Chemistry topic 10 Reaction rates and equilibrium

10 .1- 5

Define the rate of reaction

A measure of how fast a product is being made or how fast a reactant is being used up.

Explain rate of reaction with an equation and give units.

Rate of raction is the change in concentration of a reactant or product in a given time.

Equation: Rate (mol dm^-3 s^-1)= change of concentration (mol dm^-3) / time(s)

What is a concentration time graph and explain the progress of a chemical reaction as time pass. (4)

A concentration time graph have the y axis with the concentration along with the x axis with the time which is used to moniter the rate of a chemical reaction.

1. At the start of the chemical reaction, the concentration if reactants are high, the gradient is the steepest at this point.

2. In the middle of the process, the rate of reaction starts to slow down/ gradient is shallower as concentration of reactants decrease (being used up to form products)

3. Once the limiting reactant has been used up, the concentration stops changing- remain constant, the gradient turns into a straight line and the rate of reaction is zero at the end.

Explain the collision theory and the 2 conditions required for effective collisions.

The collision theory stated that for a reaction to occur, collisions must occur.

Effective collisons only occur when

1. The particles collide in the correct orientation

2. The particles have sufficient energy to overcome the activation energy barrier of the reaction for bonds to be broken.

What are the two conditions that increases the rate of reaction and describe how they can be acheived.

1. More frequent collisions- Increase the speed to particles or the number if particles present in the reaction

2. More successful collisions- Lower the activation energy or provide more energy to the particles

Describe six ways how the rate of a chemical reaction can be increased.

1. Increase temperature- increases the kenetic energy of particles

2. Increase pressure- forces particles to be more closely together for more frequent collisions

3. Increase concentration- more particles present- more frequent collisions

4. Adding catalyst- Lowers the activation energy for particles to react and collide

5. Increase surface area of solid reactants- Increase chance of collisions as more particles are exposed to react/ collide

6. Shine UV light- provides energy to break bonds

Describe the two ways of monitoring the reactions of producing gases.

1. Measuring the mass loss- where a balance is used to record gas escaped/ produced at regualr time intervals. The apparatus needed is a conical flask filled with reaction mixture and cotton wool stuck on top

2. Measuring products formed- with a gas syringe or upturned measuring cyclinder filled with water. The gas produced is recorded at regular intervals of time.

Compare and describe the graphs drawn from recorded results for both gas production monitoring methods. {better to draw out}

• Both graphs will have y axis with the volume of gass produced (cm³) or the mass of gas loss (g) and the x axis of time(s).

• Both reaction rate decreases as concentration of rractants decreases.

• The mass loss graph will have a negative non linear curve, meaning when time passes, (rate of) mass of gas loss decreases until reaction ends.

• While the graph for measuring the volume of gas produced, there is a positive non linear curve, showing as time passes, the volume of gases increases (products are made) until reaction ends.

What is the purpose of drawing tangents of these graphs? (2)

To find the instataneous rate of change at a specific point.

A straight line approximation is provided instead of a constantly changing curved line.

How do you find the initial rate (t =0) from the graph? (3)

1. Plot a tangent line starting from the origin (y=0), touching the curve without crossing it

2. Pick two points from the tangent and calculate the rate/ find the gradient by using [change in y/ change in x]

3. Make sure the units are right (either cm³ s^-1 or g s^-1)

What is a catalyst?

Something that speeds up the rate of reaction by lowering the activation energy/ provide an alternative pathway for a reaction to occur without being used up itself.

What is an intermediate that occurs in reaction mechanism when catalyst are used?

An intermediate is a specie that temporarily exist that allow reactions to proceed in multiple smaller steps instead of one large step.

Draw and label enthalpy profile diagrams for both exothermic and endothermic reactions with an addition of catalyst. (6)

• y axis labeled enthalpy, x axis labeled progress of reaction

• Exothermic reaction have reactant on a higher enthalpy level than products

• Endothermic reaction have reactant at a lower enthalpy level than products

• Ea- activation energy is always positive and labeled by an arrow going from reactants to the peak/ transition state

• Ec- activation with catalyst should be labeled lower than Ea

• Enthalpy change for exothermic reaction is negative (arrow downwards) and positive for endothermic reaction (arrows upright)

Explain the two types of catalysts and give examples for each.

1. Homogeneous- same physical state as reactants (eg. ozone depletion- chlorine radical)

2. Heterogeneous- different physical state as reactants (eg. Haber process- iron catalyst , honey comb catalytic conversters)

Explain how the honey comb catalytic converters work and give chemical formula for reaction occurs.

A honey comb construction allow large surface area to maximise gas contact with solid catalyst.

It converts incomplete combustion products like carbon monoxide into carbon dioxide that is non toxic.

NOx + CO + HC → H2O + CO2 + N2

Give 2 examples of industrial process (including balanced chemical formulas) that uses heterogeneous catalyst and name the suitable catalysts.

1. The contact process- that makes sulfur trioxide for sulfuric acid. 2SO_2(g) + O_2(g) → 2SO_3(g) - using V₂O_{5 (s)} (vanadium pandoxide)

2. Hydrogenation of alkenes- turn alkenes into alkanes. C₂H_4(g) + H_2(g) → C₂H_6(g) - using nickle (Ni _(s))

Suggest the sustainability benefits and economic importance of using catalysts. (5)

1. Reduce temperature and energy needed

2. Less electricity or fossil fuel used

3. Increase yield and profit

4. Very sustainable because fewer pollutants and fewer greenhouse gases

5. High atom economy

What is an autocatalysis?

A catalyst that is produced as a product in a chemical reaction which speeds up the rate of reaction.

Describe and label the features of a Boltzmann distribution. (4)

1. x axis= energy, y axis= number of molecules with energy

2. Area under curve = total number of molecules in a system

3. Curve starts at zero as no molecules have no energy

4. Curve does not touch the x axis at high energy as there is no maximum energy for a molecule

Describe the changes of the boltzmann distribution with the presence of a catalyst. (2)

Ec labeled on the x axis is more on the left side than the original Ea (without catalyst)

The activation energy is lower so there is a bigger area of molecules that have enough energy to start a reaction.

Describe the changes of the boltzmann distribution with a higher temperature [draw the graph and label T1 for original and T2 for a higher temp].

At T2, higher temperature, the peak is lower (flattened because the number of molecule remains unchanged) and the curve has shift to the right side where there is a greater area of moleucles that can overcome the activation energy.