Rates & Equilibrium | OCR A Level Chemistry

What is the fundamental requirement for a chemical reaction to occur according to collision theory?

Particles must collide with sufficient energy to break relevant bonds.

Define activation energy (EA).

The minimum energy required for particles to collide and start a reaction.

How does increasing concentration or pressure affect reaction rate?

It increases the number of particles per unit volume, leading to a higher frequency of effective collisions.

What is the mathematical definition of reaction rate?

The change in concentration of a substance per unit time.

How is the reaction rate determined from a concentration-time graph?

By calculating the gradient of the tangent to the curve at a specific time.

Why is 1/time used as an approximation for reaction rate in the sodium thiosulfate experiment?

It assumes the amount of sulfur produced is fixed and constant, allowing for a relative rate measurement.

What does a steeper gradient on a product-time graph indicate?

A faster rate of reaction.

Define a catalyst.

A substance that increases the rate of reaction without being consumed in the process.

How do catalysts increase reaction rates?

By providing an alternative reaction pathway with a lower activation energy.

What is the difference between a homogeneous and heterogeneous catalyst?

A homogeneous catalyst is in the same phase as the reactants, while a heterogeneous catalyst is in a different phase.

Where does a reaction involving a heterogeneous catalyst typically occur?

On the surface of the catalyst.

What are two environmental or economic benefits of using catalysts in industrial processes?

Reduced energy costs due to lower temperature/pressure requirements and decreased CO2 emissions.

What is the purpose of 'quenching' a reaction sample during titration?

To stop the reaction so the concentration of a reactant or product can be accurately measured.

Which technique is suitable for measuring the rate of a reaction where a colored product is formed?

Colorimetry.

When can electrical conductivity be used to measure reaction rate?

When there is a measurable change in the number of ions in the reaction mixture.

What does the area under a Maxwell-Boltzmann distribution curve represent?

The total number of particles present.

Why does the Maxwell-Boltzmann curve start at the origin?

Because there are no molecules with zero energy.

Why does the Maxwell-Boltzmann curve never touch the x-axis?

Because there is no theoretical maximum energy for molecules.

How do particles gain enough energy to react if the mean energy is below the activation energy?

Through collisions with other particles.

What happens to the Maxwell-Boltzmann distribution when temperature increases?

The curve shifts to the right, the peak lowers, and a greater proportion of molecules have energy above the activation energy.

Why does the total area under the Maxwell-Boltzmann curve remain constant when temperature changes?

Because the total number of particles in the system remains constant.

How does increasing surface area increase reaction rate?

It increases the frequency of collisions between reactant particles.

What is the unit for reaction rate?

mol dm-3s-1

What is the initial rate of a reaction?

The rate at the very start of the reaction, where it is at its fastest.

What is the primary advantage of using enzymes as catalysts?

They generate very specific products and operate effectively at room temperature and pressure.

What happens to the activation energy when a catalyst is added?

The activation energy is lowered, allowing more particles to have sufficient energy to react.

What is dynamic equilibrium?

A state in reversible reactions where forward and backward reactions occur at equal rates, keeping reactant and product concentrations constant.

What does it mean if the position of equilibrium is 'towards the left'?

The equilibrium mixture contains mostly reactants.

What is Le Chatelier's Principle?

If an external condition is changed, the equilibrium will shift to oppose and reverse that change.

How does increasing temperature affect an exothermic equilibrium?

The equilibrium shifts in the endothermic (backward) direction to absorb heat, decreasing the yield of products.

Why is a compromise temperature often used in industrial processes?

To balance the higher yield from lower temperatures with the faster reaction rates provided by higher temperatures.

How does increasing pressure affect equilibrium in a gaseous reaction?

The equilibrium shifts towards the side with fewer moles of gas to reduce the pressure.

What happens to the position of equilibrium if the number of moles of gas is the same on both sides?

Changing the pressure has no effect on the position of equilibrium.

What is the effect of a catalyst on the position of equilibrium?

A catalyst has no effect on the position of equilibrium; it only increases the rate at which equilibrium is achieved.

Why do industrial processes often use catalysts?

They allow for faster reaction rates at lower temperatures, reducing energy costs.

What are the conditions for the Haber process?

450°C, 200-1000 atm, and an iron catalyst.

What is the purpose of recycling unreacted reactants in industrial processes?

To improve the overall yield of the process.

What does the equilibrium constant Kc represent?

The ratio of the concentrations of products to reactants at equilibrium, each raised to the power of their stoichiometric coefficients.

Are liquids and solids included in heterogeneous Kc expressions?

No, their concentrations are considered constant.

What is the only factor that changes the value of Kc?

Temperature.

What does a large Kc value indicate?

The equilibrium mixture contains a greater amount of products.

What does a small Kc value indicate?

The equilibrium favours the reactants.

How does increasing the concentration of a reactant affect equilibrium?

The equilibrium shifts in the forward direction to remove the added reactant.

Why is high pressure expensive in industrial settings?

High electrical energy costs for pumping gases and high equipment costs to withstand the pressure.

What is the catalyst used in the Contact process?

Vanadium(V) oxide (V2O5).

What is the catalyst used in the hydration of ethene?

Concentrated phosphoric acid (H3PO4).

What is the catalyst used in the production of methanol from CO?

Chromium and zinc oxides.

What happens if pressure is decreased in a gaseous reaction?

The equilibrium shifts towards the side with more moles of gas to increase the pressure.

How does decreasing temperature affect an exothermic equilibrium?

The equilibrium shifts in the exothermic (forward) direction to release heat, increasing the yield of products.

What is the expression for Kc for the reaction N2 + 3H2 ⇌ 2NH3?

Kc = [NH3]² / ([N2] * [H2]³)

Does changing concentration or pressure affect the value of Kc?

No, Kc is independent of concentration and pressure changes.