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Chemistry > U1.1: Measuring Energy Changes > Flashcards

U1.1: Measuring Energy Changes Flashcards

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1
Q

Define energy

A

the ability to do work or cause a change

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2
Q

How can energy be transferred

A
  • Conduction: Direct Contact
  • Convection: Movement of fluids (air, water)
  • Radiation: Emission of Electromagnetic waves
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3
Q

What is chemical change?

A

Total energy before = Total energy after as energy is never created nor destroyed

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4
Q

What is physical change

A

Change in physical states

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5
Q

What is the system

A

The chemical reaction itself

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6
Q

What is the surrounding

A

Everything not the system itself

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7
Q

Properties of exothermic reactions (5)

A
  • More heat is released to surroundings than absorbed by the system
  • Surrounding gets hotter
  • Products are more stable than reactants
  • delta H is negative
  • Bonds form as removing heat brings atoms closer
  • Bonds are more stable
  • Reactant enthalpy > Product enthalpy
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8
Q

Properties of endothermic reactions (5)

A
  • More heat is absorbed from surroundings than released by the system
  • Surroundings get cooler
  • Products are less stable than reactants
  • delta H is positive
  • Bonds break as adding heat separates atoms
  • Bonds are less stable
  • Product enthalpy > Reactant enthalpy
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9
Q

What are axis labelled in energy profile diagrams

A
  • X axis: time (s)
  • Y axis: Potential Energy (kJ)
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10
Q

How does the energy profile for exothermic reactions look?

A
  • Reactants higher than products
  • EA is the distance from the reactant base to the peak
  • delta H is negative and it is the arrow below the EA (longer) from the base of Products
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11
Q

How does the energy profile for endothermic reactions look?

A
  • Products higher than reactants
  • EA is the distance from the reactant base to the top of the peak (longer)
  • delta H is positive and it is the arrow from the reactant base to the base of the products (shorter)
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12
Q

Define heat

A

A measure of total kinetic energy of particles in a substance

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13
Q

Define temperature

A

A measure of the average kinetic energy of particles in a substance

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14
Q

Define enthalpy

A

Amount of heat energy contained in a substance

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15
Q

State the 2 formulas used in the calculation of the enthalpy change of a reaction (and their units)

A
  • Q=mc(delta t): Q is energy (J), m is mass (g), c is SHC (J/G*C), t can be *C or K
  • Delta H = - (Q/n): Delta H is change in enthalpy (kJ mol^-1), Q is energy (J), n is moles (mol)
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16
Q

c of water

A

4.18 J K^-1 g^-1

17
Q

m of water

A

v in cm^3

18
Q

What are the 2 types of calorimetry

A

Flame and Polystyrene Cup calorimetry

19
Q

Outline what flame calorimetry is

A
  • To measure enthalpy of combustion reactions
  • Burning the material and see how much energy’s released to heat up the water
20
Q

Outline what polystyrene cup calorimetry is

A
  • To measure the enthalpy of reactions in solutions
  • Combining different substances and measure the change in temperature of the solutions
21
Q

In polystyrene cup enthalpy, what are the 3 types of enthalpy of solutions

A
  • Enthalpy of hydrations (dissolving in water)
  • Enthalpy of displacement reactions
  • Enthalpy of neutralisation
22
Q

in polystyrene cup enthalpy, what is the source of error and the assumption? State the 3 ways to minimise the error

A
  • Heat energy can be wasted
    1. Stirring the mixture to ensure even distribution of heat and so that the reaction proceeds faster
    2. Use insulated containers
    3. Use a lid
23
Q

What is the enthalpy graph for the heat of neutralisation and where is the maximum temperature found

A
  • Two lines drawn (one positive and one negative gradient) and where they intersect is the maximum temperature
  • X axis is volume in mL
  • Y axis is Temp (*C)
24
Q

What is the enthalpy graph in general? Where is the maximum temperature found and what does the gradient represent

A
  • Maximum temperature is where the line starts declining back down
  • Line declining back down shows reaction is over and the mixture is cooling
  • X axis is time in s
  • Y axis is Temp (*C)
25
Q

Define Specific Heat Capacity

A

The heat needed to increase the temperature of a unit mass of a substance by 1K or 1*C

26
Q

Define Hess’s Law

A
  • Enthalpy change for a reaction is independent of the pathway between the initial and final states
27
Q

What does the Hess’s Law Energy Cycle look like? State the symbol too

A
  • Arrow pointing from reactants to products
  • Arrow pointing from Reactants to Elements (+)
  • Arrow pointing from Elements to Products (-)
28
Q

Hess’s Law formula?

A
  • Delta H1 = Delta H2 + Delta H3
29
Q

What happens when the arrow switch sides

A

The symbol switches too

30
Q

When adding two reactions where one reaction had to swap its reactants with its products to make the overall reaction, what happens to the delta H value?

A
  • Symbol switches

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