Chapter 10 Flashcards
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 3 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
- Use an inverted measuring cylinder and trough filled with water, and record the volume of displaced water
How is rate of reaction usually shown on a graph?
- Time (s) 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 of
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 some limitations when choosing conditions for reversible reactions in industry?
- High pressures are expensive and dangerous
- Low temperatures can sometimes increase the yield of product, but the rate of reaction would be too slow
What is Kc, and what does it show?
- An equilibrium constant
- The position of equilibrium in homogeneous reactions
What do square brackets mean?
- The concentration of the substance inside the brackets
How is the equilibrium constant calculated?
- If your reaction is aA + bB <-> cC + dD then Kc=[C]^c[D]^d / [A]^a[B]^b
What does the calculated value of Kc tell us?
- If it less than 1, the position of equilibrium is towards the reactants
- If it is equal to 1, it is halfway between the reactants and the products
- If it greater than 1, it is towards the products
