what drives chemical reactions

Entropy is a measure of the dispersal or distribution of matter and/or energy in a system.

Entropy measures how disordered or chaotic a system is.

False. An increase in entropy means a system becomes energetically more stable.

During the thermal decomposition of calcium carbonate, the entropy of the system increases.

The entropy increases when a solid melts because the particles become more disordered in the liquid state.

Standard molar entropy (S°) is the entropy value of one mole of a pure substance under standard conditions of temperature and pressure.

The units of entropy are joules per Kelvin per mole, J K⁻¹ mol⁻¹.

False. The entropy of a substance changes depending on its state of matter.

The entropy of water vapor is higher than that of liquid water.

The symbol for standard molar entropy is S°.

CuCO3 (s) → CuO (s) + CO2 (g) There is an increase in entropy when copper carbonate undergoes thermal decomposition because:

False. There is a decrease in entropy when water vapour is cooled to form water.

2NaHCO3 (s) → Na2CO3 (s) + CO2 (g) + H2O (g) The reaction shows an increase in entropy.

C3H8 (g) + 5O2 (g) → 3CO2 (g) + 4H2O (g)  The reaction shows an increase in entropy because 6 moles of gaseous reactant form 7 moles of gaseous product.

CH4 (g) + H2O (g) CO (g) + 3H2 (g)   The reverse reaction shows a decrease in entropy because 4 moles of gas form 2 moles of gas.

AgNO3 (aq) + NaBr (aq) → NaNO3 (aq) + AgBr (s)  The entropy change of the following precipitation reaction is negative because 2 moles of aqueous reactants form 1 mole of aqueous product and 1 mole of solid.

The equation for calculating standard entropy change is: ΔS°(reaction) = ΣS°(products) - ΣS°(reactants)

ΔS° represents the standard entropy change of a reaction.

True. When calculating ΔS°, the coefficients from the balanced equation must be applied.

Standard conditions are 298 K (25°C) and 1 atm pressure.

False. The entropy of a system can increase or decrease during a chemical reaction, depending on the nature of the reactants and products.

A positive ΔS° value indicates an increase in the disorder of the system / entropy.

A negative ΔS° value indicates a decrease in the disorder of the system / entropy.

As temperature increases, the entropy of a substance generally increases due to increased particle motion and disorder.

False. The standard entropy change for water vapour condensing is -119 J K-1 mol-1. H2O (g) → H2O (l)

The standard entropy change for the hydrogenation of propene is: C3H6 (g) + H2 (g) → C3H8 (g)

Gibbs free energy is a thermodynamic concept that combines enthalpy change and entropy change to determine the feasibility of a reaction.

The Gibbs free energy equation, in terms of enthalpy and entropy, is: ΔGo = ΔHo – TΔSo

The Gibbs free energy equation, in terms of ΔGo values, is: ΔGo = ΣΔGproductso – ΣΔGreactantso

The units of ΔG are kilojoules per mole, kJ mol⁻¹.

False. ΔG can be calculated using ΔH and ΔS values, along with the temperature.

A negative ΔG value indicates that a reaction is spontaneous or feasible.

A positive ΔG value indicates that a reaction is not spontaneous or feasible.

The relationship between ΔG and temperature is inverse, as shown in the equation ΔG = ΔH – TΔS.

True. The units of ΔS must be converted from J K⁻¹ mol⁻¹ to kJ K⁻¹ mol⁻¹ when calculating ΔG.

False. The units of ΔH and ΔG are kJ mol–1**. The units of ΔS J K-1 mol–1**.

The free energy change for the decomposition of sodium hydrogen carbonate, NaHCO3 (s), at 500 K is:

A spontaneous reaction is a reaction that occurs without external influence and has a negative Gibbs free energy change (ΔG < 0).

For a reaction to be spontaneous, the Gibbs free energy change (ΔG) must be negative or equal to zero.

False. Not all exothermic reactions are spontaneous. The spontaneity of a chemical reaction also depends on the entropy change.

The factors that determine the spontaneity of a reaction are:

For endothermic reactions with positive ΔS, increasing temperature makes the reaction more likely to be spontaneous.

False. Endothermic reactions with negative ΔS are never spontaneous, regardless of temperature.

The equation to determine the temperature at which a reaction becomes spontaneous is: T = ΔHꝋ / ΔSꝋ

Exothermic reactions with positive ΔS are always spontaneous, regardless of temperature.

As temperature increases, an exothermic reaction with negative ΔS becomes less feasible and may become non-spontaneous at very high temperatures.

The reaction is spontaneous because is ΔG negative.

Reactions become spontaneous when ΔHꝋ = 0, which means that T = ΔHꝋ / ΔSꝋ. The temperature at which the reaction of carbon monoxide with water becomes spontaneous is:

The equilibrium constant (K) is a value that indicates the ratio of products to reactants at equilibrium for a given reaction.

The reaction quotient, Q, represents the ratio of product concentrations to reactant concentrations at any point in a reaction, not necessarily at equilibrium.

True. At equilibrium, Q is equal to K.

As a reaction progresses towards equilibrium, the Gibbs free energy decreases until it reaches its minimum value at equilibrium.

When a reaction reaches equilibrium, ΔG becomes zero.

True. A large positive value of K indicates that products are favored at equilibrium.

If Q < K, the reaction will proceed forward; if Q > K, the reaction will proceed in reverse; if Q = K, the reaction is at equilibrium.

ΔG° determines the value of K:

The standard Gibbs free energy change, ΔG°, for the Haber Process at 298 K is:

The equation constant is:

The change in Gibbs free energy, ΔG, for the Haber process at 298 K is: This means that the reaction is not spontaneous as ΔG is positive.