Chapter 10
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How does concentration/ pressure affect the rate of reaction?
• The higher the concentration/ pressure, the more molecules per unit volume, which increases the frequency of collisions
What are 2 ways of monitoring the rate of reaction of reactions that produce gas?
• Use a gas syringe to measure the volume of gas produced • Place the conical flask (with wool at the top to prevent any splashing) on a balance, and record the change in mass
How is rate of reaction usually shown on a graph?
• Time against concentration/ mass/ volume of gas, etc. • The rate of reaction is the gradient
What do catalysts do?
• A catalyst provides an alternative reaction route with a lower activation energy • This causes a greater proportion of the molecules to have the activation energy or more • This increases the rate of the reaction • They themselves are not used up
What needs to be included on an enthalpy profile (including the use of a catalyst)?
• x-axis: progress of reaction • y-axis: enthalpy • ΔH • Ea • Ec
What is a heterogeneous catalyst?
• A catalyst that has a different physical state from the reactants
What is a homogeneous catalyst?
• A catalyst that has the same physical state as the reactants
What are 2 benefits of using a catalyst?
• Lower temperatures and pressures can be used • This decreases the combustion of fossil fuels and therefore carbon dioxide emissions
What does the Boltzmann distribution show?
• The spread of molecular energies in gases
When drawing the Boltzmann distribution, what are 7 things to include?
• x-axis: energy • y-axis: number of molecules • Starts at origin • Peak • Lower gradient on right • Ea • Does not meet x-axis at the end
Why does the graph cross at the origin?
• None of the molecules have 0 energy
What does the area under the graph show?
• The total number of molecules
Why does the curve never meet the x-axis?
• There is no maximum energy of a molecule- the curve would need to reach infinite energy for it to meet the x-axis
How does temperature affect a Boltzmann distribution curve?
• The peak is lower and to the right • The curve still has a higher gradient on the left side rather than the right side, and the Ea is in the same position • The area under the curve is the same
How does increasing the temperature increase the rate of reaction?
• At higher temperatures, more molecules have energy greater than or equal to the activation energy
How does a catalyst affect a Boltzmann distribution curve?
• There is a new, lower activation energy; Ec
When does dynamic equilibrium exist?
• In a closed system where the rate of the forwards and backwards reactions are equal, and the concentrations of the products and reactants don’t change
What is Le Chatelier’s principle?
• The position of a dynamic equilibrium shifts to minimise the effect of any change
How do changes in concentration affect equilibrium?
• If there is an increase in the concentration of the reactants or a decrease in the concentration of the products, equilibrium shifts to the right (so that more of the products are formed) • If there is an increase in the concentration of the products or a decrease in the concentration of the reactants, equilibrium shifts to the left
How does temperature affect the position of equilibrium?
• Increasing the temperature shifts the position of equilibrium towards the products of the endothermic reaction • Decreasing the temperature shifts the position of equilibrium away from the products of the endothermic reaction
How does pressure affect equilibrium?
• Increasing pressure shifts equilibrium towards the side with fewer moles of gas • Decreasing the pressure does the opposite
How does a catalyst affect equilibrium?
• It doesn’t have an effect on the position of equilibrium • It increases the rate of both the forward and backward reactions by the same amount
What are the limitations when choosing conditions for reversible reactions in industry?
• High pressures are expensive and dangerous • Low temperatures cause a low rate of reaction • High temperatures use a lot of energy • Low pressures lead to a low rate of reaction
What is Kc, and what does it show?
• An equilibrium constant • The position of equilibrium in homogeneous reactions