Module 5: Section 1 Flashcards
Describe reaction rate
Change in amount of reactants or products per unit time
Give 4 ways to follow the rate of reaction
- Measure the vol. of gas evolved
- Measure the loss in mass as a gas is evolved.
- Use colorimetry to measure colour change of a reaction.
- Measure pH of a reaction.
Give 2 ways you can use experimental data to work out the order.
- Continuously monitor the change in concentration of A against time to construct a rate-conc graph.
- Use an initial rates method to find out how the initial rate changes as you vary the conc of A
Describe the Conc-time graphs for a reaction with:
- Zero order
- First order
- Diagonal line decreasing in conc over time
2. Curved decreasing line
Describe the Rate-conc graphs for a reaction with:
- Zero order
- First order
- Second order.
- Straight horizontal line at top of y-axis (rate)
- Straight diagonal line increasing evenly
- Curved increasing line.
If the rate constant (K) is large, what does this tell you about the reaction?
The larger the rate constant, the faster the reaction.
Define half life.
Time it takes for half of the reactant to be used up.
In a first order reaction, how is half life affected by changes in concentration?
The half-life is independent of the concentration, so half-life stays the same.
Give the equation linking half-life and the rate constant.
K = ln2 / half life
How does changing the temperature change the rate constant?
The rate constant applies to a particular reaction at a certain temperature. At a higher temperature, the reaction will have a higher rate constant.
Give the Arrhenius equation and state what all the variables stand for.
K = A e ^ -Ea / RT
K = rate constant Ea = activation energy T = temperature (K) R = gas constant (8.314) A = pre-exponential factor
State Arrhenius equation in logarithmic form.
lnK = - Ea / RT + lnA
On a graph where you create an Arrhenius plot, what are the axis labels?
What is the gradient and y-intercept?
Y axis = lnK
X axis = 1/T
Gradient = -Ea / RT Y-intercept = lnA
Give 2 features of a dynamic equilibrium.
- Rate of the forward reaction is the same as the rate of the reverse reaction.
- There is no overall change in the concentrations of the reactants and products.
What condition are necessary for a dynamic equilibrium to be set up?
Dynamic equilibrium can only happen in a closed system at a constant pressure.
Define total pressure of a gas mixture.
Total pressure of a gas mixture is the sum of all the partial pressures of the individual gases.
How do you calculate partial pressures?
- Calculate mole fraction;
no of mols of gas X / total no of mols of gas - Calculate partial pressure;
mole fraction x total pressure of the mixture.
The equilibrium constant Kp is always measured in what?
Partial pressures.
Kc is always measured in concentrations.
When writing Kp, what dont you include?
You do not include any solids or liquids for a heterogeneous equilibrium.
State La Chatelier’s principle.
If a system in dynamic equilibrium is subject to an external change in conditions, it will move to minimise the change.
What does a large Kc value tell you?
The larger the value of Kc, the further to the right the equilibrium lies and the more products there will be, relative to reactants.
What does a small Kc value tell you?
The smaller the value of Kc, the further to the left the equilibrium lies and the more reactants there will be relative to products.
Only one condition causes the equilibrium constant to change, what is it?
Temperature.
Changing the temperature alters the position of equilibrium and the value of the equilibrium constant.
What affect do catalysts have on the position of equilibrium?
Catalysts have no affect on the position of equilibrium, they only cause equilibrium to be approached faster.
Define a Bronsted-Lowry acid.
They are proton donors.
Define a Bronsted-Lowry base.
They are proton acceptors.
Describe a monobasic acid, and give an example.
Acids that can only donate one proton.
HCl, HNO3
Describe a dibasic acid, and give an example.
Acids that can donate 2 protons.
H2SO4
Describe a tribasic acid, and give an example.
Acids that can donate 3 protons.
H3PO4
What is formed when you react Metals and Acids.
Metal + Acid —> Salt + Hydrogen
What is formed when you react Carbonates and Acids.
Carbonate + Acid —> Carbon dioxide + Water
How do you calculate pH of strong acids?
Strong acids fully ionise, so the [H+] is the same as the acid concentration.
Give the Kw expression.
H2O(l) —> OH-(aq) + H+(aq)
How does changing the concentration of [H+] or [OH-] affect the value of Kw and why
It has no effect on the value of Kw, as the equilibrium will shift, changing the concentrations of the other substances to keep the value of Kw the same.
How does changing the temperature of the solution affect the value of Kw and why.
Value of Kw changes as dissociation of water is an endothermic process;
For Example, if it is warmed, the equilibrium moves to the right and Kw increases.
How do you calculate pH of strong bases?
Use the Kw expression.
Strong bases fully ionise in water and donate one mole of OH- ions per mole of base. This means [OH-] is the same as the concentration of the base.
How do you calculate the pH of weak acids?
Set up a Ka expression;
Ka = [H+] [A-] / [HA]
You can assume that all the H+ ions in the solution come from the acid, so;
Ka = [H+]^2 / [HA]
How do you calculate pKa from Ka
pKa = -log10 Ka
How do you calculate Ka from pKa
Ka = 10^-pKa
Define a buffer.
A buffer is a solution that minimises changes in pH when small amounts of acid or base are added.
How does a buffer minimise pH changes if a small amount of acid is added.
If you add a small amount of acid, the [H+] increases. H+ react with CH3COO- ions to form CH3COOH. Equilibrium shifts to the left, reducing the [H+] to its original value, minimising changes in pH.
How does a buffer minimise pH changes if a small amount of alkali is added?
If a small amount of alkali is added, the [OH-] increases. It reacts with H+ to form water, removing the H+ ions.
OH- + H+ —> H2O
More CH3COOH dissociates to form H+ ions and so equilibrium shifts to the right. [H+] increases until its close to its original value, minimising changes in pH.
How can you measure pH of a solution?
Use a pH meter.
What does a small pKa value tell you?
small pKa = strong acid
