DF.1

Question 1

What is thermochemistry?

Answer 1

Thermochemistry is the study of the energy and heat associated with chemical reactions.

Question 2

What happens in an exothermic reaction?

Answer 2

Energy is released during the reaction and temperature of the surroundings increase.

Question 3

What happens in an endothermic reaction?

Answer 3

Energy is taken in during the reaction from the surroundings, and therefore the temperature of the surroundings decrease.

Question 4

How does the products’ energy store compare to the reactants’ store in an exothermic reaction?

Answer 4

The products has less energy than the reactants.

Question 5

How does the products’ energy store compare to the reactants’ store in an endothermic reaction?

Answer 5

The product has more energy than the reactants.

Question 6

What is enthalpy change the measure of?

Answer 6

The energy transferred to and from the surroundings. It is the difference between the reactants’ and products’ energy stores.

Question 7

What is the symbol representation for enthalpy change?

Answer 7

ΔH

Question 8

What sign would the enthalpy change have for an exothermic reaction?

Answer 8

• negative, because energy is released to the surroundings.

Question 9

What sign would the enthalpy change have for an endothermic reaction?

Answer 9

+ postive, because energy is taken in from the surroundings.

Question 10

What is the equation that represents what enthalpy change is measuring?

Answer 10

ΔH = H(products) - H(reactants)

Question 11

What does H represent?

Answer 11

Enthalpy

Question 12

What is the typical unit for enthalpy change?

Answer 12

kJmol-1

Question 13

What does system refer to in a chemical reaction?

Answer 13

The reactants and the products of the reaction.

Question 14

What does the surroundings refer to in a chemical reaction?

Answer 14

The rest of the world, e.g. the test tube, you, the air.

Question 15

What are some conditions that might affect the ΔH value of a reaction?

Answer 15

• temperature
• pressure
• concentrations of solutions

Question 16

Because ΔH varies with different conditions, it is important to keep a standard when measuring. What is the standard condition that has been set for measuring and comparing enthalpy changes?

Answer 16

• Temperature = 25℃ / 298K
• Pressure = 1 atmosphere
• Concentration of solution = 1 moldm-3

Question 17

What is the symbol representation for an enthalpy change measured under standard conditions?

Answer 17

ΔH⦵

Question 18

How many Kelvin is 0℃?

Answer 18

273K

Question 19

What is the symbol representation for standard enthalpy change for a reaction?

Answer 19

ΔHr⦵, where r refers to reaction.

Question 20

What is the standard enthalpy change for a reaction?

Answer 20

The enthalpy change when molar quantities of reactants as stated in the equation react together under standard conditions.

Question 21

How can enthalpy changes of a reaction be measured in the lab?

Answer 21

By arranging for the energy involved in a reaction to be transferred to or from water surrounding the reaction vessel. If the reaction is exothermic, the water would get hotter and if the reaction is endothermic the water would get cooler. By measuring the temperature of the water, and knowing the mass and specific heat capacity of water, you can calculate the amount of energy that was transferred to or from the water during the reaction.

Question 22

What is the equation used to calculate the energy transferred to water?

Answer 22

Energy transferred (J) = specific heat capacity (of water 4.18Jg-1K-1) x mass (g) x temperature change

Question 23

What is the density of water?

Answer 23

1gcm-1

Question 24

How do you calculate the mass of the water from volume?

Answer 24

mass = density x volume.

Because density of water is 1, therefore its mass will be the same as its volume in cm3, numerically.

Question 25

What is the standard enthalpy change of combustion?

Answer 25

The enthalpy change that occurs when one mole of a substance is burnt completely in oxygen under standard conditions in standard states.

Question 26

What is the symbol representation for standard enthalpy change of combustion?

Answer 26

ΔHc⦵, where c refers to combustion.

Question 27

Is it possible to measure enthalpy change of combustion under standard conditions in practice?

Answer 27

It is impossible, therefore adjustments are made to allow for the non standard conditions.

Question 28

What sign would the enthalpy change of combustion always have?

Answer 28

- negative, because all combustion reactions are exothermic.

Question 29

What is the standard enthalpy change of formation?

Answer 29

The enthalpy change when one mole of a compound is formed from its elements under standard conditions in standard states.

Question 30

What is the symbol representation of standard enthalpy change of formation?

Answer 30

ΔHf⦵, where f refers to formation.

Question 31

It is often impossible to measure enthalpy changes of formation directly, so how do we find out the values?

Answer 31

They can be found indirectly by making use of measurable quantities and incorporating them into enthalpy cycles.

Question 32

What is the standard enthalpy change of neutralization?

Answer 32

The enthalpy change when one mole of hydrogen ions react with one mole of hydroxide ions to form one mole of water under standard conditions.

Question 33

What is the symbol representation of standard enthalpy change of neutralization?

Answer 33

ΔHneut⦵, where neut refers to neutralization.

Question 34

When setting up a practical for measuring the enthalpy change of combustion of a fuel, why is a copper can used to hold the water?

Answer 34

Because the specific heat capacity of metal is very low therefore very little energy goes into heating up the can, so it affect the end result of the enthalpy change only by a little bit.

Question 35

Why is the real enthalpy change measured in practice (in lab practicals), never exactly the same as the theoretical/ proposed value?

Answer 35

1. Incomplete combustion taking place. 2. Energy lost to the surrounding and apparatus. 3. Not done in standard conditions.