3B enthalpy change

What is enthalpy

The measure of the heat energy in a chemical system

What is enthalpy change

The difference in enthalpy between reactants and products in chemical reactions

How to know if enthalpy reaction is exothermic

If reactants is higher than products, energy is released into surroundings as heat. This means it is exothermic

ΔH for exothermic is negative

Making bonds is exothermic

How to know if an enthalpy reaction is endothermic

If the reactants are lower than the products, energy will be taken in from surroundings

ΔH For endothermic is positive

Breaking bonds is endothermic

energy profile diagrams for both endothermic and exothermic enthalpy

What is activation energy

Minimum energy required for a reaction to take place, it is required to break bonds in reactants

what is the formula q= mcΔT

Q = energy exchanged around surroundings (in joules)

M = mass of substance heated (in grams)

C = specific heat capacity (in J g -1 C -1 or with K) usually 4.18

T = temperature change (can be in either Celsius or kelvin)

ΔH What is enthalpy change formula and what is its formula

- q in kj / mol

Units = Kj mol -1

What are the 2 methods for measuring heat transfer

1. For aqueous solution

   Methods

   •carry out reaction, using known amount of reactant , use unknown volume of water to absorb/provide energy, measure temperature change

   measurements - mass /volume of starting material

   volume of water / solution

   temperature

   procedure

   •Heat loss to surroundings

   •energy to heat up equipment

2. Method - for combustion

   •burn a known amount of reactant

   •use known volume of water to absorb energy

   •measure temperature change

   measurements = measure material burned, volume of water, temperature change

   procedure = heat loss to surroundings, energy to heat up equipment, incomplete combustion

What is standard enthalpy change of formation

The enthalpy change when one mole of a compound is formed from its elements under standard conditions; both compounds and elements are in standard states

What is standard enthalpy change of combustion

The enthalpy change when one mole of an element or compound reacts completely with oxygen Under standard conditions

What is standard enthalpy change of neutralisation

The enthalpy change when 1 mole of water is formed from neutralisation of hydrogen ions by hydroxide ions under standard conditions

What is standard enthalpy change of atomisation

The enthalpy change when one mole of gaseous atoms is formed from the element in its standard state

What is average bond enthalpy

breaking of one mole of bonds

Into gaseous molecules

What does homolytically mean

Equally

In bond enthalpy what sign is endothermic reaction and why

+ve

Bond breaking requires an input of energy

In bond enthalpy what sign is exothermic reaction and why

-ve

Forming bonds releases energy

How do we do bond enthalpy calculations

What is the formula for enthalpy change of neutralisation

How do we work out enthalpy change of neutralisation

Formula =

Delta H = -m x c x change in temperature

Add the volumes as they are equal to mass and divide by 1000

Use 4.18

Use temperature

Gives you value which is enthalpy of reaction

Work out moles of water using moles= conc x volume

Moles of acid is = moles of water

Enthalpy value/moles of water = enthalpy of neutralisation

How do you calculate maximum temperature reached

Calculate moles by doing concentration x volume

Find energy released by doing delta H (given) x moles

change in temp = Q (just worked out) / mass x 4.18 (C)

Gives you temp value

Maximum temp = new temp value + temp value in question

How to work out enthalpy of combustion or enthalpy of formation

Enthalpy of formation/combustion = reactants - products