Download Lab Activity: Determining Specific Heat of a Metal

Name: ____________________________________
Thermochemistry Guided Notes
Date: ____________________
FIVE-STEP DIAGRAM
Big Equation #1:
ΔH =
m=
Cp =
ΔT =
Big Equation #2:
ΔH =
mol =
ΔHphase change =
THERMOCHEMISTRY INTRO
 At freezing and boiling points, the temperature _______________. Thus, when an ice cube is actually
in the process of melting, it’s not getting any warmer.

Also there is no such thing as _______________. There is only heat and the absence of heat.

Heat – represented with two different symbols, q and H (often written as ΔH; Δ means
_______________).
o q is for heat; ΔH is for change in heat.

Enthalpy, ΔH – the term for heat _______________ during a chemical reaction.

Skillet example: When you touch a hot skillet, heat is transferred ______________________________
_______________. The skillet lost heat; you gained heat.
o The heat transfer continues until both items reach the same temperature.
o Both the _______________ and the _______________ experience a change in enthalpy.

Endothermic reaction, +ΔH –
o Think of the SKILLET EXAMPLE (your hand gained heat when you touched the skillet)

The amount of heat gained/lost by the system must be equal to that gained/lost by the
surroundings according to the ____________________________________.
o So in the skillet example, the heat that the skillet lost was equal to the amount of heat that
you gained by touching it.

Exothermic reaction, –ΔH –
o Think of the ICE CUBE TRAY EXAMPLE (the ice cubes lost heat when they froze)

Exothermic vs. Endothermic Reactions
o In exothermic reactions, energy is _________________ so that the surroundings get
_______________ and the system _______________. There is _________ energy in the system after
the reaction than there was before the reaction.
o In endothermic reactions, energy is _________________ so that the surroundings _________
__________ and the system _______________________. There is _________ energy in the system after
the reaction than there was before the reaction.
o Classify the following reactions as exothermic or endothermic:

A system gives off heat over time. _______________

The surroundings get warmer. _______________

The change in heat is positive. _______________

There is a –ΔH. _______________

The test tube feels warm to the touch. _______________

The energy of the products is greater than the energy of the reactants. ______________
o SUMMARY:
ENDOTHERMIC
EXOTHERMIC
Energy (heat) is absorbed
Energy (heat) is emitted
Heat enters the system
Heat exits the system
Heat is a reactant:
Reactants + heat 
Products
Heat is a product:
Reactants  Products +
heat
+ΔH
–ΔH
HEAT ENERGY, MEASURING HEAT

Energy –

***HEAT ENERGY IS NOT THE SAME THING AS TEMPERATURE***
o Chemists measure heat in either _____________ or ______________ a – NOT in °F, °C, or K.

calorie – the amount of energy needed to raise the temperature of 1 g (or 1 mL) of water by 1°C.

Calorie, kilocalorie – 1000 calories

Calorimeter –

CONVERSIONS: 1 calorie = 4.184 J
1 kJ = 1000 J
1 kcal = 1000 cal = 1 Cal
o How many calories are in 100.0 J?
o How many Joules are in 522 calories?
HEAT CAPACITY, SPECIFIC HEAT

Heat capacity –
o Objects of different sizes and/or materials have different heat capacities, making heat
capacity difficult to use for comparison purposes
o Objects with LOW heat capacities __________________________________________ than those with
HIGH heat capacities.

Specific heat, Cp –
o Since this eliminates the “bigger objects take longer to heat” concept, we can now
compare different substances of any size.
o Cp units are usually: J/g•°C or cal/g•°C or J/g•K or cal/g•K
BIG EQUATION #1

ΔH = m*Cp*ΔT

Practice Problem: What is the specific heat of copper if 95.4 grams absorbs 849 J while rising
(ΔH is usually in units of Joules)
from 25°C to 48°C?
BIG EQUATION #2

On Steps B and D of the five-step diagram the phase is changing so the temperature is constant –
that means we can’t use Big Equation #1 (because ΔT = 0!).

The heat involved in phase changes is called _________________.

Molar heat of fusion, ΔHfus –

Molar heat of solidification, ΔHsolid –

Molar heat of vaporization, ΔHvap –

Molar heat of condensation, ΔHcond –

ΔH = mol*ΔHphase change

Practice Problem: The molar heat of fusion of water is 6.009 kJ/mol. How much energy is needed
(ΔH is usually in units of kilojoules)
to convert 60.00 grams of ice at 0°C to liquid water at 0°C?
CALCULATING ΔH ACROSS MULTIPLE STEPS

When a problem asks you to calculate heat evolved over multiple steps (phase changes and
temperature changes), use both equations as you would normally. Then, at the end, make sure
all equations are in the same units and then add them together.

Decide which of the two equations apply for each step:
o Enthalpy change, no phase change (5-Step Diagram Steps A, C, and E)

ΔH = m*Cp*ΔT
(answer typically in J for ΔH)
o Enthalpy change, no temperature change (5-Step Diagram Steps B and D)

ΔH = mol*ΔHphase change
(answer typically in kJ for ΔH)
THERMOCHEMICAL EQUATIONS

Thermochemical equation –

Example: The combustion of 1 mole of methane (CH4) releases 890 kJ of heat energy
CH4(g) + 2O2(g)  2CO2(g) + 2H2O(g)
OR:
ΔH = –890 kJ
CH4(g) + 2O2(g)  2CO2(g) + 2H2O(g) + 890 kJ

In EXOTHERMIC rxns heat is given off (–ΔH) so it is written positively on the _____________ side.

In ENDOTHERMIC rxns heat is absorbed (+ΔH) so it is written positively on the ____________ side.

Practice Problem: Sodium bicarbonate decomposes when heated…
2NaHCO3(s) + 129 kJ  Na2CO3(s) + H2O(g) + CO2(g)
o How much heat (in kJ) is needed to decompose 2.24 moles of sodium bicarbonate?