Energetics- enthalpy and bond energies

Endothermic reaction:

1. why does an endothermic reaction take in heat

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

3. an endothermic reaction is bond…

4. and endothermic reaction…energy

5. is deltaH positive or negative for an endothermic reaction

6. in an endothermic reaction there is a … in temperature

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

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

3. breaking

4. requires

5. positive

6. decrease

Exothermic reaction:

1. why does an exothermic reaction release heat

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

3. an exothermic reaction is bond…

4. and exothermic reaction…energy

5. is deltaH positive or negative for an exothermic reaction

6. in an exothermic reaction, there is an … in temperature

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

2. okay

3. making

4. releases

5. negative

6. increase

how do you calculate bond enthalpy change

BURP

ΔH= reactants - products
ΔH = energy required to break bonds - energy released from bonds formed

give 2 examples of exothermic reactions

1. combustion

2. respiration

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

define enthalpy change

heat change under constant pressure

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 ΔT
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 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

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 conditions for standard enthalpy change

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

give the equation to find enthalpy changes from enthalpy changes of formation

∆ H reaction = Σ ∆ fH products - Σ ∆ fH reactants

give the equation to find enthalpy changes from enthalpy changes of combustion

Δ H reaction = Σ ΔcH reactants - Σ ΔcH products

define mean bond energy

The mean bond energy is the enthalpy needed to break the covalent bond into gaseous atoms, averaged over different molecules

In general (if all substances are gases)

how do you calculate ΔH when given bond energies

ΔH = Σ bond energies broken - Σ bond energies made

why will ΔH results be significantly lower if they are callused experimentally vs using a calorimeter

• significant heat loss to surroundings

• also incomplete combustion will occur, leading to less heat energy being released

• need to draw a graph and extrapolate

what is the unit for enthalpy change

kJ/mol