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Chemistry > 1.7: Chemical Kinetics > Flashcards

1.7: Chemical Kinetics Flashcards

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1
Q

what does the rate of reaction show us?

A

How quickly a chemical reactant is used up or how quickly a chemical product is produced.

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2
Q

What is the rate of reaction?

A

The change in concentration of a reactant or a product in a given time
Rate: conc / time
units: moldm-³ S-¹

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3
Q

What must happen for a successful reaction to occur?

A

Bonds must generally broken. In order to break these bonds, energy must be put into the system to overcome the attractive force of the bonds

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4
Q

What is activation energy?

A

The minimum energy required to start a reaction by the breaking of bonds.

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5
Q

What do reaction profile diagrams show?

A

show the energy level of the chemical substances during the course of reaction

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6
Q

How is a reaction profile shown for an exothermic reaction?

A

Gives out heat energy to the surroundings, so the energy of the chemicals goes down

The reaction needs an initial input of energy to overcome the activation
energy for some particles, but the reaction would then be self-sustaining, as
energy is released as the products are formed.

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7
Q

How is a reaction profile shown for an endothermic reaction?

A

It takes in heat from the surroundings so the energy of the chemicals goes up

If the reaction is endothermic, the profile would be reversed, and the reaction
will require continuous energy input, but the activation energy is still likely
to be higher than the energy of the products.

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8
Q

What is activation energy shown as on a reaction profile?

A

Seen as an energy hump (or peak)

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9
Q

What factors can affect the rate of reaction?

A
  • concentration
  • pressure
  • catalyst
  • temperature
  • surface area
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10
Q

What must particles of in order for a chemical reaction to occur?

A
  • collide
  • have energy equal to or greater than activation energy
  • collide with correct orientation
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11
Q

What is the effect of concentration/pressure on RoR?

A

As the conc increases, there are more particles in a specific volume. Meaning particles will collide more frequently resulting an increase in RoR

The conc of gases can be increased by applying greater pressure to the system.

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12
Q

What is the effect of temperature on RoR?

A
  • more particles have energy greater than the activation energy, E>Ea
  • more successful collisions
  • increase rate of reaction
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13
Q

What is the Maxwell-Boltzmann Distribution of Molecular Energies?

A

The Maxwell-Boltzmann distribution curve shows the amount of energy that gaseous molecules have in a set amount of gas.

A few particles have little energy, most particles have an average amount of energy and a few particles have a very high amount of energy.
This is the theoretical energy the particles contain within their bonds and as kinetic energy.
The area under the curve represents the number of particles.

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14
Q

What are the two main types of catalysts?

A

heterogeneous and homogeneous.

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15
Q

What happens in a heterogeneous reaction?

A

The catalyst is in a different phase from the reactants

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16
Q

What happens in a homogeneous reaction?

A

The catalyst is in the same phase as the reactants.

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17
Q

What is a phase?

A

Solids, liquids (also liquids which are immiscible), and gases
E.g. Oil and water.

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18
Q

What is an example of a homogeneous catalyst?

A
  1. Esters can be formed by the reaction of a carboxylic acid and an alcohol. acts as a catalyst for the reaction.
19
Q

What are enzymes?

A
  • biological catalysts
  • Enzymes increase the rate of a specific chemical reaction, as the substrate is reacted in an active site which has a particular
    shape. Enzymes only work at a specific range of temperatures, with the optimum temperature being around 40 ° C, as enzymes are made up of proteins which become denatured at higher temperatures and are also affected by changes in pH
20
Q

What are examples of heterogeneous catalysts?

A
  1. In the Haber process nitrogen and hydrogen gas is passed over a solid catalyst.
  2. A catalytic converter platinum, rhodium and palladium.
21
Q

What is a catalytic converter

A

These metals catalyse a range of reaction which ultimately result in unburnt
hydrocarbons, carbon monoxide and nitrogen oxides being converted into
nitrogen, carbon dioxide and water.

22
Q

How does a heterogeneous catalyst work?

A
23
Q

How does Adsorption work?

A

Reactant molecules are adsorbed, bond onto catalyst surface

24
Q

How does reaction work as a heterogeneous catalyst?

A

Reactant held in right orientation and bonds in the reactants weaken.
Reaction takes place.

25
Q

What occurs in desorption in heterogeneous catalysts?

A

Products desorb leaving site vacant ready for other reactant molecules.

26
Q

What is rate defined as in Rate of reaction?

A

Rate is defined as a change in concentration with time.

27
Q

What is an example of a rate of reaction experiment?

A

Disappearing cross, As the reaction occurs, a solid precipitate of sulfur is formed, so we can see the formation of the solid. We can determine the rate of the reaction by timing how long it takes for an amount
of solid to form. Most simply – we look through the solution to a cross and time how long it takes for the cross to disappear

28
Q

What is the symbol equation for the disappearing cross reaction?

A

Na2S2O3(aq) + HCl(aq)  NaCl(aq) + H2O(l) + SO2(aq) + S(s)

29
Q

What is the equation for the reaction used to test RoR (Magnesium with hydrochloric acid)

A

Mg(s) + 2HCl(aq)  MgCl2(aq) + H2(g)

30
Q

How could the magnesium with hydrochloric acid reaction be measured?

A

Hydrogen gas is produced so the volume of gas could be measured over a period of time.

OR
the mass of the reaction mixture could be measured over a period of time. The mass will decrease as the hydrogen gas escapes.

31
Q

How could you determine the initial rate of reaction?

A

Plot a graph of volume or mass against time.
Draw a tangent to the curve at the start of the reaction, 0 seconds.
Measure the gradient of the tangent.
The initial gradient = initial rate of reaction.

32
Q

What is rate proportional to?

A

The concentration of the reactant raised to some power.

33
Q

What is the power in the concentration of the reactant called?

A

order of reaction

34
Q

what happens when you change conc of reactant?

A

Changing the concentration of a reactant can have a profound effect on the
rate of reaction

35
Q

What happens when you double the conc of a reactant?

A

would be expected to produce a doubling in the rate of reaction. This may be the case but not necessarily. The rate of reaction may increase by a factor of four or the rate may not change at all.

36
Q

What does this Rate ∝ [A]x mean?

A
  • where ∝ means proportional to,
  • [A] means concentration (e.g. of A) in moldm-3
  • and x is the order of reaction.
37
Q

what does this ‘Rate ∝ [A]x [B]y’ mean?

A

combination of 2 meanings of rate for reactant A and B

38
Q

what can proportionality be converted into?

A

equality by the introduction of a constant.

39
Q

What can ‘Rate ∝ [A]x [B]y’ be converted into?

A

an equality by the addition of k, the specific rate constant.

40
Q

What is the rate equation?

A

Rate = k [A]x [B]y

41
Q

What does a rate equation tell us?

A

The particles which are involved in the slowest step of a reaction mechanism. This is called the rate determining step

42
Q

What does the order of reaction do?

A

tells us how many molecules are involved in the rate determining step.

43
Q

What do we say if one, two, no molecule is involved?

A
  • If one molecule is involved, then we say that the order is first order.
  • If two molecules are involved, then we say that the order is second order.
  • If the molecule is not involved, then the order is zero order.
44
Q

How do we determine orders?

A

Using experimental data, which changes the concentration of individual reactants and looks at the effect on initial rate of reaction.

Chemistry (8 decks)

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