teaching-notes — Chemistry (Chemical Kinetics)

Topic: Chemical Kinetics

Subtopic: Rates of Chemical Reactions

Chemistry is the study of matter and how it changes. Chemical reactions involve substances changing into new ones. Some reactions happen very quickly, like an explosion, while others happen very slowly, like rusting. The speed at which a chemical reaction takes place is called its rate of reaction.

When chemical reactions occur, there is always an energy change. Energy is either given out or taken in. Based on this energy change, reactions can be classified as either endothermic or exothermic.

1. Exothermic Reactions

An exothermic reaction is a chemical reaction that releases energy, usually in the form of heat or light, into the surroundings. This causes the temperature of the surroundings to increase. You will feel the container or reaction mixture get hot.

Examples of exothermic reactions:

• Burning of fuels (combustion), e.g., burning charcoal or firewood.
• Neutralisation reactions, e.g., mixing an acid and a base.
• Respiration in living organisms.
• Setting of cement.

2. Endothermic Reactions

An endothermic reaction is a chemical reaction that absorbs energy, usually in the form of heat, from the surroundings. This causes the temperature of the surroundings to decrease. You will feel the container or reaction mixture get cold.

Examples of endothermic reactions:

• Photosynthesis in plants.
• Melting of ice.
• Dissolving ammonium chloride in water.
• Decomposition of calcium carbonate (limestone) by heating.

Quick Check:

When you mix baking soda (sodium bicarbonate) with vinegar (acetic acid), the mixture feels cold. Is this an endothermic or exothermic reaction? Explain why.

The rate of a chemical reaction is a measure of how fast the reactants are used up or how fast the products are formed over a period of time. In simpler terms, it tells us how quickly a reaction happens.

A fast reaction means a large amount of product is formed in a short time, or a large amount of reactant is used up in a short time. A slow reaction means a small amount of product is formed or a small amount of reactant is used up over a longer time.

The rate of reaction can be calculated using the formula:

Rate of reaction = Change in quantity of reactant or productTime taken for the change

The "quantity" can be mass, volume, or concentration. The unit for time is usually seconds (s) or minutes (min).

Therefore, the units for the rate of reaction can be:

• grams per second (g s^-1)
• cm³ per second (cm³ s^-1)
• moles per litre per second (mol dm^-3 s^-1)

Example:

In a reaction, 50 cm³ of hydrogen gas was produced in 20 seconds. Calculate the rate of reaction.

Formula: Rate = Change in volume of gasTime taken

Substitution: Rate = 50 cm³20 s

Answer: Rate = 2.5 cm³ s^-1

To study the rate of a chemical reaction, we need to measure how a certain property of the reaction changes over time. This property could be the volume of gas produced, the mass of reactants used up, or the time taken for a visible change to occur.

Here are some common experimental methods:

1. Measuring the volume of gas produced:

Many reactions produce a gas. We can collect this gas in a gas syringe or by displacing water in an inverted measuring cylinder and record the volume at regular time intervals. The faster the gas is produced, the faster the reaction rate.

Example: Reaction of magnesium with dilute hydrochloric acid.

Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

Hydrogen gas (H₂) is produced and can be collected and measured.

2. Measuring the change in mass:

If a reaction produces a gas that escapes, the total mass of the reaction mixture will decrease. We can place the reaction flask on an electronic balance and record the mass at regular time intervals. The faster the mass decreases, the faster the reaction rate.

Example: Reaction of calcium carbonate (marble chips) with dilute hydrochloric acid.

CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)

Carbon dioxide gas (CO₂) escapes, causing a decrease in mass.

3. Measuring the time for a precipitate to form (Disappearing Cross Experiment):

Some reactions produce a cloudy precipitate. We can place the reaction mixture over a marked cross on a piece of paper and measure the time it takes for the cross to become invisible due to the formation of the precipitate. The shorter the time, the faster the reaction rate.

Example: Reaction between sodium thiosulphate and dilute hydrochloric acid.

Na₂S₂O₃(aq) + 2HCl(aq) → 2NaCl(aq) + SO₂(g) + H₂O(l) + S(s)

Sulphur (S) is a yellow solid precipitate that makes the solution cloudy.

Understanding and controlling the rates of chemical reactions is very important in many aspects of our daily lives and in industries.

1. Food Preservation:

• Refrigeration and Freezing: Lowering the temperature slows down the chemical reactions that cause food to spoil (e.g., bacterial growth, enzymatic reactions). This is why we keep food in fridges and freezers.
• Salting and Sugaring: Adding salt or sugar to food (e.g., dried fish, jams) reduces water activity, which slows down microbial growth and spoilage reactions.
• Pickling: Using vinegar (acetic acid) to preserve vegetables creates an acidic environment that inhibits the growth of spoilage microorganisms.

2. Cooking:

• Heating: Increasing the temperature speeds up chemical reactions, which helps food cook faster. For example, boiling water cooks vegetables quickly.
• Pressure Cooking: Cooking under pressure increases the boiling point of water, allowing food to cook at higher temperatures and thus faster.

3. Industrial Processes:

• Catalysts: Industries use catalysts to speed up desired chemical reactions without being used up themselves. This saves energy and time, making production more efficient and cheaper. For example, in the manufacture of ammonia (Haber process) or sulphuric acid (Contact process).
• Controlling Temperature and Pressure: Many industrial reactions are carried out at specific temperatures and pressures to achieve the optimal reaction rate and yield of products.

4. Medicine and Biology:

• Drug Action: The rate at which drugs dissolve and react in the body affects how quickly they take effect and how long their effects last.
• Enzymes: In living organisms, enzymes act as biological catalysts, speeding up vital biochemical reactions (e.g., digestion, respiration) at body temperature.

5. Safety and Environment:

• Fire Safety: Understanding combustion rates helps in designing fire-resistant materials and fire suppression systems.
• Rusting: Preventing or slowing down the rusting of iron (e.g., by painting, oiling, galvanising) is crucial for maintaining structures and vehicles.

Quick Check:

Explain how keeping meat in a refrigerator helps to preserve it for longer.

The rate of a chemical reaction tells us how quickly reactants are converted into products. Reactions can be classified based on their energy changes: exothermic reactions release heat and cause the surroundings to warm up, while endothermic reactions absorb heat and cause the surroundings to cool down. We can measure reaction rates by observing changes in quantities like volume of gas produced, mass of reactants, or the time taken for a visible change (like precipitate formation). Understanding reaction rates is crucial for various real-life applications, including food preservation, cooking, industrial manufacturing, and medical treatments.

1. Define the term 'rate of a chemical reaction'.

2. Distinguish between an endothermic and an exothermic reaction, giving one example for each.

3. A reaction produces 120 cm³ of carbon dioxide gas in 30 seconds. Calculate the rate of reaction in cm³ s^-1.

4. State two common experimental methods used to measure the rate of a chemical reaction.

5. Explain why food spoils faster at room temperature than in a refrigerator.

6. Give two examples of how understanding reaction rates is applied in industrial processes.

7. When ammonium nitrate dissolves in water, the solution feels cold. Is this an endothermic or exothermic process?

8. Name the gas produced when magnesium reacts with dilute hydrochloric acid.

1. Classify the following reactions as either endothermic or exothermic:

a) Burning of methane gas.

b) Photosynthesis.

c) Dissolving potassium iodide in water (solution gets colder).

d) Neutralisation of sodium hydroxide with hydrochloric acid.

2. In an experiment, 45 g of a reactant was consumed in 90 seconds. Calculate the rate of reaction in g s^-1.

3. Describe how you would set up an experiment to measure the rate of reaction between marble chips (calcium carbonate) and dilute hydrochloric acid by collecting the gas produced.

4. Suggest two ways to slow down the rate of rusting of an iron gate.

5. A student carried out an experiment where a precipitate formed, making a cross disappear in 40 seconds. In a second experiment with different conditions, the cross disappeared in 25 seconds. Which experiment had a faster rate of reaction? Explain your answer.

6. Explain why catalysts are important in many industrial chemical processes.

1. Classify the following reactions as either endothermic or exothermic:

a) Burning of methane gas: Exothermic

b) Photosynthesis: Endothermic

c) Dissolving potassium iodide in water (solution gets colder): Endothermic

d) Neutralisation of sodium hydroxide with hydrochloric acid: Exothermic

2. In an experiment, 45 g of a reactant was consumed in 90 seconds. Calculate the rate of reaction in g s^-1.

Formula: Rate = Change in massTime taken

Substitution: Rate = 45 g90 s

Answer: Rate = 0.5 g s^-1

3. Describe how you would set up an experiment to measure the rate of reaction between marble chips (calcium carbonate) and dilute hydrochloric acid by collecting the gas produced.

Steps:

• Place a known mass of marble chips into a conical flask.
• Add a known volume of dilute hydrochloric acid to the flask.
• Immediately seal the flask with a stopper fitted with a delivery tube.
• Connect the delivery tube to a gas syringe.
• Start a stopwatch at the moment the acid is added.
• Record the volume of carbon dioxide gas collected in the gas syringe at regular time intervals (e.g., every 10 seconds) until the reaction stops or slows down significantly.
• Plot a graph of volume of gas against time to determine the rate.

4. Suggest two ways to slow down the rate of rusting of an iron gate.

• Painting: Creates a barrier that prevents oxygen and water from reaching the iron surface.
• Oiling/Greasing: Forms a protective layer that prevents contact with oxygen and water.
• Galvanising: Coating the iron with a layer of zinc, which acts as a sacrificial metal.

5. A student carried out an experiment where a precipitate formed, making a cross disappear in 40 seconds. In a second experiment with different conditions, the cross disappeared in 25 seconds. Which experiment had a faster rate of reaction? Explain your answer.

The second experiment had a faster rate of reaction. This is because the cross disappeared in a shorter amount of time (25 seconds compared to 40 seconds), meaning the precipitate formed more quickly.

6. Explain why catalysts are important in many industrial chemical processes.

Catalysts are important because they speed up the rate of chemical reactions without being used up themselves. This allows industries to:

• Produce products faster, increasing efficiency.
• Operate at lower temperatures and pressures, saving energy and reducing costs.
• Improve the yield of desired products.