All Free Chemistry MCQs with Answers

• A. pressure
• B. absolute temperature
• C. volume
• D. molar mass

Explanation: Average kinetic energy equals three halves of kT, so it is fixed by absolute temperature alone.

• A. high temperature and low pressure
• B. low temperature and high pressure
• C. high temperature and high pressure
• D. standard temperature and pressure

Explanation: At high pressure the molecules are close enough that their own volume is no longer negligible, and at low temperature they move slowly…

• A. the volume occupied by the gas molecules
• B. the intermolecular forces of attraction
• C. the number of moles present
• D. the temperature of the gas

Explanation: The term involving a raises the measured pressure to what it would have been without attractions pulling molecules away from the wall, so…

• A. 5.6 dm3
• B. 11.2 dm3
• C. 22.4 dm3
• D. 44.8 dm3

Explanation: One mole of any ideal gas occupies 22.4 dm3 at STP, so half a mole occupies 11.2 dm3.

• A. both reactions have stopped completely
• B. the forward and reverse reactions continue at equal rates
• C. the amounts of reactants and products are equal
• D. all the reactants have been converted to products

Explanation: The word dynamic means both reactions are still running; they simply cancel out, so the concentrations stay constant while molecules…

• A. [N2][H2]^3 divided by [NH3]^2
• B. [NH3] divided by [N2][H2]
• C. [NH3]^2 divided by [N2][H2]^3
• D. 2[NH3] divided by [N2] + 3[H2]

Explanation: Kc is written as the product concentrations over the reactant concentrations, each raised to the power of its coefficient in the balanced…

• A. the reaction reaches equilibrium very quickly
• B. the reaction is exothermic
• C. the reactants are favoured at equilibrium
• D. the products are strongly favoured at equilibrium

Explanation: Kc compares product to reactant concentrations, so a large value means the equilibrium position lies far to the right and the reaction…

• A. shifts in the direction that relieves the stress
• B. shifts in the direction that increases the stress
• C. stops reacting altogether
• D. always shifts towards the products

Explanation: The position of equilibrium moves so as to partly oppose the change imposed, whether that change is in concentration, pressure or…

• A. raising the temperature and lowering the pressure
• B. raising the pressure and lowering the temperature
• C. adding a catalyst
• D. removing nitrogen from the mixture

Explanation: There are four moles of gas on the left and two on the right, so high pressure pushes the equilibrium towards the smaller volume, and…

• A. the reaction is exothermic
• B. hydrogen iodide is a gas
• C. the number of moles of gas is the same on both sides
• D. the reaction has no catalyst

Explanation: Pressure changes shift an equilibrium only when the two sides differ in the number of gas molecules, and here there are two moles on each…

• A. shifts the equilibrium towards the products
• B. shifts the equilibrium towards the reactants
• C. increases the value of Kc
• D. speeds up the forward and reverse reactions equally, so equilibrium is reached sooner but is unchanged

Explanation: A catalyst lowers the activation energy of both directions by the same amount, so both rates rise in proportion and they still become…

• A. finely divided iron with promoters
• B. platinum
• C. vanadium pentoxide
• D. nickel

Explanation: Iron with small amounts of potassium and aluminium oxide as promoters is used because it is effective and cheap enough for a process…

• A. 1000 degrees Celsius and 1 atmosphere
• B. 25 degrees Celsius and 1000 atmospheres
• C. 450 degrees Celsius and 200 atmospheres
• D. 450 degrees Celsius and 1 atmosphere

Explanation: A moderately high temperature is a compromise, since a low temperature would give a better yield but far too slowly for industry, while…

• A. the total mass of the salt dissolved
• B. the product of the concentrations of its ions in a saturated solution, each raised to the power of its coefficient
• C. the concentration of the undissolved solid
• D. the ratio of dissolved to undissolved salt

Explanation: For a saturated solution in contact with excess solid, an equilibrium exists between the solid and its ions, and Ksp is the equilibrium…

• A. Ksp = [Ag+] + [Cl-]
• B. Ksp = [AgCl] divided by [Ag+][Cl-]
• C. Ksp = [Ag+][Cl-] divided by [AgCl]
• D. Ksp = [Ag+][Cl-]

Explanation: Silver chloride dissociates into one silver ion and one chloride ion, so Ksp is simply the product of the two concentrations, and the…

• A. the common ion effect
• B. the buffer action
• C. hydrolysis
• D. the catalytic effect

Explanation: Sodium chloride supplies extra chloride ions, and since the ionic product must not exceed Ksp the equilibrium shifts towards the…

• A. sodium acetate is a strong acid
• B. the added acetate ions shift the ionisation equilibrium back towards the un-ionised acid
• C. sodium ions react with the acid
• D. the temperature of the solution falls

Explanation: Sodium acetate is fully ionised and floods the solution with acetate, the common ion, so by Le Chatelier's principle the acid dissociates…

• A. has a pH of exactly 7
• B. neutralises all acids completely
• C. resists a change in pH when a small amount of acid or base is added
• D. conducts electricity better than water

Explanation: A buffer contains a reservoir of both a weak acid and its conjugate base, so added hydrogen ions are mopped up by the base and added…

• A. Hydrochloric acid and sodium chloride
• B. Sodium hydroxide and sodium chloride
• C. Ammonia and ammonium chloride
• D. Acetic acid and sodium acetate

Explanation: An acidic buffer needs a weak acid together with the salt of that acid, and acetic acid with sodium acetate is the standard example.

• A. concentration of the reactants
• B. pressure of the system
• C. temperature
• D. the catalyst used

Explanation: Concentration and pressure changes shift the position of equilibrium so that the same value of Kc is restored, and a catalyst affects only…