Chemistry Yr13 Mock: Enthalpy Calculations

Questions and Answers

Calculate the value for the enthalpy of lattice formation for magnesium oxide using the given values.

-338

Calculate the partial pressure of nitrogen (N2) and hydrogen (H2) in the equilibrium mixture when total pressure is 150 kPa and mole fraction of NH3 is 0.80.

N2 = 0.05, H2 = 0.15

Give an expression for the equilibrium constant (Kp) for the reaction N2(g) + 3H2(g) <=> 2NH3(g).

(pp NH3)² / (pp N2)(pp H2)³

Calculate the value for the equilibrium constant (Kp) for the reaction N2(g) + 3H2(g) <=> 2NH3(g) given the partial pressures.

4.90, units = kPa² / (kPa kPa³) Signup and view all the answers

State the effect of an increase in temperature on the value of Kp for the reaction with an enthalpy change of -92 kJ mol-1.

down Signup and view all the answers

Calculate the entropy change for the reaction between ammonia and oxygen.

181 JK-1 mol-1 Signup and view all the answers

Calculate the Gibbs free-energy change (ΔG) for the reaction between ammonia and oxygen at 600°C.

-1090 Signup and view all the answers

Explain how an increase in temperature affects the value of ΔG for the reaction between ammonia and oxygen.

ΔG is more negative because TΔS is more negative. Signup and view all the answers

Describe the stages of the alternative route provided by platinum as a heterogeneous catalyst in the reaction between ammonia and oxygen.

Adsorption, Bond weakening, Desorption Signup and view all the answers

Deduce the change in oxidation state of nitrogen when NH3 is oxidised to NO.

-3 to +2 Signup and view all the answers

Give an equation for the reaction when ammonia reacts with oxygen to produce nitrous oxide (N2O).

2NH3 + 2O2 → N2O + 3H2 Signup and view all the answers

Give the full electron configuration for the calcium ion, Ca 2+.

1s² 2s² 2p⁶ 3s² 3p⁶ Signup and view all the answers

Identify the S block metal that has the highest first ionisation energy.

Be Signup and view all the answers

Give the formula of the hydroxide of the element in Group 2 (from Mg to Ba) that is least soluble in water.

Mg(OH)2 Signup and view all the answers

Calculate the length of the flight tube in meters for a 137Ba+ ion given the kinetic energy and time taken.

1.54 Signup and view all the answers

State the meaning of the term strong acid.

Fully dissociated Signup and view all the answers

Calculate the pH of the solution formed from 10.35 cm3 of 0.100 mol dm⁻³ hydrochloric acid and 25.0 of 0.150 mol dm⁻³ barium hydroxide at 30°C.

Provide calculations; exact answer not included. Signup and view all the answers

Give a reason why water is neutral at 30°C when the pH is 6.92.

[OH-] = [H+] Signup and view all the answers

Identify the oxide that could react with water to form a solution with pH = 2.

SO2 Signup and view all the answers

Give the expression for the acid dissociation constant (Ka) for acetic acid (CH3COOH).

[CH3COO-] [H+] / [CH3COOH] Signup and view all the answers

Explain why sodium bromide has a higher melting point than sodium and sodium iodide.

Sodium has metallic bonding, NaBr has ionic bonding. NaBr has greater electrostatic forces due to smaller Br-. Signup and view all the answers

Name the shape of the NH2- ion and predict the bond angle.

Shape = bent, bond angle = 105.5 Signup and view all the answers

Study Notes

Enthalpy of Lattice Formation

Formula for enthalpy of lattice formation: -602 - (+844 + 142 - 1450 - 736 - 150 - (496/2)) = -338 kJ/mol.

Partial Pressure Calculation

Equilibrium total pressure = 150 kPa, with NH3 mole fraction = 0.80; remaining mole fraction for N2 and H2 = 0.20.
Partial pressures calculated as follows: N2 = 0.05 and H2 = 0.15.

Equilibrium Constant (Kp) Expression

For the reaction N2(g) + 3H2(g) ⇌ 2NH3(g):
- Kp = (pp NH3)² / (pp N2)(pp H2)³.

Calculating Kp Value

Substitute partial pressures:
- Kp = (1.1 x 10³)² / (1.2 x 10²)(1.5 x 10²)³.
Result: Kp = 4.90 with units kPa² / (kPa * kPa³).

Effect of Temperature on Kp

Increasing temperature causes a decrease in Kp.
Justification: Equilibrium shifts left to counteract the temperature increase.

Entropy Change Calculation

For the reaction 4NH3(g) + 5O2(g) ⇌ 4NO(g) + 6H2O(g):
- Total entropy of products = 1978 J/K/mol, total entropy of reactants = 1797 J/K/mol.
- ΔS = 1978 - 1797 = 181 J/K/mol.

Gibbs Free Energy Change (ΔG)

Calculate ΔG using the formula ΔG = ΔH - TΔS:
- At 600°C: ΔH = -905 kJ/mol, ΔS = 211 J/K/mol; ΔG = -1090 kJ/mol.

Impact of Temperature on ΔG

Higher temperature leads to a more negative ΔG.
Explanation: TΔS becomes more substantial, contributing to a more negative overall ΔG.

Stages of Catalysis by Platinum

Heterogeneous catalysis involves three stages:
- Adsorption of reactants onto the catalyst surface.
- Bond weakening of reactants, facilitating reaction.
- Desorption of products, freeing the catalyst for more reactions.

Oxidation State Change

Oxidation of nitrogen in NH3 to NO involves an oxidation state change from -3 to +2.

Reaction Equation for Formation of N2O

The reaction producing nitrous oxide:
- 2NH3 + 2O2 → N2O + 3H2.

Calcium Ion Electron Configuration

Full electron configuration for Ca²⁺: 1s² 2s² 2p⁶ 3s² 3p⁶.

Highest Ionisation Energy in S Block

Beryllium (Be) exhibits the highest first ionisation energy among S block metals.

Least Soluble Hydroxide in Group 2

Magnesium hydroxide: Mg(OH)₂ is the least soluble in water among Group 2 hydroxides.

TOF Mass Spectrometer Calculation

Given KE of Ba⁺ ion = 3.65 x 10⁻¹⁶ J, time to detector = 2.71 x 10⁻⁵ s.
Calculate length of flight tube to find d = 1.54 m.

Definition of Strong Acid

A strong acid is defined as one that is fully dissociated in solution.

pH Calculation from Reaction

For reaction of HCl with Ba(OH)₂, balance and calculate pH at 30°C; gives two decimal places.

Water Neutrality at 30°C

Water is neutral at 30°C because [OH⁻] = [H⁺].

Acidic Oxide Identification

Sulfur dioxide (SO₂) reacts with water to produce a solution with pH = 2.

Acid Dissociation Constant (Ka) Expression

For acetic acid (CH₃COOH):
- Ka = [CH₃COO⁻][H⁺] / [CH₃COOH].

Sodium Bromide Melting Point Explanation

Sodium bromide (NaBr) has a higher melting point than sodium (Na) and sodium iodide (NaI) due to stronger ionic bonding and smaller ion size of Br⁻.

NH2- Ion Geometry and Prediction

NH2⁻ ion shape: bent, with a bond angle of 105.5° due to lone pair repulsion dominating over bonding pairs.

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Description

Test your understanding of enthalpy of lattice formation with these mock questions for Year 13 chemistry. This quiz includes calculations involving the enthalpy values for magnesium oxide and its components. Perfect for preparation before your exams!