Energy changes

What is bond energy

the energy absorbed in breaking a bond or energy given out in making a bond

what makes a reaction exothermic?

the energy given out during the forming of bonds between the ____ is greater than the energy absorbed during the breaking of bonds in the ____ (reactant particles)

what makes a reaction endothermic?

the energy given out during the forming of bonds between the ____ is lesser than the energy absorbed during the breaking of bonds in the ____ (reactant particles)

What is enthalpy change

the amount of heat energy given out or taken in during a reaction

(Overall energy change (after reaction) vs (before reaction))

Where does the energy come from in exothermic reactions?

It comes from the energy released when new ones are formed

‘symptoms’ of endothermic reactions

• decrease in temp (except for melting and boiling)

• (Products have more energy than reactants)

Where does the energy go to in an endothermic reaction?

energy is used to break the bonds in the reactant molecules

What is the activation energy?

the minimum energy that molecules must possess during their collisions in order for a chemical reaction to occur

What is the activation energy used for?

breka the bonds in the reactants before new bonds can form

what must occur before a reaction can take place?

• reactant molecules have a combined energy that is more than the activation energy

why must some substances require heating before reaction starts?

so that they may gain sufficient energy to overcome the activation energy

what is the standard enthalpy change

the enthalpy change when reactants in their normal states react to form products in their normal states under standard conditions of temperature and pressure

What do standard conditions refer to

• temperature at 25ºc / 298K

• Pressure at 1 atm

• (If solutions) solutions of concentration of 1 mol/dm3

what is the standard enthalpy change of formation of a substance?

the change in enthalpy that accompanies the formation of 1 mol of the substance from its elements, with all substances in their standard states

what must we take note for a standard enthalpy of formation equation

• 1 mol of substance formed

• Reactants only consist of elements that make up the compound

• Coefficients of reactants can be in fractions

• All reactant and products are in their standard states

What is the standard enthalpy of combustion of a substance?

• the enthalpy change when 1 mol of substance in its normal state is completely burned in oxygen to form products in their normal states under standard conditions of temperature and pressure

what should we note for equations regarding standard enthalpy of combustion

• 1 mole of substance reacting with oxygen

• Coefficient of OXYGEN can be in fraction

• All the reactants and products are in their standard states

what is the standard enthalpy of neutralisation

the enthalpy change when 1 mol of water in its normal state is formed from the reaction between solutions of an aid and an alkali at 1 mol/dm3 under standard conditions of temperature and pressure

what should we take note for equations of standard enthalpy of neutralisation reactions

• 1 mole of water formed

• Coefficients of reactants an be in fractions

• all reactants are in their standard states

what should we include for the energy level diagrams?

1. arrow of enthalpy change. the ends of the arrow must touch both levels

2. chemical formula of reactants on top of their levels

3. label for how much enthalpy there is + signs

what should we include for energy profile diagrams showing activation energy?

1. an upwards curve

2. two distinct levels

3. reactants labelled on each level

4. overall arrow showing direction of enthalpy change from one level to the next

5. label for how much enthalpy there is + signs

what is their favourite MCQ endothermic process?

dissolving ammonium chloride in water

FORMAT: explain why this reaction is exothermic in terms of bond breaking and bond forming

1. name the reactants and products

2. comparison between energy absorbed and released during bond forming and breaking