Section 3: Physical Chemistry

Question 1

What is the difference between an endothermic and exothermic reaction?

Answer 1

Exothermic - releases heat to surroundings, increases in temperature.

Endothermic - absorbs heat from surroundings, decreases in temperature

Question 2

Describe an experiment for a combustion reaction (calorimetry)

Answer 2

Put 50g of water in a copper can, then record its temperature. Weigh a spirit burner with lid, then place the burner under the copper can and light the wick. Place the burner under the water and let it heat up and constantly stir the water until it reaches 50 °C. Put the flame out by using the lid and measure the final tmeperature of the water. Then weigh the burner and lid again. Calculate enthalpy change

Question 3

Describe an experiment for a dissolving reaction (calorimetry)

Answer 3

Place 25 cm3 of dilute hydrochlric acid in a polysyrene cup and record the temperature of the acid. Put 25cm3 of dilute sodium hydroxide solution in a measuring cylinder and record the temperature and record its temperature. Add the alkali to the acid and stir. Take the temperature every 30 seconds and record the highest temperature it reaches.

Question 4

Describe a experiment for a neutralisation reaction (calorimetry)

Answer 4

Place 25 cm3 of dilute hydrochlric acid in a polystyrene cup and record the temperature of the acid. Put 25cm3 of dilute sodium hydroxide solution in a measuring cylinder and record the temperature and record its temperature. Add the alkali at 5cm3 intervals to the acid and stir. Record the maixmum temperature it reaches after each addition.

Question 5

How is the molar enthalpy change calculated? What does each part of the equation represent

Answer 5

Step 1: Work out energy change: E = m x c x ΔT.

Step 2 Work out moles of the limiting reagent

Step 3 E/n = ΔH.

E - energy, m - mass of surrounds (usually mass of water = volume of water), c - specific heat capacity - 4.2, n - number of moles of reactant

Question 6

50cm3 of 0.10moldm-3 silver nitrate solution was put in a calorimeter, 0.2g of zinc powder was added. Temp of solution rose by 4.3 °C. Calculate energy change, moles of zinc reacted and molar ethalpy change

Answer 6

1. 50 x 4.2 x 4.3 = 903/1000= 0.903
2. n=0.2/65= 0.003mol
3. Δ H =0.903/0.003 = -307kJ/mol

Question 7

Excess sodium hydrogencarbonate was added to 30.0 cm3 of 2moldm-3 hydrochloric acid. The temp fell 10.3°C. Calculate the energy change,moles of zinc reacted, molar enthalpy change

Answer 7

1. 30 x 4.2 x 10.3=1297.8/1000=1.2978
2. n=2/36.5=0.055
3. ΔH = 1.298/0.0055= +23.6kJ/mol

Question 8

The rate of reaction depends on what 5 things?

Answer 8

temperature, concentration, catalyst, surface area, pressure (for gases only)

Question 9

On a rate graph what tells you about the rate of reaction

Answer 9

The steeper the line the faster the rate, it will go flat the when the reaction has stopped

Question 10

Rate of reaction formula

Answer 10

Amount of reactant used or amount of product formed / Time

Question 11

3 ways to measure the speed of a reaction

Answer 11

Precipitation, Change in mass, Volume of gas given off

Question 12

What is Collision Theory

Answer 12

Rate of reaction depends on how often and how hard the reacting particles collide with each other. Particles need to collide with enough energy to be successful and react. The activation energy is required to break bonds.

Question 13

What does a catalyst do

Answer 13

Lowers the activation energy, is not used up, provides an alternative reaction pathway

Question 14

Complete the following equations:
magnesium + hydrochloric acid ——>
calcium carbonate + hydrochloric acid ——–>
sodium thiosulfate + hydrochloric acid ———->

Answer 14

magnesium + hydrochloric acid ——> magnesum chloride + hydrogen

calcium carbonate + hydrochloric acid ——–> calcium chloride + carbon dioxide + water

sodium thiosulfate + hydrochloric acid ———-> sodium chloride + sulfur+ sulfur dioxide + water

Question 15

Why does rate change with surface area?

Answer 15

As the particle size decreases the surface area increases. The increased surface area means there is more surface for collisions to take place. There are more frequent collisions. There are more frequent successful collisions. The rate increases.

Question 16

Why does rate change with temperature?

Answer 16

The temperature increases. Particles have more energy and move faster. There are more frequent collisions and more frequent successful collisions. The rate increases. The rate also increases because….The particles have more energy so when they collide a greater proportion of the collisions are successful

Question 17

Why does rate change with concentration?

Answer 17

As concentration increases…The frequency of collisions increases…The frequency of successful collisions increases, The rate increases

Question 18

Why does rate change with a catalyst?

Answer 18

With a catalyst the proportion of collisions that are successful is increased, the frequenct of collisions is not affected

Question 19

Describe the difference in the enthalpy profile diagrams for an endothermic and exothermic reaction

Answer 19

Exothermic - product line below reactant line ΔH line points downwards and is negative. Endothermic - product line above reactant line ΔH line points upwards and is positive

Question 20

What is the energy change associated with bond breaking and bond making

Answer 20

Bond breaking - energy absorbed, endothermic process. Bond making - energy released, exothermic process

Question 21

How do you calculate enthalpy change from bond energies? (4 steps)

Answer 21

Step 1: Draw out molecules based on the balanced equation.

Step 2: Work out the number of each type of bond you are breaking and bond you are making.

Step 3: Use the bond energies to work out the total energy absorbed to break the bonds and energy released from making bonds.

Step 4: Bonds broken - Bonds made = Enthalpy change in kJ/mol

Question 22

Sketch an energy level diagram to show activation energy. Add a line to show the activation energy if a catalyst was present also

Answer 22

Curved line going up and then down to reactants. Activation energy line goes from level with reactants to the top of the curve. Another line but with a lower peak for the catalysed reaction, activation arrow is therefore shorter

Question 23

Calculate the enthalpy change for the following reaction: 2H2O2 -> 2H2O + O2. Bond energies (O-H = 464, O-O = 146, 0=0 = 498)

Answer 23

-206 kJ/mol

Question 24

Calculate the enthalpy change for the following reaction: 2H2 + O2 -> 2H2O Bond energies (O-H = 464, H-H = 436, 0=0 = 498)

Answer 24

-486 kJ/mol

Question 25

What does it mean if a reaction is reversible? How does it effect the symbol used in equations?

Answer 25

The reactants react to produce the products, but the products can react back to form the reactants again. The symbol is two arrows

Question 26

Describe the reverisble reactions involving copper sulphate and ammonium chloride

Answer 26

Copper sulphate: hydrated copper sulphate (blue crystals) can be heated to form anhydrous copper sulphate (white) and water. Water can be added to the white solid and it will go back to blue.

CuSO4.5H2O CuSO4 + 5H2O.

Ammonium chloride: when heated the white solid of ammonium chloride will decompose to form the colourless gases hydrogen chloride and ammonia. These gases can react back to form the white solid again. Usually done in a boiling tube where some of the solid disappears at the bottom but then reforms near the top.

NH4Cl(s) NH3(g) + HCl(g)

Question 27

What are four features of a dynamic equilibrium

Answer 27

A dynamic equilibrium happens when:

the forward and backward reaction are happening at the same time.

The rates of the forward and backward reaction are the same.

The concentrations of the reactants and products no longer change.

The system is a closed system

Question 28

Describe the energy changes for a reversible reaction

Answer 28

In one direction it is exothermic, in the other direction it is endothermic.

Question 29

How does temperature affect the position of an equilibrium?

Answer 29

Increasing the temperature shifts the equilibrium in the endothermic direction. Decreasing the temperature shifts the equilibrium in the exothermic direction

Question 30

How does pressure affect the position of equilibrium?

Answer 30

Increasing the pressure shifts the equilibrium towards the side with fewer moles of gas. Decreasing the pressure shifts the equilibrium towards the side with more moles of gas.

Question 31

Define:
Acid
Base

Answer 31

Acid - Proton donor

Base - Proton Acceptor