Enthalpy exercises help

[caliefleurette]

Hello everyone,

I'm very stuck on the following exercises... could someone help please?

1) Cu+(1/2)O₂-->CuO Enthalpy=-156

2Cu+(1/2)O₂​-->Cu₂O Enthalpy=-170

What is the value of the enthalpy of 2CuO-->Cu₂O+(1/2)O₂

I thought I could do +156-170 but it's not one of the possible choice

a) 142

b) 15

c)-15

d)-142

2) The mass M (in g) of a substance of specific heat capacity c (in J/g/K) increases by t°C. What is the heat change in J?

I'm very confused of how to do it, even though I think Q=M*CH2O*t°C is involved

Thanks!

[Emmi]

1. This question uses Hess's Law to find the overall enthalpy value. They give you the equation you want to use, 2CuO --> Cu₂O + 0.5O₂ and two starting equations. You can transform those first two equations into the desired one by reversing the order (reverse reaction instead of forward) or multiplying and dividing by a number. Note that whatever you do to the equation, you also have to do to the enthalpy value also. It can be summed up as:

- If you reverse the reaction, change the sign of the enthalpy value (positive to negative, or vice versa)

- If you multiply an equation by a value n, multiply the enthalpy value by that number

- If you divide an equation by a value n, divide the enthalpy value by that number

Try it out

2. Use the equation Q = m * Cp * t, although the specific heat isn't necessarily the specific heat of water. You have a mass m, a C, and a temperature change, so just add in your knowns, cancel units, and you should get an answer. Note that the specific heat has temperature in Kelvin, and your temperature change is in Celsius.

[caliefleurette]

1. This question uses Hess's Law to find the overall enthalpy value. They give you the equation you want to use, 2CuO --> Cu₂O + 0.5O₂ and two starting equations. You can transform those first two equations into the desired one by reversing the order (reverse reaction instead of forward) or multiplying and dividing by a number. Note that whatever you do to the equation, you also have to do to the enthalpy value also. It can be summed up as:

- If you reverse the reaction, change the sign of the enthalpy value (positive to negative, or vice versa)

- If you multiply an equation by a value n, multiply the enthalpy value by that number

- If you divide an equation by a value n, divide the enthalpy value by that number

Try it out

2. Use the equation Q = m * Cp * t, although the specific heat isn't necessarily the specific heat of water. You have a mass m, a C, and a temperature change, so just add in your knowns, cancel units, and you should get an answer. Note that the specific heat has temperature in Kelvin, and your temperature change is in Celsius.

Okay so for question 1 the answer would be -500?

[Emmi]

1. This question uses Hess's Law to find the overall enthalpy value. They give you the equation you want to use, 2CuO --> Cu₂O + 0.5O₂ and two starting equations. You can transform those first two equations into the desired one by reversing the order (reverse reaction instead of forward) or multiplying and dividing by a number. Note that whatever you do to the equation, you also have to do to the enthalpy value also. It can be summed up as:

- If you reverse the reaction, change the sign of the enthalpy value (positive to negative, or vice versa)

- If you multiply an equation by a value n, multiply the enthalpy value by that number

- If you divide an equation by a value n, divide the enthalpy value by that number

Try it out

2. Use the equation Q = m * Cp * t, although the specific heat isn't necessarily the specific heat of water. You have a mass m, a C, and a temperature change, so just add in your knowns, cancel units, and you should get an answer. Note that the specific heat has temperature in Kelvin, and your temperature change is in Celsius.

Okay so for question 1 the answer would be -500?

No, it should be 142. Check your math again, you might have forgotten to reverse a sign somewhere.

[caliefleurette]

I got it thanks!