Chemical Changes and Structure - Collision Theory, Reaction Profiles, and Periodicity

A complete Q&A flashcard set covering Chemical Changes and Structure, Collision Theory, Reaction Profiles, Potential Energy Diagrams, Catalysts, and Periodicity trends based on the lecture notes.

When is the reaction rate formula $\text{Rate} = \frac{1}{t}$ used?

It is used when there is no change in mass, volume, or concentration measured during a reaction, such as in a color change reaction.

According to collision theory, what conditions are required for a chemical reaction to occur?

A chemical reaction can only occur if there is a successful collision between reactant molecules that possess sufficient kinetic energy ($E_a$) and correct collision geometry.

What four factors can be altered to speed up a chemical reaction?

1. Decreasing particle size (increasing surface area)
2. Increasing concentration of reactant
3. Increasing temperature
4. Adding a catalyst

Why is collision geometry important in a chemical reaction?

Reactant particles must be geometrically aligned to collide successfully; if they are not aligned, no activated complex can form, making the collision unsuccessful.

In the Iodine Clock Reaction experiment, how is the reaction rate accurately controlled?

By producing a calibration curve and altering the concentration of the potassium iodide/water solution while keeping the total volume constant at $25\,ml$.

How does increasing reactant concentration affect the rate of reaction?

Higher concentration means more particles in a fixed space, leading to a greater chance of collisions and a higher number of successful collisions per unit time, which increases the reaction rate.

How does decreasing particle size affect the rate of reaction?

Smaller particle size increases the surface area, which increases the number of collisions that can occur at any time, resulting in more successful collisions and a faster reaction rate.

How does increasing temperature affect particle movement and reaction rate?

Higher temperature gives particles higher kinetic energy so they move faster. This increases the chance of collisions occurring with energy equal to or greater than the activation energy ($E_a$), increasing the rate of reaction.

What are the energy characteristics of an exothermic reaction?

Reactants have more energy than products, energy is released to the surroundings (causing temperature to increase), and the enthalpy change ($\Delta H$) is negative.

What are the energy characteristics of an endothermic reaction?

Reactants have less energy than products, energy is absorbed from the surroundings (causing temperature to decrease), and the enthalpy change ($\Delta H$) is positive.

Why do reacting particles experience repulsion as they approach each other?

Because all atoms and molecules are surrounded by negatively charged electrons, which repel each other as they get closer together.

What is activation energy ($E_a$)?

The minimum energy required by colliding particles to form an activated complex (or overcome the electrostatic repulsion energy barrier).

What is an activated complex?

An unstable arrangement of atoms formed at the maximum of the potential energy barrier during a reaction, where reactant bonds are partially broken and product bonds are partially formed.

How is enthalpy change ($\Delta H$) defined?

The energy difference between the products and the reactants ($\Delta H = E_{\text{products}} - E_{\text{reactants}}$).

For a potential energy diagram with reactants at $30\,kJ\,mol^{-1}$, products at $10\,kJ\,mol^{-1}$, and an energy peak at $90\,kJ\,mol^{-1}$, calculate $E_a$ and $\Delta H$ for the forward reaction.

Activation Energy ($E_a$) = $90 - 30 = 60\,kJ\,mol^{-1}$ Change in Enthalpy ($\Delta H$) = $10 - 30 = -20\,kJ\,mol^{-1}$ (Exothermic)

For a potential energy diagram with reactants at $30\,kJ\,mol^{-1}$, products at $10\,kJ\,mol^{-1}$, and an energy peak at $90\,kJ\,mol^{-1}$, calculate $E_a$ and $\Delta H$ for the backward reaction.

Activation Energy ($E_a$) = $90 - 10 = 80\,kJ\,mol^{-1}$ Change in Enthalpy ($\Delta H$) = $30 - 10 = 20\,kJ\,mol^{-1}$ (Endothermic)

What is a catalyst and how does it speed up a reaction?

A catalyst is a substance that speeds up a chemical reaction without getting used up or changed itself; it works by providing an alternative pathway with a lower activation energy ($E_a$).

What are the two main categories of catalysts?

Heterogeneous and Homogeneous catalysts.

On a kinetic energy distribution graph, what does the area under the curve to the right of $E_a$ represent?

The total number of particles/collisions that possess energy in excess of the activation energy ($E_a$) required for successful collisions.

How does increasing temperature alter the energy distribution curve and activation energy ($E_a$)?

Increasing temperature shifts the curve to the right and lowers its peak, significantly increasing the proportion of molecules with energy above $E_a$, while $E_a$ itself remains unchanged.

How does increasing reactant concentration affect the energy distribution curve and activation energy ($E_a$)?

Increasing concentration increases the height of the curve (total number of molecules) and increases the number of successful collisions, while $E_a$ remains unchanged.

How are the first 20 elements categorized by bonding and structure?

• Metallic bonding: Li, Be, Na, Mg, Al, K, Ca
• Covalent network: B, C (except buckminsterfullerenes), Si
• Discrete covalent molecules: H, N, O, F, Cl (diatomic), P, S (other small molecules)
• Monatomic elements: He, Ne, Ar

What is atomic size (covalent atomic radius)?

Half the distance between the nuclei of two bonded atoms of the same element.

If the covalent radii of hydrogen and chlorine are $32\,pm$ and $100\,pm$ respectively, what is the bond length in $\text{HCl}$?

$\text{Bond length} = 32\,pm + 100\,pm = 132\,pm$ (picometers).

What is the trend in covalent radius across a period, and why?

Covalent radius decreases across a period because nuclear charge increases while the number of electron shells stays the same, holding outer electrons more tightly.

What is the trend in covalent radius down a group, and why?

Covalent radius increases down a group because the number of electron shells increases, and inner full shells shield the outer electrons from the nuclear charge (screening effect).

What is first ionisation energy ($1^{\text{st}} IE$)?

The energy required to remove one mole of electrons from one mole of gaseous atoms. It is measured in $kJ\,mol^{-1}$.

What is the chemical equation for the first ionisation energy of sodium ($\text{Na}$)?

$\text{Na}(g) \rightarrow \text{Na}^+(g) + e^-$ ($1^{\text{st}} IE = 496\,kJ\,mol^{-1}$).

Why does ionisation energy increase across a period?

Increased nuclear charge holds outer electrons more closely and tightly, requiring more energy to remove an electron.

Why does ionisation energy decrease down a group?

Due to the screening effect of extra electron shells, outer electrons are further from the nucleus and experience less attraction, requiring less energy to remove an electron.