Physical Chemistry

Question 1

What is the conservation of
energy principle?

Answer 1

Energy is conserved in chemical reactions. The amount of energy in the universe
at the end of a chemical reaction is the same as before the reaction takes place.

Question 2

What is an exothermic
reaction? Give examples

Answer 2

A reaction where energy is transferred to the surroundings so that the
surroundings temperature increases – combustion, oxidation reactions and
neutralisation (acid + alkali) reactions. Negative sign of energy change.

Question 3

What is an endothermic
reaction? Give examples

Answer 3

A reaction where energy is taken in from the surroundings so the surroundings
temperature decreases – thermal decomposition, reaction of citric acid and
sodium hydrogencarbonate. Positive sign of energy change.

Question 4

What is activation energy?

Answer 4

Minimum amount of energy that particles need to react.

Question 5

What is a reaction profile?

Answer 5

Reaction profile is a graph which shows the relative energies of reactants and
products, as well as the activation energy of the reaction.

Question 6

What occurs in a chemical reaction in terms of bond energies? Describe exothermic and endothermic reactions in terms of bond breaking/forming.

Answer 6

Energy is supplied to break bonds and energy is released when bonds are made; exothermic – energy released from forming bonds is greater than that needed to break the bonds; endothermic – energy needed to break bonds is greater than energy released making them.

Question 7

What is the equation to find
enthalpy change in terms of
bond energies?

Answer 7

Energy of reaction = sum of bond energies of bonds broken – sum of bond energies of bonds made

Question 8

How could you find the enthalpy change for a reaction involving aqueous solutions?

Answer 8

• a) Mix known quantities of the reagents (concentrations, volumes) in a polystyrene cup (a decent insulator, so as to minimise heat losses to surroundings).
• b) Measure the temperature change with a thermometer (ΔT)
• c) Assume the solution to have a density of H2O. Find the mass of the solution (m).
• d) Knowing 4.2 J are required to increase the temperature of 1 g of H2O by 1 degree,
  calculate the heat change (c, specific heat capacity of water).
• e) Enthalpy change (ΔH) = cmΔT. Convert to kJ/mol by including the stoichiometry of your
  reagents
• f) Include the sign of the enthalpy change, e.g. if the temperature decreased, the sign is
  +ve (endothermic process).

Question 9

How is rate of reaction
calculated?

Answer 9

Rate of reaction = amount (e.g. grams, cm3 ) of reactant used or product formed / time
Rate of reaction (mol/s) = Moles of reactant used or product formed / time

Question 10

What are the various units for
rate of reaction?

Answer 10

E.g. g/s or cm3
/s or mol/s
Generally, mass/time, volume/time, moles/time

Question 11

Name three common ways of
measuring rate of reaction

Answer 11

Loss in mass of reactants, volume of gas produced, time for a solution to become
opaque (precipitation).

Question 12

Describe measuring the rate
by monitoring mass loss

Answer 12

Place the reaction flask on a balance. In these reactions (e.g. metal carbonate +
acid) a gas is given off, so record the decrease in mass in time intervals (note
hydrogen is too light). Plot a graph of mass vs time.

Question 13

Describe measuring the rate by
monitoring the volume of a gas

Answer 13

Connect a gas syringe to a reaction flask and measure the volume of a gas
formed in time intervals. Plot a graph of volume vs time

Question 14

Describe measuring the rate by
monitoring the disappearance
of a cross.

Answer 14

Take a piece of paper and mark a cross (X) on it. Put the reaction flask on this cross. Mix the reagents, and measure how long it takes for a cloudy mixture to conceal a cross.

Question 15

How to find a rate of reaction at some time, t, from a graph of amount of reactant vs time?

Answer 15

Pick a point corresponding to the time t, and find the tangent to the curve at this
point.
The tangent is the gradient of this graph - it tells you how fast the reaction proceeds at this point. The steeper the tangent line, the faster the rate. Gradient of tangent can be expressed in change in y values / change in x values.

Question 16

State five factors affecting the
rate of a chemical reaction

Answer 16

• Concentration of reactants
• Pressure of gases (volume)
• Surface area
• Temperature
• Catalysts

Question 17

What is the collision theory?

Answer 17

Chemical reactions can occur only when reacting particles collide with each other with sufficient energy (more than or equal to activation energy).

Question 18

Describe and explain the effect of increasing temperature on the rate of reaction

Answer 18

• T increases = faster reaction
• As T increases, kinetic energy of particles increases, i.e. more energetic collisions
• Also, they move faster, so they collide more frequently
• However, there is no straight line relationship between rate and temperature, i.e. they are not directly proportional to each other

Question 19

Describe and explain the effect of increasing concentration on the rate of reaction

Answer 19

• Conc. increases = faster reaction,
• More reactants = more frequent collisions

Question 20

Describe and explain the effect of increasing pressure of a gas on the rate of reaction

Answer 20

• Increasing the pressure of reacting gases, is the same as increasing concentration.
• It increases the number of gas molecules in the same volume and so increases the frequency of collisions and therefore increases the rate of reaction.

Note that volume and pressure are inversely proportional to each other. Increasing the volume slows down the reaction.

Question 21

Describe and explain the
effect of increasing surface
area

Answer 21

• If solid reactants are in smaller pieces, they have a greater surface area.
• Increasing the surface area of solid reactants increases the frequency of collisions and so increases the rate of reaction, e.g. block of magnesium reacts slower with acid then magnesium powder.

Question 22

What is a catalyst and how does it work?
How does it affect the reaction profile?

Answer 22

A catalyst changes the rate of reaction but is not used up. It increases rate of reaction by providing a different pathway for the reaction that has a lower activation energy. The reaction profile for a catalysed reaction will have a lower maximum of the curve (lower activation energy).

Question 23

What is an enzyme?

Answer 23

An enzyme is a molecule that acts as a catalyst in a biological system.

Question 24

What is a reversible reaction?

Answer 24

A reversible reaction occurs when the products of a reaction can react backwards to produce the original reactants.

Question 25

When is dynamic equilibrium reached?

Answer 25

In a closed system, when the forward and reverse reactions occur at the same rate and the concentrations of reactants and products remain constant.

Question 26

Describe Le Chatelier’s Principle

Answer 26

If a system is at equilibrium and a change is made to any of the conditions, then
the system responds to counteract change and restore the equilibrium.

Question 27

Describe the effect of changing the concentration of reactant and product on the position of the equilibrium

Answer 27

• If the concentration of one of the reactants or products is changed, the system is no longer at equilibrium and the concentrations of all the substances will change until equilibrium is reached again.
• If the concentration of a reactant is increased, more products will be formed until equilibrium is reached again.
• If the concentration of a product is decreased, more reactants will react until equilibrium is reached again.

Question 28

Describe the effect of changing temperature on the position of the equilibrium

Answer 28

- If the temperature of a system at equilibrium is increased:
- The relative amount of products at equilibrium increases for an endothermic
reaction.
- The relative amount of products at equilibrium decreases for an exothermic
reaction.
* If the temperature of a system at equilibrium is decreased:
- The relative amount of products at equilibrium increases for an exothermic
reaction.
- The relative amount of products at equilibrium decreases for an endothermic
reaction.

Question 29

Describe the effect of changing pressure on the position of the equilibrium

Answer 29

This applies to equilibria that involve gases
- An increase in pressure causes the equilibrium position to shift towards the side with the smaller number of molecules as shown by the symbol equation for that reaction.
- A decrease in pressure causes the equilibrium position to shift towards the side with the larger number of molecules as shown by the symbol equation for that reaction. Pressure has no effect on the reactions where the numbers of gas molecules are equal on both sides of the equation

Question 30

Describe the effect of a catalyst on the position of the equilibrium

Answer 30

• No effect.
• It just speeds up both forward and backward reactions equally.
• i.e. equilibrium is achieved faster.