Sciences - Co-ordinated (Double) (0654) | Chemical reactions

Hello, Future Chemists! Getting Started with Chemical Reactions (C6)

Welcome to the chapter on Chemical Reactions! This is the heart of Chemistry. Everything around you—from cooking an egg to generating electricity—involves chemical change.

In this chapter, we will learn how to tell a simple change from a true chemical reaction, how fast these reactions happen (and how to speed them up!), and look at a very important type of reaction called Redox. Understanding these concepts will help you explain much of the world around you. Let's dive in!

C6.1 Physical vs. Chemical Changes

Before studying reactions, we need to know the difference between changing the physical appearance of a substance and changing its chemical identity.

What is a Physical Change?

A Physical Change is a change in the physical properties of a substance (like shape, state, or size) but no new chemical substance is formed. These changes are usually reversible.

• Example: Melting ice. Ice (solid water) becomes liquid water. It's still H₂O; no new substance is made.
• Example: Dissolving salt in water. You still have salt and water, just mixed together.

What is a Chemical Change (Reaction)?

A Chemical Change (or reaction) is a process where one or more substances (reactants) are converted into one or more new substances (products). These changes are often irreversible.

• Example: Burning wood. Wood turns into ash, smoke, and gases (CO₂, H₂O vapor). You cannot turn the ash back into wood.
• Example: Rusting iron. Iron reacts with oxygen and water to form iron oxide (rust), a completely new chemical compound.

How to Spot a Chemical Change

Look for these signs that a chemical reaction has occurred:

1. Gas production (often seen as bubbling or fizzing).
2. Energy change (the mixture gets hot, exothermic, or cold, endothermic).
3. Colour change (a new substance is formed with a different colour).
4. Precipitate formation (two clear liquids mix and produce an insoluble solid).
5. It is difficult to reverse the change.

C6.2 Rate of Reaction

The Rate of Reaction tells us how fast the reactants are used up or how fast the products are formed over time.

A slow reaction might take hours (like rusting), while a fast reaction might take less than a second (like an explosion).

Factors Affecting the Rate of Reaction (Core Content)

We can change the rate of a reaction by adjusting five key factors:

1. Concentration of Solutions: Increasing the concentration increases the rate. (More reactant particles packed into the same volume).
2. Pressure of Gases: Increasing the pressure increases the rate. (The gas particles are squeezed closer together).
3. Surface Area of Solids: Increasing the surface area increases the rate. (Grinding a lump into powder is a great way to increase surface area).
4. Temperature: Increasing the temperature dramatically increases the rate. (Particles move faster and collide harder).
5. Catalyst: Adding a catalyst increases the rate without being used up itself.

Practical Methods for Investigating Rate

How do scientists measure how fast a reaction is going? We measure a property that changes consistently throughout the reaction, usually focusing on either the change in mass or the formation of a gas.

Method 1: Change in Mass

If a gas is produced during a reaction, the mass of the reactants decreases as the gas escapes.

• Setup: Place the reactants (e.g., marble chips and acid) in a flask on a digital balance. Place cotton wool in the neck of the flask (to stop liquid splashing out but allow gas to escape).
• Measurement: Record the mass at regular time intervals (e.g., every 30 seconds).
• Analysis: Plot a graph of mass lost (y-axis) against time (x-axis). A steeper slope means a faster reaction rate.

Method 2: Formation of a Gas (Collecting Volume)

If the gas produced is collected, we can measure its volume over time.

• Setup: Collect the gas produced using a measuring cylinder inverted in water or, more accurately, a gas syringe.
• Measurement: Record the total volume of gas collected at regular time intervals.
• Analysis: Plot a graph of volume of gas produced (y-axis) against time (x-axis).

Did you know? Both methods produce graphs that start steeply, curve and become less steep, and eventually flatten out. The reaction stops when the graph flattens because one of the reactants has been completely used up (it has become the limiting reactant).

C6.2 Collision Theory and Catalysts (Supplement Content)

To explain why those factors affect the rate, we use Collision Theory.

Collision theory states that for particles to react, they must collide with each other, and they must meet two conditions:

1. They must collide with the correct orientation.
2. They must collide with enough energy.

Explaining Rate Factors using Collision Theory

1. Concentration and Pressure

Increasing concentration (in solutions) or pressure (in gases) means there is a higher number of particles per unit volume.

• Analogy: Imagine a dance hall. If only 10 people are dancing (low concentration), collisions are rare. If 100 people are crammed in (high concentration), the frequency of collisions between particles increases dramatically, meaning more chances to react.

2. Surface Area

Only the particles on the surface of a solid can react. Breaking a solid into smaller pieces exposes more particles.

• This increases the surface area, leading to a higher frequency of collisions between the reactant particles and the solid surface.

3. Temperature

Temperature is the measurement of the average kinetic energy of particles.

• When temperature increases, particles move much faster.
• This leads to more frequent collisions AND, more importantly, a larger proportion of collisions having energy equal to or greater than the activation energy (E$_{a}$).

4. The Role of Catalysts

A catalyst is a substance that speeds up a chemical reaction but remains chemically unchanged itself at the end of the reaction.

How do they work?

• A catalyst provides an alternative reaction pathway that has a lower activation energy, E$_{a}$.
• The activation energy (E$_{a}$) is the minimum energy that colliding particles must have in order to react. Think of it as an "energy hurdle."
• By lowering this hurdle, more of the existing collisions (even those at lower kinetic energy) become successful, increasing the reaction rate.

C6.3 Redox Reactions

Redox Reactions are one of the most fundamental types of chemical reactions. The term 'Redox' is short for Reduction-Oxidation. These two processes always occur simultaneously (at the same time).

Definitions using Oxygen

Historically, oxidation and reduction were defined by oxygen transfer:

• Oxidation: The gain of oxygen.
• Reduction: The loss of oxygen.

Example (Core): In the reaction: \(2\text{CuO} + \text{C} \rightarrow 2\text{Cu} + \text{CO}_2\)

• Copper(II) oxide (CuO) loses oxygen to form copper (Cu) $\rightarrow$ It is reduced.
• Carbon (C) gains oxygen to form carbon dioxide (CO₂) $\rightarrow$ It is oxidised.

The substance that causes reduction (C, in this case) is the reducing agent. The substance that causes oxidation (CuO) is the oxidising agent.

Definitions using Electrons (Supplement)

This is the modern, more useful definition of oxidation and reduction that applies to all reactions, not just those involving oxygen.

Memory Aid: OIL RIG

To remember which is which, use this mnemonic:

• Oxidation Is Loss (of electrons).
• Reduction Is Gain (of electrons).

Electron Definitions

• Oxidation is the loss of electrons.
• Reduction is the gain of electrons.

Example: Formation of an ionic compound, NaCl.

• Sodium atom (\(\text{Na}\)) loses an electron to become a positive ion (\(\text{Na}^+\)). $\rightarrow$ $\text{Na} \rightarrow \text{Na}^+ + \text{e}^-$. Sodium is oxidised.
• Chlorine atom (\(\text{Cl}\)) gains an electron to become a negative ion (\(\text{Cl}^-\)). $\rightarrow$ $\text{Cl} + \text{e}^- \rightarrow \text{Cl}^-$. Chlorine is reduced.

Ionic Half-Equations (Supplement)

We use ionic half-equations to show clearly the gain or loss of electrons. Reduction reactions are shown by the reactant gaining electrons:

Example (Reduction at the cathode during electrolysis):

$$\text{Cu}^{2+} + 2\text{e}^- \rightarrow \text{Cu}$$

The copper ion gains electrons (is reduced) to become a copper atom.

Oxidation Numbers (for Naming Ions - Core)

The syllabus mentions the use of oxidation numbers only to help name ions, like iron(II) and iron(III) or copper(II).

• Iron(II) ion ($\text{Fe}^{2+}$) has a lower positive charge (oxidation state).
• Iron(III) ion ($\text{Fe}^{3+}$) has a higher positive charge (oxidation state).
• When an $\text{Fe}^{2+}$ ion changes to an $\text{Fe}^{3+}$ ion, it has lost an electron ($\text{Fe}^{2+} \rightarrow \text{Fe}^{3+} + \text{e}^-$). According to OIL RIG, this is oxidation.

(Note: You are only required to identify that oxidation is an increase in oxidation number and reduction is a decrease, but you do NOT need to calculate oxidation numbers for complex compounds.)