Enthalpy Change Flashcards

(29 cards)

1
Q

What is enthalpy change?

A

The amount of heat energy transferred during a chemical reaction at a constant pressure.

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2
Q

units for ΔH

A

Kj mol-1

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3
Q

What are the standard conditions?

A

100kPa and 298K

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4
Q

Exothermic

A

gives out energy

ΔH is negative

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5
Q

Give an example of exothermic reaction:

A
  1. combustion of methane
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6
Q

Endothermic reactions

A

Absorb energy
ΔH is positive

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7
Q

What reaction is bond breaking?

A

endothermic- you need energy to break bonds

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8
Q

What reaction is once the bonds are formed?

A

Exothermic as energy is released.

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9
Q

What is bond enthalpy?

A

Bond enthalpy is the energy required to break bonds.

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10
Q

Define standard enthalpy of formation

A

the enthalpy change when 1 mole of a compound is formed from its elements in their standard state under standard conditions.

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11
Q

Define standard enthalpy of combustion.

A

the enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions.

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12
Q

Define specific heat capacity

A

The amount of energy required to raise the temperature of 1g by 1k.

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13
Q

If the surroundings have a lower specific heat capacity what happened

A

An exothermic reaction will cause a bigger increase in the temperature of the surroundings

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14
Q

What are the units for specific heat capacity

A

Jk-1g-1

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15
Q

Equation for enthalpy change

A

Q= mcAt

Q- amount of heat energy given out or taken in (J)
M- mass of the surrounding (g)
c- specific heat capacity

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16
Q

If the reaction is exothermic

A

We add a negative sign at the front

17
Q

If the reaction is endothermic

A

Add a + sign to the front

18
Q

Simple flame calorimetry

A

Burn water using a flame.
Measure the temperature change of the water

19
Q

What are some problems with the simple flame calorimeter

A

Some of the energy will be lost.e.g the water will give off vapours into the air.
To prevent this, increase the mass of the water but this can cause such a small change in temperature.

20
Q

How do you find the molar enthalpy?

A

Divide the enthalpy change by the moles

21
Q

What element are found as gases under standard state?

A

Nitrogen
oxygen
chlorine
fluorine
hydrogen
And all noble gases

22
Q

What elements are found as liquid under their standard state?

A

Bromine and mercury

23
Q

If an element is under standard state what is their enthalpy change?

24
Q

When a metal reacts with oxygen, what will it form?

A

Metal+ oxgyen——> metal oxide

25
Define enthalpy of combustion
The standard enthalpy of combustion is when one mole of a substance is fully burned in oxygen under standard conditions with all reactants and products under their standard state
26
Define Hess’s law
The enthalpy change in a reaction at constant pressure is independent of the route taken
27
Arrows for enthalpy of formation
The both face upwards so arrow one needs to be flipped
28
How is an exothermic reaction shown from a thermometer?
A rise in temperature
29
How is a endothermic reaction shown by a thermometer?
Temperature drops