Rates of Reactions (Junior Cert Science): Revision Notes

Rates of Reactions

Introduction to chemical reaction rates

Chemical reactions happen all around us, but they don't all occur at the same speed. Some reactions are incredibly fast, whilst others take a very long time to complete.

Examples of reaction speeds:

• Very fast reactions: Explosions produce a large volume of gas in a fraction of a second. When hydrogen gas is ignited, a chemical reaction occurs almost instantly, creating a loud bang.
• Slow reactions: Other changes occur much more slowly. For example, an apple ripening, concrete setting, or iron rusting can take hours, days, or even longer.

Understanding what controls the speed of chemical reactions is essential in chemistry. It helps us make reactions happen faster when we need them to, or slow them down when that's more useful.

What is rate of reaction?

The rate of reaction tells us how quickly a chemical reaction occurs. We can think of it like the speed of a car - just as speed measures how fast a car travels, rate measures how fast a reaction proceeds.

We can write this as:

$\text{rate of reaction} = \frac{\text{change in concentration of any one reactant or product}}{\text{time taken}}$

This is similar to how we calculate the speed of a moving object:

$\text{speed} = \frac{\text{distance travelled}}{\text{time taken}}$

For example, if a car travels $20$ metres in $10$ seconds, it has an average speed of $2 \text{ m/s}$.

Measuring rate of reaction

To measure how fast a reaction occurs, we need to track how the concentration of reactants or products changes over time. There are several ways to do this:

• Measuring gas volume - If a gas is produced, we can collect it and measure its volume at regular time intervals
• Measuring mass change - If a gas escapes, the total mass decreases, which we can measure
• Observing colour changes - Some reactions produce coloured products we can monitor

Understanding rate graphs

When we plot the volume of gas produced against time, we get a graph that tells us a lot about the reaction.

Key features of a rate graph:

1. Steepest at the start - The reaction is fastest near the beginning because there are lots of reactant particles available to react
2. Gradually levels off - The reaction slows down as reactants get used up, so there are fewer particles available to collide and react
3. Reaches a plateau - The reaction has finished when no more gas is being produced. The line becomes flat (horizontal)

Average rate of reaction

We often want to know the overall speed of a reaction. To find this, we calculate the average rate.

Factors that affect the rate of a reaction

Through experiments, chemists have discovered that five main factors control how fast chemical reactions occur:

1. Types of reactants - different substances react at different speeds
2. Particle size (surface area) - smaller particles react faster
3. Concentration - more concentrated solutions react faster
4. Temperature - hotter reactants react faster
5. Catalysts - special substances that speed up reactions

Let's investigate each factor in detail.

Factor 1: Types of reactants

Different substances react at different rates, even when reacting with the same chemical. This is because different elements and compounds have different chemical properties.

Experiment: Comparing magnesium and zinc

We can investigate this by reacting two different metals with the same acid.

The chemical reactions:

Both metals react with hydrochloric acid to produce hydrogen gas:

$\text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2$

magnesium + hydrochloric acid → magnesium chloride + hydrogen

Similarly for zinc:

$\text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2$

zinc + hydrochloric acid → zinc chloride + hydrogen

Results:

The volume of hydrogen gas can be recorded in a table:

Time (mins)	0	0.5	1.0	1.5	2.0	2.5	3.0	3.5
Volume of H₂ (cm³)

When we plot graphs for both metals, we can compare their reaction rates:

What the graph shows:

• The steep slope for magnesium shows it reacts quickly with dilute hydrochloric acid
• The gentle slope for zinc shows it reacts slowly with dilute hydrochloric acid
• Magnesium is more reactive than zinc, so it reacts much faster with the acid

Test for hydrogen gas

To confirm that the gas being produced is hydrogen, we can perform a simple test:

Procedure: Bring a lighted taper (wooden splint) near the mouth of a test tube containing hydrogen gas.

Result: The hydrogen burns with a pop sound. This is a characteristic test for hydrogen gas.

Factor 2: Particle size (surface area)

The size of solid reactant particles has a significant effect on reaction rate. Smaller particles react faster than larger ones.

Why does particle size matter?

Chemical reactions happen when particles collide with each other. For a solid to react, particles from another substance (like molecules in a solution) must collide with the solid's surface.

• Large particles have a relatively small surface area exposed to other reactants
• Small particles have a much larger total surface area exposed, allowing many more collisions to occur

Experiment: Marble chips of different sizes

We can investigate how particle size affects reaction rate by reacting marble chips of different sizes with acid.

Background: Marble is mainly calcium carbonate (CaCO₃), which reacts with hydrochloric acid to produce carbon dioxide gas.

$\text{CaCO}_3 + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{H}_2\text{O} + \text{CO}_2$

calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide

Results:

The data can be recorded in a table:

Time (mins)	0	0.5	1.0	1.5	2.0	2.5	3.0	3.5
Total mass (g)
Loss in mass (g)

We can plot two types of graph:

Graph 1: Total mass vs time

• All three curves decrease as carbon dioxide escapes
• Small chips show the steepest decrease (fastest reaction)
• Large chips show the gentlest decrease (slowest reaction)

Graph 2: Loss in mass vs time

• All three curves increase as more mass is lost
• Small chips show the steepest increase (fastest mass loss)
• Large chips show the gentlest increase (slowest mass loss)

Test for carbon dioxide

To confirm that carbon dioxide is being produced, we can perform the limewater test:

Procedure: Bubble the gas through limewater (a clear solution of calcium hydroxide).

Result: Carbon dioxide turns limewater milky. This is a characteristic test for carbon dioxide gas.

Factor 3: Concentration

The concentration of a solution tells us how much solute is dissolved in a given volume of solvent. Changing the concentration of reactants affects the reaction rate.

How concentration affects rate

When we increase the concentration of a solution:

• There are more reactant particles in the same volume
• This means particles are more crowded together
• More frequent collisions occur between reactant particles
• The reaction proceeds faster

Experiment: Different concentrations of acid

We can investigate how concentration affects rate by reacting magnesium with hydrochloric acid of different concentrations.

The reaction:

$\text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2$

magnesium + hydrochloric acid → magnesium chloride + hydrogen

Results:

Time (mins)	0	0.5	1.0	1.5	2.0	2.5	3.0	3.5
Volume of H₂ (cm³)

When we plot the results:

What the graph shows:

• The $1.5$ M HCl (highest concentration) produces the steepest curve - the reaction is fastest
• The $1.0$ M HCl (medium concentration) produces a medium-steepness curve
• The $0.5$ M HCl (lowest concentration) produces the gentlest curve - the reaction is slowest

Factor 4: Temperature

Temperature has a dramatic effect on reaction rates. Most chemical reactions occur much faster at higher temperatures.

How temperature affects rate

When we increase the temperature:

• Reactant particles gain more kinetic energy
• They move around faster
• Collisions between particles occur more frequently
• Collisions have more energy, making them more likely to result in a reaction

Everyday example

Consider food storage: we keep food fresh for longer by storing it in a refrigerator. This is because the chemical reactions that cause food to go bad occur much more slowly at lower temperatures.

Experiment: Reaction at different temperatures

We can investigate how temperature affects rate by carrying out the same reaction at different temperatures.

Safety note:

The reaction:

$\text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2$

magnesium + hydrochloric acid → magnesium chloride + hydrogen

Results:

Time (mins)	0	0.5	1.0	1.5	2.0	2.5	3.0	3.5
Volume of H₂ (cm³)

When we plot the results:

What the graph shows:

• At $60°\text{C}$ (highest temperature), the curve is steepest - the reaction is fastest
• At $40°\text{C}$ (medium temperature), the curve has a medium steepness
• At $20°\text{C}$ (lowest temperature), the curve is gentlest - the reaction is slowest

All three reactions finish when the same total volume of hydrogen has been produced (because we used the same amount of magnesium in each case). However, the reaction at $60°\text{C}$ reaches this point much sooner than the reaction at $20°\text{C}$.

Factor 5: Catalysts

A catalyst is a special substance that speeds up a chemical reaction without being used up in the process. This is a very important property - the catalyst is still there at the end of the reaction, unchanged.

Definition

Catalysts are incredibly useful in chemistry and industry because:

• They can be used over and over again
• They speed up reactions that would otherwise be too slow
• They can make processes more economical and efficient

Experiment: Effect of a catalyst

We can investigate how a catalyst affects reaction rate by examining the decomposition of hydrogen peroxide.

Background: Hydrogen peroxide naturally breaks down into water and oxygen, but this happens very slowly - it can take several months.

$2\text{H}_2\text{O}_2 \xrightarrow{\text{MnO}_2} 2\text{H}_2\text{O} + \text{O}_2$

hydrogen peroxide → water + oxygen

However, when we add manganese dioxide (MnO₂) as a catalyst, the reaction speeds up dramatically.

Results:

Time (mins)	0	0.5	1.0	1.5	2.0	2.5	3.0	3.5
Volume of O₂ (cm³)

Comparing with and without a catalyst:

When we compare the decomposition with and without manganese dioxide:

• With catalyst (MnO₂): The reaction is very fast, producing oxygen gas rapidly. The graph shows a steep curve, and the reaction completes in just a few minutes.
• Without catalyst: The decomposition occurs extremely slowly. It would take many months for the hydrogen peroxide to completely break down. The graph would show a very gentle slope.

Test for oxygen gas

To confirm that oxygen is being produced:

Procedure: Insert a glowing wooden splint (one that has just been blown out) into a test tube containing the gas.

Result: Oxygen relights the glowing splint. This is a characteristic test for oxygen gas.

Biochemical reactions

Chemical reactions don't just happen in test tubes - thousands of them are occurring in your body right now. These reactions in living organisms are called biochemical reactions.

Definition

Examples of biochemical reactions

Biochemistry is the branch of science that studies the chemical reactions occurring in living things. Here are some important examples:

1. Cellular respiration - This occurs in every living cell. Glucose (a sugar) undergoes biochemical reactions to release energy that the organism can use. This process is essential for life.
2. Photosynthesis - This occurs in green plants. Carbon dioxide and water react in the presence of sunlight to form glucose molecules. This process provides food for plants and produces the oxygen we breathe.
3. Digestion - This occurs in our digestive system. Large food molecules are broken down into smaller substances that our bodies can use for energy, growth, and repair.

Enzymes: Biological catalysts

Biochemical reactions are controlled by special proteins called enzymes. An enzyme is a biological catalyst - it speeds up biochemical reactions without being used up.

Key properties of enzymes:

• They are very specific - each enzyme catalyses only one particular reaction
• The substance an enzyme acts on is called the substrate
• They work best at particular temperatures and pH levels
• Like all catalysts, they are not consumed in the reaction

Factors affecting biochemical reactions

The rates of biochemical reactions are affected by several factors:

1. Concentration of enzymes and substrates - higher concentrations generally increase the rate
2. Temperature - biochemical reactions typically occur best at body temperature (around $37°\text{C}$ for humans). If temperature is too high, enzymes can be damaged (denatured) and stop working
3. pH - many enzymes work only within specific pH ranges. Outside these ranges, they become inactive

Summary of factors affecting rate

We've now investigated five key factors that control reaction rates:

Factor	Effect on rate	Explanation
Type of reactant	Different substances react at different speeds	Chemical properties vary between substances
Particle size	Smaller particles → faster reaction	Greater surface area means more collisions
Concentration	Higher concentration → faster reaction	More particles in same volume means more collisions
Temperature	Higher temperature → faster reaction	Particles move faster and collide more often with more energy
Catalyst	Increases rate without being used up	Provides alternative pathway requiring less energy