Kinetics

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Question 1The reaction between excess magnesium carbonate and hydrochloric

acid can be followed by measuring the volume of carbon dioxide produced with time. The results of one such reaction are shown below. How does the rate of this reaction change with time and what is the main reason for this change?

The rate increases with time because the acid becomes more dilute.

The rate decreases with time because the acid becomes more dilute.

The rate decreases with time because the magnesium carbonate particls get smaller.

The rate increases with time because the magnesium carbonate particls get smaller.

Time0

Wrong Answer!The magnesium carbonate is present in excess, hence there is little change in surface area.

average rate of production of hydrogen gas

Question 2Zinc reacts with sulfuric acid as shown below.

Zn(s) + H2SO4(aq) ZnSO4(aq) + H2(g)

Two identical samples of zinc powder were reacted with separate samples of excess acid as follows:

Reaction 1 : zinc added to 0.5 moldm-3 sulfuric acid

Reaction 2 : zinc added to 2.0 moldm-3 sulfuric acid

What is the same for reaction 1 and 2?

?total mass of hydrogen formed

total reaction time

initial reaction rate

Wrong Answer!Zinc is the limiting reagent and hence determines the volume of the hydroge gas. The 2 samples of zinc have the same mass.

Question 3

Which of the following is true for a catalyst ?

Changes the sign of the enthalpy, ΔH, for the

reaction.

Provides a new mechanism or pathway with lower activation energy needed in a reaction.

Decreases the activation energy Ea ,for the forward reaction only.

Increases the enthalpy change ΔH, for the reaction.

Wrong Answer!Catalysts lower the activation energy for the forward and backward reactions. They provide a new mechanism or pathway.

Question 4

The reaction between nitrogen and chlorine in the atmosphere under normal conditions is extremely slow. Which statement best explains this?

The concentration of nitrogen is too high, which slows the reaction.

The molar mass of nitrogen molecules is less than that of chlorine molecules.

Very few nitrogen and chlorine molecules have sufficient kinetic energy to react.

Nitrogen and chlorine molecules are both non-polar molecules.

Wrong Answer!Very few colliding molecules have kinetic energies equal to or in excess of the activation energy.

Question 5The rate of a reaction between two gases increases when

the temperature is increased and a catalyst is added. Which statements are correct for the effect of these changes on the reaction?

collision frequency activation energyincreases increases

activation energy activation energyincreases does not change

activation energy collision frequencyincreases increases

activation energy activation energydoes not change decreases

Increasing the temperature Adding a catalyst

Wrong Answer!Activation energy does not vary with temperature.

Question 6

What are the usual units for rate?

mol dm-3 s-1 mol (dm-3)2 s-1

mol s-1mol dm-3

Wrong Answer!Rate is defined as change in concentration (mol dm-3 ) with time (s)

Question 7

For the reaction:

2HCl(aq) + CaCO3(s) CaCl2(aq) + H2O(l) + CO2(g)

Which gives the fastest rate?

1.0 moldm-3 HCl and CaCO3 chips

2.0 moldm-3 HCl and CaCO3 chips

4.0 moldm-3 HCl and CaCO3 powder

2.0 moldm-3 HCl and CaCO3 powder

Wrong Answer!This combination has acid of the highest concentration and the greatest surface area for the calcium carbonate. Both these factors maximise the collision rate.

Question 8Excess magnesium was added to a beaker of aqueous hydrochloric acid

on a balance. A graph of the mass of the beaker and contents was plotted against time (line 1).

I only II only

None of the aboveIII only

Time

Line 1

Line 2

What change in the experiment could give line 2?I.The same mass of magnesium but in smaller pieces.II. The same volume of a more concentrated solution of hydrochloric acidIII. A lower temperature

Wrong Answer!The addition of hydrochloric acid of higher concentration means that the reaction will go faster and hence the loss in mass will be faster. In addition, since the acid is the limiting reagent then a greater total volume of gas is lost and a greater mass loss results.

Question 9

Which statement explains why the speed of some chemical reactions increased when the surface area of the reactants is increased?

This change increases the density of the reactant particles.

This change increases the concentration of the reactant.

This change alters the electrical conductivity of the reactant particles.

This change exposes more reactant particles to a possible collision.

Wrong Answer!The reaction takes place on the surface. The collision rate will be directly proportional to the rate.

Question 10

Which of the following reactions do not occur rapidly?

Rusting of iron Sodium placed in water

Reaction between reactive metals with acid

Reaction between hydrogen and oxygen

Wrong Answer!The reaction between reactive metals (including sodium) with water and acid is generally very fast. Rusting is a slow process.

Question 11

The rate of many reactions increase rapidly with small increase in temperature. Which of the following best accounts for this behaviour?

The bonds become weaker.

The activation energy decreases.

The number of molecules with the necessary kinetic energy increases.

The number of molecular collisions increases.

Wrong Answer!An increase in temperature does increase the collision rate, but the rise in proportion of molecules colliding with kinetic energies equal to or in excess of the activation increases is the major underlying reason for the rapid increase in rate.

Question 12

In the reaction shown below, which species may be acting as a catalyst?

O3 + Br BrO + O2

BrO + O2 Br + O2

O2 Br

BrOO

Wrong Answer!The bromine atom is an intermediate and behaves as both a reactant and a product.

Question 13

For which one of the following reactions would pressure measurements be the least sensitive for measuring the initial rate of reaction?

2H2O2(l) 2H2O(l) + O2(g) N2O4(g) 2NO2(g)

2NCl3 (g) N2(g) + 3CI2(g)H2(g) + I2(g) 2HI(g)

Wrong Answer!Two volumes of gas form two volumes of gas. Hence, there is no change in pressure.

Question 14

All the following statements are correct except

Increasing the pressure often decreases the rates of reaction involving gases.

The smaller the size of the reacting particles, the faster the reaction.

The rates of most chemical reactions derease with time.

The rate of a chemical reaction can be decreased by decreasing the temperature.

Wrong Answer!Increasing the pressure increases the concentration of reacting gases and hence generally increases the rate of reaction.

Question 15When 100cm3 of 1.0 moldm-3 methanoic

acid,HCOOH, are added to 1 gram of magnesium turnings at 200C, hydrogen gas is slowly produced. All the following will increase the initial rate of hydrogen production except

Substituting 1.0 moldm-3 hydrochloric acid for methanoic acid.

Substituting powdered magnesium in the place

for magnesium turnings.

Using 300cm3 of the 1.0 moldm-3 methanoic acid instead of 100cm3.

Increasing the temperature of the 1.0 moldm-3 methanoic acid at 600C.

Wrong Answer!Tripling the volume of the acid will not alter the initial rate of reaction since there is no change in concentration and hence there will be no effect on the collision rate between magnesium atoms and hydrogen ions from the methanoic acid.

Question 16When ammonia is manufactured commercially a catalyst is used. What

is the effect of this catalyst?

N2(g) + 3H2(g) 3NH3(g) ΔH = -92kJ

I. To shift the position of equilibrium and increase the yield of ammonia.

II. To increase the rate of only the forward reaction.

III. To increase the rates of the forward and reverse reactions.

I only II only

I and III onlyIII only

Wrong Answer!A catalyst will increase the rate at which equilibrium is reached, as it will speed up both the forward and reverse reactions equally, but it will have no effect on the position of the equilibrium and hence the value of Kc.

Congratulations!