Kinetics - Chemistry Physical

Collision Theory

• particles need to collide with energy < or = activating energy

• In right orientation to react successfully

• More frequent and successful collision = faster rate of reaction

What 4 factors affect rate of reaction

4 factors that affect rate of reaction are

• Change in concentration/pressure

• Change in Temperature

• Change in Surface Area

• Addition of Catalyst

what 3 things happen when there’s an increase in concentration/pressure

• collision frequency increases

• Collision energy stays the same

• Activation energy stays the same

• NO CHANGE IN MAXWELL BOTZMAN DISTRIBUTION

What 3 things happen when there’s increase in temperature

• increase in number of molecules with activation energy

• More frequency of collisions

• More successful collisions

  • increase in most probable energy

What 3 things happen when surface area increases

• collision frequency increases

• Collision energy stays the same

• Activation energy stays the same

What 3 things happen when catalyst is added

• Provides an alternative route

• collision Frequency stays the same

• Collision Energy stays the same

• Activation energy decreases

• NO CHANGE IN SHAPE OF MAXWELL BOLTZMANN

Maxwell-Boltzmann Distribution

• all molecules in a substance don’t have the same amount of energy they are distributed in a pattern called Maxwell

• Total area under curve represents number of molecules, therefore must remain constant

Explain process that causes some molecules to have very low energies

• collisions

• Cause molecules to slow down or loose energy

What is activation energy?

Minimum energy required for a reaction to occur

What is the Form of Rate Equation

Rate = k[A]^m [B]^m

What is the orders of reaction

Order of reaction is the power of concentration term in rate of reaction.

• if reactant is doubled and rate stays same - 0

• If reactant doubles and rate doubles - 1

• If reactant doubles rate quadruples - 2

Why is initial rates of reaction used to determine these orders rather than rates of reaction at different times during experiment

Concentrations are known

What is the overall order

it is the sum of the reactants orders

What Units of Rate constant

• concentration - mol/dm3

• Rate - mol dm^-3 S^-1

• K depends on orders of reactants

What do the graphs look like for concentration against time

• zero order - downwards straight line

• First order - downwards curve

• Second order - steeper than first order

What do the graphs for Rate against concentration look like

• zero - straight horizontal line (y is constant)

• First order - x=y

• Second - upwards curve y=x²

What is the Reaction Mechanism

Reaction mechanism is the series of steps that make up the overall reaction

What is the rate determining step

Rate determining step is the slowest step in the reaction mechanism

How do we find the rate determining Step

• Series if rate experiments are done to derive rate equation

• If reactant appears in rate equation it’s involved in rate determining step

• Order tells you number of molecules involved

How do you know a substance is a catalyst

Appears in rate equation

Not in reaction equation

A mixture of 2dm3 of hydrogen 1 dm3 oxygen is at RTP. What happens

a) No reaction to form water because molecules do not collide with sufficient energy

b) molecules do not collide with sufficient energy

C) mean velocity of H is less than O

D) partial pressure of each gas is same

a) No reaction to form water because molecules do not collide with sufficient energy

Choose correct statement : if distribution curve of molecular energies is an ideal gas at a given temperature

A) no molecules with 0 energy

B) curve is symmetrical about maximum

C) changing temp as no effect on position of maximum

D) most molecules have mean energy

A) no molecules with 0 energy

Addition of catalyst to an equilibrium mixture choose correct answer :

A) activation energy for reverse action increases

B) equilibrium constant for forward reaction increases

C) Rate of reverse reaction increases

D) enthalpy change for forward reaction decreases

C) Rate of reverse reaction increases

In a given temperature the average kinetic energy for molecules in gas is??

Constant

Which statement is correct for the distribution curve of molecular energies in a gas?A. The curve is symmetrical about the maximum.

B. There are always some molecules with zero energy.

C. The position of the maximum of the curve is not dependent on the temperature.

D. The mean energy of the molecules is greater than the most probable energy of the molecules.

D) mean energy of molecules is greater than most probable energy

When using larger conical flask how is the time affected

Time will increase

Collision theory In endothermic reaction

• as concentration increases amount of heat given out increases

• More Successful Collisions

• Increases rate of reaction

Why it's important to clean the exposed magnesium ribbon before reacting it with hydrochloric acid

• it’s coated with MgO

• The corrosion product will react not Mg

Two differences between the reactions of magnesium with hot water and steam

• reaction is faster with steam

• MgO produce with water Mg(OH)2 with steam

• Bubbling with water and bright white light with steam

Rate of reaction

Measured change in concentration of a substance in unit time

When is it possible to use 1/T to find rate of reaction

When measured change is fixed or constant

In a typical procedure, a mixture of 1.00 g of propanone, 5.00 g of ethane-1,2-diol and 0.100 g of benzenesulphonic acid, C6H5SO3H, is heated under reflux in an inert solvent. Benzenesulphonic acid is a strong acid.When the concentration of benzenesulphonic acid is doubled, the rate of the reaction doubles. It can be deduced that:A. the reaction is first order overall.

B. the reaction is third order overall.

C. the reaction is acid-catalysed.

D. units for the rate constant, k, are mol-2dm6s-1.

C. Reaction is acid catalysed

( not enough informations about the other reactant orders for it to be A/B)

The rate equation for the hydrogenation of ethene (C₂H₄(g) + H₂(g) → C₂H₆(g)) is Rate = k[C₂H₄][H₂]. At a fixed temperature, the reaction mixture is compressed to triple the original pressure. What is the factor by which the rate of reaction changes?

9

What is the rate Equation where Conc of propane is 100x that of iodine and hydrochloric acid if iodine is order 0

K[H+]. The concentration change of propane is negligible as its constant so has a constant effect on rate and therefore is also order 0

A general equation for a reaction is shown.

A(aq)+B(aq)+C(aq)→D(aq)+E(aq)

In aqueous solution, A, B, C and D are all colourless but E is dark blue.

A reagent (X) is available that reacts rapidly with E. This means that, if a small amount of X

is included in the initial reaction mixture, it will react with any E produced until all of the X

has been used up.

Explain, giving brief experimental details, how you could use a series of experiments to

determine the order of this reaction with respect to A. In each experiment you should obtain

a measure of the initial rate of reaction

Stage 1: Preparation1a. Measure known volumes of reagents.

1b. Measure a known amount of reagent X or use a colorimeter.

1c. Combine reagents in separate containers (up to two reagents and X can be mixed together, but X should not be added last)Stage 2: Procedure2a. Start a timer when mixing reagents (unless only two reagents are mixed)

2b. Record the time for a color change (blue), colorimeter reading, or disappearing cross.

2c. Use the same concentration of B and C, the same total volume, and the same amount of X.

2d. Maintain a constant temperature (using a water bath, if necessary)

2e. Repeat the experiment with different concentrations of A (this can be achieved by varying the volume of A while keeping the total volume constant)Stage 3: Use of Results3a. Calculate the rate as 1/time.

3b. Plot 1/time against the concentration/volume of A, or plot log(1/time) against log(concentration/volume of A)

3c. Interpret the graph to determine the order of the reaction (the shape of the graph, the gradient of the log plot, or the ratio of changes in concentration and rate can be used to determine the order)

What does it mean by term order of reaction with respect to A

Power of concentration term

Why Doubling temperature has a much greater effect on rate of reaction than doubling concentration

Reaction occurs when energy is greater or equal to activation energy

When doubling temperature many more particles reach activation energy or more

But when doubling concentration of E it only doubles the number of particles with this concentration