Module 5 - Physical Chemistry and Transition Elements - Exam Questions Flashcards
A proposed mechanism for a reaction takes place in several steps. Suggest two reasons why the reaction couldn’t take place in one step (2)
Stoichiometry in rate equation does not match stoichiometry in overall equation
Collision unlikely with more than 2 ions/species/particles
What is meant by the half life of a reaction? (1)
Time for concentration of reactant to fall to half of the original value
How can a concentration against time graph for a reactant be used to work out order of reaction? (2)
Show two half-lives on graph
Zero - half life decreases
First - half life is constant
Second - half life increases
If concentration is altered, is the half life effected on a first order reactant? (1)
No
What effect on Kc does using a smaller container for the reaction have? (1)
No effect
What effect does adding a catalyst have on Kc? (1)
No effect
State three large scale uses of hydrogen (3)
Choose three from: Extraction of some metals from ores Fuel cells Hydrogenation of alkenes Haber process Making HCl Making methanol
The forward reaction is exothermic, if the temperature is raised, what effect is there on Kc? (1)
Kc is smaller
2 mols LHS, 2 mols RHS, if pressure is increased, what effect is there on Kc? (1)
Kc is same
Kc is not changed by pressure
4 mols LHS, 3 mols RHS, forward reaction exothermic. A chemist increases the temperature and pressure, explain why it can be difficult to predict the change to equilibrium position (2)
Increased temperature shifts to the left
Increased pressure shifts to the right as fewer moles of gas on RHS
Difficult to predict relative contributions of two opposing factors
Define Brønsted-Lowry acid (1)
Proton donor
Define weak acid (1)
Partially dissociates
What name is given to Kw? (1)
Ionic product of water
What is the expression for Kw? (1)
Kw = [H+(aq)] [OH–(aq)]
What is meant by the rate-determining step? (1)
The slowest step
What information is provided by Ka values? (1)
Strength of acid
H2PO4(-) can act as an acid or a base. Why can Ca(H2PO4)2 in water act as a buffer solution, using equations? (3)
H2PO4- produced from Ca(H2PO4)2
H2PO4(-) + H(+) H3PO4
H2PO4(-) H(+) + HPO4(2-)
State what is meant by a buffer solution (1)
A solution that minimises pH changes
Suggest a chemical that could be added to methanoic acid to prepare a buffer solution. Explain (2)
HCOONa Supplies HCOO(-)
What factors determine the pH of a buffer solution? (2)
Ka
Temperature
Write an equation for the reaction of nitric acid with limestone (2)
2HNO3 + CaCO3 -> Ca(NO3)2 +CO2 + H2O
Why is the pH scale a more convenient measurement for measuring acid concentrations than [H+]? (1)
pH makes numbers manageable
Define a strong acid (1)
Completely dissociates
A solution mixes NaOH and CH3COOH (less moles of NaOH than CH3COOH), why is it a buffer solution? (2)
Solution is a mixture of CH3COOH and CH3COONa
Has an excess of acid
A solution mixes NaOH and CH3COOH (same number of moles), why is it not a buffer solution? (2)
Contains only CH3COONa
Therefore it’s neutral
The pH of water at 25°C is 7.0, and 6.7 at 40°C. What does this show about water’s tendency to ionise with temperature? (2)
[H+] increases
H2O H+ + OH-
Equilibrium moves to the right
Why is second electron affinity an endothermic process? (2)
Negative ion repels electron being added
Suggest why the entropy of water is zero at 0K (1)
Particles are in maximum state of order