Chemistry Paper 1 Flashcards
Ionic equation for the formation of NH3 gas
NH4+ + OH- —> NH3 + H2O
The enthalpy of combustion determined experimentally is less exothermic than that calculated using enthalpies of formation.
Give one possible reason for this, other than heat loss
Incomplete combustion
The student rinses the burette with deionised water before filling with sodium hydroxide solution.
State and explain the effect, if any, that this rinsing will have on the value of the titre.
- Titre value will increase
- Because NaOH solution would be more dilute
This question is about ionisation energies of Group 2 elements.
Explain why the first ionisation energy of the Group 2 elements decreases down the group.
- Shielding increases
- So weaker attraction between nucleus and outer electrons
Give an equation, including state symbols, to represent the process that occurs when the third ionisation energy of magnesium is measured
Mg2+ —> Mg3+ + e-
Explain why the third ionisation energy of magnesium is much higher than the second ionisation energy of magnesium.
- Electron removed from 2p subshell
- Electron removed has less shielding than 3s
The student uses this method to complete a titration.
Rinse a burette with distilled water.
Fill the burette with sodium hydroxide solution..
Use a measuring cylinder to transfer 25 cm³ of the citric acid solution into a
conical flask.
Add 5 cm³ of indicator.
Slowly add the sodium hydroxide solution from the burette into the conical flask Add the sodium hydroxide solution dropwise near the end point until the indicator
just changes colour.
Repeat the titration to get concordant results.
The method used by the student includes three practical steps that will lead to an inaccurate final result
For each of these three steps
identify the mistake
explain why it is a mistake
suggest how the mistake can be overcome
Stage 1:
a. Problem - using a measuring cylinder
b. Explanation - large uncertainty / not accurate enough
c. Improvement - use a (volumetric) pipette (Not dropping pipette)
Stage 2:
a. Problem too much indicator
b. Explanation - may react and affect the endpoint reading
c. Improvement - use a smaller volume (2-6 drops)
Stage 3:
a. Problem-rinsing the burette with distilled or deionised water
b. Explanation - will slightly dilute the alkali solution
c. Improvement - rinse the burette with alkali solution
Explain why CBr4 is not a polar molecule
- the molecule is completely symmetrical
- the dipoles cancel out
In acidic solution, 10, ions oxidise iodide ions to iodine.
10+51+ 6H 312 + 3H2O
Give a half-equation for the oxidation of iodide ions to iodine.
Deduce the half-equation to show the reduction process in this reaction
Oxidation:
2I- —> I2 + 2e-
Reduction:
2IO3- + 12H+ + 10e- —> I2 + 6H2O
When iodide ions are oxidised using concentrated sulfuric acid, sulfur dioxide, a yellow solid and a foul-smelling gas are all formed.
Give an equation to show the reaction between iodide ions and concentrated sulfuric acid to form the yellow solid.
Identify the foul-smelling gas.
Foul smelling gas: H2S
Equation:
H2SO4 + 6H+ + 6I- —> S + 3I- + 4H2O
Explain why using a catalyst has no effect on the % yield
Increases rate of forward and reverse reactions equally
Test for CO2
Gas is bubbled through limewater and a white ppt is formed
Describe a series of tests that can show a solution contains ammonium sulfate
- Warm with NaOH and damp red litmus paper turns blue
- Add BaCl2 and a white ppt forms
What is the importance of percentage yield?
How can it be improved?
Increases the amount of product that is made from the reactants.
% Y can only be improved by making improvements in experimental technique.
What is the importance of atom economy?
How can it be improved?
Reduces the amount of waste by products.
AT can only be improved by finding a different reaction to produce the desired product, which also produces fewer by products.
The atom economy of reaction 1 is 88.20%
The atom economy of reaction 2 is 68 45%
Give a reason for this difference.
An additional (named by product e.g CO2) is made aswell.
What is a mean bond enthalpy?
Is the mean energy required to break one molecule of covalent bonds under standard conditions with everything in a gas state, average out for the bond across a range of molecules
Why do mean bond enthalpies given or calculated differ from those determined using Hess’s law calculations?
The value as according to Hess’s law in only approximate, averaged out over a range of molecules. Whereas, in a specific compound, a bond will have a specific enthalpy which may differ from the mean value
Explain with the use of an equation, why a solution containing [Al(H2O)6]3+ has a pH of <7
Eq: [Al(H2O)6]3+ ≈ [Al(H2O)5(OH)]2+ + H+
Al3+ has a large charge and a small size
Weakens the OH bond, releasing H+ ions
State one observation when solid sodium chloride reacts with concentrated sulfuric acid.
Give an equation for the reaction.
State the role of the chloride ions in the reaction
Observation: misty fumes
Equation: NaCl + H2SO4 –> NaHSO4 + HCl
Role: base or proton acceptor
Give an equation for the redox reaction between solid sodium bromide and concentrated sulfuric acid.
Explain, using oxidation states, why this is a redox reaction
2NaBr + H2SO4 –> SO2 + Na2SO4 + Br2 + H2O
Br changes oxidation state from -1 to 0 and is oxidised
S changes oxidation state from +6 to +4 and is reduced
The Enthalpy of hydration of Ca2+(g) is -1650KJmol-1
Suggest why this value is less exothermic than that of Mg2+(g)
1) Ca2+ is bigger
2) therefore, a weaker attraction to the s- O in H2O
