rate of reaction examples Archives

How do you calculate the reaction rate?

Methods to measure the rate of reaction

The rate of reaction can be measured in two ways:
(a) Average rate of reaction
(b) Rate of reaction at a given time
The average rate of reaction is the average value of the rate of reaction within a specified period of time.
Example: 0.2 g of a magnesium ribbon reacts completely with dilute hydrochloric acid in 40 seconds. What is the rate of reaction?
Solution:
(a) The rate of reaction determined in above Example is known as the average rate of reaction.
(b) This is because it gives the average value of the rate of reaction within the 40 seconds.
The average rate of reaction does not show the actual rate of reaction at a particular instant. Only the rate of reaction at a given time can be used to reflect the actual rate at that instant.
Definition:
The rate of reaction at a given time is the actual rate of reaction at that instant.
The rate of reaction at a given time is also known as the instantaneous rate of reaction.

People also ask

Calculating the rate of reaction for a reaction that liberates a gas:

For a reaction involving the liberation of a gas, the rate of reaction can be determined through the following ways.

An experiment is carried out to measure the volume of gas collected at regular time intervals.
A graph of the volume of gas collected against time is plotted.
The graph plotted has the shape as shown in Figure.
The graph can be used to determine the average rate of reaction and rate of reaction at a given time.

Determining the average rate of reaction from the graph:
How do you calculate the reaction rate 3
From Figure,
the average rate of reaction in the first t₁, second

Determining the rate of reaction at a given time from the graph:
How do you calculate the reaction rate 5
From Figure,
the gradient of the tangent at any point on the curve

This is the rate of reaction at that given time.
Hence,
Rate of reaction at a given time = gradient of the curve at that instant
The rate of reaction at a given time, t, can be calculated through the following steps.
Step 1: Draw a tangent (line AB) on the curve at the time t.
Step 2: Draw the right-angled triangle ABC.
Step 3: Measure the lengths of AC and BC.
Step 4: Calculate the gradient of the tangent AB.
Rate of reaction at time t
= gradient of the curve at time t
= gradient of the tangent AB
How do you calculate the reaction rate 7

Shapes of curves using different selected quantities to measure the rate of reaction
Assuming a reaction is represented by the following equation:
R + T → W + X
The following changes occur as the reaction proceeds:

Mass or concentration of the reactants R and T decreases with time.
Mass or concentration of the products W and X increases with time.

For example, a reaction is carried out by mixing excess marble with dilute nitric acid. The chemical equation for the reaction is represented by the following equation:
CaCO₃(s) + 2HNO₃(aq) → Ca(NO₃)₂(aq) + CO₂(g) + H₂O(l)

If the concentration of nitric acid against time is plotted, the following graph will be obtained.
How do you calculate the reaction rate 8
The above graph is obtained based on the following facts:

Concentration of nitric acid decreases with time.
At the end of the reaction, all the nitric acid is completely reacted and its concentration becomes zero. This is due to the marble used in the reaction is in excess.

If the mass of marble against time is plotted, the following graph will be obtained.
How do you calculate the reaction rate 9
The above graph is obtained based on the following facts:

Mass of marble decreases with time.
At the end of the reaction, marble is in excess and m grams of marble remains unreacted.

If the concentration of calcium nitrate produced against time is plotted, the graph in Figure 1.8 is obtained.
How do you calculate the reaction rate 10
The above graph is obtained based on the following facts:

As more and more calcium nitrate is produced, the concentration of calcium nitrate increases with time.
When the reaction stops at time t, the concentration of calcium nitrate attains a maximum value, c mol dm^-3.

If the volume of carbon dioxide gas produced against time is plotted, the following graph is obtained.
How do you calculate the reaction rate 11
The above graph is obtained based on the following facts:

As more and more carbon dioxide gas is produced, volume of carbon dioxide gas increases with time.
When the reaction stops at time t, the total volume of carbon dioxide gas achieves a maximum value, V cm³.

An experiment is carried out using the apparatus set-up as shown in Figure.
How do you calculate the reaction rate 12
Chemical equation for the reaction is
Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)
If the mass of the conical flask and its contents against time is plotted, the graph in Figure is obtained.

The above graph is obtained based on the following facts:

As the reaction proceeds, more and more hydrogen gas is produced. The gas escapes to the surroundings.
Thus, the total mass of the conical flask and its contents decreases with time.
When the reaction stops at time t, no more hydrogen gas is produced. Hence, the total mass of the conical flask and its contents achieves a minimum value, w g.

How do you calculate the reaction rate experiment

Aim: To determine the average rate of reaction and the instantaneous rate of reaction.
Materials: Granulated zinc, 0.1 mol dm^-3 hydrochloric acid, water.
Apparatus: 150 cm³ conical flask, burette, stopper fitted with a delivery tube, retort stand and clamp, stopwatch, basin, electronic balance, 50 cm³ measuring cylinder.
Procedure:

25 cm³ of 0.1 mol dm^-3 hydrochloric acid is measured and poured into a conical flask.
A burette is filled with water until it is full. It is then inverted over water in a basin and clamped vertically using a retort stand.
The water level in the burette is adjusted and the initial burette reading is recorded.
The apparatus as shown in Figure is set up.
5 g of granulated zinc is weighed using an electronic balance.
The granulated zinc is added into the conical flask.
The conical flask is closed immediately with a stopper fitted with a delivery tube as shown in Figure and the stopwatch is started at the same time.
The conical flask is shaken steadily throughout the experiment.
The volume of gas collected in the burette is recorded at intervals of 30 seconds for a period of 6 minutes.
The results are tabulated in a table.

Results:

Time (s)	0	30	60	90	120	150	180	210	240	270	300	330	360
Burette reading (cm³)	48.90	41.40	37.15	34.40	32.40	30.65	29.15	27.90	27.15	26.40	25.65	25.15	24.90
Volume of gas liberated (cm³)	0	7.50	11.75	14.50	16.50	18.25	19.75	21.00	21.75	22.50	23.25	23.75	24.00

Interpreting data:

A graph of the volume of hydrogen gas liberated against time is plotted.
To calculate the average rate of reaction
To calculate the instantaneous rate of reaction

Discussion:

The instantaneous rate of reaction at 30 seconds (0.19 cm³ s^-1) is higher than the instantaneous rate of reaction at 90 seconds (0.08 cm³ s^-1).
The difference in rate is due to
(i) the concentration of hydrochloric acid at 90 seconds is lower than the concentration of hydrochloric acid at 30 seconds.
(ii) the total surface area of solid zinc at 90 seconds is smaller than the total surface area of solid zinc at 30 seconds.
From the graph plotted, it can be seen that the gradient of the curve decreases with time. Hence, the rate of reaction decreases with time.
As the reaction proceeds, the total surface area of zinc and the concentration of hydrochloric acid decrease with time. Hence, the rate of reaction also decreases with time. The rate of reaction will finally become zero, that is, the reaction stops when one or both the reactants are completely reacted.
The chemical equation for the reaction in this experiment is:
Zn(s) + 2HCl → ZnCl₂(aq) + H₂(g)
Another method that can be used to measure the rate of reaction between zinc and hydrochloric acid is by measuring the change in mass of the conical flask and its contents against time.

Conclusion:
As a reaction proceeds, the rate of reaction decreases with time until it becomes zero, that is, the reaction finally stops.

Rate of Reaction Calculation

Solving numerical problems
Examples about the calculation of the average rate of reaction and instantaneous rate of reaction are shown below.

1. 0.1 g of calcium carbonate is added to excess hydrochloric acid. The reaction stops after 15 seconds. 24.0 cm³ of carbon dioxide gas is collected. Calculate the average rate of reaction in
(a) g s^-1
(b) mol s^-1
(c) cm³ s^-1.
[Relative atomic mass: C, 12; O, 16; Ca, 40]
Solution:

2. In an experiment, one spatulaful of manganese(IV) oxide powder (as a catalyst) is added to 50 cm³ of sodium chlorate solution.

The oxygen gas liberated is collected in a burette by downward displacement of water. Its volume is recorded at intervals of 2

(s)	0	20	40	60	80	100	120	140	160	180	200	220	240	260	280
Volume of oxygen gas (cm³)	0.00	12.00	20.50	26.50	31.50	35.50	39.00	41.50	43.50	45.50	46.50	47.50	48.50	48.50	48.50

(a) Plot a graph of the volume of oxygen gas collected against time.
(b) Calculate the average rate of reaction
(i) in the first 2 minutes.
(ii) in the second minute.
(iii) for the overall reaction.
(c) Calculate the instantaneous rate of reaction
(i) at 20 seconds.
(ii) at 80 seconds.
Solution:
(a)
How do you calculate the reaction rate 21

0 seconds for a period of 280 seconds. The results are shown in Table.

Time

What is the rate of the reaction?

Rate of reaction measures the speed at which the reactants are converted to the products in a chemical reaction.
For a reaction that occurs rapidly, the rate of reaction is high. Conversely, for a reaction that occurs slowly, the rate of reaction is low.
The time taken for a fast reaction is short, whereas the time taken for a slow reaction is long.
Hence, the rate of a particular reaction is inversely proportional to the time taken for the reaction.
Different chemical reactions occur at different rates. Some examples are illustrated in Table.

Type of reaction	Fast reaction	Slow reaction
Reaction involving liberation of a gas	Bubbles of carbon dioxide gas liberate rapidly when sodium carbonate powder reacts with dilute hydrochloric acid. Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + CO₂(g) + H₂O(l)	In photosynthesis, carbon dioxide reacts with water very slowly in the presence of sunlight and chlorophyll to produce glucose and oxygen gas. 6CO₂(g) + 6H₂O(l) → C₆H₁₂O₆(s) + 6O₂(g)
Precipitation reaction	When silver nitrate solution is added to sodium chloride solution, a white precipitate of silver chloride is formed immediately. AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)	When dilute hydrochloric acid is added to sodium thiosulphate solution, a yellow precipitate of sulphur appears only after a few seconds. Na₂S₂O₃(aq) + 2HCl(aq) → 2NaCl(aq) + S(s) + SO₂(g) + H₂O(l)
Heating a metal in air	When a small piece of potassium is heated in air, it burns rapidly to form a white solid of potassium oxide. 4K(s) + O₂(g) → 2K₂O(s)	When a small piece of copper is heated in air, it reacts slowly with oxygen in the air to form a black solid of copper(II) oxide. 2Cu(s) + O₂(g) → 2CuO(s)

People also ask

What does the rate of reaction measure?

Observable changes for measuring the rate of reaction:

1. When a reaction occurs, two obvious changes that occur are:

the quantity of a reactant decreases with time
the quantity of a product increases with time

2. The quantity of a reactant/product can be the

number of moles of a substance
mass of a solid
volume of a gas
concentration of a solution

3. If the changes of any of these quantities are visible and measurable during a reaction, then it can be used to measure the rate of that reaction.

4. Suitable measurable visible changes in a chemical reaction are:

volume of a gas liberated
formation of a precipitate
changes in the mass during a reaction
colour changes
changes in the electrical conductivity of the solution
temperature changes
pressure changes
changes in concentration of the solution of a reactant
pH changes

5. One of these measurable visible changes can be selected as a suitable quantity to determine the rate of a particular reaction.
The changes in this selected quantity can be measured by carrying out an experiment and the results are then analysed to determine the rate of that reaction.

6. Definition: Rate of reaction is defined as the change in a selected quantity during a reaction per unit time whereby the selected quantity can be any of the measurable visible changes in the reaction.
What is the rate of the reaction 3

7. Two examples to illustrate the meaning of rate of reaction.
Reaction between magnesium and dilute sulphuric acid
Mg(s) + H₂SO₄(aq) → MgSO₄(aq) + H₂(g)

In the reaction between dilute sulphuric acid and a magnesium ribbon, the following two changes are observed:
- The mass of magnesium (the reactant) decreases with time.
- The volume of hydrogen gas (the product) increases with time.
Hence, the rate of reaction between dilute sulphuric acid and magnesium can be determined by measuring the change in the mass of magnesium or the volume of hydrogen gas per unit time.
Quantitatively,
Reaction between ethanedioic (oxalic) acid, H₂C₂O₄and acidified potassium manganate(VII) solution.
- When excess aqueous ethanedioic acid is added to acidified potassium manganate(VII) solution, the purple colour of the solution slowly decolourises at room temperature.
- By measuring the time taken for the purple colour to decolourise, the rate of reaction can be determined.
- Rate of reaction is inversely proportional to the time taken for the purple colour to decolourise.
- Rate of reaction is reflected by the value of 1/time taken. The larger the value time taken of 1/time taken, the higher the rate of reaction.

Table shows the units for the rate of reaction measured in different ways.

Change in a selected quantity per unit time	Units for the rate of reaction
Change in mass per unit time	g s^-1 or g min^-1
Change in volume of a gas liberated per unit time	cm³ s^-1 or cm³ min^-1
Change in concentration of a reactant per unit time	mol dm^-3 s^-1 or mol dm^-3min^-1
Change in number of moles of a reactant per unit time	mol s^-1or mol min^-1

Rate of Reaction Experiment

Aim: To compare the rates of a few reactions.
Materials: Marble chips, 2 mol dm^-3 hydrochloric acid, 0.1 mol dm^-3 sodium thiosulphate solution, 1 mol dm^-3 lead(ll) nitrate solution.
Apparatus: 50 cm³ beakers, test tubes.

Procedure:

5 cm³ of 2 mol dm^-3 hydrochloric acid is poured into each of the three test tubes on a rack.
The test tubes are labelled I to III respectively.
One piece of marble chip is added into test tube I.
About 2 cm³ of 0.1 mol dm^-3 sodium thiosulphate solution is poured into test tube II and the mixture is shaken well.
About 2 cm³ of 1 mol dm^-3 lead(II) nitrate solution is poured into test tube III and the mixture is shaken well.
The changes are observed carefully. The rates of reactions in the three test tubes are compared.

Observations:

Test tube	Reactants	Observation
I	Marble chip and hydrochloric acid	Bubbles of a colourless gas are liberated rapidly, that is, effervescence occurs rapidly.
II	Sodium thiosulphate solution and hydrochloric acid	A yellow precipitate appears only after about 12 seconds.
III	Lead(II) nitrate solution and hydrochloric acid	A white precipitate is formed immediately.

Inferences:

The reaction between lead(ll) nitrate solution and hydrochloric acid is very fast.
The reaction between the marble chip and hydrochloric acid is moderately fast.
The reaction between sodium thiosulphate solution and hydrochloric acid is slow.

Discussion:

The chemical equation for the reaction in
The rate of reaction in ascending order is: rate in test tube II < rate in test tube I < rate in test tube III
The observable change that can be used to compare the rate of reaction in
(a) test tube I is the time taken for the effervescence to stop completely.
(b) test tube II is the time taken for the appearance of a yellow precipitate.
(c) test tube III is the time taken for the appearance of a white precipitate.

Conclusion:

The rate of reaction between sodium thiosulphate solution and hydrochloric acid is the lowest.
The rate of reaction between the marble chip and hydrochloric acid is moderately high.
The rate of reaction between lead(II) nitrate solution and hydrochloric acid is the highest.