional to 1/time. The clock reaction is repeated several times with different concentrations and values of 1/time is calculated for each run.

What is an iodine clock reaction?

A common type of clock reaction relies on the formation of iodine. As aqueous iodine is coloured orange-brown, the time from the start of the reaction and the appearance of the iodine colour can be measured. Starch is usually added because it forms a complex with iodine, which is an intense dark blue-black colour. The colour change can be delayed by adding a small amount of another chemical (e.g. sodium thiosulfate) which removes the iodine as it forms. Once this chemical is all used up, the blue-black colour appears.

What does a Rate-concentration graph look like for a first order reactant?

A first order reactant produces a straight line graph through the origin. The rate constant can be determined by measuring the gradient of the straight line of the graph, if there are no other reactants.

What does a concentration-time graph look like for a first order reactant?

A first order reaction produces a downwards curve with a decreasing gradient over time (exponential). In a first order concentration-time graph, the time taken for the concentration of the reactant to halve is constant. This time is called the half like and the rate constant can be determined using the equation halflife = ln 2 / k

What is first order?

A reaction is first order with respect to a reactant when the rate depends on its concentration raised to the power of one. There is a direct correlation between rate and concentration of the reactant. If the concentration increases by a factor, the rate increases by the same factor also.

What is second order?

A reaction is second order with respect to a reactant when the rate depends on its concentration raised to the power of two. If the concentration increases by a factor, the rate increases by the factor squared.

What does a Rate-concentration graph look like for a second order reactant?

A second order reactant produces an upwards exponential curve with increasing gradient. By plotting a second graph of the rate against concentration squared, the result is a straight line through the origin. If there are no other reactants, the gradient is equal to the rate constant.

How does temperature increase affect the rate constant?

As temperature increases, the rate increases and the value of the rate constant k will also increase. This is because of increasing temperature shifting the Boltzmann distribution to the right, increasing the proportion of particles that exceed the activation enthalpy. It is also because as temperature increases, particles move faster and collide more frequency. However, it is mainly determined by activation energy.

For consistency, chemists generally measure rate using what formula?

Change in concentration / time

In an experiment, how do you measure rate?

Quantity reacted or produced / time

How can you calculate the rate constant from the rate?

Rearrange the formula Rate = k [A]' [B]'' to equal k = Rate / [A]' [B]'' Then input the data.

How can you express the Arrhenius equation as a graph?

The Arrhenius equation can be expressed as a logarithmic relationship, which can be plotted as a straight line of ln k against 1/T. It is useful as it allows the activation energy and A to be determined graphically. The gradient is -Ea/R and the intercept is ln A.

What does a concentration-time graph look like for a second order reactant?

The graph for a second order is also a downwards exponential curve, steeper at the start but tailing off more slowly.

What is the overall order?

The overall order of reaction gives the overall effect of the concentrations of all the reactants on the rate of reaction. It is calculated as follows: Overall order = sum of all orders with respect to each reactant.

What is the rate constant?

The rate constant k is the proportionality constant in the rate equation (rate = k [A]' [B]''). It is the number that mathematically converts between the rate of reaction and concentration in respect to order.