sem 2 chem 4.06

A 10 g gold coin is heated from 25°C to 50°C (CAu is 0.13 J/g-°C). What is the ΔH?

+32.5 J

There are two key points about enthalpy that are related to chemistry:

- Most chemical reactions occur at constant pressure and volume, so in chemistry—and in this course—we will refer to enthalpy as it relates to the thermal energy of the system (not work). - Enthalpy cannot be measured directly.

The latent heat in a substance has a negative value. Which of the following situations could be true?

A liquid is solidifying at the freezing point

Calculate the delta HΔH when 100 g of liquid water cools from 100ºC to 0ºC (CWCW is 4.18 J/g-ºC). What is delta HΔH?

Given: m = 100 g C = 4.18 J/g-ºC delta TΔT = (0ºC-100ºC) = -100ºC Unknown: delta H equals ?ΔH=? Input the variables into the equation and solve the problem. ΔH = q = mCΔT ΔH = (100g)(4.18 J/g-0C)(-1000C) ΔH=−41.8kJ The delta HΔH is negative. The reaction is exothermic. This makes sense because the water is releasing heat as it cools.

Which of the following symbols represents enthalpy?

H

Ice melting is

a phase change

Changes in state, like chemical reactions, involve

changes in enthalpy

Exothermic reactions

have a negative ΔH because the products have less enthalpy than the reactants.

Endothermic reactions

have a positive ΔH because the products have more enthalpy than the reactants

The heat content (thermal energy content) of any system—a candle or a specific amount of gasoline—is called

its enthalpy

there must be a change in enthalpy that is "hidden," or not reflected by a change in temperature. This hidden enthalpy change is called

latent heat

This diagram would represent the enthalpy change in which of the following? https://static.k12.com/calms_media/media/1107500_1108000/1107866/1/cbaa4a121d9f4cdbcf317a37b3cca29ccbb7e482/3835585_VHS_CHEM_S2_04_07__110_reactants_products.gif

liquid water freezing

Enthalpy is measured in

terms of change in heat between the products and the reactants

What is enthalpy?

the heat content of a system at constant pressure

enthalpy

the thermal energy content of a chemical or physical system

Enthalpy is

the total heat content of a system at constant pressure