4.1 a/b/c: Heat and Enthalpy

Heat and Enthalpy

4.1

Syllabus dot points

conduct practical investigations to measure temperature changes in examples of endothermic and exothermic reactions, including: – combustion – dissociation of ionic substances in aqueous solution

investigate enthalpy changes in reactions using calorimetry and 𝑞 = 𝑚𝑐𝛥𝑇 (heat capacity formula) to calculate, analyse and compare experimental results with reliable secondary-sourced data, and to explain any differences

construct energy profile diagrams to represent and analyse the enthalpy changes and activation energy associated with a chemical reaction

What we will cover

• enthalpy and heat

• endothermic vs exothermic

• specific heat capacity formula

• calorimetry

Multiple Choice

The graph above is from which type of reaction?

• Endothermic reaction

• Exothermic Reaction

Energy Profile Diagrams

• shows the change in energy within a reacting system, as a function of the progress of the reaction

• we can determine endo or exo, what the enthalpy change is, what the activation energy is

Endo vs Exothermic

• Endothermic = absorbs heat = products will have more energy than the reactants --> e.g. photosynthesis

• Exothermic = releases heat = reactants have more energy than products --> any combustion rxn

• We talk about the system and the surroundings

• Endo = decrease in measured temp. Exo = increase

Heat, Temp, Changes

• Temperature = is a measure of degree of hotness or coldness of an object/substance

  • ​measure of the average kinetic energy of the atoms

• Amount of heat is different from temperature

​

• Specific heat capacity (c) = amount of heat required to increase the temperature of a substance by 1 degree (J/K/kg or J/K/g)

  • can use these to calculate quantities of heat ​

  • higher SHC indicates that more energy is required to raise the temperature of this substance

• We measure and calculate heat changes through calorimetry

​Calculating quantities of heat

​Use the calculation from data sheet:

q = mcΔ​T

​

Where:

q = the quantity of heat

m = mass of substance

c = specific heat capacity of substance ​

​Δ​T = change in temperature (final - initial)

​Calculating quantities of heat

​Use the equation from data sheet:

q = mcΔ​T

(trickiest part is figuring out the units) ​

​

How much heat is required to warm 275g of water from 349K to 360K?

​

m= 275

c = 4.18

Δ​T = 11

​

​

Multiple Choice

What type of reaction is this?

• Exothermic

• Endothermic

• Energy Producing

• No way to tell

Heat (q)

• energy that flows from a system to its surrounding

• measured in J

• q = positive = heat released from system into surroundings = exothermic

• q = negative = heat absorbed into system from surroundings = endothermic

• sig figs become very important here (as we often only have 2, due to temp - don't round early!)

Enthalpy (H)

• measure of the total stored energy (potential energy) in the bonds of a chemical system

• We measure the CHANGES in enthalpy, not enthalpy itself (ΔH), measured in kJ/mol

• ΔH = H_products - H_reactants

• If ΔH = positive = endothermic

• If ΔH = negative = exothermic

• ΔH_soln = heat absorbed when 1 mole of substance dissolves in water

Calculation for molar enthalpy changes

for calculating the heat released or absorbed per mole

Multiple Choice

Define 'Enthalpy'

• a) Energy stored in the movement of molecules in a substance

• b) The opposite of temperature

• c) The temperature of a molecule

• d) Energy stored in the chemical bonds in a substance

Multiple Choice

How do you calculate Enthalpy of a reaction?

• ΔH = ΔH_products - ΔH_reactants

• ΔT = q / mC

• ΔG = ΔH -TΔS

• E = mc²

Multiple Choice

ΔH value in an endothermic reaction is a positive number.

• True
• False

Multiple Choice

Temperature is a measure of average [blank] energy of individual atoms.

• heat
• potential
• mechanical
• kinetic

Specific Heat Capacity

• ability of a a substance to change its temperature

• water has very high - stable - biological stability

• often is water in enthalpy calculations, can find it on the data sheet

​Heat of Combustion

• ​the heat released when 1 mole of substance undergoes complete combustion

• this is just the negative of the enthalpy change - because it is heat lost rather than heat gained

• it is always exothermic - so always negative Δ​H

• can be measured indirectly by having the combustion heat water and measuring the temperature increase

  • law of conservation of energy

  • heat released by combustion reaction = heat absorbed by the water ​

Calorimetry

• experimental process of measuring the amount of heat released or absorbed during a reaction in a system

• determining exo or endo

• usually done in water (the surroundings) and the dissolved substances are the system

• law of conservation of energy - has to go somewhere!

Multiple Choice

Which statement is true regarding endothermic reactions?

• a) Temperature change is positive and enthalpy change is positive
• b) Temperature change is positive and enthalpy change is negative
• c) Temperature change is negative and enthalpy change is positive
• d) Temperature change is negative and enthalpy change is negative

Multiple Choice

 _?_  is the amount of energy required to raise the temperature of 1 gram of any substance 1^oC.

• Specific heat
• A calorie
• Thermal energy
• Conduction

Multiple Choice

The potential energy diagram of a reaction is shown. Which statement below is correct relating to this reaction?

• #1 represents the enthalpy change for this exothermic reaction.

• #2 represents the enthalpy change for this endothermic reaction.

• #3 represents the enthalpy change for this endothermic reaction.

• #4 represents the enthalpy change for this exothermic reaction.