Experiment 12- Hess' Law

What overall reaction do we want to study?

• Mg + H₂O--> MgO + H₂

• Mg + 2HCl ---> MgCl₂ + H₂

• MgO + 2HCl --> MgCl₂ + H₂O

For the reaction of interest Mg + H₂O--> MgO + H₂

How do we make the 2 reactions we perform add up to the reaction of interest

RXN 1: Mg + 2HCl ---> MgCl₂ + H₂

RXN 2: MgO + 2HCl --> MgCl₂ + H₂O

• Just add RXN 1 + 2

• Flip RXN 2 and add RXN 1

• Flip RXN 1 and add RXN 2

To make 450 mL of 1M HCl,

We need to dilute ____ mL of 3M HCl to 450mL

• 100

• 125

• 150

• 175

How do you find the moles of HCl used in this experiment

• It is equal to the amount of moles of Mg

• It is already given

• Convert the volume of HCl used to moles

What temperatures are you measuring? (Choose 2)

• The initial temperature of the HCl once you add in metal

• The final temperature: The temperature the thermometer reaches after resting for a bit (consistent temperature)

• The final temperature: The maximum temperature the thermometer reaches

• The initial temperature of the HCl before you add in metal

What values do you use to calculate Heat?

• Heat= (Sp heat _water)(mass _water)(ΔT)

• Heat= (Sp heat _water)(mass _mg)(ΔT)

• Heat= (Sp heat _mg)(mass _mg)(ΔT)

To calculate the mass of water from volume, we need to know

• It is given

• we need to weigh it

• 1g of water= 1mL of water

ΔH of reaction is found by taking heat and dividing it by:

• Moles of water

• Moles of mg

• Moles of HCl

• Moles of O₂