Energetics

why does an endothermic reaction take in heat

more energy is needed to break the bonds than is released when bonds are formed

learn how to draw and label a diagram for an endothermic reaction

okay

an endothermic reaction is bond…

breaking

and endothermic reaction…energy

requires

is an endothermic reaction positive delta H or negative delta H

positive delta H

learn how to draw and label a diagram for an exothermic reaction

okay

and exothermic reaction…energy

releases

an exothermic reaction is bond…

making

why does an exothermic reaction give out energy

less energy is needed to break the bonds than is released when the bonds are formed

is an exothermic reaction positive delta H or negative delta H

negative delta H

how do you calculate energy change (delta H)

energy change= energy required to break bonds - energy released from bonds formed

give 2 examples of exothermic reactions

1. combustion

2. respiration

in an exothermic reaction, there is an … in temperature

increase

in an endothermic reaction there is a … in temperature

decrease

give an example of an endothermic reaction

photosynthesis

what are the 3 stages to any reaction

1. chemical bonds are broken

2. atoms rearrange themselves into new positions

3. new chemical bonds are formed, creating the product

define activation energy

minimum energy needed for a successful collision

what is meant by the enthalpy change of a reaction

transfer of thermal energy

learn enthalpy change calculations using bond energy diagrams

okay

what does a -730 delta H mean in a chemical reaction

the reaction has given out 730 kJ/mol

exothermic reaction

how do you calculate heat energy change (Q)

Q= mc delta T
Heat energy change = mass of liquid being heated x 4.2 x change in temp

how do you calculate (molar) enthalpy change

ΔH = Q (kJ) / moles

what is the symbol for heat energy change

Q

what is the symbol for enthalpy change

ΔH

how can you adapt equipment to reduce heat energy loss

add a lid

example question: explain in terms of bonds broken and bonds made, why (this) reaction is exothermic MARK POINTS

mp1. less energy is needed to break the bonds in (H2) and (O2)

mp2. than is released when the new (H2O) bonds are made

mp3. be specific to bonds in reaction

what type of process is bond breaking

endothermic

what type of process is bond making

exothermic

A Level flashcards start here:

okay

what does the size of the enthalpy change of a particular chemical reaction depend on

• temperature

• pressure

• physical state 

• amount of reactants 

name the standard conditions for

1. temperature 

2. pressure 

3. solution concentration 

1. 298 K

2. 100 kPa

3. 1 mol / dm³

define the standard enthalpy of formation

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

by definition, what is the standard enthalpy of formation of all elements

0 kJ/ mol

define the standard enthalpy of combustion

the enthalpy change when 1 mole of a substance in its standard state reacts completely with an excess of oxygen under standard conditions 

define the standard enthalpy change of neutralisation 

the enthalpy change when solutions of an acid and an alkali react together under standard conditions to produce 1 mole of water 

state Hess’s Law

the total enthalpy change in a chemical reaction is independent of the route by which the chemical reaction take place as long as the initial and final conditions are the same