Enthalpy&measuring change Flashcards Preview

Y12 AS Chemistry > Enthalpy&measuring change > Flashcards

Flashcards in Enthalpy&measuring change Deck (22)
Loading flashcards...
1
Q

What is enthalpy change?

A

The heat transferred in a reaction at a constant pressure

2
Q

What are units of enthalpy change?

A

kJ mol -1

3
Q

What are standard conditions?

A

100kPa

Stated temp usually 298K

4
Q

What is an exothermic reaction?

A

energy given out to surroundings, temperature goes up

5
Q

Is the enthalpy change negative or positive in an exothermic reaction?

A

Negative

6
Q

What is an example of an exothermic reaction?

A

Combustion

7
Q

What is an endothermic reaction?

A

energy taken in from surroundings, temperature falls

8
Q

Is the enthalpy change negative or positive in an endothermic reaction?

A

Positive

9
Q

What technique is used to measure enthalpy change?

A

Calorimetry

10
Q

How does calorimetry work to measure enthalpy change?

A

As the fuel burns, it heats the water and you can work out how much energy has been absorbed by the water if you know the mass, the temperature change and the specific heat capacity

11
Q

What two equations do you need to work out enthalpy change?

A

q=m x c x Δt

ΔH= q/n

12
Q

What is mass measured in?

A

g

13
Q

What is temperature measured in?

A

K

14
Q

What is the specific heat capacity?

A

4.18 j g-1 K-1

15
Q

What is ‘q’ in the equation?

A

Heat lost or gained in joules

16
Q

How do you convert q from joules into kilojoules?

A

divide by 1000

17
Q

What do you divide q by?

A

the number of moles of the fuel that caused the enthalpy change

18
Q

How do you calculate the number of moles?

A

Moles = Mass/Mr

19
Q

What would you put in front of the answer for enthalpy change if the temperature has gone up?

A

Negative symbol as it is an exothermic reaction

20
Q

What would you put in front of the answer for enthalpy change if the temperature has gone down?

A

Postive symbol as it is an endothermic reaction

21
Q

What are some experimental problems with calorimetry?

A
  • some heat absorbed by container

- some heat lost to surroundings

22
Q

What are some experimental problems which only apply to flammable liquid calorimetry?

A
  • Some combustion may be incomplete - less energy given out

- Some flammable liquid may escape by evaporation