topic 5/15- energetics

Question 1

symbol and description of standard conditions for enthalpy change

Answer 1

• pressure of 100 kPa
• concentration of 1 mol dm-3 for all solutions
• temperature is usually 298K
• each substance involved in the reaction is in its standard state (solid, gas or liquid)
  ΔH°

Question 2

what is heat?

Answer 2

a form of energy, sometimes referred to as thermal energy

Question 3

what is temperature?

Answer 3

a measure of the average kinetic energy of the particles

Question 4

total energy is —- in chemical reactions. Why?

Answer 4

conserved; the first law of thermodynamics states that energy cannot be created/destroyed

Question 5

describe an endothermic reaction

Answer 5

• heat is taken into the system from the surroundings
• the temperature of the surroundings decreases
• the enthalpy change of the system is positive

Question 6

describe an exothermic reaction

Answer 6

• heat is given out from the system to the surroundings
• the temperature of the surroundings increases
• the enthalpy change of the system is negative.

Question 7

describe enthalpy

Answer 7

the heat stored by a substance

Question 8

what three things make up a system?

Answer 8

• the chemical reaction
• the reactants
• the products

Question 9

what two things make up the surroundings?

Answer 9

the air or solvent around the reactant and product molecules

Question 10

give the equation for heat change

Answer 10

q=mcΔT

Question 11

the density and specific heat capacities of aqueous solutions are assumed to be…

Answer 11

equal to those of water

Question 12

what is ΔH meaured in?

Answer 12

kJ/mol

Question 13

define standard enthalpy of formation

Answer 13

the enthalpy change when 1 mole of a substance is formed from its elements in their standard states, under standard conditions,

Question 14

look at how to form standard enthalpy of formation/combustion equations

Answer 14

-

Question 15

define standard enthalpy of combustion

Answer 15

the enthalpy change when 1 mole of a substance combusts completely in oxygen, under standard conditions

Question 16

what does standard state refer to?

Answer 16

the normal, most pure stable state of a substance measured at 100kPa

Question 17

describe how temperature fits into the definition of standard state

Answer 17

temperature is not a part of the definition of standard state, but 298K is commonly given as the temperature of interest

Question 18

what is one of the main limitations of calculations of specific heat capacity?

Answer 18

we assume that the density and specific heat capacities of aqueous solutions are equal to those of water

Question 19

what is the temperature of interest usually given as part of the definition of standard state?

Answer 19

298K

Question 20

give 2 limitations of enthalpy change experiments

Answer 20

• heat losses to the environment
• heat capacity of the calorimeter

Question 21

the enthalpy change of a reaction that is carried out in a series of steps is equal to

Answer 21

the sum of the enthalpy changes for the individual steps

Question 22

look over how to construct a hess cycle/solve those types of equations

Answer 22

-

Question 23

hess’ law

Answer 23

the enthalpy change of a reaction is independent of the route between the initial and final states

Question 24

the standard enthalpy change of formation of any element in its standard state is

Answer 24

zero

Question 25

give the equation for the use of hess’ law to calculate the enthalpy change of a reaction

Answer 25

ΔH=Σ(ΔHf products)-Σ(ΔHf reactants)