energetics

Question 1

what is enthalpy change

Answer 1

the heat energy transferred in a reaction at constant pressure

Question 2

when in the enthalpy change negative

Answer 2

in exothermic reactions as the products end up with less energy then the reactants started with

Question 3

when is the enthalpy change positive

Answer 3

in endothermic reactions as the products end up with more energy then then reactants started with

Question 4

what is bond enthalpy

Answer 4

the energy needed to break a bond

Question 5

is bond breaking endo or exo

Answer 5

endo

Question 6

why is mean bond enthalpy important

Answer 6

The bond enthalpy between two atoms changes depends on the compounds and its slightly different in all of them so mean bond enthalpy takes that into account

Question 7

what is the equation for enthalpy change in terms of energy

Answer 7

enthalpy change = energy absorbed - energy released

Question 8

what is standard enthalpy change of formation

Answer 8

the enthalpy change when 1 mole of a compound is formed from its elements in their standard stated under standard conditions

Question 9

what is standard enthalpy change of combustion

Answer 9

the enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions with all the products and reactants in their standard conditions

Question 10

what is the standard enthalpy change of reaction

Answer 10

the enthalpy change when a reaction occurs in the molar quantities shown in the chemical equation under standard conditions with all the reactant and products in their standard states

Question 11

what is the standard enthalpy change of neutralisation

Answer 11

the energy change when one mole of water is formed by the reaction of an acid and an alkali

Question 12

how do you find the enthalpy change for neutralisation

Answer 12

1. add a known volume of an acid to a polystyrene cup and record the temperature
2. add a known volume of alkali and record the temperature at regular intervals while stirring it so the solution is evenly heated
3. work out the temperature change then work out the heat energy given out by using q= mc(delta)T

Question 13

how do you find the enthalpy change in a dissolution reaction

Answer 13

1. add a known mass of the solid being dissolved to a known volume of water polystyrene cup and record the temperature
2. the temperature at regular intervals while stirring it so the solution is evenly heated
3. work out the temperature change then work out the heat energy given out by using q= mc(delta)T making sure that you divide the heat energy change by the number of moles of solid

Question 14

what is a calorimetry experiment

Answer 14

experiments that measure heat given off

Question 15

how do you find the enthalpy of combustion of a flammable liquid and why is it often inaccurate

Answer 15

burn it in a calorimeter

some heat will be lost to the surroundings
some incomplete combustion will be taking place
some might evaporate as flammable liquids are often volatile

Question 16

what is the equation to calculate enthalpy change and units for each

Answer 16

q = mc(delta)T

q= joules
m- grams
c= 4.18JgK
T= kelvin

Question 17

what does Hess’s Law say

Answer 17

the total enthalpy change for a reaction is independent of the route taken